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I found a cheap mildly built motor that I want to put a supercharger on but I dont know what compression ratio it is. Is there any way to calculate compression ratio by using and compression tester? Thanks
 

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Measure the compression PSI and divide by 14.7 and that will give you a good approximation. Remove all of the spark plugs to get it to crank as fast as possible and give the most accurate number. stock 9:1 compression motors usually crank around 125 psi, much more than that is getting to be a bit too much for a supercharger on pump gas.
 

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Measure the compression PSI and divide by 14.7 and that will give you a good approximation. Remove all of the spark plugs to get it to crank as fast as possible and give the most accurate number. stock 9:1 compression motors usually crank around 125 psi, much more than that is getting to be a bit too much for a supercharger on pump gas.
My last build cranked out 150 across the board(give or take a few).

Completely stock motor.

So 150/14.7 = 10.20:1 No way thats accurate. 1 point of compression is a big jump.

Here is what you need to accurately measure Compression Ratio

http://www.ford-trucks.com/article/idx/14/102/article/Determining_Compression_Ratio.html
 

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well, it's a good appproximation at any rate, but yes, cranking PSI is greatly effected by things like the scavenging effect of the cam, valve overlap, and what rpm your're cranking at. the fact that you're also injecting fuel into the cylinders, and liquids don't compress, will also effect the measured cranking compression.

however, when you want to know exactly what will and will not run on pump gas, measured cylinder pressure is more accurate than calculated compression ratio. motors with up to 10.5:1 compression can be made to run on pump gas if the cam is correct to allow the motor to bleed off cylinder pressure at low rpm's and avoid detonation.
 

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99-01 Cobras come with 10.5 CR, I can just imagine all the dumbass owners using 87 octane to save some $$, luckily Ford has the timing set quite conservative in the stock ECU Flash!
 

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Thanks for all the input guys. At least I know that if its higher than 150 its likely that it is not the stock 9:1 ratio. I will also disconnect the fuel pump relay so fuel will not affect the measurements.
 

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if you want to know your compression, your going to need to know the stroke of you motor, piston top and any volume mods, and cc of your heads, and thickness of your gasket, if you call Holcomb motorsports or Weisco or any big name engine builder they can input it in a computer program and tell you what your looking at for compression, dont try back yard methods, find out exact parts you have and call it in. Trust me they know what to ask you for.
 

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You can NOT measure pressure and divide by 14.7. That is absolutely wrong. The thing that screws that up is cam lobe timing. It would ONLY work if the cam had zero overlap, was installed straight up, the lifters had zero lash, and they lifted off the seat and closed back onto the seat when the piston was at exact BDC and TDC. This is never the case.

There really is nothing you can do at home to measure compression with the engine together. If it is apart you can measure the volume above the piston and the volume in the head chamber with a liquid, and estimate head gasket volume. If you know all that and bore and stroke you can find compression.

Or you could turn the engine on it's side so a spark plug hole is the highest point, remove the rocker arms, and measure the volume change of a liquid when filling the cylinder with the piston BDC, emptying it totally, and filling it again to the same level at TDC.

Using pressure won't work well at all.

Tom
 

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Tom I tried to tell them. I don't care what they do. I was just pointing out mis information, so others do start to think the same way.
 

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Tom I tried to tell them. I don't care what they do. I was just pointing out mis information, so others do start to think the same way.

I see this all the time in electronics. When something is impossible to measure with a simple test, human nature is to use a simple wrong or meaningless method.

Whoever invented the formula of cranking pressure /14.7 could not have understood the effects of valve timing on an internal combustion engine's cylinder pressure.

I can come just as close by saying "all 302 engines are 10.25 : 1".:D Then you don't have to measure any useless numbers or perform any useless math and you have an exact answer to two decimal points.:hihi:

Tom
 

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Thanks for all the input guys. At least I know that if its higher than 150 its likely that it is not the stock 9:1 ratio. I will also disconnect the fuel pump relay so fuel will not affect the measurements.
Dude you look at it this way. You could have a 12:1 CR and only have 125 PSI cranking compression. Compression ratio is a fixed Number. It doesn't change unless you modify stuff like the Combustion chamber, head gasket thickness,Piston compression Hieght, Bore size. Your thinking is totally wrong.
 
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