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Jacking up the compression is probably the oldest hot-rod trick in the book. The payoff is usually quicker acceleration, better throttle responses and maybe even some more top end power. Years ago, taking your V-8 car’s head down to the machine shop to remove some metal from the gasket surface was both quick and cheap, until you got radical and valves started interfering with pistons, and holes were burnt from detonation. Everything had its limits. The same can be said for 2-stroke snowmobile engines.
Easy to work on?
The charm with snowmobile 2-strokes has always been that they were easy motors to work on, at least in the early years of free-air engines and one-piece hoods. Everything was right in front of you. It’s probably why vintage racing is so popular, it takes you back to simpler days when tinkering with your sled was almost a daily affair.
Today’s 2-stroke engines are still relatively easy to work on, but there have been obvious advances like fuel injection and a host of other technology. Good performance can be had by re-doing your heads right, but there is now more items to watch than ever. There are several important areas which we work on, combustion chamber volume and shape, and squish area clearances. These can be machined for more power.
Math camp for speed freaks!
To calculate the compression ratio you measure the full volume above the piston when it is at bottom dead center, and divide by the volume above the piston in the combustion chamber at top dead center. To measure you place the piston at top dead center with a little grease around the rings to prevent the measuring fluid from leaking down, and then you pour the measuring fluid down the spark plug hole with a metered buret until it tops out at the top of the spark plug hole.
Let’s say you had a 440 engine with two cylinders. Each cylinder has a displacement of 220cc, and your measurement showed that it took 21.8cc to fill up the chamber. Since the spark plug hole has a plug in it when the engine is running, you have to subtract 1.8cc from the measured volume to get the actual combustion chamber volume.
You are then left with a chamber volume of 20cc. At bottom dead center the total volume above the piston would then be 220 + 20 which totals 240cc.
Divide this number by the volume at top dead center (20cc) and you get a ‘geometric’ compression ratio of 240/20 or 12:1. I have always called it a ‘geometric’ ratio because there is some confusion with Japanese compression ratio practices. The Japanese manufacturers would only measure the volume above the piston after the piston closed for the exhaust port. Since most of the ports close about half way up the cylinder wall, they reasoned that the actual volume being compressed would then be 110 + 20 = 130cc. Divide this number by 20 and you get a Japanese ratio of 6.5:1.
This creates confusion because you would think the engine was a really low compression engine, when in reality the geometric ratio was 12:1. When you then start porting cylinders and moving the top of the exhaust port up, you might think you lost compression ratio when actually the geometric ratio is still the same.
Engine builders use only the geometric ratio as a tool to measure the compression ratio, otherwise the whole thing gets too confusing. After all, you don’t see 4-stroke builders measuring the compression ratio after the inlet valve closes as the piston moves up.
Some may argue the Japanese system is correct, because when you pull the engine over after you grind your port up, the compression pressure is lower. This is true because the engine is not running, but as soon as the engine runs and the resonant exhaust pulses start working, there is a positive pressure at the exhaust port at bottom dead center. So, the actual ratio is probably closer to a geometric number than a Japanese number when the engine is actually running. This is why the Japanese system hasn’t been adopted.