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post #1 of 25 (permalink) Old Jul 14th, 08, 9:36 PM Thread Starter
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Compression test...

Does anyone know where I can find some kind of conversion chart that will tell me the compression of my car if I am doing a standard compression test with a lbs gauge.

Or does anyone know what kind of reading I would get say if it were 10:1,11:1,12:1,12:.5,etc...

Thanks
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post #2 of 25 (permalink) Old Jul 14th, 08, 9:55 PM
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That totaly depends on the cam/duration etc ,you can have 11:0 comp with with 150 psi cranking compression and 9:5 comp with 180 psi cranking comp depending on the cam.

Just curious,what size motor/what heads/what pistons/and what are the cam specs?

Scott

SCOTT
1969 CHEVELLE SS396,ORIGINAL #'S MATCH,GOT IN 1978,(In 2001 rblt/bored original 396 .030 to 402)/M20/12BOLT/3:31'S
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post #3 of 25 (permalink) Old Jul 14th, 08, 9:58 PM
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Re: Compression test...

You cannot calculate what the cranking compression will be (PSI) from knowing what the static compression ratio is due to their being too many variables, however I have seen an online DYNAMIC Compression ratio calculator which tells you what the cranking compression of your engine will be, but you have to punch in the exact camshaft specifications, and if you don't know what they are then it won't work right. Furthermore, if you perform a compression test on an engine that has worn rings, worn or improperly seated valves, then your PSI readings on the compression guage will be reduced, and that will make it more difficult to accurately determine your actual compression ratio from such a test. There really are a lot of variables. When I get home, I can post a link to a dynamic compression ratio calculator. Although there are a number of them on the net, I've only found one which gives you the PSI reading of what the cranking pressure should be.

70 Chevelle SS clone (632 CI powered).
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post #4 of 25 (permalink) Old Jul 14th, 08, 9:59 PM Thread Starter
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Re: Compression test...

Hey Scott,

Chevy 454, bored .60 over, Merlin Rectangular Port Heads 219 188, Mechanical Comp Cam 637 lift 270 Duration. Need more info? Not a problem, I could really use any imput.

Thanks,

Drew
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post #5 of 25 (permalink) Old Jul 14th, 08, 10:03 PM
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Re: Compression test...

Any idea what pistons are in it or if the block was decked?

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post #6 of 25 (permalink) Old Jul 14th, 08, 10:20 PM Thread Starter
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Re: Compression test...

TRW - 454ci Open Chamber Head Race Piston, .060'' Overbore
Piston Dia.: 4.310''
Piston Top: 50cc Dome
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post #7 of 25 (permalink) Old Jul 15th, 08, 12:17 AM
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Re: Compression test...

i had 11.7 to 1 bbc 396 with a solid ZL-1 cam specs 262/273 @ .050" it was cranking out 150 psi in the cylinder.

396 bbc with 7.7 to 1 compression and a 272 crane cam it too was 150 cranking psi

454 with 50 cc dome pistons 14.5 to 1 compression and herbert solid roller cam 254/268 @ .050" it was 270 psi.

little off beat 350ci with 11.4 to 1 compression and 284 crane cam had 220 psi.

thought i would just throw those out there to show some examples of some of the engines i have put together.

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post #8 of 25 (permalink) Old Jul 15th, 08, 12:43 AM
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Re: Compression test...

my old 10.3:1 327 with a 248/254@,050 sild cam was only 150psi. Can`t go by cylinder pressure to measure static compression.
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post #9 of 25 (permalink) Old Jul 15th, 08, 1:38 AM
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Re: Compression test...

Here you go.... thelink below will bring you to a site where you must punch in all the cam info as well as the bore and stroke of your engine. You can then leave the "volumetric efficiency" box at 94 and the boost at 0 as well as the altidude at 0 and then click the box right below it that has the flames in it. A window will pop up with your "Dynamic" compression ratio and also the dynamic cranking cylinder pressire in PSI. And the second link below is a good calculator for static compression ratio in case you don't know what it is in your engine. You're going to need the exact static compression ratio figure for the the first one for finding your true dynamic compression ratio.

http://www.speedwaybids.com/DynamicC...Calculator.php

http://www.rbracing-rsr.com/compstaticcalc.html

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post #10 of 25 (permalink) Old Jul 15th, 08, 8:42 AM Thread Starter
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Re: Compression test...

Quote:
Originally Posted by BillyGman View Post
Here you go.... thelink below will bring you to a site where you must punch in all the cam info as well as the bore and stroke of your engine. You can then leave the "volumetric efficiency" box at 94 and the boost at 0 as well as the altidude at 0 and then click the box right below it that has the flames in it. A window will pop up with your "Dynamic" compression ratio and also the dynamic cranking cylinder pressire in PSI. And the second link below is a good calculator for static compression ratio in case you don't know what it is in your engine. You're going to need the exact static compression ratio figure for the the first one for finding your true dynamic compression ratio.

http://www.speedwaybids.com/DynamicC...Calculator.php

http://www.rbracing-rsr.com/compstaticcalc.html
Thanks BillyGman!
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post #11 of 25 (permalink) Old Jul 15th, 08, 11:05 AM
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Re: Compression test...

My 12.8:1 468(according to je spec piston) with a 278/282 @.050 cam was 190-195 psi.

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post #12 of 25 (permalink) Old Jul 15th, 08, 11:28 AM
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Re: Compression test...

Quote:
Originally Posted by dd69ss View Post
Thanks BillyGman!
You're quite welcome.

70 Chevelle SS clone (632 CI powered).
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post #13 of 25 (permalink) Old Jul 15th, 08, 12:05 PM
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Re: Compression test...

BTW,FWIW, the famous David Vizzard claims that engines with dynamic cranking pressures of less than 200 PSI will run on 93 octane pump gas without pinging, and when you go above the 200 PSI mark, you're usually getting too close to pinging on pump gas, and race gas is usually required to avoid it. I hope this helps.

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post #14 of 25 (permalink) Old Jul 15th, 08, 12:29 PM
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Re: Compression test...

That would be awesome if I could run mine on pump gas. A little scared to try it though...

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post #15 of 25 (permalink) Old Jul 15th, 08, 8:19 PM Thread Starter
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Re: Compression test...

Quote:
Originally Posted by BillyGman View Post
BTW,FWIW, the famous David Vizzard claims that engines with dynamic cranking pressures of less than 200 PSI will run on 93 octane pump gas without pinging, and when you go above the 200 PSI mark, you're usually getting too close to pinging on pump gas, and race gas is usually required to avoid it. I hope this helps.
Yeah I did a dynamic pressure test today, came up 220 psi.

I am thinking of just running race fuel and re-adjusting the timing because obviously it's retarded in order to run on pump gas.

she still runs like an animal though, without pinging or any noise.
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