Listen To Everything

The question - I know that cleaning up and smoothing out the exhaust port is very helpful, but what about opening it up? Does that change any of the port timing etc.?

The response - It depends on which direction you go grinding. If you open the port upward, then yes, you are changing the timing. The downward traveling piston crown will by-pass the hole sooner and on it's upward swing, it will close the port later - exactly what is meant by the word duration. This will move the powerband's strength to higher rpms. But the pipe really does all the work, so make sure the pipe will gladly "accept" these new timings. The general rule is to pipe to the port, not port to the pipe. But it can still be done. If you don't have the correct tools (Foredom,etc.) to cut on the transfers (the angle MUST not be screwed up) then don't go blindly bringing the exhaust port to much, if at all, taller. The exhaust may already be high enough in relation to the transfers and all you will end up doing is narrowing your powerband. Now, if you simply widen the exhaust port, you will surely gain - especially upper-mid/top. But, care needs to be taken here. If you go too wide, there will be some of the exhaust port open when the piston is at TDC. If you look at the piston, you will notice that the sleeve squares off on each side around the gudgeon pin. The best way I have found to find max width is to very lightly scribe the piston. With the cylinder bolted down over the assembled piston/crank/motor, and looking into exhaust - use a scratch awl or nail, and as the piston travels upward/downward while you are rotating the crank, scribe the piston as far left and as far right as possible. Now disassemble and measure the distance from the scribe mark to the flat edge on the piston's skirt. Subtract about 1.8 mm - 2.4 mm and this is how far you can cut width-wise. Good luck and remember: when in doubt - don't do it!   Well I have another 5 minutes, so here goes. I would hesitate to simply opening the port between the cylinder sleeve window and exhaust manifold. A smaller passage keeps the exhaust gases hotter and gases flow much easier and readily when hotter. However, there is a threshold here - so don't think that the smaller the hole / the better. The secret is to finding the "primo" size. Some pipes even come with "stuffers" that actually make the exhaust manifold a little smaller. You would be better off just making sure there are no kinks or tight bends. If you notice some carbon buildup in a particular area in the exhaust port-manifold, you can rest assured that exhaust is not flowing well in that area. Eddying of gases like this hampers flow so try you best to remove the obstacle, or tight bend that is causing it. Trax310   Some cylinders have an exhaust port bridge (TRX 250R and others); a separation in the port used to make the port wider overall. Because of the bridge, exhaust port widths can become wider than the old rule of thumb which was usually 70% of bore size - maximum. In an engine with a single large exhaust port (Banshee or Blaster and others) it is more important to pay attention to this ratio. That bridge performs a couple of duties - some good and some bad. While it allows a wider port overall, it also adds a restriction in the port, reducing the effective size - kind of a catch 22, though its benefit outweighs its disadvantages. It lets the port area be largely increased. Primarily it is there to support the piston and more importantly the piston ring. Without it the ring would surely bulge into the port too far and snap the ring.   Narrowing the bridge to increase the effective port size can be helpful but it too has drawbacks. Since this area of the port receives the first blast of hot gasses, the narrower bridge can swell into the cylinder sooner and cause a seize in that area. Giving the bridge some additional clearance will make this not happen as much. Use rubber abrasives and a inside diameter micrometer to check its clearance. An additional .001" to .002" in the bridge area ONLY will do what's needed. It is probably best to widen an exhaust port by working the outside of the port walls before working the bridge. Many times just cleaning the casting flaws on the bridge is good enough. The final finish of the part of the port which extends outward from the liner to the exhaust flange should be smooth but it is not necessary to polish it unless you have extra time. Very little benefit will be realized from a micro-shine. Fine grit rubber abrasives serve this purpose well.   The shape of the roof of the port can effect the powerband too. Exhaust ports with a flat top - straight across - will pop open and give a more peaky power delivery. Ports that are more rounded at the top will open slower and smooth out the delivery somewhat and are generally favored.   When working with exhaust ports with a bridge, consider the two ports as one - that is - if you're going to make the top rounded, make the right port low on the right side and higher toward the left side and make the left port low on the left side and higher toward the right side. I'm explaining this as though I'm inside the cylinder looking out the exhaust port with the barrel in the standard upright position.   Bridged ports can be widened MUCH further than 70% of bore. How much exactly depends on how well the port bevels are created and how well the bridge is clearanced. It is not impossible to widen them to 85-95% of bore. This kind of width puts the edges of the exhaust port well over the top of the main transfer ports.   The same shape rules are true for single round exhaust ports. Give them a flat top and you get peaky power, make them rounder and you get a smoother delivery. Be sure to give the port a nice bevel so the ring can be gently guided back into its ring land. No matter what the port width - all rings bulge into their respective ports.   It has been said that simply rounding the cylinder liner to the size of the port you would normally cut it to can be of almost the same benefit. That is - cutting the liner with a very round radius to create attachment of the gasses to the cylinder wall. Doing so creates improved flow and keeps port velocity high.   Many times in motors that have larger pistons installed or big bore kits installed - with larger sleeves as well, it is impossible to cut the exhaust port high enough to give the motor the same port timing that the stock motor had. This is because the exhaust port is angled downwards. When you bore out a cylinder you lower its port and therefore its port timing - and raised the cylinder compression. This also includes transfer port timing - they too will be reduced a bit because of their angle to the bore.   In extreme cases there is the possibility of cutting through the top of the port - into the water jacket - if the exhaust port roof is raised too far. In these cases raising it to a safe height, then adding a large radius on the top edge of the port - the cylinder liner only - can produce the desired timing effect needed. It seems to work very well.   Like Trax310 said - it is always a good idea to scribe lines in port walls to give indication of where to cut to. Using machinists dye can make the lines more visible too. With the piston installed on the bottom end and the barrel placed on - without the rings - you can also scribe the bridge (if it has one). This allows you to drill oil lubricating holes in the piston directly in the area of the bridge - a good idea on motors that have exhaust port modifications.   Some new pistons come this way; Wiseco and some Pro-X, but others don't. Even with the cast Pro-X pistons it won't hurt to add a couple of oil holes here. The first one should be located about 1/2" down from the piston edge and a second 1/2" below that. Be sure to take your time, locate the hole carefully by lightly punching the piston with a sharp tool to center the drill. You don't want to drill a hole in the piston which will let crankcase mixture out into the port when they are crossing the bridge area. They are to be located directly and completely within the bridge area.   Consider this about raising the exhaust port roof - there must be about 30 to 35 degrees of blowdown - the duration the exhaust port is open all by itself - to make sure there the exhaust can escape out of the port and to allow an adequate pressure drop so the transfer ports can let in the new mixture. The earlier the exhaust port opens, the hotter the gasses are that cross the leading edge of the piston. This would make the piston hotter which could lead to problems with detonation. Care must be taken to not create more heat that necessary by widening first, then raising.   When raising the exhaust port the static compression of the motor is lowered. Since there is less volume to compress - it will be somewhat lower. This can be determined by doing a compression test on the motor before and after the exhaust port height has been cut. You can also calculate this figure using some simple math - though real world results vary because of the variables involved.   To get back the compression which was lost during porting it is necessary to reduce the volume of the head. Without doing this step you can not fully realize the effects of the new port timing. Consider it a necessary step. It is most important to get the squish band clearance right. Do it first. Depending of the motor it is usually between .030" and .040". When that is right, work on the dome to make the volume correct.   Additionally - when the exhaust port is raised the effective power stroke is reduced. Even a few degrees here can make a substantial difference. Considering that a 2 stroke motor delivers all of its power in about 85 to 95 degrees of crank duration - as compared to a 4 stroke where there is almost a complete stroke to accomplish this. Fortunately the 2 stroke will recover its loss by making 2 power strokes in 2 crankshaft revolutions equaling about 180 degrees of duration.   C.C. Specialty in Lawrenceburg TN (615-762-6995) sells all the appropriate porting tools - including the Foredom brand Trax310 mentioned. That's where I got mine. I have not seen them selling the diamond impregnated cutting tools though. The Dremel company makes a line on diamond tools which produce an excellent finish on port walls. I use these tools on the intake and transfer ports only. It gives them a "tooth" without making them unnecessarily rough. All of the major cutting must be done prior to using them though since they don't remove material that fast. A light touch with constant movement of the tool creates a wonderful finish. Expect to pay about $10 - $15 for each one of these 3/16" diameter 2" long pieces. They come in various head shapes and a couple of them are also great for beveling the ports prior to honing the cylinder. I can only complain that they could be a little longer to reach further into transfer ports. I don't think Dremel thought that engine builders would discover these perfect for use in this kind of work. With the appropriate hand piece I am always able to get where I need with them - even though they are short.   I always go over the port bevels with 400 or 600 grit wet/dry paper with WD-40 on it or fine grit rubber abrasives before I'm done. I hate the thought of a scratch being created on a piston or ring because of a little burr I could not see or feel.  Rick   Master, you must be exhausted (no pun) after that dissertation, and I hate to even question your knowledge. However, I always hear of polishing after porting the intake, but you mentioned the following:   "I use these tools on the intake and transfer ports only. It gives them a "tooth" without making them unnecessarily rough."   Did you polish after you ported? If not, why? Trying to be a good student, master. Thanks!! hehe   Grasshopper       I think that Trax and Rick will both concur that polishing of the intake and transfer passages is essentially an utter waste of time. Polishing of the piston crown, exhaust port and combustion chamber will marginally reduce surface area and thus heat absorption and carbon accumulation... but it is still high effort, lots of time for micro benefits. Chiefly a cosmetic endeavor. Ceramic coatings will reduce heat absorption and improve combustion chamber burn to a much higher level.... and even those are only minutely beneficial in extreme applications. Time can be much better spent insuring accurate and symmetrical metal removal yielding areas and angles that match the entire target of the whole engine package, intake and exhaust system. I leave an 80X finish on intake and transfers and believe, if anything, it aids fuel atomization which is extremely important to maintaining better combustion efficiency and fuel economy. Effective porting alterations are keyed to dimensional accuracy and integrity. High luster finishes show virtually no improvement on a flow bench.   High Output       Grasshopper - Please DO question anything at all! Yes High Output - you hit it right on the head. While polished pieces look great there must be a valuable reason to do so. I have seen no difference and therefore do not polish those pieces even in my own engines. As far as ceramic coatings go - they do have a place... and that place would be IN PLACE of polishing. Coatings on the piston crown and combustion chamber act as insulation against heat and allow you to run leaner mixtures, higher compression and perhaps more timing. This will make more power than the same pieces either polished or natural.   One of the keys to great power is turbulence in the combustion chamber - this will promote a better fuel/air mixture and a more complete burn. The best way to achieve this is to correctly set the squish band areas depth and width. For those of you who want to learn a little bit more about squish, please read the article "The Art of Squishing Things Till They Give (Power)" a 3 part series by Dale Alexander. It is a very good article.   http://www.gs.cornell.edu/mikeh/rdheat/RD-heat1.htm   Rick     High Output, I just always seem to agree with you. I guess that's the safest and surest way to go though! Grasshopper, hehe, on exhaust gas, we want to get rid of these gases fast and can care less how they get there, so by smoothing out everything associated with exhaust, better air flow is netted and the exhaust system benefits. On the intake side of things, we want the gas to mix into the air and become as small as possible (atomize). So, from the time the air/gas mixture forms (from gas pouring out of a small hole in the carb into a stream of air going by laterally) until it reaches the piston, we want the air & gas to be stirring/mixing and turbulence can help with this. So, by leaving the intake side of things a little rough, mixing is enhanced. Now, that's about as basic sounding as I am allowed to make it.   On the polishing thing. While polishing does nothing really to enhance horses, scratches do account for more surface area. So if the exhaust is scratched deeply and severely, although still smooth, more heat will be absorbed into the cylinder because of the added surface area that the heat is applied to. And we don't want the heat in the cylinder - we want it in the pipe.   Trax310     Really well written and explained, guys! The only thing I would add is that, in my humble opinion and experience, I find it is not always necessary (or desirable) to increase the static compression ratio after raising an exhaust port. Yes, after only raising an exhaust port and then viewing the compression on a pressure gage at kickover speeds, the indicated pressure will drop if the "trapped volume" at TDC has not been further reduced. And yes, when you do the math and calculate the "corrected" compression ratio (volume at exhaust port closing vs. volume at TDC) the ratio is reduced when the exhaust port is raised without also reducing chamber volume. The problem with this is that the higher exhaust port will raise the powerband and the engine speed at which trapping efficiency once again rises to an equivalent level before the port was raised. IF the port timing increase is beneficial and was chosen wisely, then the actual VOLUME of air trapped at the now higher engine speed will be increased due to a "ram" or mild supercharging effect. The result CAN BE a much higher dynamically trapped volume over a now narrower and higher engine RPM range. Thus the dynamic compression ratio over a narrow high RPM range can be increased even though the static "corrected" compression ratio has mathematically DECREASED. This means that while an 8.5:1 "corrected" compression ratio (for example) may be safe in a motor with a 180° duration exhaust peaking at, let's say, 8200 RPM.... but it may be far too high to preclude detonation in the same motor with a 205° exhaust duration peaking at 10,500 RPM. This is the reason I plan compression ratios using the "uncorrected" method (volume at BDC vs. volume at TDC) and have devised an accompanying exhaust duration/squish velocity/bore area/RPM chart for planning what UCCR is appropriate for the application. I use cranking pressure readings only as a reference to detect when optimum ring seal has been achieved (break-in complete) and as sort of a "blood pressure" check periodically to detect when something is failing. And on that note, I feel that squish area vs. bore area, angle of divergence (with respect to piston crown... which is most often a radius, further complicating things...), squish actual angle and the mean velocity that they will generate at target RPM's are all very critical in determining what squish clearance is best without generating too little or too much squish and combustion chamber pocket turbulence. That's my two cents, for what it's worth... Great discussion guys! Thanks for the thought provocation!   High Output       Compression is one of the things that comes as FREE power. If the exhaust pipe is tuned correctly there will be an increase in volume trapped - hopefully at the target RPM. However, like you said, increasing the exhaust port time/area would increase the trapping efficiency at a higher RPM over a narrower RPM range.   Increased compression produces increased power. This is a static relationship. Friction not included - if you raise the compression of an engine 5%, the motor will notice a power increase of 5% - increase the compression 10%, and the power level increases 10%. This power will be available throughout the whole RPM range - making more power available above and below the target RPM. Having said that - reducing compression the same amount will reduce the power by the same factor.   When raising the exhaust port - the compression lost must be regained. On mild port jobs this may not be as noticeable or even needed. But on motors where everything matters - it is a MUST DO. It is entirely possible that without changing the compression back to the original it will make the motor produce its peak power at a higher RPM only - and NOT INCREASE its peak power at all because the power stroke is shortened there less available thrust.   I have found it worthwhile to check the static compression of engines - like you do - to find out its relative state. Additionally it is necessary to calculate it very closely in order to help create the target purpose the engine will used for.   I have found it wise to cut some of the crown off the piston tops and instead give them an angular cut in the squish area. Them finish off its top by lathing it flat. I use a synthetic or real diamond tool for this procedure. The final product is of much higher quality than any metal tool can produce. This also lightens the piece by a small amount.     Since most of the heat contained in the piston is transferred to the cylinder walls through the piston rings it is wise to not over wear the rings and check the compression regularly. Replace them when ever compression drops 5-7%. This goes deeper into engine building than many people go, but everything must be figured into the equation. Add to this an angular squish on the dome and what is created is a constant equal distance squish area with less trapped unburned gasses floating around which will not ignite. Squish is probably the single most important area of combustion chamber creation and it contributes more to creating power than perhaps any other single modification made to a motor.   I have also made a few flat top TRX motors and a few flat top Banshee motors. The TRX pistons can be bought (CR250 parts) but the Banshee pieces have to be made. Flat top piston motors have to have their heads completely reworked since it is so different than what they came with. I usually cut the squish on the dome to a 3 or 4 degree angle - depending on the bore size. Quite a bit of material has to be removed from the gasket surface of the head to accomplish this. It is easier to use Pro-Design Cool Heads for this since I can order the domes blank and cut them myself, though they will cut them for a few bucks more.   Flat top pistons have a greater mechanical efficiency over crowned top pistons. They aid in creating a better fill space for the incoming charge and promote mixture attachment - which is favored over just letting the mixture fly out in the dead air space. Since the CR250 pieces are of the single ring design - more maintenance is required. More ring changes and more careful inspection of the piston to bore clearances.   Here's one thing that bugs me about people after they get motor modifications done:   Many people, after getting their engine work done, lower (higher number) their final drive gear ratio - because they think the lower gearing will move them faster - this is widely accepted in the automobile drag racing scene. In most cases this is not indicated for bike or ATV motors - and actually is opposite of what should be done.   If the stock bike had a 13 front sprocket and a 38 rear (2.92 ratio), they would put on a 12 in the front (3.16) or even 11 (3.45). Sometimes additionally increasing the rear to say 39 (3.54). When in fact the gearing should have gone the other way to perhaps a 14 on the front (2.71) and maybe down a tooth on the rear (2.64). To take advantage of an engine that is producing more power you must gear accordingly. It has been my experience that along with increased port timing and compression switching to a higher (lower number) final drive ratio moves the bike faster. There will be fewer shifts and it allows the motor to pull the bike through the gears.   It is better to under-rev an engine between gear shifts while drag racing than it is to over-rev it and miss the power band. The idea here is to completely use the power range available. While it may sound very fast to be at the top of the RPM range all the time - there will be no increased acceleration if the motor is not shifted at the proper time. That time is at the RPM where the motor "comes on the pipe" to just before it falls off - without overrevving it except perhaps in top gear. Let the motor do the work. It takes practice to get it right. Once dialed in and shifting correctly the bike will move more swiftly with less effort on the drivers part. Rick     I just want to comment on the compression thing. I know that you, MacDizzy, stated that the compression MUST be upped when the exhaust is raised. However, I tend to agree with High Output, (and I also follow the UCCR method) - when a motor is ported for hi-rev, one of the sure things to do is raise the exhaust port. However, when the pipe works with the port, the returning surge wave from the pipe will force some exhaust/unburned escaped mixture back into the cylinder just prior to the piston's closing off the port. The net result is that when the squish occurs and "on the pipe", the compression will probably be even higher than before it was ported. So, if the compression was brought up after being ported and detonation is not occurring, then it is just luck on your side, and the motor was even further away from detonating in its "before ported" state of tune. I'm sure that you also know that on two-stroke motor, it is able to compress more volume than the total swept volume - almost twice as much in extreme cases - all due to the supercharging effect that the return wave produces. And these extreme cases is when it is ported for VERY hi-rev. Sure, when not on the pipe, you will lose performance as compared to "before ported", because there is not only a loss of "power distance" in the stroke, but there is also more time for an "untimed" piston closing/returning wave to breathe back down out of the cylinder. But that's a price you pay for flying on the other end. Also, while I am chatting here... On the squish angle thing. Have you dyno'd the difference between simply cutting the entire crown of the piston to the 3-4 degree angle as opposed to cutting just the squish area of the piston to 3-4 degrees followed by flattening the rest of the crown inside this barrier? The dome, of course, being cut to the 3-4 degree squish band and clearance in the .035" range for both. Thanks for the input...and keep up the GREAT posts!   Trax310     I am not a fan of peaky power over narrow ranges. I find that kind of motor useless for my purposes. Therefore the opinions I express tend to lean toward creating engines using a lose less/gain more at the same time philosophy.   My figures show a 1986 TRX motor to have 178 to 180 degrees of exhaust duration - stock. Lets do some Bench racing. Make a TRX motor without increasing the exhaust port area. Only increase the static compression and set its squish properly (say .035"). Increase compression to the max you can (say 225-240 PSI) using race gas (say 110 octane) without the onset of detonation. Gear the bike for this change. Give the motor a torque type pipe. No other mods.   Make another TRX motor with 194 degrees of exhaust duration, but don't widen the port. Raise it only. Make no changes to its static compression - don't bring it back from what was lost during the port height change. Set the squish properly to the same as the motor above. Let it run the same race gas as above. Gear the bike for this change. Give the motor the same torque type pipe. No other mods.   Forget the dyno. Take them to a sand hill and race them up it. Who wins? Why? Take them to a flat sand drag and race them through top gear. Who wins? Why? Take them to a flat track and race them around it. Who wins? Why?   Flat top pistons seem to work best when the dome is cut to 3 - 4 degrees. This makes a small wedge in that area. It is possible to make this area parallel but I feel its necessary to create a small positive angle there, just like crowned pistons. You always want to create a positive angle to direct the charge at TDC. The reason I don't cut the angle across the whole piston crown is twofold. First I can make the part lighter. And second a flat top piston has greater mechanical efficiency. I should dyno these things - but like everything - time has to permit...   Rick       MacDizzy, have we gotten deep enough into this topic yet or what ?:) I just love talking this stuff! On your two motors in the example - that's very tricky because you said same torque pipe on both. The problem is that the standard DG, FMF, torque pipes etc. are aimed more towards a stock cylinder. So, when mildly ported and bumped compression, a 250R will fly. But, when severe porting is done, there is really no pipe out there on the market made for this - although we all try to buy a pipe to meet our ports. So, you are ABSOLUTELY right. If you go cutting the exhaust upward and put a low-revving pipe on, you will surely slow down. However, put a high revving pipe on, and see what happens. Take 4 examples. Take your two, then take your two again but replace the pipe with a hi-rev more aimed at the raised exhaust motor. I would bet my bike on the later hi-rev pipe on the raised exhaust in a drag. I drag race every weekend and have been there / done that. I had a LRD super-cool adjustable pipe on my stock WAY hi-rev ported cylinder(never bored). It seemed slow until I bumped the compression to 255 lb. (at below sea level myself). The thing was awesome and I won a few reputable races! But, when I got the Whale pipe and did some minor mods to the pipe, I was detonating even on C12. Why? Hmmmm... I did NOTHING at all to the motor, but now I am detonating! I later figured out that it was because the Whale pipe was more in tune with my port job. Like I said in a previous post - always pipe to the port. If you run a long torque pipe with a hi-rev port, the return pulse wave will NEVER make it back in time to be trapped in the cylinder upon the piston's closing it. I'm gonna stop here. But, this is interesting conversation. Your turn....   Trax310     I'll still take issue with that. I'll give you the high revving pipe. Have a pipe custom made just for this setup. Give it MORE exhaust duration if you wish. Just give me what I listed above as increasing compression, perfecting squish and making gear changes only.   Like I said my figures show the 86 TRX to have 178 to 180 degrees of exhaust duration stock. It also has 30 degrees of blow down stock. Additionally it has about 165 lb. of cranking compression stock. And 13/39 gears (3.00). If you cut the exhaust to 194 degrees you'll likely lose 10 to 12 PSI static compression or more. That would put it around 150 PSI static - UGH! Not to mention the blow down would be FAR to long - 37 degrees? UGH!   How can the custom pipe make up that much of a difference? It can send the pressure waves at the right time and push everything back like it should but it won't be able to take full advantage of the race gas because it has to be compressed higher that what the pipe alone can induce to ignite. It can not possibly rev high enough to allow that many more power pulses to make up the difference - the motor won't rev over about 9500 no matter what you do. The ignition is too limited. It's BMEP will be lower. I doubt any gear change would help out much except perhaps on a flat smooth track.   Though the stock bike with its (lame) 178-180 degrees of exhaust with 225-240 PSI static compression and a perfect squish is producing more work per stroke, has a higher BMEP and is able to raise the gear ratio (to 13/37 [2.84] or 13/36 [2.76] - easily and more likely to 14/37 [2.64]) to take advantage of a flat, wide power spread. A motor like this will still turn 8300 RPM.   Like I said before - increasing static compression is FREE power. I know you're saying you can make that compression up with a pipe that is tuned that way, but I'm saying that it is of more benefit to have it built in to the motor and not have to depend on the pipe for it. It's nice to get a boost from a pipe but no pipe can make this motor have the same BMEP at its peak HP that equals the BMEP the higher static/lower exhaust timed motor has. It simply started out way too weak. Rick     I agree with you as to which one would be faster, Rick, but I don't understand why you think anyone with even a basic understanding would build an engine in the manner which you describe for the "high revver". Why would a person just raise an exhaust port and not widen it, too? (Actually, first!) Why would a person raise an exhaust port and not alter transfer area, duration and quite possibly direction of entry into the upper cylinder as well? Otherwise, of course the blowdown period would be excessive... And certainly the compression ratio should be raised to a somewhat higher level than before taking into consideration pipe efficiency, tuned length and diffuser/baffle included angles of divergence/convergence. I certainly didn't mean to suggest earlier that a stock compression ratio should be utilized with a longer duration exhaust.... I just find the "trapped" method quite misleading and not easily used when considering all affecting items. And why would you try to use a torque pipe with its' typically mellow angles and longer tuned length with greater than standard exhaust duration? (unless it was significantly deficient in the first place...) Furthermore, an ignition change to compliment and enable the high rev package would be mandatory (and I know you know that). The whole package you describe is mismatched. Your recipe for the standard ported cylinder is excellent and works well, but the high rev conglomeration is way off. Who builds motors like that with any success? And increases in static compression are not "free power"... They come at the cost of increased pumping losses and considerable increases in heat generation such that a 10% increase in compression will net only 2 or 3% increase in power at upper RPM's. The biggest benefit will be to the low and middle RPM's where, ideally, you may see 4-6% IF all other engine parameters are optimized. Lastly, I was under the impression that flywheels rotate rather than reciprocate. Not trying to start a "peeing match" and greatly enjoy your contributions... I just respectfully disagree.   High Output     Oops - Thank you for pointing that out, yes the flywheels DO go round and round, not back and forth...   You're right, increased compression has losses associated with it. It will do all the things you mentioned - and more. The reason I call it "free" is because there is no other modification I can think of that will boost power throughout the engines whole operating range as easily, cheaply or dramatically. Dollar for dollar it may be the cheapest way to boost power.   Perhaps my biggest gripe with high compression is getting the engine to carburet properly. I'll take the additional heat, frictional losses, pumping deficit and shortened bearing life when shooting for maximum power. Overcoming the characteristics of radical internal modifications become a normal procedure and can be easily overcome when given proper attention to.   I made the 2 bench racing engine examples as similar as I could - I should have given the high rev engine a high rev pipe right from the start, but it probably wouldn't matter anyway. There were too many things wrong with that motor package. The whole point of the bench racing example was to make what you noticed obvious. It IS a terrible combination, and I hoped it looked that way. I wanted to point out how silly it is to only work one part of the motor (in this case the exhaust port) and do almost no other modifications to it. You pointed out exactly what the problems were - and then some. All the areas of an engine have to be working together to work well. No one should build an engine like the high-rev model I described. But I have seen it done.   Rick     Wow, I just read your and High Outputs posts below. Nice posts gentlemen!!! However, I have been desperately seeking someone who knows a bunch about 250R's and it seems that you do. I've got an invaluable source for motor work and tuning, but need a good source for chassis and gearing setup. Here's what caught my eye. You mentioned that when motor work is done gearing changes are required, towards a lower ratio. Currently I'm running a 13/41 (3.00) and tried switching to a 14/41, for a final ratio of I think (w/o the chart I'm guessing) 2.78 It's a dog off the line, the sand was wet, as it always is on the Oregon coast in Nov. But here's the dilemma I launch in 2nd gear with the 3.00 setup, my as tuned peak is at 7950, and I can't get out of 2nd gear fast enough before it falls off the top (no ext. swing arm, I'm on the tank, but gets squirley sometimes), whereas I can't always get a good 3rd gear launch. Do you have any suggestions???? Is there any advantage of running larger sprockets for more mech. advantage. but with the same final ratio. Thanks for any help.   I thought that perhaps I should add in a few other factors. First, the nut behind the wheel isn't too light 6'4" 250 lb.. My usable power kicks in around 5750 rpms with, as stated, the as tuned "now" peak at 7950, with somewhat decent overrev to about low 8100's, any more it stays flat but rev's to 8400. Ultimately my port grinder and I wish to see my peak at about 8400-8600, with a custom pipe. I'm working with a FMF hi-rev H-26 pipe, but have got my eye on the CT Whale, I've got Trax310 measureing his so that we're able to assess it to see how it's characteristics are as it pertains to my motor. We've got plans to whack up the FMF, but tuning weather is sparse this time of year in western WA. you can email me or post back here whichever, I check it all about three times a day. Thanks.!!   Backcountry     Thanks, I have found the conversations interesting and informative. Obviously there is more than one way to make HP! Actually I said that switching to a lower gear ratio is INCORRECT. What is needed is a higher final gear ratio. While I'm thinking of it please see my final gear ratio chart on my glamis web page - http://www.macdizzy.com/glamis.htm - you'll need to scroll down the page a bit, but it's there. A 13/41 is a 3.15 final ratio. That is lower than stock which was either 13/39 (3.00) for the 1986 TRX or 13/38 (2.92) for the later model years. The 14/41(2.92) you switched to is the same ratio as 13/38 (exactly).   With my Pro-X 310 kit (73.5 mm bore) and 14/38 (2.71) or 14/37 (2.64) gearing I am able to get great 3rd gear launches and I only have to shift 2 times up the hill. At the top I'm pulling 8500 RPM in 5th (its peak power is at 8100 RPM).   With my 270 cc and 250 cc Honda barrel motors (both produce peak power at 8300 RPM) I can use either 13/38, 13/37 or 13/36 (depending on the sand moisture content) and still get a great 2nd gear launch but I am unable to launch it successfully in 3rd with any of those ratios. I would attribute this to less displacement and a little less compression. When I reach the top I'm turning 9300 RPM but I'm only in 4th gear - it will not pull 5th gear well enough. This is because of the slightly lower compression and lighter engine parts. The piston alone is quite lighter.   Now, which is faster up the hill? Though the 310 is in 5th gear at 8500 rpm, and the 270 in is 4th at 9300, the 310 is quicker to the top because of the superior launch. Since 3rd gear is a little taller than 2nd I'm, able to keep it in 3rd longer, and when I make the switch to 4th, the RPM drop is only about 1200 RPM therefore lowering it enough to be within the sweet spot of the powerband again. When I click into 5th, the RPM's drop again enough to pull hard though it won't wind out on top like the smaller motors.   Sometimes if there is a bogging problem with launching or when driving up the hill increasing the pressure of the rear tires can make them slip a bit more. This can be just enough to make the motor respond more correctly. Anytime I'm dialing in a gearing combination I always play with tire pressure first. If the pressure is too low there can be too much bite and this can make selecting the proper gearing difficult.   It sounds like your motor is right on the edge of letting you launch in third. If you have to change into 3rd very quickly you are very close to where you want to be and with a little tuning you should be able to get it to happen.   If increasing the tire pressure does nothing to help you there, you might want to increase the static compression of the motor a bit. Race gas may be necessary. I don't know the facts of your motor so I'm guessing here. I have good success starting with a static compression of between 205 and 225 PSI. That is high compared to what a lot of people run, but I always beat them up the hills so - what to say?!? That higher compression hurts the high RPM's a bit but it more than makes up for it with superior launching and pull power when shifting gears up the hill.   Rick     Yes - more information is always important! Please refresh me about your motors specs when you get a chance. Include the model year of the motor, its CC's and all the port timing and static compression. Also include any additional pieces that matter.   After looking at the RPM range you state your motor operates best within it leads me to believe that it is very close to stock. The unmodified TRX motor peaks at 7500 and revs out to about 8350. Though using that area between 7500 and 8350 is (almost) useless - there is simply not much power there. Targeting the peak RPM at 8400-8600 is good - though maybe perhaps a little high for certain types of riding/racing. Flat track or ice racing motors would do best in that range. Since they are always on the pipe and in a high gear, it seems to work well. They don't have to worry about falling off the power band because they never get the motors down that far.   It has been my experience that for sand hill racing and flat sand drag racing a wider power band is more desirable. That would make the motor peak at about 8200-8300 with over rev to 9000 or a little more. Because sand is such a high drag medium, motors need to be made to account for this.   The H-26 pipe (I have one on a shelf in my garage - though used it for quite a long time) is OK, allows some overrev but then falls flat on its face. It hits pretty well when it comes on but does not hit as soon or as hard as the H-49 which I use now. I also have a CT midrange and a Trinity midrange pipe sitting idle collecting dust. Dyno tested the H-49 out performs the other pipes, but they were tested on the 270 cc motor only. I would think that perhaps it would do as well with the 250 cc setup, but on the 310 cc motor a pipe built for its larger displacement should allow superior results. Having said that, the 310 still makes over 5 more HP than the 270 which makes 5 more than the 250.   I have not tested the CT-Whale pipe. Though I have raced and beaten several TRX's with them attached. The only TRX I can remember being beat by was using the Whale during a flat land sand drag. I thought it was VERY unusual that I would beat this guy all the way through 5th gear topped out - then he would pull 4 to 6 bike lengths on me when I switched into 6th. If I get beaten by someone while racing I like to talk to them to pick their brain. When I talked to this guy I immediately noticed a braided steel line running along the frame rail and entering his cooler on his rack. He had nitrous. As usual he told me he wasn't using it because he was out of nitrous (no one admits to using the laughing gas). I challenged him to a few more races and smoked him by more than the amount he beat me by. I knew by then he WAS out of the nitrous.   I wouldn't get the Whale for one simple reason - it sticks out so far that when you kick start the motor your foot hits the pipe. When the pipe is hot it melts your boot sole to it and it's very difficult to remove that baked on rubber. I know that's a pretty lame reason to not buy a product, but my quad is not built as a single purpose machine. Though I have it set up to produce high power, it's still tame enough to ride in the dunes and enjoy on long trips.   As far as your setup in concerned, you may find - because of your weight - added displacement is just the ticket. If you're at a 66 mm bore or there about, why not move up to 69 or 69.5 mm (69.5 mm is the largest I recommend to bore into the stock liner). Even going out to 68.5 mm leaves you 3 over bore sizes to go to. Tuned properly the Pro-X (310 cc - 72 mm) cylinder and head setup can really make a TRX fast. Though just letting some company throw one together for you is not going to get you to the finish line first. It is not in THEIR best interest to let someone beat THEM! I bought one a few years ago when they first came out - I think the number stamped on it is 34. They have improved the casting of them since then by putting more metal in the area around the rear bolt holes to make them stronger.   The 250 cc motor (66.25 mm) I rode/raced at Glamis this past Thanksgiving smoked more Pro-X barrel motors than ever. I didn't know they had become so popular - they seemed to be everywhere. Some of the people I talked to were at 330 cc (76 mm) and a couple of them had stroked cranks as well ($900 - $1400 were the prices people were telling me they paid for their top ends and more for the stroker motors). No one I beat believed I was at 66.25 mm bore. Though it does mess with their heads to see a stock Honda barrel on my motor. Even with the Pro-Design head there it doesn't seem to make sense to them.   If I'm going to give an engine builder the kind of money to make a motor fast - it better win races! It had been my experience that letting other companies build my engines wasn't good enough. So I chose to learn about these things myself and build them the way I wanted to. These days I refuse to let ANYONE else do work on my motors.   This past spring I dyno'd a 250 cc motor at Trinity. Harry was operating it. I had to wait for a guy with a ATC 3 wheeler with a 500 cc CR motor in it finish his runs. I was very impressed with the 78 rear wheel HP he got. The bikes owner said he thought it was a little low. I raised my eyebrows...   Harry was more impressed with the HP my 250 cc TRX had made. So much so he offered to buy the motor from me. I refused and told him that I enjoyed smoking his and other engine builders Banshees way too much to part with this combination. He then offered to buy just the porting specs - I declined. The only thing I told him was that there was a lot of epoxy in the motor and there were a couple more holes drilled in the cylinder to help cylinder filling. He raised his eyebrows...   The 270 cc motor specs I use are posted on my web page - and that is what they actually are. I always log the data in my FileMaker Pro database so I can locate it easily. I have not posted 250 cc or 310 cc specs there since much of it is the same and the 270 is my favorite motor.   The epoxy and the additional holes are different. I refuse to give that info out [Click Here to view it - I changed my mind- Rick] - except to say that when Paul Turner was still in Santa Cruz and actually building motors he did some things to the transfer ports that interested me in a BIG way. He was on to something. Though instead of epoxy he had plastic inserts he glued in place and ported the motors around that.   My 2 cents - for a wide power band AND additional high RPM power make the transfer ports have a very smooth arc to them. This means removing no material from the part of the barrel that is in contact with the cylinder liner. This also means removing material from the outside of the transfer port - but you don't want to see daylight!   Then, make the transfer port roofs are very flat. The front (main) transfer ports have an area of metal that must be removed to make these ports aim as far back in the cylinder as possible. That area is between the exhaust port and the original main port wall. Early TRX barrels had little done to them from the factory to make them aim back. Later model TRX barrels (1988 on and perhaps 1987 but I can remember) had much of this material removed and the port wall was aimed pretty well but still could be improved upon. The later model barrels also had additional exhaust port timing though the transfer port timing was still 120 degrees open (too short).   My 250 cc motor (66.25) produces 20 HP @ 5000 RPM, 29 HP @ 6000 RPM, 48 @ 7000 RPM, 52 @ 8000 RPM, peaks with 53 @ 8250 and has 50 @ 9000. There it drops off to 40 @ 9400 RPM. The way this looks on paper shows it to have 40 HP or over from 6600 RPM through 9400 RPM. Keep in mind a stock TRX makes about 32-34 HP peak at 7500.   Rick     Well, that was quite the post. I think that I've been very fortunate, first off I found a person to do more motor work for me here( and by accident), not to mention his tuning knowledge!!, at the BBS and a few other people that share my love for the 250R's. I've got an '86, but with an '89 rod. You're close on the cranking pressure. My cranking pressure is 240 static. 14.5:1 UCCR. Squish width is .315 ( maybe slightly more could be .35), cut at 12 deg., with clearance at .048", I require a min. of 104 octane (r+m/2) to keep it quiet. 38 PWK a/s. RAD valve carbon fiber reeds. MSD set at full retard (7 deg. initial adv.) I started thinking about tire pressure too. I just switched to 20x11-9's *8 paddle Haulers, and I did have the pressure pretty low about 3.0 maybe less.... kinda squishy. I guess I'll try and set it a tad higher. I did check out your Glamis page, nice. Very good tech articles too...   Backcountry       240 PSI static is VERY cool. I like it! I get those kind of numbers at Glamis (below sea level), but the numbers are somewhat lower where I live. Usually about 10% lower. I have had success with tightening the squish (on a fresh/tight motor) to .035". To some people this is too close for comfort with a 250 cc motor, but unless it bangs the head it's OK with me. I never go over .040" for any TRX. A tight Banshee motor can run right up to .030" with success.   12 degrees is a safe dome angle and probably will not trap much mixture out there. It's hard to match an angle cut to a radius of a piston crown without some compromise. That's why I cut the pistons tops myself. I don't like to live with things that I can easily change. I could just as easily radius the squish area of the dome but by machining the piston I accomplish a couple of good things. First it lightens it by a small amount. Lighter pistons make motors rev quicker. Second I can center the crown to the piston skirt.   We all know pistons are not just round cylindrical pieces - they are tapered from top to bottom and are cammed from front to back. So machining them is a very exacting process. Time spent here centering and truing it before making a cut will pay off big. Cast pistons LOOK as though their domes are centered - but they're not - they can be WAY off. Too many times I have put an engine together with a cast piston to check its squish only to find out there is more clearance on one side than the other, or from front to back. What's the point of taking the time to properly set the squish when you end up compromising because of this. When properly machined the squish will be almost perfect all the way around the dome - easily within .001", usually less.   Forged pistons are better as far as their dome being centered, but not perfect. There is always room for improvement. Since their tops generally have less crown to them, less material removal is necessary.   I like the long rod combination and may do it since I have a TRX motor to rebuild right now - and I have both the long and short rods on my bench... hmmm... I don't know how but there's an aluminum base spacer plate sitting there too - thinking long rod, with CR flat top piston.   I know you were having trouble launching but I found a tremendous improvement with the switch to 10 paddle haulers. The trend in paddle tires right now is to switch to a tire that is taller/narrower rather than shorted/wider. This increases the contact area of the tire to the sand and provides better floatation. Gear changes will be necessary. I still use the shorter/wider ones, but am considering a switch since the paddles are starting to tear off.   Narrowing the rear axle or buying a shorter one keeps the tires under the bikes weight for superior hook up. Notice a dragster - they keep the tires very close for this reason. I have also seen a couple of low budget tricks for accomplishing the same thing using the stock axle. There are those small spacers on the axle which can be moved to the outside of the hub, but that only brings the tires closer by about 1" total, but if you take the hubs/wheels off and run the right hub/wheel on the left side of the quad and the left hub/wheel on the right side you'll end up bringing them a few inches inches closer - though I haven't measured it exactly. Keep in mind that the lug nuts and the valve stems will be on the inside on the wheels - toward the swing arm.   The MSD ignition has interested me. I don't like the idea of having to cut part of the pulse trigger off the flywheel in order to get adjustability in the timing though - since if you want to go back you have to get a new flywheel. I know the 89 Honda TRX ignition is about as close to perfect as you can get so I haven't decided to go that route.   But with the pressures I'm making I notice a wave in the Dyno curve near the peak that can not be jetted out. I believe this is because I'm getting some "snuff out" of the spark under peak power. Perhaps it's time to bite the bullet on that one and let my motor realize its full potential. If I had to guess I'd say I could pick up 1 1/2 to 3 HP at peak HP RPM and perhaps a couple of HP more before and after the peak as well - with additional voltage to the plug.   There's one other thing on my mind. World class motorcycle road racing 250 cc twins are making more than 100 rear wheel HP with BMEP's over 200. That intimidates me. I want one of THOSE engines to play with.   Rick     Just got back from a rousing trip to the dunes. Everything went well and had some great racing against some nice Banshee's. A little tweak here and there are paying off big at the top of the hill. Anyway, I noticed that you're hesitant to buy the MSD ignition due to cutting your flywheel. You don't have to do that. Buy a Hinson adjustable counterbalance bearing holder, and adjust the initial advance to the seven degrees and go from there as recc'd by MSD. I think that you'll find a few more ponies tucked away with the far superior timing curve on the MSD unit, not to mention a hotter spark. My static compression is 240 lb., at sea level and am running a 14.5:1 UCCR, and run a .038" plug gap without ANY problems from it. I had fabricated a mounting "plate" for all the system that tucks up nicely beneath your gas tank. If you're interested I'll draw a sketch of it, include a few digital photos and send it your way. Go buy that MSD.   Backcountry     Rub it in.... It looks like I'll be going riding soon though. That is a good idea. I wonder why MSD didn't mention it when they sent me the info on that part a year ago. I've had my plug gapped to .026" to .028" for way too long. I know I'm giving up some HP everywhere, not just the peak. If you have the camera, by all means shoot me a pict of it, I'd love to see what you've made to mount it all.   How long does the battery last for you anyway? Do you have carry an extra with you? Would I have to worry about running out of battery and being dead in the dunes? Do you use the remote control race car type NiCad batteries instead of the one it comes with? I understand many people do.   By the way, how tall/long is the hill you race on there? I have to make it there one of these days. I've been by there but not when I had my TRX.[in Oregon - Rick]   Rick       Well, it was a good ride, Saturday it MONSOONED ALL DAY, but then Sunday and Monday were nice for riding. As to the battery on the MSD I can usually ride a whole weekend in the dunes without having to recharge, BUT, normally I recharge at night, it only takes and hour. I drain it fully using a wired-up pigtail with an 1157 auto. bulb, and then I bought MSD's quick charger, WA-LA we're off and riding.... MSD claims it lasts 18 to 20 hours but I'd say it's closer to 15 hours. I'm in process of more motor work, I decided to go to the 72 mm bore , and I discussed at length with my motor guy about crankcase volume and the reduced transfer volumes, and he has all the right answers as far as my knowledge allows, I trust him 100% to do the right thing, he's gained and proven that trust many times over. Go take a look see at http://www.mix-net.net/~red So a cylinder spacer, added transfer ports (much like you did to your 270), and a custom pipe are in my near future... can't wait to rip on that thing.... The hills in the Oregon dunes are small compared to the photos of Olds and Comp at Glamis I've seen, the big ones are maybe 60 to 70% the size. Racing here is actually a pretty short run, usually rev 4th out and that's it, there's a few places to race on the flats that run you up into 6th, but most like racing the hills. I think that it would be worth the trip just to see what the difference is like, I think that the almighty 270 would whoop some ass up here. There's many guys who "port" motors and get it close enough to be sufficient most of the time, until someone rolls up with a motor done right, and KILLS them. It's funny to listen to guys talk smack about their motor that they had such and such shop do their "full race port' (to me that means I really don't know what they did), and get beat badly......... But anyway I think that you'd enjoy the trip. I should be able to get to taking some photos and get them headed your way by oh maybe Saturday, Trax310 is interested in it too, so maybe I'll fire it off to the both of ya.   Backcountry

Date Last Modified: 4/19/99
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