If you were tuning it in stages and weren't encountering any issues, and then all of a sudden had an event (smoke burst), and then you're having all of this positive crankcase pressure and issues, it leads me to think that event caused the issues you're having now, not that it started out that way. If the motor was a dud when you got it, wouldn't it have shown issues like this during the stepped tuning process? I know this doesn't help or change anything at all, just thinking out loud.

Either way good luck with the rebuild - sounds like there's lots of good advice in here, and now is the right time to rebuild as a more traditional boost motor, or rebuild at high compression and stay NA.

 

I would pull the engine, tear it completely down and inspect everything and buy new rings and gap them for boost. Re-assemble and have the heads checked, it looks like detonation happened by looking at your heads.

 

I do appreciate all the advice, I am just a very curious person and engineer that feels the need to figure out the root cause of this failure as to not have it happen again. The plan is to do exactly like 97Z28SS suggested, but I really want to know what the hell happened. The engine was still running like a champ after the smoke burst. I should mention, that smoke burst was what I thought I saw, it could have totally been in my head since I was just waiting for something to blow up on a max RPM pull. I have never been in a vehicle with that much power or noise so it is a lot to take in at once, especially when you are already freaked out a bit from finally driving a $60k piece of rolling garage art. The only reason I discovered something was wrong was when I changed to a vented catch can and saw excessive vapor coming out of it at idle, the engine still ran GREAT.

This is all so dang confusing. There are too many variables for me to figure out what happened. Next time I need to limit variables by buying new or checking the engine before using it.

 

The reason ring gap is important is that when you boost or spray an engine, the heat causes the rings to expand and you need enough gap that when they expand the ends of the rings don’t touch, because if they do, they lift (like mountain ranges and the plates of the earth) and it breaks the upper ring land.


Forged pistons can bend/tolerate more than brittle cast pistons, but not for long. That’s the biggest killer of engines that guys whether LS or anything boost without regapping the rings.


It’s great that this didn’t happen to you, but you need to regap the rings before you go further. Detonation is also a huge killer, and again forged pistons will tolerate and bend more than a cast piston which breaks because it’s brittle, but that’s your next issue to address. You need cold plugs, like a BR7 or something, and either race gas or drop your compression ratio a touch, and/or get a bigger more turbo friendly cam that will drop your dynamic compression a little like was mentioned. Forged piston material does come into play after a while, but for your fairly modest goals (which I’m assuming are flywheel HP not rwhp since your baseline 485 was flywheel) you should be fine with either material since cast pistons with proper gap and no detonation can survive at 700fwhp. A different set of heads with bigger chambers, or adjusting your head gasket thickness for ideal quench and maybe dropping the CR a touch can help since selling those heads and buying bigger chamber as cast trickflows would be cheaper than swapping pistons and having to rebalance and reassemble everything. Boost is the great equalizer which can compensate for a lower flowing head, by shoving that air down its throat, so even if you are giving up a touch on flow from ported heads going to as cast trickflows, a bigger chamber and less CR can allow more boost, safely.

 

Since the engine was a used mill that was raced, I'd also consider that the marks in the heads were from a previous failure, and/or off of a different shortblock before you got it. To me, the marks look like something was digested by those cylinders. In particular, the last picture has a longer, shallower, skinny dent slightly off-center and above the intake valve. 3 o'clock to that, is a v-shaped dent.

Since the pistons and cylinder walls look fine, the damage pre-dates the pistons. Regardless, take them to a shop and have them inspected and tested for cracks.

 

I bought a 383 race prepped block from Scoggin Dickey about the same time as yours and it was fitted with -14cc JE inverted dome pistons. It was advertised as a 10.4 SCR.

From looking at your pictures:
Flat top pistons (-5cc ), 4.040" bore, zero deck and 54cc combustion chambers (LT4 heads) with a .039 head gasket yield 12.7 static compression ratio.

You probably have pistons with broken ring lands due to detonation.