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Old 07-24-2005, 10:52 PM
classic_chevy_guy's Avatar
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How to figure compression ratio......

I bought this 350 SBC and it seems pretty powerful, the guy i bought it from says its pulling some monster compression, but i have no way of knowing what the compression ratio is. Does anybody know how i could go about finding out what the compression ratio is, without tearing the block apart??? I heard that you can figure it out by using a compression tester, then looking up what compression ratio corresponds to that reading, but where would i find a table like that??? Ive searched the entire site and i cant find one ( maybe im blind ), but if anybody could help me out, id appreciate it.

thanks,
classic

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Old 07-24-2005, 11:23 PM
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Well, to get a basic idea of the compression we would need to know what the combustion chamber CC is. Is the engine all stock? Do you know if the pistons flatty's or domed? Has the block been machined (decked)? If you can answer those questions then somebody here will be able to give you an idea of what it may be. To get an accurate calculation one would need to know the following:

Bore
Stroke
Combustion Chamber CC
Head gasket CC (or head gasket Bore and Thickness)
Pistons Dome or Valve Relief CC
And also the Deck height.

I would not believe it to be true that a compression tester can tell you the static compression of a motor. That tester is reading your the cylinder pressure produced, based upon the RPM that the starter cranks at and the valve timing. You could have the same motor, same compression ration, but if you retarded the camshaft it would read a lower cylinder pressure when you do a cranking test. This is because the intake valve is closing later, letting more pressure escape at lower RPM before the valve closes.

Hope this helps!
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Old 07-25-2005, 12:02 AM
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i think i found it!!!

i just went to this other forum, like this one, and they had a whole bunch of calculators. It said it was just a ballpark estimate, but being as how i dont know all that much about the engine its better than nothing. From what he told me - the heads have been "tweeked" but are still stock, its got an aftermarket cam, the cranks been balanced, and the pistons are aftermarket. From what i can see - the heads are still stock with aftermarket springs and rockers, it definitely has an aftermarket cam, the crank is balanced, and theres got to be more than that, so i think the pistons had to have been switched out. I really dont want to tear the engine down, but i know thats the only way to tell for sure. Ive never measured the dome/dish on a piston, so how would i know what that is? The website gave me this (ballpark estimates):

8:1 - > 102.9 psi
8.5:1 - 102.9 > 110.25 psi
9:1 - 110.25 > 117.6 psi
9.5:1 - 117.6 > 124.95 psi
10:1 - 124.95 > 132.3 psi
10.5:1 - 132.3 > 139.65 psi
11:1 - 139.65 > 147 psi
11.5:1 - 147 > 154.35 psi
12:1 - 154.35 > 161.7 psi


thanks,
classic
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Old 07-25-2005, 06:48 AM
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LOL - my 8.5:1 motor makes ~150 psi.
For those numbers you have to make sense the motor would have to have a whopping big cam in it.
Static CR can be derived from cyl. pressure (I have seen dyno programs that give reasonably accurate cyl. press. est.) but I'm sure that you would also need all the specs on the cam card and a fair amount of time to sort through the formulas.
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Old 07-25-2005, 10:19 AM
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it wasnt a very good site, but thats all they had to go by. i guess id better tear it down and see for myself.

thanks,
classic
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Old 07-25-2005, 10:31 AM
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Quote:
Originally Posted by classic_chevy_guy
i just went to this other forum, like this one, and they had a whole bunch of calculators. It said it was just a ballpark estimate...
...maybe a ballpark on mars , j/k

Yea, my 10:1 makes 150 cold and 165 hot. Then there was my 9.5:1 motor that made 180 cold/190 hot!!

Are you able to get casting numbers off of the heads? Could you ask the guy that built the motor what pistons he put in?
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Old 07-25-2005, 11:21 AM
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Itīs the only way......http://carcraft.com/techarticles/982/
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Old 07-25-2005, 09:34 PM
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Not to say that i wouldnt trust the guy, but he also didnt build it, his cousin that owns a machine shop built it for him and he just told him to do whatever. It was in a Nova but theres no way of knowing if it came from that until i get the numbers. Anybody know of a really good website to go to where i can run the numbers??? And how would i go about measuring the pistons, i guess i could check at AutoZone (should be able to get something since i work there now.....woo hoo). Can anybody recomend how much boost i should run if i added a turbo and it was 10:1, about 3-4lbs right, just throwin some ideas out there.
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Old 07-26-2005, 10:39 AM
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If you turbo a 10:1 350 it will not be happy with you!! I would shoot for an 8-8.5:1 CR with boost, using a forged bottom end. It sounds like you are opening a can of worms here, you may want to rethink the whole turbo thing...at least until you can figure out compression ration
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Old 07-26-2005, 09:59 PM
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the safest amount of boost at 8-8.5:1 is about 8lbs, or at least thats just what all the manufacturers recommend. I figured if i ran it low, it would be enough to make a little bit of difference, but not do too much damage. If it is 8-8.5:1 you can bet ill boost it at around 8lbs, but if its about 10-10.5:1 then im gonna try it at about 3-4lbs. I really dont care about the engine all that much, its gonna get replaced with a 427 anyway, so.... i figured why not mess around and try stuff, its a spare anyway.
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