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'91 Honda Civic Si
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Discussion Starter · #1 · (Edited)
The 2007 rules are out and it seems like I'm going to go for Honda Challenge 2. Rules state that you have to use the motor as a whole (must be a D16A6 from the ground up). You can raise compression by milling as much as you want off of the head and using any headgasket. Cams are open, you can port match ONLY 1" INTO THE HEAD, OE valve size, aftermarket IM and TB are allowed, NO ITBs. I'm figuring 75.5mm OEM pistons, mill the crap out of it and hope to be @ 10:1 - 10.5:1 and going with Level 2 Bisi cam.

My question for the hardcore N/A guys is how much material can you remove from the A6 head (using OEM piston) to raise the compression as much as possible by ONLY this method? .030"? .040"? .050"???
 

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according to the compression calculator over at zealautowerks if you milled the head by 0.050 you would have a compression of 10.16, thats with the a6 headgasket. if you used the y8 2 layer it would be 10.74.

does this sound right to the experts, cause i am no expert on that engine, i am not even an expert on my own engine.
 

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according to the compression calculator over at zealautowerks if you milled the head by 0.050 you would have a compression of 10.16, thats with the a6 headgasket. if you used the y8 2 layer it would be 10.74.

does this sound right to the experts, cause i am no expert on that engine, i am not even an expert on my own engine.
My honda power book says not to mill more than .004. Not sure exactly why, valve clearance maybe?
 

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Discussion Starter · #6 ·
My honda power book says not to mill more than .004. Not sure exactly why, valve clearance maybe?
.004 or .040, BIG difference. Keep in mind that most books and reference material will tend to be conservative so that you can't sue them when you blow your motor up.

Big Tuna, you've got PM.

I'm thinking 75.5mm pistons, .055", 2-layer Y8 HG = 11.01:1...nice and round number 11:1
 

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.004 or .040, BIG difference. Keep in mind that most books and reference material will tend to be conservative so that you can't sue them when you blow your motor up.

Big Tuna, you've got PM.

I'm thinking 75.5mm pistons, .055", 2-layer Y8 HG = 11.01:1...nice and round number 11:1
er yeah .04 dur

I just reread it and:

4 one hundredths of an inch is what it said was suggested max but that .06 was possible
 

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Discussion Starter · #9 ·
didnt big tuna mill .079"?
That is what his build says...and that is using a P29 piston that although it sits .05" lower in the block has a huge dome to increase CR even more.
 

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I just got my PM9 head back from the machine shop after taking .030" off.

It could easily go another .030 without getting into exhaust stud issues.
 
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