I have a basically all stock 88 GTA 350. Factory says 9.3 or 9.4 with 64cc heads. How do I figure out my compression with a different cc head? I just want to get a rough estimate. I have the calculator thing but I dont know a lot of the variables, like how far the piston is away from the deck height and all. Cant I just substitue the cc numbers? The heads I am going to be getting are around 55cc or so. What will my compression be with a stock gasket and the new heads? Could you give me a simple formula?
Thanks
James

------------------
If the women don't find you handsome they should at least find you handy
Aftermarket T-56 (pain in the A$S but worth it)
Lots of things to do still

 

CR = (Cyl vol + comb volume) / comb volume

Since we know Cyl vol and CR we can solve for comb vol.

9.3 = (716.62cc + CV) / CV

CV = 86.3398

We know the stk head is 64cc's. Subtract 55 from 64 and then subtract that difference from the CV.

Your new CV = 77.3398cc and CR = 10.265

 

Thanks man. I wouldnt have been able to do that my self.

James

 

That's close enough but you should also know the deck height (how far down the piston is in the cylinder when at TDC), how thick the head gasket is and how much the valve reliefs or dome is on the piston. Only when you have all the variables can you get an acurate compression ratio.

------------------
Stephen's racing page

87 IROC-Z Pro ET Bracket Race Car
383 stroker (carbed) with double hump cast iron heads and pump gas
454 Big Block buildup now in progress for the 2001 racing season

Best results before the 383 blew up
Best ET on a time slip: 11.857 altitude corrected to 11.163
Best MPH on a time slip: 117.87 altitude corrected to 126.10
Altitude corrected rear wheel HP based on power to weight ratio: 476.5
Best 60 foot: 1.662

Racing at 3500 feet elevation but most race days it's over 5000 feet density altitude!
Member of the Calgary Drag Racing Association

87 IROC bracket car, 91 454SS daily driver, 95 Homebuilt Harley

 

James,

If you are going to be swapping heads, cams, and other internals, you really ought to bone up on your math. It'll save you a lot of pain and agony in the end. You could also spend $50 and buy Desktop Dyno 2000 if you wish. An engine compression ratio calculator is right there in graphic detail.

Some things you are going to learn are how to calculate the proper valve height, calculate intake centerline, piston to valve clearance, compressed gasket height and volume, the ramifications of tall & short deck heights, piston & ring clearances, etcetera.

These concepts will enable you to intelligently choose the proper components. Otherwise, you might end up with a broken engine, or (perhaps worse) an expensive, tempermental, sluggish engine.


------------------
Daniel Burk
http://www.isthq.com/~dan/fcar.html
'84 Trans Am WS6/L69
KB SFC, Moser axles, Torsen Diff. PST suspension, Braided stainless brake lines, Koni struts, 11-inch rear disks,Spohn Adj. torque arm,
Ported 305 heads w/1.94"intake valves, Comp Cams XE262H, Griffen alum. radiator,
Turbine Technologies 2500 stall converter, underdrive pulleys, Crane Hi-6 & more.
1.05g skidpad verified.
New best E/T! 14.039 at 100.82 MPH in 41 degree air at Stanton, Michigan.

 

Hey Dan. That's another great present I got this year. A friend of mine just got a CD burner, and he gave me a copy of Dyno2000 for free!



------------------
89RS w/350 TPI; 69RS/SS w/450 HP 350/Muncie 4-Speed "Too weird to live, too rare to die."