Engine compression math ?

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Oct 26, 2004 | 11:42 AM

SmokedTires

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Le Mans Master

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From: B'Ville NY

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St. Jude Donor '05

Engine compression math ?

I'm wondering if anyone can do some compression figuring for me. As you may know, I'm looking to install a turbo onto my C3 but I currently have high compression which isn't ideal for this type of setup.

I'm considering trying the new Cometic head gaskets as a way to lower my compression. My friend had o-ringed his heads and since removed the o-rings resurficed his heads & installed the Cometic gaskets. He was running 21 lbs of boost with his twin turbo with no problems. Te gaskets will be much less expensive than new pistons to tryout. They offer gaskets in thicknesses as high as 074 and .120.

So my current setup is 68cc head chambers, 4.155 head gasket bore diameter, .030 gasket thickness, and the pitons are .02 below the deck height.

Thanks

Oct 26, 2004 | 11:55 AM

gkull

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From: Reno Nevada

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Mike - Go to www.smokemup.com register and go to automath.

Going bigger on the head gasket increases the quench distance with has the opposite effect on detonation. It detonates easier with less compression and a taller quench.

The only way to really do it is enlarge the chamber or dish the pistons

Oct 26, 2004 | 12:03 PM

Fevre

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From: Living in the Hartland

My questions is if the gasket is a perfect match to the cyl bore then what would be the diff between that and have the piston set deeper in the block? Guess tech with a dished piston it sits in the same relative position but is hollowed out some.

Oct 26, 2004 | 12:28 PM

SmokedTires

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Le Mans Master

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From: B'Ville NY

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St. Jude Donor '05

Hmm, these are things I'm not aware of which is why I'm throwing this out for discussion. I'm with Fevre though, why would this not be an issue if the pistons are dished which is increasing the chamber area ?

Oct 26, 2004 | 01:00 PM

LemansBlue68

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From: May help you? You can sure as hell try!

When you dish the pistons you don't dish the quench portion of the piston. The dish has a D shape to it. This keeps the same quench height but gives a lower compression ratio. Quench is used to increase charge motion as the piston approaches TDC by squeezing out the A/F mixture. The increased charge motion causes the mixture to burn faster preventing detonation.

Oct 26, 2004 | 01:27 PM

Pete79L82

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From: Shawano WI

As LemansBlue69 pointed out that when you dish a piston, only a portion is dished so as to keep the quelch area constant.

The shape of the dish on a low compression piston will differ from the dish on a piston designed for forced induction. The shape of a forced induction piston dish is very critical to help reduce detonation at high boost.

If you increase the head gasket size you will increase the distance from the quelch area of the piston and the quelch area of the head. This is OK if your goal is only to lower compression, but if you do this an then run forced induction you will be severly limited on the amount of boost you can use.

With your combination now you have 10.446-1CR
With a .075 gasket you will have 9.461-1CR
With a .120 gasket you will have 8.630-1CR