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Supercharged Subaru: Bank 2 AFR leaning at a certain RPM range WOTh

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Hi guys,

I have a 2005 Subaru Outback (Liberty) SOHC EJ25 with a Vortech centrifugal supercharger.

The factory ecu is setup with a front AFR sensor on each head which are generally considered to be accurate down to 11:1 AFR. The o2 sensor is right on the flange of the exhaust basically immediately after the exhaust port of the head.

I have an unequal length headers with an UEGO after both heads combine.

Here is a picture of the exhaust for reference:


What I am seeing is under WOT conditions, bank 2 (left side of the car) leans out from around 6000-6800rpm with a peak lean condition around 6400rpm. The lean condition starts around 6000 and peaks at 6400rpm, then trails off by 6800rpm where it goes back to normal expected AFRs.

My target gasoline+methanol injection AFR is around 11:1.

On bank 2 I will see as lean as 12.5:1 at the max lean out condition which is pretty scary if it is accurate.

I have tried swapping AFR sensors from left to right and separately swapped injectors left to right with no change in the results.

I am now also on a new motor with the same results.

I can see no exhaust leaks anywhere.

Fuel system is stock Subaru design which starts on the left side of the car with cylinder 4, then goes to 2, then over to the other side of the vehicle before going into the FPR and back to the tank.

Does anyone have any thoughts as to what this could be caused by?

I've heard some theories about exhaust pressure, perhaps related to the unequal length headers but nothing is conclusive to me.

Log showing af #2 leaning out to 12.46 @ 6401rpm.


Do you know someone with equal length headers you could borrow to see if it is just the pipework?