compression test

[fly944]

I think Im going to do a compression test on my motor while Im waiting for my por-15 fuel tank sealant to arrive. Does anyone have any tips on this procedure and what sort of numbers are exceptable. Just did my 86 944 w/ 205,00 miles on original motor and got 178-180 psi.

76 911 s w/ upgrades and rebuild

[Tyson Schmidt]

Perform the test with the engine warm, all the plugs out, and the throttle wide open. Disable the ignition system first. I recommend doing this by unplugging the CD box.

The number values are less important than the consistency of those numbers from cylinder to cylinder. Preferably within 10% of one another.

[Wayne 962]

From Chapter One of the New Engine Rebuild Book:

Compression Tests

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 911 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, Porsche specifically recommends against this. 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. Now, disconnect the cable from the capacitive discharge (CD) unit (1969-1983 911 and all Turbos). If you’re working on a 1965-68 911, then simply disconnect the power line (+) from the coil. If you’re testing a 1984-89 911 Carrera, then remove the small square DME relay from under the passenger seat. Doing this will disable the car’s ignition system, and prevent the spark plug wires from firing. It’s also a wise idea to remove the fuel pump relay at this time (for 1969-83 911s). You are going to be cranking the engine over several times, and you don’t want raw fuel to be dumped into the system.

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. An early 911S engine, for example (with its high-overlap cams ) has a tendency to give lower compression readings than the 911 CIS engines (1974-83), despite having a higher compression ratio. This is caused by the aggressive overlap of the camshaft.

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. For the complete procedure on adjusting your valves with the engine inside the car, take a look at Project 18 in the book, “101 Projects for Your Porsche 911.” 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 common on the 911 engine.

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.


-Wayne

[fly944]

thanks a bunch. when will this book be out again.

[Doug E]

Based on the quality of Chapter 1, I hope your book has 500 chapters because I won't be able to put it down.

Great information!

[john walker's workshop]

hey tyson, when ya comin back? had enough yet?

[Wayne 962]

It will have six Chapters:

- When to Rebuild?
- Engine Drop (on Bonus CD-ROM)
- Engine Teardown
- Machine Shop
- Performance and Reliability Enhancements
- Assembly (broken down into ten sections)
- Engine Break-in

-Wayne

[Superman]

Yeah, Tyson. Are those sunny days and beautiful women getting to you yet? There's still plenty of beer and coffee here, so just let us know when you're coming.

[Tyson Schmidt]

Ha! Well, to be honest, I sure don't love everything about L.A.

As to when I'm coming back? I have no idea. I do miss Seattle a lot though. Find me an amazing job up there and I'll be home in a heartbeat.

So how's the Seattle gang doing?