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Old 06-21-2004, 11:30 AM   #1 (permalink)
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Thumbs up Procedure and Math behind a compression test: Please Add Your 2 cents :)

I was recently asked how so here it is.
after reading your spark plugs, put all but one in (generally start with number 1) using the hose of the gauge, install the adaptor in the spark plug hole. crank the engine 2x and record the peak number. Do this 3x per cylinder. All the measurements recorded must be within at most 5% of one another for the measurements to be valid. Average the three into one and compare over the four cylinders. None of the measurements should be more than 10% off each other. If they are, your engine is fighting itself and not producing the best power it can so its not just about high vs. low. Then, average the 4 cylinders and compare to every individual measurement (12 comparisons) and Ideally no variation greater than 5% is observed.

So how do you know what your gauge ought to read? take the atmospheric pressure ( some barometers will be nice enough to provide you with psi) and get the amount of vaccum your engine is operating under (This is where I need help, please post vaccum at idle and the mechanical cr of your ride) and subtract that from atmospheric pressure. Finally, multiply this number in psi by the final compression factor of your engine (9.2 stock) to obtain what the gauge should read. Even variation over all 4 cylinders that average less than 5% are permisible.
14.7 psi is 29.92 inches of mercury
1 psi is 2.0354 inches of mercury
N psi divided by 2.0354 is N inches of mercury
N in Hg multiplied by 0.4913 is N psi

Again, please add your two cents and include vaccum at idle and mechanical cr as well as any compression test results you have. I will have some new ones once this storm goes by here in Denver.
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Old 06-21-2004, 12:50 PM   #2 (permalink)
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Looks like a good topic for discussion. We'll Sticky it for awhile.
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Old 06-21-2004, 02:46 PM   #3 (permalink)
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i would assume one would need this particualr "guage tool" with a "hose", how much do these run for. Is it cheaper to buy one or simply take your car in for a check up? Considering the fact that one doesnt take compression tests once a week.

i mean unless you rebuild engines daily is this method viable for the average DIY'fer?
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Old 06-22-2004, 12:00 AM   #4 (permalink)
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Originally Posted by 7thGear
i would assume one would need this particualr "guage tool" with a "hose", how much do these run for. Is it cheaper to buy one or simply take your car in for a check up? Considering the fact that one doesnt take compression tests once a week.

i mean unless you rebuild engines daily is this method viable for the average DIY'fer?
The gauge set is fairly inexpensive. While you want something accurate, there is no need for a snap on for most of us. Mine has a neat little burp button that allows it to hold the reading and then clear with no more effort than a stopwatch would require and I think it was 35 bucks or so at autozone.
One can speculate that regular tests might be informative because the amount of air that is compressed is dependent on what can come in so if you know your engine is in good shape then variations from measurement set to measurement set must be due to restrictions upstream. While this is not a reason to go out and buy one, it would be interesting to see if one can increase the volume and post compression numbers beyond the "expected" number.

also, I came across this formula for determining the final compression factor of an engine:

[((pi/4) X (B^2) X S)+ Vc) / Vc

pi =3.1415
B = Bore
S = Stroke
Vc is the volume of the combustion chamber

This formula works only if all the units are the same so if you use mm for the bore and cm for the stroke, its going to get messy. The Vc is in mm^3 and if you use mm everywhere it works good.
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Old 06-22-2004, 12:39 AM   #5 (permalink)
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im sure bone has something to say ... if he shows up !
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Old 06-24-2004, 12:06 PM   #6 (permalink)
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Quote:
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im sure bone has something to say ... if he shows up !
hey people, jump in!! this will be very useful reference when you need it but if you do not contribute, it wont work.
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Old 06-24-2004, 09:27 PM   #7 (permalink)
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A leakdown test could be a better diagnostic tool because it can reveal and pinpoint more problems. Here is an article I found on http://www.dsmtuners.com/forums/showthread.php?t=97401

If you find yourself needing to do a leakdown test yet can't find a shop to do it let alone one that knows what it is you can easily do it yourself at home if you have the proper compressor and compression tester gauge.

1. Take the small valve at the end of the hose on the compression tester out, it should have a little tool to unscrew it. This way air can go out without the valve stopping it.

2. Make sure the engine is at TDC for either cyl 1-4 or 2-3 and take all spark plugs out.

3. Now thread the compression tester onto one of the cylinder that are at TDC and pop off the gauge.

4. Now comes the compressor, most compressors have a dial to adjust the pressure from 0-125psi or so. Turn it to 0 and hook the hose up to the compression tester hose. When all is hooked up slowly increase the pressure to about 30-40psi and listen for air leaks, too much pressure and the engine will crank over since the plugs are out.

Air in the exhaust means exhaust valves (my car's problem), intake hose or sound in the upper ic pipe means intake valves. Lots of air coming out of the oil caps means rings and bubbling in the coolant means headgasket.

5. Redo above steps on other cylinder at TDC, then check other cylinders once you have them at TDC. Once all are done you should have found what your problem is.


Note: your compressor or tester may be different from what I used and may not work or may give different results. This is not a true leak down test but will give you similar results as it will tell you where you are losing your compression. This worked very well for me and was 100% accurate with diagnosing my low compression..bent exhaust valves.
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Old 06-26-2004, 05:48 AM   #8 (permalink)
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1 thing I noticed everyone didn't mention is that compression test need to be recorded with the throttle completely open. and there's 2 ways...with engine cold will give lower readings....run the engine for a couple of mins....not to long as it will get hot to the touch....but the oil on the cylinder walls will provide the seal as if the motor was running and provide more accurate #'s. like my motor cold will make 185 psi +or-....but after it's run fer a few mins that number changes to 210 +or-
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