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Big Cams STI Dual AVCS (Cobb)

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Good Morning!

I've been tuning Subaru awhile but never really had the chance to tune with bigger cams.

Got a couple builds now with GSC s2 and Bc280 Quad AVCS to tune and wonder if anybody have pointers.

I'm looking mostly for pointers to smooth out the driveability between 2200 and 3000 rpm.

For example, now I'm road tuning a 2012 Sti with GSC S2 Cam on MAF for the break-in Period. Car has 1700x Injectors with Adjustable FPR, billet rails and an6 lines mounted in parallel setup. (Mentioned to eliminate the usual Subaru stumble issue as a diagnostic)

I will use Hybrid or SD on the dyno tune once the breaking period will be done.

Idle is pretty smooth and my fuel Learning and correction up to 3.5V are under 3%. But if I hold the Throttle open at a complete stop around 2000 2200 Rpm I'ts pretty rough and rpm bounces. it will even throw misfires codes on all cylinders if I hold it there for a bit.

During normal cruising low load between 2200 and 3000 Rpm, you can feel some slight hesitation (almost like a phantom misfire).

I have a similar Issue with a 2013 STI on BC 280 quad Avcs build were but the problem is even more noticeable and harder on this one between 2700 and 3000 rpm. Similar fuel setup.

I heard that Enriching around 13.5 AFR is often some kind of patch used to help this phenomena. Is it true?

Also if that is the case, in Cobb There no Closed Loop Target table. So what is the way to enriching your idle? (cheating o2 calibration?)

Even in SD the Closed loop seems to keep control of the Idle Target.

Thanks for your help

Adding some fuel at idle area normally solves the problem. Anywhere from 14-13.5 afr is very effective ratio to be used in problem zone. You can also add a couple of degrees of ignition timing to compensate for bigger cams.


with bigger cams, The stock avcs tables in the very low load/crusing areas are sometime a little too much. Try reducing the intake avcs table in your problem area. iIf your fueling is done it should take care of your issue.

Let me know how it goes.

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