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Old 01-04-2005, 12:07 PM
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compression ratio compared to compression PSI

Is there a relationship between compression ratio and compression PSI checked at the plug hole? I bought this motor from a friend and he thinks it is 12:1 or better and doesn't remember cam specs other then it was Isky. It is a 350 chev with 202 heads. I have 185lbs at the plug holes and 13 lb of vacum at 1000rpm idle.

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Old 01-04-2005, 12:13 PM
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there's no relation. It depends more on the cam than anything else-

K
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Old 01-04-2005, 12:16 PM
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COMPRESSION

If you can get the cam spec's i can back figure it for you pretty close. With out them it's like wizzing up a rope!!!!

Keith
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Old 01-04-2005, 12:20 PM
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compression

Is this due to valve overlap? Is there a way to measure with a dial indicator to get cam specs. Has solid lifters.
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Old 01-04-2005, 12:21 PM
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Compression Comparison

In a word, NO. Compression ratio is based on squeezing the swept area into the unswept area of the cylinder. Compression ratio is figured on the gross combustion area in the cylinder,when the piston is at bottom dead center, divided by the unswept area, which is the area above the top of the piston when its at top dead center. This gives you the static compression ratio.
Cylinder PSI is the actual pressure built in the cylinder, when the mixture is totally compressed.
There are a lot of factors governing PSI, cam overlap, cam timing, are two major factors.

For example; a 64 289 2 bbl engine with 9:1 compression, spec'd at 150 PSI.
A 64 260 2 bbl engine with 8.7:1 spec'd at 150 PSI, and a 200 cube 6, with 8.7:1 compression spec'd at 170 PSI.
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Old 01-04-2005, 12:46 PM
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compression

I was just thinking about this all i really need in the intake closing degrees @.050 lift. If the engine is on a stand then you can use a degree wheel. Get it set to true tdc. Then let me know what the intake valve is at degree wise when the valve is at .050 before closing.

I need bore size, stroke, rod length,and the altitude your at.

Keith
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Old 01-04-2005, 12:53 PM
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PSI

The above PSI specs I used earlier, are, as they are for all engines, based on controlled environment, with atmospheric pressure set at sea level, mean temperatures etc. However, those figures are mostly affected by altitude when all other factors are the same.
150 PSI at sea level, will equate to about 120 PSI at 6000 ft elevation. (approx. figure).

If you have the spec sheet for your cam, it may well tell what the valve overlap is.
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Old 01-04-2005, 05:07 PM
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Having 185 PSIG and 13" vac tells me a couple of things. You have pretty stiff static compression and a pretty healty cam. I don't think you are running more than 12:1 though. I run just under 200 PSIG and 11.5" vac with 11.85 compression and a 258/266 @.050 camshaft. I do have quite a bit more displacement (454 small block) though it at least gives you some numbers to ponder.

One thing is for sure, if you can run pump gas then you are not running 12:1 compression. Won't work with a 2.02 iron head.
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Old 01-05-2005, 12:34 AM
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ratios

All I use a compression tester for, is to see if all cylinders are equal. I got a reading of 160 psi on 3 cylinders on a 4 banger. It had a broken ring on every piston. The cylinder with 2 broken rings was 80 psi.
Compression ratio is determined by piston dome and cylinder head combustion chamber cc size.
If you take the casting #s off the head. Look them up and it will give you a pretty good idea where you are.
For example; not too many people will dump alot of money into a 78 cc smogger head. Yes, you can put a high dome piston in it, but it isn't usually done.
A 70 cc head would look better and a 64 cc head would tell you there is probably a couple of bucks in there.
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Old 01-05-2005, 08:31 AM
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KBs site that explains it.
http://kb-silvolite.com/article.php?...54ee33dbc145cf
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Old 12-28-2010, 06:58 PM
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Thanks for the good info.
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Old 12-28-2010, 10:36 PM
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Personally I do believe that cylinder pressure can be converted to compression ratio. If your compression ratio is 12:1 theoretically that means the compression is 12 times atmospheric pressure. (atmospheric pressure compressed to 1/12th its uncompressed volume) At sea level there is 14.7psi of atmospheric pressure. 14.7 x 12 = 178psi. Not far from 185 and the compression tester could be off that much.

It may not be exact but it should get you close. 178 is an awful lot of compression for a gas engine so I would believe 12:1 to be accurate or close to it.
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Old 12-28-2010, 11:15 PM
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Quote:
Originally Posted by Torque454
Personally I do believe that cylinder pressure can be converted to compression ratio. If your compression ratio is 12:1 theoretically that means the compression is 12 times atmospheric pressure. (atmospheric pressure compressed to 1/12th its uncompressed volume) At sea level there is 14.7psi of atmospheric pressure. 14.7 x 12 = 178psi. Not far from 185 and the compression tester could be off that much.

It may not be exact but it should get you close. 178 is an awful lot of compression for a gas engine so I would believe 12:1 to be accurate or close to it.
Doing it this way will be as close as throwing darts at a chart, you'll get lucky every once and a while.

Your "theory" is not even close, intake valve closing point(cam timing) can alter this figure by 40+psi either way. You don't start trapping pressure in the cylinder until the intake valve is closed, there will be a big difference in even the same engine with a either a 45 ABDC intake closure and a 70 ABDC intake closure. I've seen an 11-1 engine pump 230 psi, and I've seen 11-1 pump 150 psi....depending on the camshaft size.

This is the reason why dynamic compression ratio calculators need mechanical compression ratio, stroke length, rod length, and intake closure point to compute a dynamic compression ratio calculation. There is no other way to do it.
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Old 12-28-2010, 11:27 PM
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Quote:
Originally Posted by ericnova72
Doing it this way will be as close as throwing darts at a chart, you'll get lucky every once and a while.

Your "theory" is not even close, intake valve closing point(cam timing) can alter this figure by 40+psi either way. You don't start trapping pressure in the cylinder until the intake valve is closed, there will be a big difference in even the same engine with a either a 45 ABDC intake closure and a 70 ABDC intake closure. I've seen an 11-1 engine pump 230 psi, and I've seen 11-1 pump 150 psi....depending on the camshaft size.

This is the reason why dynamic compression ratio calculators need mechanical compression ratio, stroke length, rod length, and intake closure point to compute a dynamic compression ratio calculation. There is no other way to do it.
You're talking dynamic compression, I am talking static compression. I think he was in reference to 12:1 static compression. Yes some of that will change based on cam specs, etc.
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Old 12-28-2010, 11:36 PM
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Quote:
Originally Posted by Torque454
You're talking dynamic compression, I am talking static compression. I think he was in reference to 12:1 static compression. Yes some of that will change based on cam specs, etc.
When you test an engine with a compression gauge, you are cycling the engine, you are measuring dynamic compression at this point, no other way to do it.

You can't cycle the engine and come up with any meaningful measurement of static compression ratio, but you can take this dynamic reading, combined with the intake valve closing point in degrees After Bottom Dead Center and do the math to come up with a reasonable approximation of static compression ratio. But this can still be influence by altitude, and starter cranking speed to throw the result off.
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