More Power. Please! 

No More Over Bore This original stock 1988 LT80 liner finally ran out of over sizes. I've used all of the available after market pistons (Wiseco makes them to 2 mm over size - 52 mm's) for this engine. Those bore size increases took it from 82 cc's when it was at the stock 50 mm bore to 89 cc's with the larger 52 mm piece - an 8% increase of displacement. Increasing the bore size adds quite a bit of power to a small engine - especially an over square engine like this one (50 mm x 42 mm bore and stroke). In this case the increased displacement adds about 1.2 hp all on its own - about a 20% power increase. In stock form the little LT opens its exhaust port at 105 degrees ATDC for a duration of 150 degrees. The transfer ports open at 123 degrees ATDC for a duration of 114 degrees. The blowdown is 18 degrees of crankshaft rotation. The volume above the piston (when at TDC) of 8.9 cc's makes the uncorrected compression ratio (UCCR) 10.27:1 - it's 7.4:1 corrected and has a trapped volume of 57 cc's (measured from when the exhaust port closes to TDC). The exhaust port is 28.5 mm's wide (57% of the bore width) and the static compression is 163 psi @ sea level. Suzuki lists the stock engine as being able to produce a maximum of 6.1 hp @ 6500 rpm (SAE corrected) and maximum torque of 0.75 kgm @ 4000 rpm. The software I use to double check these figures and to assist me in predicting power output indicates a slightly higher output number. It shows 6.49 hp @ 6500 at the rear wheels (7.37 at the crank). This cylinder/head had been modified for more upper mid-range power. I did this one quite some time ago. Though not radical in appearance or design the porting and head modifications help deliver much more useable power. The exhaust port has been reworked and a lot of attention was given to the transfer ports. The porting numbers I choose to use when modifying this barrel are quite different from above. The exhaust port is cut to open at 95 degrees ATDC for a duration of 170 degrees - an increase of 20 degrees. The transfer ports are cut to open at 119 degrees ATDC for a duration of 122 degrees - an increase of 8 degrees. With these changes the blowdown comes in at 24 degrees - an increase of 6 degrees. The exhaust port is widened to 65% of bore width - an increase of 8% (+ 5.4 mm increase in port width). Even though the displacement has been increased by the larger piston, the 3.5 mm exhaust port height change lowered the trapped volume to 54.1 cc's - a reduction of 2.9 cc's. Together the reduction of those two parameters significantly weaken the off idle and low speed output - less trapped volume to turn into power and a shorter power stroke. They both take from the work cycle. This will be compensated for by an increase in compression ratio, increased rpm's and the addition of a tuned exhaust pipe and clutch work. To complement the pipe and new engine speed operating range a 20 mm Keihin round slide carburetor was fitted. The fan cooling system was also modified so it will pass its air more easily with less restriction. To achieve the targeted UCCR of 12.7:1 (92 octane pump gas) the volume above the piston @ TDC needed to be reduced by 2.02 cc's. This could be achieved by removing .037" (.95 mm) from the deck surface (if it had enough) or a combination of deck surface and cylinder head surface skimming, though to reach the target I chose to deck the top of the barrel by .012" (.30 mm) which reduced the volume by .65 cc's and left me an uncut head with plenty of material to rechamber it to reduce its volume by the additional 1.37 cc's I needed. During that process I designed a head shape and squish band width/velocity that accompanied these engine modifications. With the volume above the piston @ TDC of 7.6 cc's the targeted UCCR fell into place, the corrected compression ratio moved to 8.1:1 and the static compression changed to 178 psi. The redesigned engine now produces it's peak power at 9500 rpm's. The ports, pipe, head shape, carburation, clutching and cooling system have all been targeted to work together at that peak rev level. The software program now shows the output power to be 12.68 hp @ 9500 at the rear wheels and 14.41 hp @ 9500 at the crankshaft - an increase of 6.19 hp when using the software output numbers - a 95.5% increase. Though the power peak is fairly high up the rev range, the engine still makes great power elsewhere. Low speed maneuvers and drivability are greatly improved as is mid speed acceleration. Power in the mid range is very strong and moves the machine and rider very well no matter what the terrain or obstacle. The driven clutch shoes have been lightened to work with the advanced port timing too - the stall speed is increased to 2400-2500 rpm. I couldn't locate a set of replacement springs which would have accomplished the same thing so I was forced to lighten he parts to allow them react later to the inertia of their spinning weight. In addition to allowing the machine to accelerate from a standing start with much more authority (yes it spins the paddles) the modification also helps when starting on hills since the stock engagement was too low to allow for a take off unless headed down hill or practically flat. This LT easily negotiates the terrain of Competition Hill or Oldsmobile Hill at the Glamis Sand Dunes with a 105 lb. rider on board, though it does sound busy doing so. There is a replacement sleeve available for the barrel to bring it back down to 50 mm's for those who want to use OEM size or after market pistons, but I choose the big bore route instead. The warn out sleeve is pushed out and the cylinder is bored to accept the big bore sleeve. It will accept a 54 mm Pro-X piston and increases the displacement to 96 cc's. Using the same hardware and porting specs as the modified barrel above I can estimate the power to be 13.68 hp @ 9500 rpm at the rear wheel and 15.54 hp @ 9500 at the crankshaft.
These shots show the difference in the size/shape of the ports between the old and new sleeves pretty well. The original liner was originally ported for housing a 50 mm piston. The new liner starts with smaller ports than the other barrel, and it starts with a larger 54 mm piston. With the liner removed it is easy to "rough in" the ports. It saves time doing it that way and it comes out better.
The stock LT80 barrel is conservatively ported. It was not intended to be a racer. It seems like it was intended to be used by a wide variety of people including young people who might be intimidated by it if the power delivery was too spicy. The last thing a manufacturer would want to do is scare a rider so that he/she wouldn't want to ride the machine. The stock LT has a mellow power band but there's still enough speed on tap to allow riders the opportunity to test and improve their riding skill. When its rider is ready the power output can be substantially increased using standard engine building practices and a few after market bolt ons and it retains the reliability of the stock machine. That's quite a mismatch between the liner and the aluminum of the transfer ports. In addition to being poorly lined up the ports are on the small side. A great deal of time must be spent there to allow them to flow well enough to make good power as much as 3000 rpm's higher up the scale. Even with careful port carving the engine needs assistance from a tuned exhaust pipe.
The larger carburetor matches much better to the modified reed cage inlet. Care has to be taken in order to preserve the built in o-ring around its mouth. The bridge between the reed ports has been narrowed. The main passage has been opened up to let the 20 mm Keihin breathe. The stock cage design is good for the lower revving engine, spring steel reeds and tiny carburetor.
A replacement reed petal can be cut from a piece of glass fiber - I used a piece that was .48 mm thick. It responds much better than the heavy steel petals and last a very long time. The tapered inlet tube allows cleaner jetting and it lets the carburetor breathe much better than if it were fitted with a straight wall "radiator type" tube. This part does not fit the stock carburetor without major modification.
From the right side of the engine - Notice the removal of the carburetor air box and associated plumbing. The incoming cooling air is grabbed up high by the shortened inlet snorkel and travels through fewer obstructions before doing its job. That's a K&N air cleaner attached to the carburetor - under the filter sock. The big carburetor delivers the fluids. The inlet noise is a bit bothersome - I'll be working on making it a bit quieter in the near future. From the left side of the engine - Notice the large oval cooling air outlet. When positioned like this it doesn't blow against the ground creating dust which then could be sucked into the cooling inlet. The air tube that connects to the clutch cover received a Uni filter with a light oil on its surface. Out back the exhaust pipe receives a SuperTrap to keep its sound in check.
In the future I'll see if I can take and post some pictures of the modified clutch drive and driven rollers/shoes as well as the modified cylinder head. -Rick