Low compression cylinder, blow by, no visible damage?

[6barrel-gtx]

I have admired and read your site for years, I just never needed to be a member before, due to most of my questions could be answered by reading the threads, but this is a different one.

After completing some basic mods, Exhaust, 16g,fuel inj, fuel pump, and DSM link, I had driven the car for a couple of months and then developed a rough idle. I was playing around trying to diagnois the issue, while heavy load in second the car shut down hard. I pushed the clutch in calmed down and slowed down then slowly let the cluth out. The car re-fired and I drove it home, but it was smoking. The car smoke under slight load, but did not smoke at idle, but it did develope blow by and was smokeing out of the valve cover breather.

I thought: great! I busted a piston or burned a hole in it. I did a compression check cylinder 1,2,3,4, (135,130,90,135). Hole number three was light. I tore down the top of the engine and discovered no signs of damage to the piston. I am now assuming that it must be a ring or ring land busted. I also found considerable oil in the intake from the turbo as well and this was brand new grant you. ( The rough idle was due to a cracked vacuum fitting that once fixed the roughness was gone of course)

So my questions are 1. Is it common to see a ring or ring land fail on a 4g63 engine? ( I was running 12 psi boost) 2. If it is a ring land or piston, would you just replace one piston and check the weight of the assembly, or is the balance so sensitive that I would need to replace all 4 pistons? 3. This is the big question. If the turbo puked or the ring failed how could these to item cause such a violent shutdown with no signs of damage?

Thank you for taking the time to review and think about this.

 

[6barrel-gtx]

If anyone is interested! I pulled the pistions and 3 out of 4 pistons where busted. The break was between the upper and lower compression rings. One land was busted out and I pulled about 80 degrees of circumference of the piston out like a seperate ring Anyway that severe of damage had to be overboost and I am replacing all four pistons anyway.

 

[Dorifto]

Hmm, at least I'm not the only one suffering from the same issue. The previous owner of my GSX severely overboosted the engine [a small-16g on a stock A/T GSX is a bit overkill], causing insane detonation, which blew a chunk out of piston #2. It's bad enough in my case to require boring the cylinders out .020. Everything else, including the head, is fine though. What initially tipped me off was the oil in my turbo [less than 500 miles on it], the intake, and breather system, as well as the light knock from piston slap. My headgasket went at the same time, and was found by the tell-tale white cloudy substance on the oil cap.

That kind of damage doesn't happen at 12psi, so I would look into what caused the issue. You've narrowed it down to overboosting, so I'd check lines and wastegate actuation. I'd fix the problem as quickly as possible, as it would make no sense to clean up the mess of an overboost, without fixing the culprit of the issue.

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^ My Christmas present, hah. A replacement stock shortblock is on its' way, and the current damaged block will be torn down and prepped for an overbore, stroker crank, and Wisecos.

Best of luck to you on your buildup. As a forum owner myself, it's good to see the lurkers join the members.