#5 went lean why?

[Turbo Cobra]

So we had been running the same setup for a while 750 passes with only fairly minor changes.

Setup:
Stock 04 Cobra Longblock,
Stock Manifolds
GT47-88 19-35psi
Sullivan/Hpp intake
BS3 w LS1 truck coils tuned/cyl
Ford/Bosh NG 150/160's E98 and NGK B8EFS plugs

Bigger changes:
Went from a Lentech 4r70w to a Glide many passes does not
seem to be a factor.

Went from a small moroso 40 micron fuel filter to a much larger russel 10 micron filter after the pumps per weldon (AFR in downpipe still the same after the change).

One day on the way to the track I did a high gear pull 60-1?? low boost up a small hill 3.27 gear and 30"DR drove all the way to the track fine but ran like crap after restart at the gate, burnt the
part of the tip off of #5 we had never had a problem with #5 before all other plugs perfect.

Replaced the plug car seemed fine so we pulled 2deg from and added 3% fuel to #5 made a few short test passes all 8 plugs look perfect made a few mid/low 9 sec mid boost passes checked all
8 plugs each pass still no issues drove the car home still found no problems...

Made ~15 more mid boost passes cheked all 8 plugs looked perfect turned the boost up a little car seemed a little slow to spool but ran ok till 1000' lost power melted piston #5 bad (looks like there was some fuel) and AFR was good all the way till shutdown 10.7(E98 all other plugs etc look perferct).

So why do you think #5 went lean?

Injectors all checked out
Sent BS3 Box in it checked out (they think it may be the main BS3 ground it was not to the battery).
Coil did not test

So I added a #8 or #10 ground from the battery to the main chassis ground
Replaced the Injector
And replaced the Coil

Any other ideas?

Could it be a fuel delivery issue? I Y into the front of the rails #5 is the front cyl and is a longer run... this was never an issue in the past even at higher boost levels and dont think it is my thinking is if it was an issue #6 would also show at least slightly lean???

[Collinsautomotive]

Pictures of the piston ? plug etc. That 8 plug is a bit to hot for ethanol.

[Turbo Cobra]

Originally Posted by Collinsautomotive

Pictures of the piston ? plug etc. That 8 plug is a bit to hot for ethanol.

Run the 8's for low and mid boost 9's for high do you think a 9 would be better for mid boost?

[Collinsautomotive]

can you find all of the ring gaps or did they all line up on the same side ?

[Turbo Cobra]

Originally Posted by Collinsautomotive

can you find all of the ring gaps or did they all line up on the same side ?

There is no gap now but I see the ends they were offset to the left of the major damage about an inch apart.

[Collinsautomotive]

So the ring gaps are all on the same side of the piston nearly lined up or you can find the gaps for the top and second ring ?

[Turbo Cobra]

Put a white mark at each gap

[Collinsautomotive]

Ok trying to rule out ring problems. To me it looks like oil got by the rings somehow. Could also be a case where the piston simply just fialed. How do the other slugs look ?

[Turbo Cobra]

Originally Posted by Collinsautomotive

Ok trying to rule out ring problems. To me it looks like oil got by the rings somehow. Could also be a case where the piston simply just fialed. How do the other slugs look ?

Perfect

[trbosvt]

no EGT's? sullivan with an HPP elbow feeding from the passenger side? i did something similar to mine and it was number 5 also, but mine feeds from the rear of the car. single gt4780 with a turbo 400.... 22lbs on gas a/f at 11.1 and 13 degrees. over 100 passes on it and had never hurt anything, then i freshened it up and put cometics in it, 4 passes and it trenched the head. after that i bought egt's. I went through everything, and didn't find anything wrong with ring pack, injectors, fuel pump etc. Motor is in another car now with fel-pro's in it, same tune up and a 91, over 40 passes and still runs like a champ. I will tell you this, when i pulle the plug in the one that burned up, the plug had a nick on the electrode from when i had gapped it...it was pretty notcieable. After talking to a couple of people....some said it could have created a hot spot, some said that i was over thinking it. You never know...the sullivan manifold might have bad distribution and might be feeding that cylinder more than the others.

[Turbo Cobra]

Hi Eddie I met you at a Vegas PSCA race a few years ago
No EGT's
Mine feeds from the drivers side



What egts did you use?
Hmm dont think I gapped the plugs they come at .027 I think will check

[Turbo Cobra]

14mm Thread Size, 17.55mm (.691") Reach, 5/8" (16mm) Hex Size, Tapered Seat, Non-Resistor, Removable Terminal Nut, .027" (0.7mm) Gap, Heat Range 8

[redvictor]

You burnt the tip off no5 plug is where it all started. That means the cylinder got hot,which means it detonated. That in turn would have damaged the piston/piston ringland. It possibly could have trapped the top ring.This would pull oil into the cylinder and detonate even more.It's a downward spiral from there. You need to creep up on it next time by plug electrode reading.
Lean usually burns the centre out of the piston,not the edge.

[Turbo Cobra]

This was only a midboost pass same setup / tuneup we ran 750 passes never had an issue with #5 before this so why did we burn the tip off 5 this pass 5 was not a hot cyl b4 and we pulled 2 degrees out and added 3% fuel plugs read good

[mikee]

You better check the valve springs if you really have even half the passes you claim the springs could be shot leading into valve float ,piston to valve could have started the problem.