Dummie question about Compression on M20B25

[Oldemar Gutierrez]

Hi All,

I am new writing here but I´ve been reading around for a while now. So I bought my E30 which is a 1991 325i USDM about a year ago. Before buying the guy I went to measure the compression (psi per piston).

What is the supposed psi per piston I should get? This buddy must have 150k miles I beleive (can´t exactly say cuz the odometer is not working and don´t how long it has been that way)

The 6 pistons were average, 4 of them did like 140psi and 2 of them 138psi.

I wanna know in order to know what will be the north on this project, to see if I should invest some money refreshing the engine or if I can go forward with any other ideas for him. I want to stick the car to the original shape and not willing to make crazy things to the engine.

Thanks!

[20gmr]

Some compression testers are not as accurate as others, so instead of looking at peak numbers focus more on variance between cylinders. You should be concerned if one cylinder makes 150 and another makes 100. I'd say by your numbers you have a very healthy engine!

I'm sure someone on Here knows the actual spec but for instance my Evo tested about 140 across the board with one tester yet another tester said 120.

[20gmr]

Rule of thumb is you want them within atleast 10% from the highest to lowest

[Oldemar Gutierrez]

Cool Thanks! Good news then!

I have to make a tune up, inyectors clean up and some other stuff soon. I am planning to retest again.

BTW: I want to put in a new exhaust and I am looking for options on the size of the exhaust