What are OK compression numbers?

#1

I’m going to look at a potential parts car today which is supposed to be running OK. I’m going to look at it to salvage the front end parts for my recent accident.

If the engine is good, I might be able to sell it an recoup some of the cost. I’ll take it out for a drive but wonder if a quick compression test would help determine the condition of the engine. What range of numbers is considered good?

The car is located far from any BMW shop and wonder if any decent auto repair shop can do this test?

Thanks for the feedback.

Keith

#2

Kieth, time to saddle-up and learn to do the compression and leakdown testting for your future endeavors.The price of the equipment is less than one visit to a shop. Compression numbers are easy to do. Above 150 and you’ve got a good one.

Good Luck, Robert Patton

#3

Just to expand on that a little…

Depending on the quality of the compression tester you buy, the absolute numbers (too a point, obviously 120psi would be bad) may not be the ultimate answer for “good” or “bad” compression. The tools you buy from a parts store probably don’t carry a valid calibration, but will give you a pretty good idea of the health. Also important is uniformity. I’ve always used 10% total range for all cylinders as a rule of thumb.

#4

I’ve been doing compression tests and leakdown tests on my motors lately as I try to figure out what is good and what isn’t. Have learned a lot. One of the things I learned is that leakdown tests are far more reliable. I’d blow off the compression test and go right to the leakdown.

The leakdown test is not infantry simple because there’s lots of different gauge types. And don’t buy a crappy leakdown gauge, that will just add to confusion. I did 3 leakdown tests last night. Once I seemed to figure out how to do it, I just kept at until the test results started looking similar.

You will need an air compressor with a decent storage volume. My 3gal volume isn’t ideal.

One thing I found is that my leakdown test numbers did not synch with my shop’s numbers.

Tonight I’m going to do it all over again, after “spurting” a little oil in each cylinder. Then if the numbers change, you know you have a ring problem. So I’ve read, anyhow.

#5

OK, I test drove the donor car candidate this afternoon. I took it up to redline in 1st,2nd and 3rd and it pulled strong up to 100mph. That’s all I was comfortable with not knowing the condition of anything else, including a driver’s seat back that wouldn’t stay upright!

Haven’t done any engine testing but it felt strong and pulled smoothly to redline with no flat spots or hesitation.

I think I’ll take a chance on this one. I don’t need the engine so I might have an available engine if someone needs one.

If I get it, I may take it by Motorsports Connection and let them test the engine before I start to strip all the front-end parts.

Keith

#6

Kieth, hang around and watch Billy do his check of the engine. The results and your data will make the engine more valuable (hoping for good leakdown numbers).

Gress is learning lots of “valuable” stuff which will better his game in the paddock. I wonder how affordable this hobby would be if I could not do the work…wait, I know the answer.

Regards, Robert Patton

#7

Robert,

I’ll definitely try and “participate” in the testing once I get this thing home, just wondering if it sounds likely the engine would be good based on the road testing I did?

Keith

#8

smithk3933 wrote:

[quote]Robert,

I’ll definitely try and “participate” in the testing once I get this thing home, just wondering if it sounds likely the engine would be good based on the road testing I did?

Keith[/quote]

Your butt dyno will tell you if it runs. We tend to fool ourselves into thinking our butt dyno is more rigorous then that. I took my car on the highway last week and the motor seemed nice and strong. Then I did my leakdown test and found that the motor was as weak as my CMP times indicated.