Snowmobile World banner

1 - 5 of 5 Posts

·
Registered
Joined
·
2 Posts
Discussion Starter #1
I just assembled my ZRT800 with a 992cc big bore and checked the compression. It has 185psi!! so I had a machine shop open the squishband .020 deeper and it is still 175psi. Do I need to use thundercat 900 heads? what can I get away with on pump gas? thanks!
 

·
Registered
Joined
·
67 Posts
You've increased your swept volume considerably. Calculate your compression ratio then maybe try a thicker base gasket or use a head with a greater combustion chamber volume.
 

·
Registered
Joined
·
2,885 Posts
That is fairly high on PSI, but that isn't a true indicator of compression on a two stroke.

The heads you have need to be machined to match the diamater of your larger pistons, STOCK HEADS WILL NOT WORK!!!!!

There are so many factors on a two stroke that come into play when the engine is running that factors into the real compression of the engine. The amount of backpressure from the exhaust, crankcase volume, port timing, squish all will factor into that.

You can run that high of a PSI, but the engine better have ALLOT of squish.
 

·
Registered
Joined
·
2,885 Posts
"suish" is found when we roll the piston up to top dead center, and measure the distance from the edge of the piston up to the bottom side of the clyinder head at the farthest most diamater of the bore.

It is easy to measure. Use a piece or .08" solder cut long enough to reach thru the spark plug hole to the outside diamater of the bore. Roll over the engine and it will come up and "squish" the solder to the thickness between the two pieces.

The distance is very critical. It relates to compression, but doesn't necessaraly change it.

It is a old trick to change the angle of the cylinder head so that it brings the outer edge of the head closer to the piston. It will raise compression SLIGHTLY, but it tends to make an engine more torquey.

Problem is, that there is limit to how thin you can run for squish. As fuel burns, the "flame front" travels from the plug outward. If the squish is too tight, the engine will detonate (ping). The fuel at the outer edge will ignite prior to the flame front getting there, and this will kill the piston.

Now, this will give you the same results as too much compression (holes in pistons, crushed looking eges of pistons), so many times it is thought to be compression, where it is mearly squish.

There are many factors to determining the correct value, but on a F7 laydown Cat, they are happy around .062" squish. But run a 800 Ski-Doo engine that tight and it will tear itself apart.

Given the nature of this 800 Cat Tripple, you can run that high of cylinder pressure, BUT it better have ALLOT of squsih, upwards of .090"-.100"
 
1 - 5 of 5 Posts
Top