Compression

[stupidslow02gt]

I don't understand why people keep telling me that this set up will net me 8.5:1 compression when this calculator is telling me otherwise. I have stock ported heads from an 02 GT with 43cc combustion chambers, -20cc dished pistons, and am trying to get my compression to 9.1:1-9.3:1. The block is 20 over.


Compressed head gasket thickness is .036in and deck volume is .012. I am using ford OEM MLS gaskets.

 

[CobraRed_96_GT]

Static compression to net 9:1 with .20 over is a 18CC piston, 17CC will get you near 9.15:1. Trust me, i'm running a .20 over shortblock with a 18CC manley with PI heads. W/out .20 over you're closer to 8.9:1, a 23cc dish manley will get you closer to 8.5:1 - but .20 over it's more like 18.6:1.

Edit: BTW it's 42cc chamber, not 43: http://www.mre-books.com/sa82/sa82_5.html
43/44 is likely where i am since mine are ported.

 

[stupidslow02gt]

Ok, so did I mis-type something into the compression calculator above? Because it is not aggreeing with you. Also, from what I've read PI heads have between 42-44cc chambers that also seems to be unconfirmed. And I have stage 3 heads on my car that still have 42cc combustion chambers, so porting doesnt necessarily mean bigger chambers.

 

[CobraRed_96_GT]

Ok, so did I mis-type something into the compression calculator above? Because it is not aggreeing with you. Also, from what I've read PI heads have between 42-44cc chambers that also seems to be unconfirmed. And I have stage 3 heads on my car that still have 42cc combustion chambers, so porting doesnt necessarily mean bigger chambers.

Porting usually means taking of the low-rpm emissions ledge by the intake valve, with smoothing account for up to 2cc of volume.
i don't really care if the calc agrees with me or not - look at gear ratios calcs, use three different ones and you'll come out with three different answers.
Use common sense, a stock 99/00 mustang came with 18.10CC piston dishes making around 8.84+:1 compression according to ford's stats. That's with PI heads. Take off .1CC of dish and add .20 of bore and you have just about 9:1 compression. Also every modular engine builder saying manley 18cc's put you at 8.9-9:1 helps too.
Not sure what else you need. I HAVE a forged 18cc shortblock and ported PI heads.

 

[stupidslow02gt]

Ok so you are telling me that 17cc is where i need to be?

 

[CobraRed_96_GT]

Ok so you are telling me that 17cc is where i need to be?

I'm saying JE/modmax/kieth black/CP 17cc pistons or mahle 16cc pistons will put you basically where you need to be. Ask a modular shop to confirm.

 

[Stan Weiss]

I have a calculator on my web site Automotive Performance Software / Interactive JavaScripts Calculators and using your inputs get 9.2082:1 CR. The problem is that I have the head gasket bore set to 3.572 inches and it is probably bigger than that. So measure the bore size of the head gasket and recalculate your CR.