Good enough compression for a head swap?

[camip]

I recently ran a compression cylinders 1, 2, 3, 5, and 6 were all about just at 90psi. Cylinder however is around the high 70s to low 80s the couple times I tested. The question is should I rebuild the bottom before I swap in a 885 head. I was planning on swapping the head seeing if I liked the combo then rebuilding everything later and possibly turbo.

[jlevie]

Those would be extremely low compression numbers. A leak down test and a wet compression test should tell if the problem is cylinder/ring wear or leaking valves.

[camip]

Now I'm not sure if I was doing it correctly. Fuck or my motor is shit.

[camip]

Are you suppose to do the compression test when the engine is warm or?

[slippery]

Warm engine.

J

[camip]

Fuck I read online and said nothing about being warm. I'll re-test and see what happens. Where should the psi be in range. I've read about 150psi.

[UserError]

150 would be good. After a first test, put a little (less than a capful) of oil into the spark plug holes, and re-run the test. If the number goes up significantly, then the rings are worn, and you'll want a full rebuild. If the number doesn't change much, then the low compression is in the head, and a head swap would do you a whole world of good.

[camip]

^ that's what I'm hoping for. Those compression numbers can't be right or my car would be a turd.

[whodwho]

Those are really low numbers and would be a turd. I would suspect a low reading compression gauge or not getting enough air into the cyl. Are you opening the throttle when cranking? How many revolutions are you cranking it for?

The low numbers may not be an issue but the fact that you have lower compression in one hole is a concern. A leakdown test should be your next approach.

[StereoInstaller1]

Make sure the throttle is fully open when cranking the motor

[camip]

I had the throttle fully open. I I'll grab another compression gauge and a leak down tester. Just sucks the car is 2.5 hours away :(

[unevolved]

The low numbers may not be an issue but the fact that you have lower compression in one hole is a concern. A leakdown test should be your next approach.

This. With as many variables as there are to a compression test, all it can reliably tell you (unless you're following a very specific procedure) is whether or not a cylinder is worse than the others.

[whodwho]

I had the throttle fully open. I I'll grab another compression gauge and a leak down tester. Just sucks the car is 2.5 hours away :(

Did you bump it thru 5-6 revs until the gauge stopped rising? And you want to keep the same count per cyl.

[camip]

Did you bump it thru 5-6 revs until the gauge stopped rising? And you want to keep the same count per cyl.

I did it about 5-6 revs but I'm not sure I did them all the same. I'll redo the test next time I'm home with scientific precision.