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The question - I'm not sure that all cylinders are like mine but I guess they are basically the same. Coming out of the reeds and into the cylinder, there is an intake port straight ahead, then two smaller ones that wrap around the sides and go into the cylinder on the sides. These ports are about the size of nickel. Can they be opened up more to provide increased flow or will this interfere with other things?

The response - Yes, those passages feed the lower crankcase, help provide lubrication to the main bearings and increase the flow of mixture to the transfer ports - particularly when the piston is at or near BDC and the exhaust is returning a suction wave. Increasing the size of them should be done carefully in order to NOT weaken the cylinder. Removing material closer to the cylinder liner is preferred over removing it from the outer side where it could weaken the area of the cylinder where the base nuts hold the cylinder down. It is doubtful that increasing the size of these ports and not doing anything else to the cylinder will produce ANY increase in power - however removing casting flaws in the cylinder ports (especially in this area) can produce noticeable results. Massive ports are not always the key to a faster bike. When you make the ports larger you also decrease the velocity of the gasses that pass through them and decrease the crankcase compression. This has to be taken into consideration and compensated for - depending on the application the motor will be used for.   Many times I port engines by using epoxy to make passages smaller to increase the velocity of the gasses traveling through them. Many engines have their port angle-area, time-area mismatched (too large) and need some adjustment to make them more effective at the target rpm of the engine. Epoxy can be used to correct dips, valleys and other casting flaws and reshape poorly designed transfer port passages.   Though porting increases the chances of fuel entering the cylinder at the right time all the parameters of the engine must be worked out ahead of time. Careful planning can produce the desired effect. It is most important to plan exactly what the engine is going to be used for and what its targeted rpm range is going to be. Rick   I guess I'll fall under the ETC. category. While I'm no expert port grinder, I believe that you are referring to the "boost" ports. Typically they are enlarged considerably from stock. The guy who did my port mod's opened them up big enough that I can stick my big 'ol thumb through it!!! I know that on 250R cylinders that's the easiest and first place to head to open up a cylinder. Watch that you don't enlarge them too much and create a "view window or skylight" into your cylinder. Then add a reed spacer to move the tips of your reed cage back to allow more flow thru the now enlarged boost ports and away you go........hope I helped and maybe High Output or Trax can elude more on this please..... Backcountry   Damn premium response, Rick. I call those extra windows "Auxiliary Intakes"....... "Boost Ports" usually refers to additional transfer ports in the rear area of the cylinder whose tops rise to the same or similar level as adjacent transfers. As Rick pointed out, Auxiliary Intakes frequently double as additional transfer "feeds" at and slightly after BDC with the aid of a functioning tuned exhaust system. The need to match time/angle/area to the rest of the engine's state of tune "package" and velocity requirements for good throttle response is well mentioned and also something frequently overlooked. As operational RPM targets rise, passages can be further enlarged to provide the required airflow and the higher RPM will create usable velocity levels. In lower RPM applications, sizes must be scaled down. There's nothing I can think of to add to that! High output   Damn, although it needn't be said, you guys are good! The only thing I can say is that on many cylinders in stock form, the manufacturer (wanting to keep it reliable) made a "weaker link." (not really on purpose) This implies that sometimes simply altering the breathability of the primary intake and its auxiliaries can be beneficial because the exhaust may be setup in a "more aggressive" state of tune. However, it may be just the opposite - it could be the exhaust's breathability holding the motor back. Then again, it could be that the exhaust port timing is very conservative when matching up with the intake. Or - opposite... Now, if you go opening up the intake and it was the exhaust hampering the motor all along, all you will do is slow down... I think you ought to research and see if you can swindle one of the Polaris pros out of their port numbers or any tips (you'll never do it!). Or send it off to a reputable digger. Don't just go cutting away on those ports though... but cleaning them up and texturing the intake side of things can't hurt. Polish the hell out of the exhaust port too... Oh well, good luck, Trax310   Excellent gentlemen..... I'm archiving that. This shows how much MORE I need to learn. I knew about large ports and velocities, but when you start speaking of port time ratios and such, I really listen..... cool stuff man. Backcountry   Me too!   A long time ago I bought a book called the Two Stroke Tuners Handbook. It was technology pertaining to the engines of its day (around the mid to late 70's - mostly piston port engines), not much information about reed valve's, but I think they talked about disc valve engines a little bit. Anyway, the point is - there is more information crammed into that little book than you would believe. It starts with the basics and builds on it from there. Most of it is still relevant today.   Anyone who wants to learn about 2 strokes and can get their hand on that book, should read it. Suddenly everything will make sense. There are many other good books on Two Stroke Cycle engines out there, but that is the one that says it all. Check the SAE directory for these technologies too. They publish some great stuff and much of it is from the leading manufacturers of motorcycles. Also do a search at http://www.amazon.com and type in "two stroke" you'll probably find some interesting reading there too. Rick   That book was written by Gordon Jennings in 1973 and while it has a large amount of very useful information and most of it does indeed still apply, it is a bit outdated in several areas. A better choice might be "Performance Tuning in Theory and Practice" written in 1983 by A. Graham Bell as well as many SAE papers including a hard bound collection "PT-33" printed in 1989 and any information written by Dr. Gordon Blair of the Queen's University of Belfast, N. Ireland. HighOutput

Date Last Modified: 4/19/99
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