Compression question 454 Chev.

# Thread: Compression question 454 Chev.

1. ## Compression question 454 Chev.

I have a stupid question.

The hydro has a stock stroke 454, maybe bored .030, I have never had it apart yet so I don,t know what exactly is in it.

My cranking pressure is 135 - 150 PSI.
The seller claimed 13:1 CR. It has rectangle, open chamber iron Chevy heads. (I do know this much for sure from when I put the Webster girdle on)

Is it possible to guess at actual CR based on this?? I am thinking that I am wasting cash on racing fuel!

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3. ## Compression guess

If the compression is accurate, then you are 8.5 or 9.0 to 1 static.

4. Tommy, You can't use cranking pressure as a guage for compression ratio. The cam with it's timing and duration will have a direct effect on cranking pressure. It is possible to have an engine with 8 to 1 c/r and have 180 lbs of cranking pressure and another engine with 13 to 1 c/r and only have 120 lbs of cranking pressure. It's all where the valves open and close. Keep buying the race gas, it very well could have 13 to 1 c/r, you don't want to find out the hard way, it gets expensive.

5.

6. X2!:d

7. ## Compression

I'm no expert, just asking. I thought I saw a formula on here where you could take a compression reading, say 180 lbs and divide that by some fixed number and come up with your approximate compression ratio...is that true or just way off base...

8. Can't be done. Your can figure dynamic compression ratio vs static, if you know the valve timing events. But there is no way to figure compression ratio based on cranking pressure.

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9. Originally Posted by Tittyman
I'm no expert, just asking. I thought I saw a formula on here where you could take a compression reading, say 180 lbs and divide that by some fixed number and come up with your approximate compression ratio...is that true or just way off base...
The others are right. And yes, you can NORMALLY be fairly close - but depending on the cam it could be way off. If you have a 106lc, it's gonna change the readings quite a bit from a 112lc.

10. Thanks guys!

It just seemed a little low to Me. I expected a higher reading.
Oh well, I have all winter to pull a head and check it out.

11. Like others have posted, it could very well be close to that compression. If it has a long duration cam in it, with a late closing intake, your are going to get low cranking pressure numbers. Besides, cranking pressure much above 125Lbs on pump gas gets little iffy. I don't think those numbers are low at all if the thing has any cam at all in it.

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12. Originally Posted by thatguy
Thanks guys!

It just seemed a little low to Me. I expected a higher reading.
Oh well, I have all winter to pull a head and check it out.
About the only thing a cranking compression test is useful for is comparing one cyl to the next but it does have its place for field testing or quick checking for things like blown head gaskets, leaking/burnt valves and holes in pistons, etc. but a leakdown test is much more useful than simple compression testing.

13. The only thing I know about the cam is that it is a solid lifter, and has ABOUT .630 lift.
I measure the lift when installing the girdles. (I set lash at .022 and .024, thats what I could see he had it at on average)

The seller gave me a cam card for a .680 Crower roller, but when I pulled the valve covers and looked down into the galley I could see that it is not a roller. In his defense He said he also bought it with the engine in it, but who knows.

I can't do the engine this winter, the Miller is priority.

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