Compression Ratio - What is it?
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From: Cameron, OK
Compression Ratio - What is it?
Ok, I hate to have to admit this but I have no idea what the compression ratio is and what is good, bad, or otherwise. I know what compression is, and I know if you bore an engine you increase the combustion chamber size... and it increases power. But does this increase or decrease your compression ratio? And I hear of guys shaving their heads (the ones on the engine for you smart @55es out there) as a performance mod but it seems that this would decrease the combustion chamber size... how does that help? Anyway, I've tried reading up on it online but I just haven't found a good explanation yet... anyone here have any input?
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From: 3rd Rock
Compression ratio is the volume above the piston at bottom-dead-center (BDC), divided by the volume above the piston at top-dead-center (TDC).
If you rebuild your engine, but all you do is bore it out slightly and install identically shaped, but bigger pistons. You do not shave the head or change the stroke, or decrease the volume of the combustion chamber. You will still increase your compression ratio.
If you rebuild your engine but all you do is add a stroker crank. And you order new pistons with the pins mounted farther up so that the piston does not intrude farther into the combustion chamber at TDC. Thus, your combustion chamber volume is unchanged - all that changes is the stroke. You will still increase your compression ratio.
Hope this helps
If you rebuild your engine, but all you do is bore it out slightly and install identically shaped, but bigger pistons. You do not shave the head or change the stroke, or decrease the volume of the combustion chamber. You will still increase your compression ratio.
If you rebuild your engine but all you do is add a stroker crank. And you order new pistons with the pins mounted farther up so that the piston does not intrude farther into the combustion chamber at TDC. Thus, your combustion chamber volume is unchanged - all that changes is the stroke. You will still increase your compression ratio.
Hope this helps
Post Fiend
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From: Topsfield, MA
Thats a little confusing for a beginner...but let me try and simplify...
Shaving heads reduces the combustion chamber volume, thus increasing compression which increases power (within reason). Most stock small blocks have compressions in the 8.5ish range. When you bore the motor out your increasing the size of the cylinders which allows more air into the motor...more air = more power.
Shaving heads reduces the combustion chamber volume, thus increasing compression which increases power (within reason). Most stock small blocks have compressions in the 8.5ish range. When you bore the motor out your increasing the size of the cylinders which allows more air into the motor...more air = more power.
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Compression ratio is Volume of combustion chamber with piston at bottom of cylinder divided by the volume of the combustion chamber with the piston at the top of the cylinder.
Now, lower CR makes less power and a less efficient engine. Gas engines run 7.5:1 to 10.5:1 Race gas engines can run up around 13:1. The higher the CR the more prone a gas engine is to detionation. Detonation can destroy an engine so Manufacturers try to reduce CR to reduce warrantee claims. Increasing octane rating can reduce detonation and high compression engines might only be compatible with high octane gas.
Forced induction engines (turbo or super charged) must have a lower compression ratio (around 7:1) because the pressurized intake charge has the effect of increasing compression ratio.
Related to above, is dynamic compression ratio. This is using cam timing and intake/exhaust overlap to increase or bleed off pressure thereby increasing or decreasing the effective CR.
Diesels for comparison run up around 20:1 CRs. This is how they ignite the fuel. Also one of the reasons they tend to be more efficient than gas engines.
Now, lower CR makes less power and a less efficient engine. Gas engines run 7.5:1 to 10.5:1 Race gas engines can run up around 13:1. The higher the CR the more prone a gas engine is to detionation. Detonation can destroy an engine so Manufacturers try to reduce CR to reduce warrantee claims. Increasing octane rating can reduce detonation and high compression engines might only be compatible with high octane gas.
Forced induction engines (turbo or super charged) must have a lower compression ratio (around 7:1) because the pressurized intake charge has the effect of increasing compression ratio.
Related to above, is dynamic compression ratio. This is using cam timing and intake/exhaust overlap to increase or bleed off pressure thereby increasing or decreasing the effective CR.
Diesels for comparison run up around 20:1 CRs. This is how they ignite the fuel. Also one of the reasons they tend to be more efficient than gas engines.
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From: Cameron, OK
Ok, that helps, thanks guys. But the actual number, say 7.5:1, what unit is that measured in... or is it? If I understand right the 7.5 is the volume in the combustion chamber at bottom dead center and the 1 is the volume at top dead center. But is that actually a volume measured in cubic inches or something or just a generic ratio. And how do you know what your compression ratio is? Is it something that you measure after you build the motor? Seems alot of guys know what CR they want and build to it rather than get it together and see what they have?
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No unit. Pure ratio. It is cubic inches divided by cubic inches so the units cancel out. The 1 represents the volume of the combustion chamber with piston at TDC so you take that volume of that, multiply it by the first number and you have the volume at BDC. If the units you use are cubic inches your answer will be cubic inches. If CCs then the answer is CCs.
There is no way to measure it on an assembled engine. You can estimate it using stock dimensions, but the only way to know for sure is to measure as you build it.
There is no way to measure it on an assembled engine. You can estimate it using stock dimensions, but the only way to know for sure is to measure as you build it.
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From: Cameron, OK
Thanks again guys for the info, I don't feel like such an idiot now. Now if I can ever get fluent in cam speak I will be ok.
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