converting psi to compresstion ratio

# Thread: converting psi to compresstion ratio

1. ## converting psi to compresstion ratio

I have #150 on the compression gauge and trying to pick out a new cam. The cam makers always want compression ratio. Is there a way to determine this? Thanks Scott

2.

3. Originally Posted by scottys
I have #150 on the compression gauge and trying to pick out a new cam. The cam makers always want compression ratio. Is there a way to determine this? Thanks Scott
Divide that compression reading by the atmospheric pressure at the moment it was measured. Use in/Hg /2 to get PSI

It's not exact but pretty close.

For example: If the atmospheric pressure was 30.00 in/Hg then divide 150 by 15. That would show 10:1 compression ratio.

4. Originally Posted by Infomaniac
Divide that compression reading by the atmospheric pressure at the moment it was measured. Use in/Hg /2 to get PSI

It's not exact but pretty close.

For example: If the atmospheric pressure was 30.00 in/Hg then divide 150 by 15. That would show 10:1 compression ratio.
isn't that kind if dependent on the cam that's currently there now? My stock Volvo 5.0 has about 175 psi when read off the gauge and I think factory specs are maybe 9 to 1. How close are we talking?

5.

6. You will be measuring the approx. effective compression ratio. True static compression ratio needs to be calculated using engine build specs.

Maybe someone else with more experience doing these types of tests will jump in. If smart will turn around and run instead of post.

7. Hey Scott,

The short answer is no. You can not get real compression ratio by checking cranking compression. But if you want to get close. Find some casting numbers on your heads and block and give us some more details of any after market parts that have been installed in the bottom end.

Also if you want to get really simple.....does the current combo run on "pump gas" with out detonation?
If so tell your camshaft company it's a "pump gas motor" and give them the head casting number and you should be in good shape.

Nic.

8. Originally Posted by scottys
I have #150 on the compression gauge and trying to pick out a new cam. The cam makers always want compression ratio. Is there a way to determine this? Thanks Scott
What is the history of the engine? Custom build? Stock? Boat? Car? BBC, BBF, B, RB, SBC, SBF??? Honda?

Wont be able to tell anything with just a compression test, but may be able to get close with more info. Any light you can shed on the subject will help point you in the right direction.

9. The motor is a 60 over 427 bbc (440) aluminum head open chamber. flat top piston w/single valve relief. Has a solid roller cam in the 760 lift range (#missing from cam rubbing on back plug) It came from a blown gas flat I had sold and am putting into a Henry J with enderly stack injection (unblown). Going for light street, strip.

10. Originally Posted by gn7
8 to 1

Might consider new pistons for a stack injection if you're interested in maximizing the combo. If not, 87 octane all day long wont be an issue.

8:1 is the number you're looking for though.

11. ## Ballpark number??

Originally Posted by Infomaniac
Divide that compression reading by the atmospheric pressure at the moment it was measured. Use in/Hg /2 to get PSI

It's not exact but pretty close.

For example: If the atmospheric pressure was 30.00 in/Hg then divide 150 by 15. That would show 10:1 compression ratio.
In this case my old way shows about the same number... Volumetric air pressure at sea level is about 14.7psi, so wouldn't 147psi cranking, compute to the same 10:1?? (or slightly higher for 150#).... (Yes, I know there is a margin of error based on variations in true volumetric pressure depending on weather conditions and altitude... but 14.7 will get you pretty close, IMO....)
Then of course the true way is to do the math, but you need ALL the numbers, including "dome volume", and an accurate combustion chamber volume.... (1 cubic inch equals 16.39 cubic centimeters).....AND, yes, cam overlap profile will bleed off compression at cranking speeds, AND, most people forget to #1, pull ALL the plugs, and #2, open butterflies wide open for the standard compression gauge test......Oh, and a good hot battery...
Ray

12. Originally Posted by Moneypit
In this case my old way shows about the same number... Volumetric air pressure at sea level is about 14.7psi, so wouldn't 147psi cranking, compute to the same 10:1?? (or slightly higher for 150#).... (Yes, I know there is a margin of error based on variations in true volumetric pressure depending on weather conditions and altitude... but 14.7 will get you pretty close, IMO....)
Then of course the true way is to do the math, but you need ALL the numbers, including "dome volume", and an accurate combustion chamber volume.... (1 cubic inch equals 16.39 cubic centimeters).....AND, yes, cam overlap profile will bleed off compression at cranking speeds, AND, most people forget to #1, pull ALL the plugs, and #2, open butterflies wide open for the standard compression gauge test......Oh, and a good hot battery...
Ray
Ray, you're on the wrong side of the cam events. Overlap has no effect on cranking pressure.

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