compression question

[fast90mini]

We are running a sbc 350 with 76cc heads, stock block, rhoads lifters with 4 valve relief pistons, I have a set of the same heads cut to 72cc, with the 76cc heads we have 150 lbs of compression, is it worth swaping the heads to the 72cc and how much gain in compression would we get? Other cars in our class have 180-190 compression and they claim they are all stock, just want to try something to keep up with the other cars.

[fast90mini]

Anybody????

[bushracing67]

compression ratio and psi on a compression tester are two completely different thing and do not necessarily correspond to one another, you need to do a volume test to determine compression ratio, which dropping 4cc would help out quite a bit, i can give you an example

a 355 zero decked with a 0cc piston (true flat top no reliefs) .020 thick 4.060 bore head gasket and 76cc head would be about 10.05:1

same combo with a 72cc head would be about 10.55:1

[bushracing67]

btw compression test (psi) is more of a test for wear, ring seal, cyl wall wear, valve seal to seat etc are all checked by psi check. either the cyl ability to build psi or leak down which is ability to hold psi, all just checking consistency from one cyl to next to verify nothing is wrong

[turner]

Duration/lobe seperation/where you have cam installed at , etc will all change psi.

[cautrell05]

I have to laugh every time I see a track with a cranking psi rule. My 360 is just over 12-1, 55 cc heads, .020 head gaskets, .014 deck height, 2 valve relief pistons. Its around 150 psi. My 89 gmc has a 98 vortec motor in it, all stock except for the tbi on it and its also around 150 psi at 9.4-1. I have pumped many 4 cylinders at work that are in the high 180s that are no more than 9-1. Good rule in theory untill you research it.

Nick

[3dinter]

I have to laugh every time I see a track with a cranking psi rule. My 360 is just over 12-1, 55 cc heads, .020 head gaskets, .014 deck height, 2 valve relief pistons. Its around 150 psi. My 89 gmc has a 98 vortec motor in it, all stock except for the tbi on it and its also around 150 psi at 9.4-1. I have pumped many 4 cylinders at work that are in the high 180s that are no more than 9-1. Good rule in theory untill you research it.

Nick

Our track has that rule, i dont mind some good research. Point me in the right direction cause it would be nice to beat it.

[perfconn]

Build a 14-1 engine and use a camshaft with a lot of overlap

[Goose]

I thought when doing a compression test for tech you were suppose to remove the pushrods for the cylinder you were testing? If that's the case then how is a 12:1 motor only pumping 150psi?

[billy5]

if you remove the pushrods and prevent the intake valve from opening,,then the only air that enters the cylinder is from under the rings and it wont build very much pressure at all.

[cautrell05]

I comp checked a quad4 a while back in a newer cavalier that was 210.

[stock car driver]

I thought when doing a compression test for tech you were suppose to remove the pushrods for the cylinder you were testing? If that's the case then how is a 12:1 motor only pumping 150psi?

That is a cubic inch tester that you remove the pushrods for. Not compression.

Its already been stated that compression cranking test psi is testing the sealing of the components for leakage... as in wear

[carowner73]

More gain thru the head gaskets than the heads themselves, run the old metal shim head gaskets with a spray on sealer.