Stumbling on transition (subaru ej255)

Hi everyone!

I'm having a Subaru Forester 2007 SG9 ej255 turbo upgraded to vf49 twinscroll turbo (from TD04), 76mm exhaust, no cat, engine is bit less CR than stock (is about 8.5) and i've got MAHLE forged pistons. The rest is stock.

ECU was tuned by our local tuners, on the road by making power ramps. However time to time i'm getting issues (car stumbling on acceleration) on 5th and more on 6th gear on low to moderate load between 2400-3200 rpm, usually it's not on the boost yet or on very low boost ~0.1-0.3 bar. While issue are presented engine even can get a detonation, but very very rare (got this while was on very cold site -26 deg.celsium).

If i try to push car hard at the same time - there is no any issues, car accelerates with no issues easily (spooling and comes to full boost around 1-1.3 bar). Some time I can feel soft stumbling even on cruising at the range of 2400-3200 rpm at the 5/6 gears.

Could you please advice the strategy to refine the tuning? What to check align first? Is there any common things can give such affect?

IMHO transition is tuned poor, but what could be my strategy to refine this?

Hi Alexey, it's very difficult to narrow down your exact issue from the description you've given. It may be that your issue is related to transient enrichment tuning or you could have issues related to the main fuel map, or alternatively you may have ignition related problems - coils or plugs. The first step in diagnosing this is to check the AFR before and after the issue occurring. If you're seeing a rich or lean spike at the point the engine stumbles, and it's always related to a throttle input then I'd suspect transient enrichment tuning is your issue. You haven't mentioned the brand of ECU fitted so it's hard to offer morespecific information, however we have a number of transient enrichment webinars in the archive you can check. Here's one for an example - https://www.hpacademy.com/previous-webinars/233-haltech-elite-transient-fuel-tuning/