Hey guys.
I am trying to figure out my CR. For simplicity's sake, I am comparing my rebuilt 357 shortblock to a new GM shortblock. They both have -13cc 4V relief pisons, stock deck height, and 58 cc heads.
The GM one gets 9.5:1 CR with the stock 4" standard bore pistons and .039" compressed thickness head gasket.
My shortblock has 4.040" pistons and i am thinking of going with a thinner .015 head gasket.
Is there a way I can figure my CR from this info?
Thanx in advance

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'88 RS (originally 2.8)
-350, .040 over, vette alum. L98s, stock tpi.
-305 injectors & chip
-TES headers & edelbrock cat back
-3.73 gears, 700r4.
Best with 2.8-17.4@77mph
Best with 305-15.0@93mph
Best with 356-13.7@101mph
on a 2.050 60 ft.
(Damn the 2.8!)

 

My calculations show that your 4.040 cylinder with a .015 head gasket and all other measurements the same on both engines will give you a compression ratio of around 10.16

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Follow my racing progress on Stephen's racing page
and check out the race car

87 IROC-Z SuperPro ET Bracket Race Car
461 naturally aspirated Big Block (times are for the current engine)

Best ET on a time slip: 11.447 altitude corrected to 10.99
Best MPH on a time slip: 119.42 altitude corrected to 124.86
Altitude corrected rear wheel HP: 493
Best 60 foot: 1.586

Racing at 3500 feet elevation with a typical race day over 5000 feet density altitude!
Member of the Calgary Drag Racing Association

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

 

Why thankyou sir.

 

you'll have problems with the shim type gaskets. usually the surface finish isn't that good for them to seal and last. coated shims are much better, but thicker. i like the standard coated comp type gaskets and you don't give up much on compression.

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ICON Motorsports
1st & 3rd
MM Black Diamond 538 F&AM

 

I get a compression ratio around 12:1 depending on the deck height. If you are using Speed pro pistons I don't think they move the pistons as close to the top of the cylinder, so that will increase your deck height thereby reducing your compression ratio. Call the piston manufacturer you plan to use and have them help you double check this.

hope this helps,
Craig Maners