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Thread: Compression Test - Altitude Factors

  1. #1
    Administrator The Altered Beast Gumba's Avatar
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    Default Compression Test - Altitude Factors

    Altitude and temperature affect compression readings. Manufacturers specifications are almost always given at a specific altitude (14.7psi sea level) and 59 degrees Fahrenheit. Both temperature and barometric pressure changes as you go up in altitude, so you will need to correct your measurements if you wish to compare it with factory specifications. The following chart provides conversion factors for correctly compensating for changes in altitude.

    Compression Altitude Chart.jpg

    A standard compression reading of about 150 psi at sea level in Los Angeles would measure significantly less in the surrounding mountains. For example at an elevation of 6000 feet, the expected reading would be 150 psi x .8359 = 125 psi. The cylinders would be reading low if compared to sea level measurements, yet perfectly fine at this altitude.

    You can determine if the rings are causing the low compression readings by squirting about a tablespoon of standard 10W-30 oil into the cylinder. Crank the engine 2-3 times to spread the oil inside the combustion chamber. Then retest the compression. If the readings shoot up significantly (45 psi or so) then the problem most likely is the piston rings seating to the cylinder wall. Squirting the oil inside the combustion chamber in this manner allows the rings to temporarily seal more than if they were dry. If the compression numbers don't change then the most likely culprit is a leaky valve.

    Time for a leak down test.
    Last edited by evoRSTroy; 12-21-2010 at 08:14 PM. Reason: fixed a spelling error

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    Platinum_Member RaceKern davek's Avatar
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    Two other things that I've heard can change the results - not sure if they're wives tales or not:

    1) aggressive cams can lower the number

    2) doing the test with throttle closed will yield a lower number than with throttle at 100% (assuming this is only if air can't leak its way into the intake manifold).

    Dave

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    Silver_MemberHigh on Boost project_skyline's Avatar
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    I haven't seen cams affect the numbers at all if any. I tested after I installed my GSC s2's(old cams were hks 272's).

    Also the throttle open or closed made no difference.

    Steve what is your source for all the above info?

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    Administrator The Altered Beast Gumba's Avatar
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    Source: How to Rebuild and Modify Your Porsche 911 Engine -Wayne R. Dempsey

    The Compression Test

    One of the most common tests that can be performed on a engine is the standard compression test. This particular test measures the amount of pressure that is built up inside the combustion chamber when the engine is turned over. The typical compression tester is a pressure gauge that is attached via a short hose to a plug that is screwed into the spark plug hole. As the engine turns over, the compression gauge will read the maximum pressure exerted within the combustion chamber. The overall value is one method of testing your engine to determine the condition of the rings or valves.

    Your car needs to be setup before you can start the compression test. With the car cold, loosen the spark plugs with a spark plug socket and extension. Then tighten them up very lightly. You want to test the engine when it’s warm, yet if the spark plugs are very tight in the heads, you can damage the threads in the heads by removing them when the engine is hot. Loosening them up a bit when the engine is cold will minimize any damage you could possibly do to the threads in the heads. Although you might think that it’s good practice to use anti-seize compound on the plug threads, its been said the anti-seize compound seems to interfere with the proper grounding of the plugs. Also, temporarily remove any heater hoses that might get in the way of removing the spark plugs.

    Warm the car up to operating temperature and then turn it off. Wait about 5 minutes or so, as head temperatures tend to spike right after you turn the engine off. At this point, the engine fan has stopped, and the heat tends to build up with no place to dissipate to. Removing the spark plugs right after turning off the engine can cause the threads in the aluminum to gall. After about five minutes, remove the spark plugs from their holes.

    Having a helper around is useful, as you can watch the gauge while he or she cranks the engine. I recommend that you attach a battery charger to your battery to avoid running it down. Don’t fire it up at 50 Amp, but instead leave it on about 10 amps, which should help it recover when it’s not cranking.

    With the engine warm, install the compression tester into the spark plug hole. A bit of patience and skill are required in order to properly manipulate and screw in the compression tester so that you don’t cross thread and damage the threads in the cylinder heads. With the compression tester installed, crank the engine over 12-16 times. Make sure that you place your foot all the way down on the throttle. This will allow maximum air flow into the engine, otherwise your compression readings will be off. The engine should make six to eight full complete compression strokes (12-16 turns of the crankshaft). You can tell when the engine is on a compression stroke because the compression gauge will jump and show an increase when the cylinder is compressed. Carefully watch how the compression tester gauge increases, and record the maximum value when you have completed the last compression stroke. The gauge will jump at first, and then increase slowly until cranking the engine over more and more has no additional effect on the reading. Remove the compression tester and repeat for each of the other cylinders.

    So what to do with the results? In general, compression tests are limited in what they can tell you. It is important to remember that different compression testers may give different readings as well. Cranking the engine faster (with a stronger battery or high powered starter) may also skew readings. The most useful piece of information that you can glean from them is how each cylinder compares to the others. All of the cylinders should give readings that are very close to each other. This would generally indicate an engine in good health. A good rule of thumb is that each cylinder should read a minimum of 85% of value of the highest cylinder. So, if the highest reading is 150 psi, then the minimum acceptable reading would be about 128 psi.

    It is important to note that this would be an acceptable figure, but not necessarily ideal. In all practicality, all of the cylinders should be very close to each other (within about 5-10 psi). On a newly assembled and run-in motor, compression numbers are usually within this range. As the engine ages and certain parts wear faster than others, one or more cylinders may experience a bit more wear than the others. This will definitely show up in the compression tests. Needless to say, if you have all of your cylinders in the 150 psi range, and one cylinder is down around 120 psi, that should give you cause for concern. The important thing is to remember is that you want to gather consistent readings across all of the cylinders, without focusing on the actual values. If a reading is significantly off, go back and test that cylinder again to make sure that the measurement was not caused by some sort of fluke, which is often the case.

    So what causes variations in compression tests, and why can’t they be used as the final word on engine rebuilds? The problem is that there are several factors that effect the final pressure read by the tester. Engines running with very aggressive camshafts have a tendency to give low compression readings. This is because there is significant overlap between the intake and the exhaust stroke on the cam. During high-rpm operation of the engine, this overlap works to give the engine more power. However, when turning the engine at a low RPM, as with a compression test, the overlap causes some of the pressure in the combustion chamber to leak out before the valve is closed.

    Altitude and temperature also affect the compression readings. Manufacturer’s specifications are almost always given at a specific altitude (14.7 psi at sea level), and 59° Fahrenheit. Both temperature and barometric pressure change as you go up in altitude, so you will need to correct your measurements if you wish to compare it with a factory specification. The following chart provides conversion factors for correctly compensating for changes in altitude:

    Compression Test Altitude Compensation Factors
    Altitude Factor
    500 0.987
    1500 0.960
    2500 0.933
    3500 0.907
    4500 0.880
    5500 0.853
    6500 0.826
    7500 0.800
    8500 0.773

    A standard compression reading of about 150 psi at sea level in Los Angeles would measure significantly less in the surrounding mountains. For example, at an elevation of 6000 feet, the expected reading would be 150 psi X .8359 = 125 psi. The cylinders would be reading low if compared to sea level measurements, yet perfectly fine at this altitude.

    Another factor that can alter compression test readings are incorrectly adjusted valves. If the valves are not opening or closing at the correct time, then one cylinder may read vastly different than another. Make sure that your valves are adjusted properly prior to performing the test. Along the same lines of thought, premature camshaft wear can also lead to variances in compression readings, however, this type of wear is not normally as common.

    You can determine if the rings are causing low compression readings by squirting about a tablespoon of standard 10-30W engine oil into the cylinder. Crank the engine 2-3 times to spread the oil around inside the combustion chamber. Then retest the compression. If the readings shoot up significantly (45 psi or so), then the problem is most likely with the piston rings seating to the cylinders. Squirting the oil inside the combustion chamber in this manner allows the rings to temporarily seal quite a bit more than they would dry. If the compression readings do not change, then most likely culprit is a leaky valve.
    Last edited by Gumba; 12-21-2010 at 09:09 PM.

  5. #5
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    I have not ever seen any changes by leaving the throttle plate closed or open.

    Aaron
    "Even though there are days I wish I could change things that happened in the past, there's a reason the rear view mirror is so small and the windshield is so big... because where you're headed is much more important than what you've left behind."

  6. #6
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    I have seen evidence of this on the Evo 4G63 with 272 cams, an SR20DE with C2 cams (Most aggressive cam you can run on this motor), SR20VE motor (VVL) and now my 2.5L Jetta. The TB open and the camshafts choice being run does make a difference on the numbers. I have personally swapped cams between stock and aftermarket on the same motor and was able to bump or lower compression based on just the cams run alone.

    YMMV

    -E

  7. #7

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    Обратите внимание на то, что вы должны ввести правильный адрес электронной почты перед активацией. На указанный вами адрес придёт письмо, содержащее ссылку для активации учётной записи.



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