Rb30Det Rebuild Compression

[hg56]

Hello, I recently built up an RB30DET with forged pistons and rods, I have done about 3000km (2000km was 3 run in oil changes, last 1000km on Mobil 1), engine runs great and got 370hp on the dyno, but I did a compression test yesterday and found cylinder 2-5 were 150psi but number 1 was 140-142psi, what I am wondering is there a chance number 1 hasnt fully bed in yet and if I leave it till 10000km it will gain those few more psi to match the othe cylinders? Or is it likely to never bed in and just get worse?

[Shoota_77]

To fix it all you need to do is put a scoop of sand in that cylinder.... Joke.

It's a worry, I can't imagine that it will get much better of it's own acord. It's probably not likely to get any worse in terms of relative to the rest of the cylinders unless there is some other drama. Were your first few oil changes mineral oil? What was your run in procedure? There's also a possibility it's something other than the rings (valve not seated properly or burnt??).

It's not bad enough that you would bother doing anything about it I wouldn't have thought. Just monitor it and make sure it doesn't get any worse (relatively).

For your sake I hope it just magically beds in all of the a sudden but wouldn't count on it. The first 15 minutes of run time are just so important. Once the bores are glazed it takes a long time to see any changes in sealing occuring.

[hg56]

Thanks for your reply, Yeah I hope something magically happens and fixes itself at 10000kms lol, ill do a leak down test but a teaspoon of oil brings it to 150 psi, so that says rings to me. As for run in procedure, I ran penrite run in oil, did 50-100km, changed oil and filter with more run in oil, then did a further 500 km, changed oil and filter again with more run in oil, did 500km the changed it with run in oil and did 1000km and finally put synthetic mobil 1 in at just over the 2000km mark, probably a bit over the top lol you think there would be any merit in changing back to run in oil till 10000km?

[Piggaz]

Did you baby it around or did you load it up to get the gases behind the rings early on?

[Shoota_77]

Doesn't sound good. The problem is and will most likely continue to be the fact that it's only the one cylinder that's significantly lower. It's just not likely to catch up unfortunately. It wouldn't hurt to go back to mineral oil for say 5000kms but I think it's too late because of the bore glaze. There's probably not enough left of the hone marks to bed the rings in any further. Anything is worth trying though!

In reality all of your comp results are low. I would have thought they should be all around the 160-165 psi mark on a run in engine?

As Piggaz is alluding to above I just don't think you gave it a hard enough time in the run in process. You've gotta hit it with as much boost as you can as early as possible which forced the rings out hard against the cylinder walls and wears the rings in super quick before the glazing commences.

[hg56]

An 8psi difference is only 5%, not too bad as long and not worth a rebuild as long as it stays where it is lol, yea I just use my gauge for reference between cylinders, dont think the readings are overly accurate, but fine to see the difference between cylinders, and I do trust it sonewhat as putting a teaspoon of oil in all other cylinders yields no extra compression so those rings must be healthy. I will put some run in oil back in and do another 5000km and I will post back whether my compression goes up or down or stays the same, Thanks for your input guys!