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Old 30-05-2008, 21:48   #1
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Is there a formula for Converting Compression Ratios

to PSI?

I am thinking that say a 10:1 Compression ratio would give a PSI of 140 or so.

Am I correct in that thinking?
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Old 30-05-2008, 21:57   #2
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The answer is EZ! Goto Compression ratio - Wikipedia, the free encyclopedia
and here Data compression ratio - Wikipedia, the free encyclopedia
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Old 30-05-2008, 22:49   #3
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The first one I looked at...it is too late in the evening for math formulae



Thsecond one looks like it is for computers.
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Old 30-05-2008, 23:14   #4
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The short answer is yes, the long answer is no... if you are checking cylinder pressure of an engine. Stop reading here, if you aren't working on an engine... (Grin... Its late and I won't sleep till its all out on paper.)

Engines have two compression ratios. Static, and dynamic.
Static, is swept area, the stroke of the connecting rod by the cylinder diameter (Which gives your base volume) over combustion chamber volume. This accounts for head gasket thickness, valve cutouts, and depth of the top ring land. Pretty straight forward.

Dynamic involves the overlap of the intake and exhaust valves. Because there is an overlap, some intake charge exits with the exhaust. The idea being that at high rpm's the exhaust gas does not have enough time to exit the chamber completely. Engines with less overlap are geared towards low rpm usage... and develop a lot of cylinder pressure, but still less than the static ratio. Engines meant for high rpm use develop less compression, but have better cylinder filling... less exhaust in the chamber.

Once you have the static compression ratio, you take the camshaft profile and calculate the dynamic compression.

But... if you were to check the cylinder pressure you still have a few kinks. That is leakage. The piston rings do not seal absolutely, and the valves may or may not seal perfectly. So... if you know the PSI of the cylinder, you can't necessarily reverse engineer the puzzle unless you know static or dynamic... and guesstimate the engine wear.

Finally getting to your question:

You take your atmospheric pressure in PSI, sea level 14.7. Then you multiply your by the number above the denominator. In your case 10. You end up with 147psi. This assumes a perfect seal at sea level... engines don't. The seals with flippers on the other hand...

Because of all the other factors, it isn't very reliable beyond a ball park figure. So... for the sake of adding to the ball park figures: A gasoline powered engine meant for low rpm use will have a range between 115-150psi. 100psi is the wear limit on the low side, less than that and no tuneup will cure what ails her. A wild machine will range from 150-250psi. Something thats routine maintenance involves removing the cylinder head may start reaching for 275psi... and push the limits of what race octane gasoline can handle.

Detonation is the reason behind all this. Increasing the cylinder pressure improves the thermal efficiency of the engine, but only increases brake output by about 2-4% a point. Low maintenance machines sacrifice potential in name of longevity. Detonation is the reason. Gasoline detonates at 1075 absolute... or 615F. This benchmark is a theoretical engine with 13:1 compression at sea level on a 90 degree day. (F to absolute = +460) Raising the compression ratio from 10 to 11 raises the cylinder pressure 147 to 161psi. (On paper!)

... and the math which is used to calculate the difference in temperature between the two compressed ratios is eluding my tired brain. Good stopping point!
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Old 31-05-2008, 06:40   #5
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Keeping it simple and logical.

First the compression ratio is the comparison of the cylinder volume when the piton is all the way down(bottom dead center, BDC) (maximum) and when the piston is all the way up (top dead center, TDC) (minimum). For our example, lets assume the compression ratio is 15:1 (diesels frequently run this high or higher)). That is the maximum volume is 15 times greater than the minimum.

So logically the gauge pressure at the bottom would be 0. In other words it is at one atmosphere, or 14.7 psi. In our example this air will be squeezed to 1/15th of it volume. That means the pressure will be 15 times greater than 14.7 or abut 220.

Now for the complications.

I assume you want to know the psi value to check for compression and your gauge is in PSI.

This means the engine will be turning at cranking speed. Pretty slow. Intake valves often aren't closed at BDC so some of the gas is pushed out as it starts up so the PSI at TDC is a little lower.

The gas heats due to compression (in a diesel it is that heat that provides ignition) so the pressure goes up.

Piston rings and valves seats, even when new, don't provide a perfect seal, so the reading goes down.

Knowing exactly what the PSI should be at cranking speed requires a shop manual for your engine.

As in the Wikipedia link in an earlier post, and only as a rule of thumb PSI is generally 15 to 20 times the compression ratio . Every engine will vary but all cylinders in the same engine should be withing a few PSI of each other.

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Old 31-05-2008, 11:12   #6
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Thank you all for your help.
Sometimes even a mechanic needs some more edjumacation
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Old 31-05-2008, 11:43   #7
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Hi Chief,
It would be a linear equation using a simple ratio. ...assuming a perfect seal at the rings and the valves and assuming the pressure inside the cylinder is at atmospheric pressure when the valves close...of course in reality for a normally aspirated engine that pressure would be slightly below atmospheric, because of resistance across the opemn valve and throught the manifold. For a supercharged or turbo'd engine the pressure would be something above atmospheric pressure....and this would of course vary with the speed of the engine. With the supercharger or turbo unit spun up, the pressure would be greater when the valves are close.

Also, when the valves close will vary with speed with a gasoline engine and with a computer controlled Diesel engine that uses solenoid controlled valves.

Its not a simple answer.

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Old 01-06-2008, 01:34   #8
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And hence all the above is why compression testing does not tell you a lot. The better test is a leak down test. This gives a constant volume of air and the leaking air is summed against the incoming and the difference is your test of compression. By listening to air escaping, you can determine if the air is going past rings or valves etc.
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Old 01-06-2008, 05:38   #9
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If you don't have a leak tester, you can squirt a little oil in the spark plug or injector hole and retest. If the rings are the culprit, the oil will help seam them and compression will jump several PSI. If valves are the culprit, the squirt of oil will produce no improvement.

Again, these are not specific identifiers, just rough indicators.

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