ins to be had from smaller ports, for high altitude compensation or for more punch for trail and snowcross riding. Simply using a thinner base gasket or by tunring-down the cylinder base on a lathe. Cometic Gasket Co. in Mentor Ohio makes graded gaskets from .25 to 1.5mm and even custom base plates for stroker engines. (www.cometic.com) Another method is by welding the perimeter of the port, although that entails replating the bore. Transfer and intake ports can be made smaller with the use of epoxy. Brand name products like DURO Master Mend or Weld-Stick are chemical resistant, easy to mold to fit, and can withstand temperatures of 400F. Master Mend is a liquid product and Weld-Stick is a semi-dry putty material. The epoxy can be applied to the roof of the ports to retard the timing and reduce the duration. It can be applied to the sides of the transfers to reduce the time-area, and it can be applied to the transfer ducts to boost the primary compression ratio (crankcase volume).
Porting for Big Bores
WISECO offers big bore piston kits for most popular snowmobiles. (www.wiseco.com) The average increase in displacement is 50cc per cylinder. This requires that the cylinder be over-bored 4-8mm. Because the ports enter the cylinder bore at angles, when the bore size is increased all the ports drop in height. The steeper the port angles the greater the port height will drop. Lower port heights mean retarded timing and reduced duration. The exhaust port gets narrower and the transfers get wider. A larger displacement cylinder will require more port-time-area. Normally the exhaust port needs to be raised higher than stock to compensate for the compression ratio. If you're adding a set of performance pipes at the same time as the big bore, you'll need to compensate the port timing to get the best gains from the pipes. It's a complicated thing. Sometimes tuners use thicker base gaskets to compensate for big bores, but ideally the port-time-area needs to be calculated before any serious porting changes are made. If you are strictly trail riding at high altitude, you can just have the cylinders bored and replated because the porting will inherently change to suit that type of application.
Big Bore Specs Link (coming soon!)
Computer Design Software
The best-kept secret in high performance tuning is the use of computer design software. These products became popular about six years ago when Tom Turner adapted the SAE programming code and added in his own empirical data from his career as a motorcycle drag racer and tuner. Tom's products are named TSR software (www.tsrsoftware.com) and available in MS-DOS format for PCs. The programs cover individual engine components. The most popular program for porting is PORTTIME 2000 and it sells for $200. It features target specs for all sorts of vehicles including snowmobiles. Basically a tuner types in engine spec dimensions like the bore and stroke plus all the individual port measurements. The program runs mathematical calculations in order to provide a simulation of how the porting changes will affect the engine's powerband. TSR's programs will get a tuner 90% to the engines potential. The next level of programs includes 1D and 3D gas dynamics simulators. Dynomation is a 1D simulator that sells for about $500 (www.audietech.com). It enables tuners to combine all the engine components together to simulate dyno runs on the computer so they can save time machining metal and swapping parts on actual dyno runs. Dr. Gordon Blair programmed the 3D simulator named Virtual Two-Stroke, the most well know two-stroke engine researcher. His program is marketed through Optimum (www.optimum-power.com) through lease programs. Priced at $12,000 a year, it's intended for use by engine manufacturers.