Low Compression Advice

[E30Chriss]

Looking for a little advice for my situation. Car is a M50b25 vanos with a turbo. ARP headstuds and Athena cutring gasket.

Compression tested all cylinders and #2 was low respectively to all other cylinders (200,130,200,220,215,190). So I proceeded to leak down test it and heard it hissing through the turbo manifold. Took it apart as my valve seals were also a bit leaky and car would smoke after idling for a while. Replaced the exhaust valves in cylinder 2, lapped and seated them and put the car back together.

The head was extremely clean and looked basically brand new. No cracks, no blowby from hg, pistons didn’t look burnt or anything, nice crosshatching on the cylinder walls, and overall just clean.

With the car back together now, I compression tested again and same result, 130psi in cylinder 2. I did the leak down test yet again and heard a lot of air in the head (65% leakage) and when I moved the camera over the dipstick tube you really heard it.

So now I’m at my current point of what is my next move

1) New rings in cylinder 2 (I thought about all of them but with their numbers I don’t see why)

2) Replace all pistons/rings (feels like overkill)

3) Buy a spare engine with good compression and swap my head, arp studs, and Athena cutring gasket onto it (have one locally with 180-195 across the board for $400)

Let me know your thoughts and how you’d move forward from here

[TobyB]

Me? I'd find an M54B30, re- ring THAT, and ditch the turbo.

But for you, take the shortblock apart, measure the pistons, and if they're ok, rings. If they're not, buy that $400 motor, and do that.
If you like having spare parts, then jump right to the spare engine.

t

[Panici]

For $400 I would be buying the spare engine.
Then build it on the bench with new HG, studs, and skimmed head.

Once it's ready to go, swap it in the car. Minimize downtime and now you have a spare engine again to rebuild at your leisure.

[Northern]

I'd grab the cheap bottom end, but also try to do a quick look at what's wrong with your current bottom end.

Not sure how much boost you're running. If there's any damage from rings butting together, I'd regap the rings on the replacement bottom end or do something to reduce how much heat the rings are seeing.
Hopefully you get a good spare block out of this that you can keep as a spare, if you have the space to store it.

[E30Chriss]

For $400 I would be buying the spare engine.
Then build it on the bench with new HG, studs, and skimmed head.

Once it's ready to go, swap it in the car. Minimize downtime and now you have a spare engine again to rebuild at your leisure.

Think this is the route I’m going to go. If I wanted to use the head that’s currently on the car (recent resurface, couple new valves, all new seals) would I need to get new cutrings and resurface the head again?

If yes, I feel like I’ll just get a running/driving tune on current engine and build the other one on the side so it’s a quick in and out once it’s all cleaned up.

[E30Chriss]

I'd grab the cheap bottom end, but also try to do a quick look at what's wrong with your current bottom end.

Not sure how much boost you're running. If there's any damage from rings butting together, I'd regap the rings on the replacement bottom end or do something to reduce how much heat the rings are seeing.
Hopefully you get a good spare block out of this that you can keep as a spare, if you have the space to store it.

Only ever seen max of 12psi. Didn’t look like there was any damage at all to the cylinder which is why I was surprised when I put it back together and still had low compression. Will likely be buying the spare Saturday and inspect it as I tear it apart (assuming leak down test is good when I show up to look at it. He claims 180-190psi across the board)

[moatilliatta]

Probably cracked ring land. 1 used piston and retention ring gaps.

[E30Chriss]

Probably cracked ring land. 1 used piston and retention ring gaps.

Yea, I just get a feeling if I start taking it fully apart then I’m just going to rebuild it completely and end up with forged rods and pistons lmaoo

the spare engine I went to go look at turned out to be no good so I am considering just seeing if the piston and cylinder are good and just replacing the rings and calling it a day so I can get it back running