Engine compression test results

[CeglA]

Hi!

Today I changed spark plugs and made engine compression test (everything work fine).

Cylinders:
1- 185psi
2- 185psi
3- 200psi
4- 200psi
5- 200psi
6- 200psi

Could someone help me to interpret the result?
Car has around 80 000miles.

Regards,

[MineralWhiteF80]

Short version:
If those are actually your numbers, I would say that your engine is perfectly healthy and that you have nothing to worry about.

Long version:
If those are actually your numbers, it is odd that four of your cylinders are EXACTLY 200psi and two of your cylinders are EXACTLY 185psi. I'm not saying that cannot happen, but normally, you do not have identical pressure readings to within 1psi across many cylinders unless you were just rounding up or down for whatever reason.

The general rule of thumb is that cylinder pressures should be within 10% of each other to be considered healthy. In your case, you are within 7.5%, which is perfectly normal.

The general rule of thumb for cylinder pressure seen during a compression test is 17 to 20 times the engine's compression ratio. Of course, this topic can get super detailed, which is why I "general rule." The S55 engine has a 10.2:1 compression, which means an expected cylinder pressure range that you might expect when doing a compression test should be somewhere in the 173 to 204 psi range. I haven't looked to see if BMW provides an actual cylinder pressure spec for a compression test, but I would assume they do.

[CeglA]

Thank you for so detailed information.
I converted my numbers from bar to psi so that why results may look a bit strange.
In bars:
1-12.8
2-12.8
3-14
4-13.8
5-13.8
6-13.8

Im a bit worried that first two cylinders have lower compression than others. But like you sad its completely fine and engine should be healthy?

Regards,
Chris

[MineralWhiteF80]

Originally Posted by CeglA

Thank you for so detailed information.
I converted my numbers from bar to psi so that why results may look a bit strange.
In bars:
1-12.8
2-12.8
3-14
4-13.8
5-13.8
6-13.8

Im a bit worried that first two cylinders have lower compression than others. But like you sad its completely fine and engine should be healthy?

Regards,
Chris

Percentage-wise, your compression numbers are within the "industry standard of 10%," and your engine is considered healthy.

There are too many variables to worry much more, and there are literal books written about the fine details. For basic examples, you need to know the gauge's tolerance and realize that the gauge can show different results at different times for the same cylinder for multiple reasons. You also need to be cranking the engine over exactly the same number of revolutions for each cylinder for the readings to be considered measured equally. The list goes on and on and you can read all you want.

But for the sake of simple troubleshooting, the 10% rule is a good place to stop if an engine is running perfectly fine.

[CeglA]

Thank you. Your post was very helpful