madeouttaglass

Has anyone toasted a forged aftermarket #7 piston?

stevieturbo

Lean mixtures or detonation can kill any type of piston.

madeouttaglass

That I understand. I'd like to know if someone has had the same problem with an aftermarket piston in the 5 or 7 cylinder. I changed mine out to forged Mahles. When I did I could see how easy it was for any of the stock pistons to let go. That hole in the top of the factory piston comes very close to the top ring land.

leojnknsC5

YOur computer will be constantly in a state of confusion from adjusting the air fuel ratio when 5 and 7 fire. The cylinder after 5 or 7 will end up too lean.

leojnknsC5

It could be the composite intake on teh LSx was designed for motors using vacuum to make power, not pressurized air.

Do the maggies have the same problem?

NassauBlue98

Can you quantify this, Peter? For instance, how much spread do you see between the AFRs from cylinder to cylinder?

Warp Factor

I'd think the opposite would be true if 5 and 7 are leaner to begin with (as Peter stated) and injectors are selected to help even this out.

Bill Reid

What about a different heat range plug in #5 and #7?

NemesisC5

leojnknsC5

The problem in 5 and 7 is the ratio changes. When they get too lean, they blow. But they work most of the time so its not a problem of them being too lean, but that they fluctuate.

Anybody who's blown 5 or 7 would be wise to pull the composite intake manifold and replace it with one that centers the incoming air to the center of the manifold, like a carbed manifold. Then see if 5 and 7 still go lean.

RED99

Can you share some of your data with us? How much spread did you see between the richest and the leanest when you tested the AFR on all 8 cylinders?
If you are showing say 11:1 AFR in the collector, about how much leaner is the #7 running?

FRCTony

427 LS1

Something else to consider is switching to a Evans Water pump and T stat, they noted much less flow on the odd side bank, their pump evens out the flow on both banks and flows alot more than stock. May help keep things more stable...

RED99

I have been thinking the same thing. Along with there special waterless coolant.

hex

I have been impressed with this setup over the years, on every engine type I have switched it to. Well worth it on a road racer.

I have had intake flow problems over the years on various engines, and they all have one or 2 cylinders running leaner than the others, and I just work the intake until I get the flow much closer to matched. 2% or less cylinder to cylinder flow variation seems to do the trick.

The guy that said do not push them to the freaking edge had a very good point, which leads to the dyno and AFR measurement arguments, as well. If your tuning data is not perfectly accurate, in combination with "tuning for numbers", these things tend to happen more often. Match the airflow, stay off the edge, use accurate tuning data, and the problem % goes way down.

Robert56@RNS

GM should have known after 200 sprayed passes, upto 250rwhp hit, that upping the shot to 285rwhp on an absolutely bonestock longblock LS6, I would finally break a ringland. See my point, these motors are monsters from the factory, and with a spot on tune can go for some time at the limits. The only reason I had problems at 285rwhp was I messed my tune up and added 4* timing instead of pulling said timing. Can you guess what cylinder went with the resulting mega temps this motor saw, and 665rwtq at about 3500rpm. Oh and the a/f was 10.8 through out the dyno run.


Robert

johncbourg

Thats exactly what gave on mine

BRETTROD

just went to the machine shop to check what the problem was on mine today. Of course, lucky number SEVEN!!!

mine isn't too bad.....looks like a slight flat on the piston spot where it may have gotten too hot and some dimples in the cylinder wall.


So what should I do on the rebuild?!?!?!?!

brett

chriswtx

rebuild forged..You might need a new block as well. Have it checked real well...