rear cylinders running lean, why?
Thread Starter
Staging Lane
Joined: Oct 2006
Posts: 76
Likes: 0
From: Graz, Austria
rear cylinders running lean, why? we made some shake down passes with a 200 shot with a hidden direct port system. All the bugs are ironed out, things pulls like crazy.
However we lost cylinder #7 and we have no clue what could have happened!
After taking heads off we realized that the exhaust valve has been burned up completely and that's why we had no compression anymore. All was fine up to that very moment, EGT are in 1200 range, AF a tad above 12.0, 25 degrees timing (9 degrees taken out via timing tuner from NA tune).
Now the thing, the first 6 spark plugs are perfectly slight brown, the rears have been white. Strange as the inlet of the fuel and nitrous is at the back and there is no too short term radii prohibiting fuel from making the turn.
what could have caused to run #8 and #7 too elan but all others at a perfect AF?
We had the injectors checked out, all have perfect spray pattern and work well - so no fault at all.
some specs:
2000 Vette, forged internals, 11.3:1 CR
running on 100 octane pump gas plus a few bottles of octane booster,
huge Bosch pump for engine
stock 26# injectors
Ferrea stainless steel valves in CNC ported racing heads
232/234 cam
dynoed at 540hp flywheel hp @6400 rpm
engine has 20.000 hard miles on it with that combo, and with that driver I mean really hard miles with long bursts over 170mph on the Autobahn or hillclimbing...
when on nitrous:
separate standalone pump system, with second huge Bosch pump set at 60 psi
200 shot
25 degrees timing
colder plugs /TR6 to exact
spraying from 3500 to 6100, shift light at 6200, shift at 6300
I am drawing a blank and unless I can figure out what "could" have happened I don't know what to look for.
However we lost cylinder #7 and we have no clue what could have happened!
After taking heads off we realized that the exhaust valve has been burned up completely and that's why we had no compression anymore. All was fine up to that very moment, EGT are in 1200 range, AF a tad above 12.0, 25 degrees timing (9 degrees taken out via timing tuner from NA tune).
Now the thing, the first 6 spark plugs are perfectly slight brown, the rears have been white. Strange as the inlet of the fuel and nitrous is at the back and there is no too short term radii prohibiting fuel from making the turn.
what could have caused to run #8 and #7 too elan but all others at a perfect AF?
We had the injectors checked out, all have perfect spray pattern and work well - so no fault at all.
some specs:
2000 Vette, forged internals, 11.3:1 CR
running on 100 octane pump gas plus a few bottles of octane booster,
huge Bosch pump for engine
stock 26# injectors
Ferrea stainless steel valves in CNC ported racing heads
232/234 cam
dynoed at 540hp flywheel hp @6400 rpm
engine has 20.000 hard miles on it with that combo, and with that driver I mean really hard miles with long bursts over 170mph on the Autobahn or hillclimbing...
when on nitrous:
separate standalone pump system, with second huge Bosch pump set at 60 psi
200 shot
25 degrees timing
colder plugs /TR6 to exact
spraying from 3500 to 6100, shift light at 6200, shift at 6300
I am drawing a blank and unless I can figure out what "could" have happened I don't know what to look for.
Originally Posted by coolchevy
we made some shake down passes with a 200 shot with a hidden direct port system. All the bugs are ironed out, things pulls like crazy.
However we lost cylinder #7 and we have no clue what could have happened!
After taking heads off we realized that the exhaust valve has been burned up completely and that's why we had no compression anymore. All was fine up to that very moment, EGT are in 1200 range, AF a tad above 12.0, 25 degrees timing (9 degrees taken out via timing tuner from NA tune).
Now the thing, the first 6 spark plugs are perfectly slight brown, the rears have been white. Strange as the inlet of the fuel and nitrous is at the back and there is no too short term radii prohibiting fuel from making the turn.
what could have caused to run #8 and #7 too elan but all others at a perfect AF?
We had the injectors checked out, all have perfect spray pattern and work well - so no fault at all.
some specs:
2000 Vette, forged internals, 11.3:1 CR
running on 100 octane pump gas plus a few bottles of octane booster,
huge Bosch pump for engine
stock 26# injectors
Ferrea stainless steel valves in CNC ported racing heads
232/234 cam
dynoed at 540hp flywheel hp @6400 rpm
engine has 20.000 hard miles on it with that combo, and with that driver I mean really hard miles with long bursts over 170mph on the Autobahn or hillclimbing...
when on nitrous:
separate standalone pump system, with second huge Bosch pump set at 60 psi
200 shot
25 degrees timing
colder plugs /TR6 to exact
spraying from 3500 to 6100, shift light at 6200, shift at 6300
I am drawing a blank and unless I can figure out what "could" have happened I don't know what to look for.
However we lost cylinder #7 and we have no clue what could have happened!
After taking heads off we realized that the exhaust valve has been burned up completely and that's why we had no compression anymore. All was fine up to that very moment, EGT are in 1200 range, AF a tad above 12.0, 25 degrees timing (9 degrees taken out via timing tuner from NA tune).
Now the thing, the first 6 spark plugs are perfectly slight brown, the rears have been white. Strange as the inlet of the fuel and nitrous is at the back and there is no too short term radii prohibiting fuel from making the turn.
what could have caused to run #8 and #7 too elan but all others at a perfect AF?
We had the injectors checked out, all have perfect spray pattern and work well - so no fault at all.
some specs:
2000 Vette, forged internals, 11.3:1 CR
running on 100 octane pump gas plus a few bottles of octane booster,
huge Bosch pump for engine
stock 26# injectors
Ferrea stainless steel valves in CNC ported racing heads
232/234 cam
dynoed at 540hp flywheel hp @6400 rpm
engine has 20.000 hard miles on it with that combo, and with that driver I mean really hard miles with long bursts over 170mph on the Autobahn or hillclimbing...
when on nitrous:
separate standalone pump system, with second huge Bosch pump set at 60 psi
200 shot
25 degrees timing
colder plugs /TR6 to exact
spraying from 3500 to 6100, shift light at 6200, shift at 6300
I am drawing a blank and unless I can figure out what "could" have happened I don't know what to look for.
Thread Starter
Staging Lane
Joined: Oct 2006
Posts: 76
Likes: 0
From: Graz, Austria
we will go down with AF, that was the first pass to see where we are. The goal is 11.0 to 11.5 indeed but first you need to see where you are, that it took out a valve was not really expected. We should have been below 12 anyway as we started with 10 psi higher fuel pressure then recommended on the stand alone fuel system.
All in all I believe the AF was perfectly fine, the absolut AF may have been wrong as the rear cylinders were too lean and in the collector you only measure average AF with an LM1. I will also move the EGT probe to the cylinder #7 which seems to be the most affected one.
Dave explained how that can be tuned out but first I need to wait until the new valves have arrived from the States (along some spare ones, haha)
what spark plugs would you recommend?
All in all I believe the AF was perfectly fine, the absolut AF may have been wrong as the rear cylinders were too lean and in the collector you only measure average AF with an LM1. I will also move the EGT probe to the cylinder #7 which seems to be the most affected one.
Dave explained how that can be tuned out but first I need to wait until the new valves have arrived from the States (along some spare ones, haha)
what spark plugs would you recommend?
Last edited by coolchevy; Jun 20, 2007 at 06:07 AM.
I agree, timing and too lean played a roll here. Yea, if your not running a EGT in #7 then you really don't know that that cylinder was even at your stated a/f. I also lost #7, on a 285rwhp shot and to much timing and tr6's, with a a/f of 10.8 across the board and 104 Trick. My new set-up has a dedicated EGT in #7. You going to do a new motor, or build what ya have?
Robert
Robert
Thread Starter
Staging Lane
Joined: Oct 2006
Posts: 76
Likes: 0
From: Graz, Austria
we just put in two new valves and put everything back together. No harm to pistons or anything. We will move the EGT to #7 and run colder plugs in the rear cylinders and richen up the mixture.
Last edited by coolchevy; Jun 21, 2007 at 06:55 AM.
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Originally Posted by coolchevy
we just put in two new valves and put everything back together. No harm to pistons or anything. We will move the EGT to #7 and run colder plugs in the rear cylinders and richen up the mixture.
Thread Starter
Staging Lane
Joined: Oct 2006
Posts: 76
Likes: 0
From: Graz, Austria
we already had good rods and pistons in it, that saved a lot of hassle.
But, I know that sight too well from my dragster. Sometimes 20psi boost and a 300hp shot won't work together too well.
One thing I learned was that greed and boost or greed and nitrous or greed and boost and nitrous don't mix ...........
But, I know that sight too well from my dragster. Sometimes 20psi boost and a 300hp shot won't work together too well.
One thing I learned was that greed and boost or greed and nitrous or greed and boost and nitrous don't mix ...........
Get yourself some #9 nkg plugs, they are alot colder, but will help preven the issues you are having. Also, I'd run teh back 1/2 of the motor a jet heavy on the fuel, #7 maybe even 2. What happens is the air gets crammed in the back of the intake and that's why #55, 6, 7, 8 run leaner then the front 4. It's a common problem, only solved by going to the carb style intake.
