Cutting out only in high gear

[Bobo]

Car just started doing this. I have made very few changes, moved O2 from header to downpipe, free'd up the exhaust a little, and went from 114 to 118 octane. The last few passes it misses in high hear ( 2nd gear for me, powerglide). It has cut out a little once or twice at the very top of 1st. AF never shows leaner than 12:1, and richest has been around 10.5:1. This is close to what it's always been before this problem. I only looked at the fuel pressure guage once, it was reading steady and slightly flucuated with the missing ( not more than maybe 2 psi). I was thinking it may be fuel delivery, but the f.p. didn't drop off and the AF didn't show lean. One thing, I leaned it out a little and added .5 degree of timing the last pass, mph was the lowest and it cut out the most. Makes me think it is lean and I just made it worse. Any ideas?

 

[Craig Smith]

Make sure your injector duty cycle isn't abnormally high. This would indicate a lack of fuel delivery and the ECU is holding the injectors open longer to compensate. If you don't see any strange readings on a data log, it's probably time to look for broken valve springs and that kinda stuff. I hate that!

Craig

 

[Bobo]

Thanks Craig. I'll put inj. duty cycle on my data logging. The a/f never went lean, O2 correction was almost 0% the whole run. It may be an ignition breakdown, Harry @ Precision Turbo told me about some stuff I've done that isn't the best way to do it. Figures
I'm gonna make sure the rockers are set right. And I'm acctually gonna turn the boost down since it seemed to run better around the low 20's. I'm second guessing the T76 q trims ability to make boost in the high 20's on a 347 ci motor. Oh the joys of tuning

 

[Lance]

With a T76 on my V6, I was never able to achieve over 26 psi boost. I suspect the back pressure was just too much or the turbo was just not able to deliver the volume.

As for the missing, you have all the tools to determine if it's a fuel delivery problem caused by the ECU or calibration. I would look at rpm, injector duty cycle, injector pulsewidth, and target spark timing (Spark BTDC). If you suspect that the miss seems to coincide with peak torque, make sure the rotor phase is correct at the spark timing you run at peak torque. This will be the first place you will see a rotor phase problem. I don't know what ignition you have but you could also try closing the plug gap up a tad.

Lance

 

[Bobo]

I've been able to run 28 psi, but the car has run better with boost in the low 20's. I believe my peak torque is acctually below my stall speed, 5500. The converter is gonna be tightened up. Converter is slipping bad too. I may go colder on the plugs, but they are gapped at around .022-.025 now. There Champion race plugs, the kind that the ground strap doesn't come out all the way over the electrode so it's kinda hard to check them. I don't know how much this hurts, and it probably has nothing to do with this miss, but my air temps in just the 1/8 are 230

 

[Lance]

I just thought of another commonly overlooked item. When running forced induction, you need to make sure your valve springs are up to the task of getting the intake valve closed under high boost conditions. Consider that you have to compensate for the 20-25 psi of pressure acting on the backside of an intake valve that has a fairly large surface area. A 2" valve can have a 3 sq. inch surface area. With 25 psi of manifold pressure, this can equate up to an additional 75 lbs of force trying to keep valve open. Of course there are more dynamics to consider but you get the idea. (BTDT)

Lance