Compression testing ... theory
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Nordschleife Master
Joined: Jan 2003
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From: North Dakota
Compression testing ... theory
I was reading another thread about testing compression and leakdown on a 993TT. There was some discussion that 135 psi was too low. Which got me to thinking, an always dangerous condition. The 993TT has a compression ratio of 8.0:1. Yes, its crazy low, which probably explains our very crappy fuel mileage. But here's where I need help. If normal atmospheric pressure is 14.7 psi, and you compress it 8.0 times, then I would expect a maximum compression pressure of 8.0 x 14.7 = 117.6 psi. And that's assuming ideal conditions.
So where have I gone wrong? No, scratch that. So what's wrong with my math?
So where have I gone wrong? No, scratch that. So what's wrong with my math?
Thread Starter
Nordschleife Master
Joined: Jan 2003
Posts: 8,498
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From: North Dakota
In fact, my arithmetic seems faulty, if you had a compression ratio of 2.0:1, then you would expect to read 14.7 on the compression gauge, not 29.4. So... according to my model, the max pressure for a 993 TT is 7.0 x 14.7 = 102.9 psi. This is clearly incorrect. Why?
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Nordschleife Master
Joined: Jan 2003
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From: North Dakota
Now this sounds right. I'm guessing that there is a formula that WILL predict the max compression pressure based upon the compression ratio plus the added pressure from the heat. And this has probably been made into a simple look-up table for mechanics to use when testing engines. Thanks for this, I'll sleep better tonight.
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From: Costa Mesa, CA
Your equation assumes that temperature doesn't change. But anyone who has put his thumb over the spark plug hole and had the engine cranked knows that the air is bloody hot. When a piston compresses the gas in a cylinder, it is usually assumed to be an isentropic compression meaning no heat is transferred and is reversible (i.e. no change in entropy). The pressure ratio for an isentropic compression is given by:
p2 / p1 = (v1 / v2) ^ (gamma) or p2 = p1* (v1 / v2) ^ (gamma)
v1/v2= compression ratio = 8, and gamma for air is 1.4
p2 = 270 psia = 255.3 psig
much higher than we see. But with a cold engine, the heat transfer assumption is probably not valid. Also, with valve overlap, some of the gas in the cylinder escapes. Both of these factors would yield lower than ideal pressure increase. So the pressure is going to be somewhere between the constant temperature assumption 102.6 psig and the isentropic assumption 255 psig. 135 psig is in that range.
p2 / p1 = (v1 / v2) ^ (gamma) or p2 = p1* (v1 / v2) ^ (gamma)
v1/v2= compression ratio = 8, and gamma for air is 1.4
p2 = 270 psia = 255.3 psig
much higher than we see. But with a cold engine, the heat transfer assumption is probably not valid. Also, with valve overlap, some of the gas in the cylinder escapes. Both of these factors would yield lower than ideal pressure increase. So the pressure is going to be somewhere between the constant temperature assumption 102.6 psig and the isentropic assumption 255 psig. 135 psig is in that range.
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From: Vancouver, BC
You just hadda ask, din't you?
As far as the numbers go, there are enough differences between the gauge accuracies, engine temperature, air temperature and just plain technique as not to worry too much about the absolute numbers unless they area grossly out of range. The delta is important though. Same goes for the leak down gauges, I am looking for the difference between the gauge detached and attached to the cylinder. The delta is the leak-down contributed by the engine.
I have a few compression gauges, and one of my older ones always reads a bit low, but it's repeatability is excellent.
I have not seen any calibration devices for these gauges (unlike torque wrenches where you can buy strain gauges now). I sometimes use compressed air set with a gauge and regulator to test my compression gauge as well as comparing two different gauges.
Cheers,
Mike
As far as the numbers go, there are enough differences between the gauge accuracies, engine temperature, air temperature and just plain technique as not to worry too much about the absolute numbers unless they area grossly out of range. The delta is important though. Same goes for the leak down gauges, I am looking for the difference between the gauge detached and attached to the cylinder. The delta is the leak-down contributed by the engine.
I have a few compression gauges, and one of my older ones always reads a bit low, but it's repeatability is excellent.
I have not seen any calibration devices for these gauges (unlike torque wrenches where you can buy strain gauges now). I sometimes use compressed air set with a gauge and regulator to test my compression gauge as well as comparing two different gauges.
Cheers,
Mike
RL Technical Advisor
Joined: May 2001
Posts: 11,870
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From: Portland Oregon
I'm guessing that there is a formula that WILL predict the max compression pressure based upon the compression ratio plus the added pressure from the heat. And this has probably been made into a simple look-up table for mechanics to use when testing engines. Thanks for this, I'll sleep better tonight.
Quite illuminating, to say the very least.
Let me if I can find it over the weekend and if so, I'll post it here.
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From: Canada & the Alps
I came across these CR and Effective CR (turbo) calculators.
Last edited by IXLR8; Jan 11, 2013 at 11:30 AM.