Comp. ratio question with 58/54cc heads on stock bottom end..
 

I just ordered a set of AFR 58cc heads. Knowing that the factory compression is 9.2:1, and the factory heads are around 63-65cc's, what will my compression be with the 58cc head, and what will it be with the heads milled to 54cc's? The guys at AFR told me that with 58cc's it would be close to 10:1, and with 54cc's it would be 10.5 to one. Others are telling me it will be close to 9.6:1 and 9.9:1 respectively. Anyone know? And what kinds of HP increases are associated with a bump in compression of say 1 point?

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95 GT convt' 5 speed, 3.73's,FMS HD clutch, Pro 5.0 shifter, ASP alum. underdrive pulleys, Bassani offroad X pipe, 2 chmbr flowmasters, K&N w/o silencer, edelbrock performer intake, FRPP 65mm T-body, FRPP shorty headers, FRPP aluminum driveshaft.Hooker 4pt chrome car bar, Xenon/Wings West ground effects, custom interior
Best ET: 13.83
Best MPH: 98.55mph with Best 60': 1.91 on Nitto DR's
Homepage: <A HREF="http://www.angelfire.com/pa2/killercanary" TARGET=_blank>http://www.angelfire.

double post here, can't delete it!!!------------------
93-LX: 5 Speed, Flow's with H-Pipe, 3.73's Sub's and K&N.

[This message has been edited by lx mike (edited 02-04-2001).]

AFR is right, it will be around 10-1 and 10.5-1 with the cc heads you mentioned. i am not sure about hp increase maybe 3-5 % for every point raise in C.R. with alum head can run more C.R seeing as they reject heat better

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93-LX: 5 Speed, Flow's with H-Pipe, 3.73's Sub's and K&N.

Is there a formula to figure this out? On edelbrocks web site they have a detailed method, but its too hard for me, you have to know gasket thickness and stuff.

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95 GT convt' 5 speed, 3.73's,FMS HD clutch, Pro 5.0 shifter, ASP alum. underdrive pulleys, Bassani offroad X pipe, 2 chmbr flowmasters, K&N w/o silencer, edelbrock performer intake, FRPP 65mm T-body, FRPP shorty headers, FRPP aluminum driveshaft.Hooker 4pt chrome car bar, Xenon/Wings West ground effects, custom interior
Best ET: 13.83
Best MPH: 98.55mph with Best 60': 1.91 on Nitto DR's
Homepage: <A HREF="http://www.angelfire.com/pa2/killercanary" TARGET=_blank>http://www.angelfire.

I would assume that the guys at AFR should know what they are talking about. And just from my general knowledge that sounds about right. 10.5:1 compression is perfect for the street too. Looks like a great combo. I would say going from 9:1 to 10.5:1 should yeild you about 35hp. They say about 25hp per point of compression, or so I've read.

Skyler

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-1989 Saleen Mustang #406-
TFS Heads, Edelbrock intake, E-303, 3.73's, 1 5/8 shortys, Offroad-H, 2chamber flows, 36psi FPR, 15degrees timing, 70mm tb, 73mm maf, 24lb inj, Crank pulley, MSD6A, Alum driveshaft.

Added: B&M Ripper, Subframes, & Turndowns.

Well-63-65 cc's seems pretty huge for stock heads-I think they are more around 61 and 9.0:1 compression. Try this website:

http://www.turbofast.com.au/Tfcomp.html

You need to know the gasket thickness and the deck height from the top of the piston at TDC. Not exactly sure what stock is but I know the thinner graphite gaskets are .038" and not sure what stock DH is either

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1988 GT-under construction