i haven't found any old posts about good compression numbers on a just-rebuilt 8.5:1 motor. (not 8.8:1). i will be testing it wednesday, just-built VT motor, no miles on it...what sort of values should i be getting to ensure that one of the rings isn't bad? the only reason i ask is that the 2 times the motor has been started since the rebuild it has been running at 3000 rpm on non-synthetic oil. would an unseated ring be indicated only by a variation of the compression numbers relative to the other 3 cylinders, or could low compression numbers on all 4 cylinders indicate that all 4 rings are not good? if so, whats the minimum pressure i should be ok with?

you need to give that motor some more time to break itself in. on my 8.8 2.0 i had about 155 psi when i comp tested it cold with 0 miles on it, after 1500 it was about 175-180 and when fully broke in it had outstanding compression. like i said though just give it time.

if you're really worried the rings arent sealing you can always perform a leak down test. just to give you some peice of mind, if VT built your engine it will be just fine.

as long as the numbers are all about the same, the rings are sealing and you're fine. its very normal to have blow by during break in.

yeah i know VT builds well, but running the motor at high rpm on it's first startup cannot be good for the rings.

Originally posted by cyan
i haven't found any old posts about good compression numbers on a just-rebuilt 8.5:1 motor. (not 8.8:1). i will be testing it wednesday, just-built VT motor, no miles on it...what sort of values should i be getting to ensure that one of the rings isn't bad? the only reason i ask is that the 2 times the motor has been started since the rebuild it has been running at 3000 rpm on non-synthetic oil. would an unseated ring be indicated only by a variation of the compression numbers relative to the other 3 cylinders, or could low compression numbers on all 4 cylinders indicate that all 4 rings are not good? if so, whats the minimum pressure i should be ok with?

You do not want to break in a motor on synthetic. The rings will not seat properly as the oil will be too slippery. You want non-synthetic. One Cylinder being lower on compression than the other three could indicate a bad ring, bad head gasket install, warped head, bad valve... there a few things.

Running at that speed shouldn't hurt it. Some engines with flat tappet cams require at first start up that you run the engine around 2500 rpms for the first 15 to 30 minutes to break in the cam. The 420a has roller cams and do not reguire that.

I have JE 8.6:1 pistons and my compression is 165 psi across the board. And I agree with CODE4 you should break it in on conventional oil.

Mine was at 180 (hot) across the board.

My old 8.8:1 Howell shortblock was around 180 PSI across all 4 after it was broken in.

Nice pic :)

I agree, conventional until the motor is broken in. I wouldn't about compression testing it until the ring are fully seated. Was the motor idling at 3000? Maybe you have a vacuum leak in there somewhere.

would the 02 sensore not being plugged in be the cause of the high idle?

Unlikely. I believe that without Oxygen sensor input, the ECU runs off of a preset map, aka limp mode. I do not think it would affect idle to that degree.

Originally posted by cyan
would the 02 sensore not being plugged in be the cause of the high idle?

No. Still sounds like a vacuum like problem to me.

which vacuum lines should i check besides the ones coming off the hrc fpr and the stock fuel rail? how many more are there (never checked those before)

look for the hoses coming off the back of the intake manifold. Follow all of those and be sure they are all plugged in.