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Post by Dave Weckl on Aug 4, 2005 7:59:25 GMT -5
Would it be possible to check for cylinder pressure to make a ballpark figure on what is the compression of an engine?
The camshaft and pistons sizes are known but the actual combustion chamber volume is unknown.This is of course without taking the engin eoff the heads..
The goal is to see if its compatible with forced induvction
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Post by Mike Stark on Aug 4, 2005 9:45:03 GMT -5
You can't get enough info from cranking pressure to properly determine compression.
There are too many variables in the equation. It it cranks low enough you may be Ok anyway.
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Post by Maximum Race Engines on Aug 4, 2005 20:55:58 GMT -5
Like Mike said, there are way too many variables. Just getting a compression test reading doesn't tell you how much compression you have, but it can tell you how far off the combination is, ie: Camshaft being too large for the compression and or combination, etc... Yes, a lower number will be good for forced induction, BUT, if the camshaft is not for the application like a blower or turbo, it will not be suitable even though the cylinder pressure is on the low side. Alot of variables. Without knowing the details of the combination, like you said not knowing any specs to figure compression or if you dont know camshaft specifications and things like, it will be tough to say wether or not it will be good for forced induction. Don't just go by the fact that the number may be low enough at cranking speeds.
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Post by dennis on Aug 16, 2005 22:35:05 GMT -5
to give you an idea, one of my 396 bbc with 7.7 to 1 compression and 272 crane cam put out 150 psi cranking compression. another 396 with 11.7 to 1 compression and a factory ZL-1 cam which is 262/273 at .050 was 155 psi cranking comp. a 396 with 10.5 to 1 compression and a 233/239 at .050 cam put out 180 psi cranking compression and my 454 with 14.6 to 1 compression and solid roller cam with 256/268 at .050 puts out 270 psi cranking compression. a stock 9to1 350 engine from 1969 malibu put out 165 psi. thought i would share
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Post by Maximum Race Engines on Aug 16, 2005 22:41:04 GMT -5
Dennis, good points. But what I would do from there with each cubic inch you listed, I would go over each combination, and make changes to bring those cranking compressions to the specified numbers that I go by. For example that 14.6 compression 454 has 270lbs of cylinder pressure. You will gain power by dropping that by 50lbs, by camming it correctly. That would actually be too much cylinder pressure, because the cam is so small, it is not allowing the engine to breathe and power falls off. You CAN have too much and you CAN have too little. It all depends upon each application and combination.
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Post by dennis on Aug 17, 2005 14:06:00 GMT -5
what is this method you speak of for figuring cranking compression verses ci. for my street driven combos i keep it pretty much under 180 psi so i can run 87 octane. even the 11.7 to 1 396 i had a few years back ran on 87 octane fuel and i would throw 92 in there at the time for 175 hp shot. that was a fun 10 sec street car all out, other wise in street trim on radials with no nos it ran low 12's.........my 454 was an opps i was shooting for 13 to 1 compression and the heads i was using came in smaller then what was thought ran it anyway for a while very responsive good for 127 mph runs in the qrt. i got it lowered to 13.5 but never ran cranking compression test nor have i got it back to the track yet.
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