compression?

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psychrunner

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In the final stages of my restoration. The resto shop called and said the engine is running strong and gave some compression numbers that they said supports this claim. I am not a mechanic and wondered if they sounded okay to you experts on the board. All pistons(?) with the exception of one are running in the 145 range (if that makes sense) and the other in the 135 range. What do these numbers mean and does it sound okay for an original m30 csi engine (mechanical fuel injection with D jetronic)?
Thanks
Randy
 
Sounds like a healthy engine. Basically the readings are in pounds per square inch (psi). The highest and lowest should be within 10%. The 135 may be a little low, but its still in the acceptable range.
 
Nope, not really. It can still burn oil even if the compression test is good. There are three separate rings on each piston -- two seal the piston in the cylinder and the third ring (which is really three separate rings in one groove) is for oil control. All the compression test really means is that the valves and rings are sealing well enough to give a good test result. However, it doesn't necessarily mean that the oil control rings are doing their job. Bad valve guides/seals will also cause it to burn oil. There's no way to really know without running it.
 
compression readings

I have a slightly different take on the compression readings you mentioned. I may have missed other information on your engine, but the readings seem on the low side of acceptable.

If, for example you have a fresh csi engine with 9.5/1 compression, I would expect to see 180 lbs/in2 or even higher readings in each cylinder. 145 lbs/in2 suggests lower compression in the 8/1 neighborhood. Of course there are several possible explanations for these readings, including the use of a gauge that reads low or an engine with a high overlapping cam or even a deliberate use of low compression pistons for use in a turbo application.:wink: Other possibilities, which we hope is not the case, include leaky rings and/or valves. :cry:

I concur with AndyM regarding the 10 per cent rule, but all things being ideal, - each cylinder should produce identical readings and to a certain extent, the higher the compression the better. :wink:

Good luck

P.S. for your benefit, you might want to look at this site. It is hardly authoritative, but it is nevertheless illustrative of the process. http://www.geocities.com/dsmgrrrl/FAQs/compression.htm
 
I would suspect that the compression gauge is not calibrated, generally you can only get relative readings with an inexpensive gauge. The test should be run with all plugs out and the engine turning as fast as possible. If you've had a recent valve job or have new (unseated) rings, the reading will be bogus until they seat (the starter can't turn the engine fast enough to overcome air leakage - a wet test would address this). Even taking this into account, it is usually worthwhile running the test multiple times (even on different days) before jumping to any conclusions. The 10% relative rule has been common garage wisdom for many years; hard to say if it is a good rule but it has been used to justify more than a few rebuilds. I wouldn't tear into my engine unless it was burning oil badly or no amount of tuning would get it to run smoothly.

As for the reading of 145psi, isn't this exactly right? 1 bar = 14.5psi. With a compression ratio of 10:1 I would expect a reading of exactly 145psi. The reading of 135psi would be about 9.3:1. Have I missed something here?

Remember, your car has also been sitting basically idle for 2 years. The M30 and M20 engines don't like being run at low RPM for extended periods; they tend build up crud on the intake valves which can cause them to not seat perfectly (and mess up compression readings). If you can tune the engine and get a good idle and power then the odds are in your favor. I recommend you simply drive the car and see what happens. Try doing a compression test later this summer after a nice invogorating drive.
 
Good points. A compression test is most useful if you have some type of baseline. For instance, I should take compression readings on my rebuilt motor now that it has about 10k on it. Then keep track of it about once a year and compare the figures using the same gauge.

I agree -- drive it and see how it does. That's really all that matters.
 
Did they do the test with the engine hot or cold? With a hot engine, on a 10:1 engine (a Nikasil one at that, with 130k) got ~180 on all. Cold, it would be much less.
 
compression readings

Although I rarely use them, I have a number of different compression gauges, some of which translate pressure readings into "approximate" compression ratios. I just seem to recall that 155 lb/sq.in., is translated/depicted on one of the gauges as being slightly above 8:1.

As noted, temperature may make a difference in readings as might the condition of the battery for cranking speed.

FWIW, I asked a friend to look at the "technical data" from the blue notebooks. He read the following: "good above 11(bar)/156 ; normal 10 - 11 (bar) /142-156; poor below 10(bar)/142". Again, as noted, different gauges are calibrated differently and even old-expensive gauges can be out of spec. I suspect there are some explanatory footnotes to these numbers - but they put the subject engines readings in the ballpark.

Parenthetically, the last time I checked the compression in my e9 I obtained 190 -195 lb/in2 readings. I used aftermarket pistons advertised at 10:1 compression ratio. I thought that was a bit high, and repeated the test with a gauge designed for diesels and obtained the same results. I can't really explain it except that different gauges may provide different results. As you gents pointed out, consistency between cylinders is every bit as important as high numbers. :roll:
 
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