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Cylinder Base Gaskets

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mstull

R.I.P.
Joined
Jun 23, 2005
Messages
1,263
Location
West Texas
I tried leaving out the cylinder base gaskets to increase the compression ratio and change the port timing, favoring lower RPMs on my de-rated Kawasaki 340 engine.* Those gaskets are only about 0.5 mm thick, so it's just an incremental change. I used sealant instead of the gaskets.

The improvement is tremendous at low and mid throttle settings.* It didn't change the full (de-rated) throttle RPM, EGT, or carb tuning.* But it gained a lot of power and efficiency at mid and low throttle settings, using about 10% less throttle to turn the same RPM in cruise and burning about 11% less fuel.

The snowmobile manual says it will burn regular gas. Burning premium, it doesn't knock with the slightly higher compression.* The fact that I'm de-rating the engine (to keep within the legal speed limit) with a small (TM-24 mm Mikuni) carb also helps make it safe. The small carb and prop choice limits the engine to about 6,400 RPM.

I don't recommend this change for anyone else. I present it here just for educational purposes. Rest assured Kawasaki (and other engine manufacturers) tried every tuning trick to get the most high RPM power while keeping a strong mid-range and reliability.

This change could cause a catastrophic engine failure with a full sized carburetor. At higher RPMs this modification would likely decrease power. Engines need a lot of "breathing" time at high RPMs, and this modification decreases breathing (port open duration) time.

Moving the cylinder lower, relative to the piston, opens the exhaust and transfer ports later at the end of the power stroke, making the power stroke longer... and closes the ports sooner, trapping and compressing slightly more intake charge at lower RPMs. Higher compression is known for increasing engine performance, as long as the engine doesn't knock and can dissipate the heat.

Normally, we tune our 2-stroke engines with different exhaust pipes and carb jetting. Everything else is kinda factory set and usually best left alone. I've wanted to try this modification for some time. I'm amazed how much difference it made.

I like to run the engine on the rich side to reduce carbon build up on the piston rings. But I like the fuel economy of running a little leaner. Now I can run the engine on the rich side and still get excellent fuel economy.
 
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