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post #1 of 8 (permalink) Old 04-27-04, 06:29 PM Thread Starter
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Decompression benefits

Anyone know what benefits there are from running lower compression in an EA.
What performance gains would you expect by fitting a 4.0 head to a 3.9?
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post #2 of 8 (permalink) Old 04-27-04, 06:50 PM
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Re: Decompression benefits

There are no benefits. Higher compression (to a point) is what you want.

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post #3 of 8 (permalink) Old 04-27-04, 08:23 PM
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Re: Decompression benefits

you want lower compression if you
go forced

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post #4 of 8 (permalink) Old 04-27-04, 09:16 PM Thread Starter
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Re: Decompression benefits

So what gains can you get by raising the compression?
Is it just a matter off shaving the head?
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post #5 of 8 (permalink) Old 04-27-04, 09:30 PM
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Re: Decompression benefits

thinner head gasket will raise compression
shaving the head will do it also

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post #6 of 8 (permalink) Old 04-27-04, 09:34 PM
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Re: Decompression benefits

http://www.sdsefi.com/techmods.htm

did a bit of searchin though got this stuff in books

Increasing the compression ratio is another way to increase power. It also increases fuel mileage. Unfortunately, the pump fuel available in most areas limits the compression ratio useable on the street to under 10.5 to 1 on most engines. The difference in power is minimal going from say 9 to 10.5 to 1 and it is a lot of work to shave the head or install new pistons. Again, if you get stupid and try to run an 12 to 1 CR on 92 octane fuel, you will suffer with lots of pinging and eventual failure. Many high compression street engines must have their timing severely retarded to avoid detonation which reduces the power right back to stock levels. Don't raise the compression ratio too high!

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post #7 of 8 (permalink) Old 04-28-04, 03:36 AM
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Re: Decompression benefits

i have also read that due to the length of the stroke in the I6 the power increase you recieve from increasing the compression ratio above about 10:1 is offset by the power required to shove the piston up the bore against compression pressure,(due to the pressure acting against such a long 'throw' of the crank, so i personally would not go any more than 10:1 in a N/A application for a falc I6.

did that make sence?

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post #8 of 8 (permalink) Old 04-28-04, 05:35 AM
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Re: Decompression benefits

yes i have read the same thing rollin, apparently there is little to be gained from a higher compression ratio in the I6, however I guess every little bit helps

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