Calculating Compression Ratio - The 1947 - Present Chevrolet & GMC Truck Message Board Network

orange72350 · 03-15-2004, 04:42 PM

I have a rebuilt 350. the long block was put together by the rebuild company, who has since gone out of bussiness. i know that i have flat top pistons, but i cant get the numbers off the heads right now. i know that they are stock 1972 chevy pickup heads. i would really like to know the compression ratio on this engine if at all possible. thanx guyz. let me know if u need more info about the heads.
thanx
dale

Bowtie67 · 03-15-2004, 05:04 PM

Flattops come in different flavors, generally they are 9.1, but can range all the way up to 11.1. Usually in most cases 9.1. Also most early chevy stock trucks heads are 76cc. So with this combo you should have at least 9.1CR. The only true way of knowing for sure is pulling heads and checking pistons, and getting the numbers off the heads. or find the slip there should be some reference to what part number pistons or CR engine was setup to.

bpmcgee · 03-15-2004, 05:07 PM

If you have the engine apart, then you can do the only REAL way of checking, which is to actually measure the displacement.

The actually size of your combustion chamber will vary depending on how much porting has been done. Also, you have to take into account the quench area -- the height below the deck of the piston at TDC. Lastly, you need to know the compressed thickness of your gaskets. Without these three numbers your compression estimate will always be too high.

Brian

orange72350 · 03-15-2004, 05:11 PM

thanx man. good stuff to know.

03-15-2004, 04:42 PM	#1
orange72350 Registered User Join Date: Jan 2003 Location: Cold, Snowy High Level, Alberta Canada Eh Posts: 394	Calculating Compression Ratio I have a rebuilt 350. the long block was put together by the rebuild company, who has since gone out of bussiness. i know that i have flat top pistons, but i cant get the numbers off the heads right now. i know that they are stock 1972 chevy pickup heads. i would really like to know the compression ratio on this engine if at all possible. thanx guyz. let me know if u need more info about the heads. thanx dale __________________ 2003 yamaha raptor. Stock, but it still hauls!!!! 2000 skiddo mxz 700. Camoplast 1.5 inch track, jaws twin pipes, b+b can, custom built and weighted clutch, wiseco piston, high comp heads. its very fast. wow. 72 c-10 longbox. Bright Orange. Brand new 350, rv cam, performer intake, qjet, headers, 2 1/2 inch exhaust into hooker areo chamber mufflers. Body work coming soon. "dont let yer mind wander, its to little to go places by itself." "Forget the herse, cause i'll never die"-ACDC Back In Black "If It's To Loud, Yer Just Too Old" "I Love Anything Fast Enough To Do Something Stupid In" "Instant idiot, just add beer!!""

03-15-2004, 05:11 PM	#4
orange72350 Registered User Join Date: Jan 2003 Location: Cold, Snowy High Level, Alberta Canada Eh Posts: 394	thanx man. good stuff to know. __________________ 2003 yamaha raptor. Stock, but it still hauls!!!! 2000 skiddo mxz 700. Camoplast 1.5 inch track, jaws twin pipes, b+b can, custom built and weighted clutch, wiseco piston, high comp heads. its very fast. wow. 72 c-10 longbox. Bright Orange. Brand new 350, rv cam, performer intake, qjet, headers, 2 1/2 inch exhaust into hooker areo chamber mufflers. Body work coming soon. "dont let yer mind wander, its to little to go places by itself." "Forget the herse, cause i'll never die"-ACDC Back In Black "If It's To Loud, Yer Just Too Old" "I Love Anything Fast Enough To Do Something Stupid In" "Instant idiot, just add beer!!""

03-15-2004, 05:04 PM	#2
Bowtie67 Registered User Join Date: Jan 2004 Location: NH Posts: 2,266	Flattops come in different flavors, generally they are 9.1, but can range all the way up to 11.1. Usually in most cases 9.1. Also most early chevy stock trucks heads are 76cc. So with this combo you should have at least 9.1CR. The only true way of knowing for sure is pulling heads and checking pistons, and getting the numbers off the heads. or find the slip there should be some reference to what part number pistons or CR engine was setup to.

03-15-2004, 05:07 PM	#3
bpmcgee Registered User Join Date: Mar 2003 Location: Northern Illinois Posts: 2,181	If you have the engine apart, then you can do the only REAL way of checking, which is to actually measure the displacement. The actually size of your combustion chamber will vary depending on how much porting has been done. Also, you have to take into account the quench area -- the height below the deck of the piston at TDC. Lastly, you need to know the compressed thickness of your gaskets. Without these three numbers your compression estimate will always be too high. Brian



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