lucxy

So my HCI LS7 failed at the track and it looks like cylinder 7 piston is damaged or melted. We dont know for sure yet until the motor is taken apart.

This setup went fine for about 5K miles and 3 track events. The failure occured @ watkins glen.
I dont know yet what caused it. My theory is cylinder 7 leaned out at the track, causing high temps and subsequently piston failure.


here is the sequence of events:
(BTW, I checked oil and coolant temp rigth beiore failure: oil 250F, coolant 220F). Running Mobile 1 0-40.
1) Low oil pressure at the end of the run (30 min lap)
2) Check gauges come on (oil pressure is at 5-3PSI)
3) Followed by blue smoke from exhaust (oil being burned)
4) Oil temps and coolant suddenly went up to 300F
5) Engine stalled
6) No leaks (coolant or oil) on the floor, no apparent holes in the block

This all happened with 30 seconds during a cooldown lap.

Sent car to my shop and they scoped the cylinders: cylinder 7 piston damage. No dropped valves, springs+rockers are intact.

The car was tuned to 12.3 AFR @ WOT. Is that too lean for track use?
Also what are my options now? Rebuild with forged pistons or stock bottom end with much richer AFR ~11.5. Opinions?

Or maybe just go back to stock LS7 altogether?

Thank you.

Vito.A

What year Z06? I ask because the 2012-13 models have had issues with the polymer bearings.

First you need to tear it down to see what the failure was and what parts are useable. If the block, crankshaft, and connecting rods are intact, then yes a rebuild with forged pistons, new bearings, and new rings would be the way to go. If the bearings damaged the crank/rods, then you may need a new reciprocating assembly.
Good luck!

lucxy

2009 z06 but this motor is a 2012 motor (2nd motor) with 40k on it. First failed at 15K with oil pump failure.

Vito.A

Bearing failure in a 2012 engine is very likely. If you do rebuild it, make sure you use quality Tri-Metal bearings.

lucxy

What is strange is that after a few hours of sitting around, the car started with 50 PSI of oil pressure and cylinder 7 misfiring and no clunking, metal noise.

I dont understand what led to the pistin melting. Obviously temps were too high? but why? Did the oil temp was much higher then indicated on display? Should I have upgraded cooling?

I am puzzled as to why cylinder 7 overheated so much and what the fix is to avoid it in the future.

Dan_the_C5_Man

I am totally on-board with the 2012+ main bearings being an issue, but from the description we have so far, it really sounds like a broken ring land (if so, "lucxy"should change his screen name to "lucky" in that it didn't take out a valve).

For the OP, as far as AF / ignition timing goes - there is a reason a tune picks up significant power over stock - it is because GM knows that for durability reasons, it needs to run fat - you'll see the AFR dip into the 11's on a stock tune..

All said, it sounds like you dodged a real bullet here, and it won't be too costly to repair.

Ps. YIKES! I was JUST going to say "..and DON'T RUN THE MOTOR UNTIL IT IS TORN DOWN!.."

Man, don't tempt fate twice!

The other rule of thumb I have is to dump a bottle of (good) octane booster in the tank when doing a track day - just extra cheap insurance.

lucxy

So should I tune the to run in the 11s @WOT for track use, upgrade cooling? I am not exactly sure how to avoid this in the future.

Dirty Howie

Your temps were fine after 30 minute session.


DH

Dan_the_C5_Man

Cooler is always better, but I agree with your initial assessment - due to the intake manifold design, #7 always tend to run lean on LSx engines - if you consider that a dyno tune (especially a tune performed on a Dynojet) is very close, but not exactly the same as on-road condiftions, and the AFR you are referring to is an AVERAGE, you can imagine you might have been running lean enough on #7 to cause this failure - on the next tune, error on the side of caution, not HP numbers.

Michael_D

Were you actually in PE mode? You can be running 4500rpm and still be in closed loop - 14.7 afr. Several permissives need to be met to enable PE.

Really need to tear her down before concluding the cause.

Bill Dearborn

I have heard this since the LS1 came out. Not sure how it works though. If the intake design is causing #7 to run lean that means the intake is funneling more air into that cylinder than the others which doesn't seem likely given the distance from the inlet and resistance to air flow inside the manifold. The more likely result would be less air getting into #7 and the cylinder running rich compared to others.

Bill

Dan_the_C5_Man

That topic has been beat to death, all you need to do is spend 5 min with the search engines of LS1Tech and CF, and you'll find many, many examples of #7 issues, yet I can't recall a similar failure mode tied to cylinders 1-6 or 8.

The data doesn't lie, it is what it is..

Racingswh

Can you expand upon what you mean? What permissives exactly? Does throttle position make this difference? I am extremely interested and just need to learn more about all the issues we potentially face.

Dan_the_C5_Man

Throttle Position / MAP are one of the more significant drivers to trigger Power Enrichment (it is safe to assume that if you are tracking the car, coming out of a corner and on to a straight-away at anything more than 80% throttle (typically you'd be at 100%), the PCM is commanding PE).

I know you track your car frequently - get a Dashlogic, and keep it hooked up. Also install a Wide-Band O2 sensor, so you can watch AFR. With this combo you can monitor PE mode, all your temps (including transmission), pressures and other parameters while you are hammering at the track. You can also set up alerts for certain conditions - the only way to go IMO.

c5racr1

#7 cylinder runs much hotter then the others which causes the piston problem. This has been around since the ls1. Some people put cross over tubes in the rear of the head to prevent air pockets around that #7 cylinder. With the cross over tube being in the front now, it doesn't help.

I have front and rear crossover tubes because of this known issue. Do a search on ls1 tech you will see what I'm referring too.

Michael_D

Sure.... Might be easier to just post a couple screen shots. The first is the HPT / PE tab, and permissive of a "stock" / un-modified, or "tuned", 2007 Z06 tune file.... The second screen shot is how I have my tune file, PE configured..... The permissives need to be met, in order for PE to be activated.

To further illustrate this, I've posted another screen shot of a log I took yesterday. It is a log of my car. My PE tables are a bit different than the OE calibration. I've modified them to enable PE much quicker than the OE settings, because my fuel is shiit, and in addition to having to remove a lot of spark, I need this sucker to enter PE mode quickly, to stay away from knock retard.

I have marked the log by the little white vertical line. All the data on the left represents the horizontal lines of the same color. The vertical line marks that data, kinda like a time stamp, you can take logs and review the data, then make calibration changes..... Anyway, you will see that the engine is running 4694 rpm, but commanded AFR was still 14.7. TPS is 34%. MAP is 33kPa.... This particular screen shot was during a slow and controlled rpm increase, as I was trying to collect a lot of data after making a cal change.

Racingswh

Ours is tuned rich to be safer supposedly. Who knows if that helps? I have seen them tuned / run in the 12's and we are much fatter than that. Mid 11's afr. Colder plugs. Always fresh gas.

I don't think going back to stock is an option at this point but it may be if you feel it's the only way to maintain the level of reliability you're looking for. I personally couldn't do it. I am addicted to the way this thing delivers power now.

Thanks Dan for your suggestions. I use the Race Keeper to capture the on board diagnostic info with every run. In that regard I am pleased with what I am seeing. I don't know if Lucxy is using anything. I know now that you can get a Blue Tooth OBDII plug that will send diagnostic info to your phone. Seems like an economical way to capture the info. I need a wideband gauge for sure.

I am curious if the Vararam would cause a lean condition at high speeds? Something you don't see really on the dyno.

Also and I don't know if this makes any difference to the OP's issue but I am out of injector over 6k. Car makes 540/510 and with my stock injectors they are at 100% duty cycle over 5900. We could not make any more power than that with the stock injectors. When I went to the track the highest I revved it was 5700 rpm. Still stupid fast.

Todd I would like to see photos of those cross over tubes you speak about if you could. I am just assuming you're talking about the LS7 and when I went to LS1 tech and for me it was tough to get clarity on what they were discussing.

Thanks Michael for your help. I am going to look at the PE table in mine. So in your case you have lowered your PE rpm to 0. What exactly does PE DELAV stand for? Why would stock be set to what reads to me like 5,000 rpm? The enrichment ramp in looks higher in your PE table than the stock table 1.5 yours compared to 1.0 for stock. Am I seeing that correctly?

You also have boost enrichment whereas stock has no values there. Reason?

In slower traffic on the track I can see high RPM and only partial throttle. Is it possible that the 4694 rpm yet still closed loop 14.7 afr condition you recreated may exist in that scenario or is that just guessing? I certainly can datalog run sessions and see what happens during them. EFI Live which we use would presumably allow us to see everything?

Thanks for everyone's input.

I hope you are able to get your car back together quickly Lucxy. Sucks it happened at basically the beginning of the season.

lucxy

I do have dashlogic as well as blue tooth OBDII scanner. I did not log anything on that run. My bad. I guess I did not expect any failures since it was my 3rd track event this year without issues.

I ran full tank of 93 pump gas mixed with 6 gallons of 100 octane race gas.

My shop will be sending my oil for analysis and the engine wil be taken apart next week. More details will follow.

It really sucks since the car maybe down now for the whole season.

Dan_the_C5_Man

It is PE Delay - it infulences when PE will kick in. A lot of folks set this to 1500 RPM, but clearly Michael is not fooling around.

I like the Dashlogic solution because I can display the critical data on the HUD and DIC, real-time..

There can be little argument that running richer, although giving up peak HP and Tq, can only help durability.

That is crazy on your injector duty cycles.. Man, get that fixed when you can.

lucxy

What is really strange is low oil pressure warning.

I guess the question is: did the piston failure cause loss of oil pressure or loss of oil pressure caused overheating and subsequent piston failure?

I dont know if taking engine apart will answer this.
The sequence of low oil followed by blue some soon after may indicate that it is the loss if oil pressure that caused piston failure.

Anyway I am speculating here.