My EF XR6 automatic wagon is running a self-tuned J3 chip, 2800-rpm stall torque converter, cam grind one step up from XR6, mild head skim, headers, SS Inductions and BA induction tube, and it generally runs superbly. But I am getting a weird RPM effect down the quarter-mile in 3rd gear only (see log graph).

It starts to rev normally after the 2-3 shift, but after hitting about 4300 rpm the revs dip to about 4200 rpm and gradually come back up over several seconds, hitting about 4300 rpm again crossing the line. The car is still accelerating, though it feels weaker than it should be. Other automatic Falcon sixes (same 3.45 gearing) start pulling away once I hit third gear, so I know I'm losing some ET and MPH at top end. What could be causing this?

- The ignition timing is the same in each gear, so that can't be it.
- It is well below the speed limiter (160kmh in the wagon), so that's not it. The speed continues to increase anyway.
- Is it a torque converter or transmission issue? It's well above stall rpm, so it shouldn't be changes in torque converter slip. Could it be the TC trying to lock up, and dropping the revs?
- The engine temperature does increase a little down the quarter and continues increasing down the return lane (heat soak?). It's normally at the bottom of the L in NORMAL, but increases to about the middle of the A, and takes about 5 minutes or so to come back down. The engine has been flushed, and the radiator, high-flow 87C thermostat and coolant are new. It has an extra-large external transmission cooler (in line with the one in the radiator side tank to keep trans fluid at engine coolant temperature).
- The AFR is about 12.5 at peak torque, rising to about 13.0 at redline - reasonably safe and normal. Because the revs don't really increase in 3rd, the AFR stays at about 12.4, though it is noticeably richer than in the other gears. It doesn't make sense that it could be a misfire, as that would leave unburned oxygen, causing the O2 sensor to read lean, not rich.
- Is this a detonation issue? That is, is the knock sensor retarding the ignition in 3rd gear? If so, why? It's not running significantly hot, and always has 98-octane in the tank. Besides, it doesn't do it in 1st or 2nd gear. Also, I would have thought 12.4 AFR should be rich enough to suppress detonation in an N/A engine if it is starting to creep in in 3rd gear. And I can't hear any detonation, but at 145 kmh I'm not sure that I would be able to hear it even if it was happening.
- It's not losing coolant noticeably.
- The bottom end has 325,000 kms on it, so the rings will be a bit tired, though is burns almost no oil. The head was done 80,000 kms ago.
- I checked the coilpack and leads, and the resistance is at the bottom end of spec.
- This may be unrelated, but it also produces a moderate amount of visible smoke (greyish) while this is happening. I suspect it's probably caused by running a bit over-rich in 3rd.

I can't figure out what it is for the life of me. It's not a major, as the car performs nicely otherwise. But it burns me that I could be giving up a tenth or two unnecessarily. Any suggestions would be much appreciated.

EF/EL's will lock the TC in third, with '3' selected on the quadrant - EB/ED's don't. From memory I found the EF's would do it much higher in the rev range than the EL's.

What you can do to monitor the TC lockup is back probe the S7 line, run an LED and resistor - LED lights up means 12v and TC lockup.

Hey, snap0964. Sorry for the late reply.

I think you have nailed it. As I have optimised the 1-2 and 2-3 shift points in 'Power' mode in the tune, I have been leaving it in 3 or 4/D and letting it shift automatically. That works well for me in DYO racing. But I haven't been paying much attention to whether it's in 3 or 4/D. That would explain why the problem is intermittent.

I checked the tune, and even at full throttle in 3rd, the torque converter is programmed to lock up at 3315 tailshaft RPM (about 3500-3600 engine RPM unlocked). Your explanation explains everything. I feel stupid that it wasn't obvious to me.

I will make sure to do my runs in 4/D. I have changed the 3rd Power full-throttle TC lockup point to 5000 RPM in case I forget. The drags are back on next week Friday, so I will be able to confirm that it has worked.

Thanks again for your help.

Yeah best of luck with it.

Confirmed. It was the converter locking up. Thanks for all the help.