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Engine Specific Info and Questions => IDI Engine => Topic started by: dieselpower on September 15, 2004, 10:56:34 pm
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I heard that lowering compression will help get more power out of the engine. I kno that lowering compression all on its own will not improve power but running higher boost and fueling levels will. has anyone ever lowered their compression and what were the results.
also how did you do it?
is it worth it to have bad cold starts for the extra power potential. im tyring to build a custom twin turbo setup and want to run 30 or so psi with a modded 1.9 pump. these are my plans anyway. so bottom line is it worth the time and money to lower compression?
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Having run 17.5:1 compression with a modded pump and 35 psi, I can tell you that it's intolerably sluggish below 2500 rpm and 20 psi. Above those numbers it's almost as good as a stock compression engine running 20 psi. I've read 18:1 is the magic number. Having also tried 16.5:1, and feeling the improvement in gaining 1 compression point to 17.5:1, half that difference again to get 18:1 isn't going to improve performance enough to make it work. There is roughly half a compression point difference between each headgasket thickness.
My next engine is being built to set factory compression in the 1 notch head gasket range with the lowest piston projection stated in the bentley-provided range for that gasket thickness. Then, if my EGT's indicate too high, I'll have the ability to run 22.5:1 with a two notch, or 22:1 with a 3 notch. I have a strong feeling that less than this is too little. Ceramic coating the combustion chambers, valves, exhaust ports, manifold and turbo would help the engine tolerate high temps as well.
my 2 cents.
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The proper way (the way that would maximize efficiency) to reduce compression is the bore out the prechamber to increase it's radius, and machine deeper the "cloverleaf" relief pockets in the pistons. Reducing it by machining uniformly off the tops of the pistons or increasing headgasket thickness increases volume in the quench area where combustion doesn't happen very well, so the air will not being utilized as well (combustion quality will be adversely effected.)
What's the point of forcing in extra air with running higher boost pressures if your motor can't effectively mix fuel with that extra air and burn it?
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doesn't all the mixing happen in the prechamber? I kinda figured all that quench theory didn't apply to IDI engines in quite the same way as gas engines.