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Reducing Open Throttle Turbo Surge

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Are there any tips on reducing full throttle turbo surge?

I have improved the tune of the engine significantly after putting the training courses info into action on my car (500BHP JDM Impreza with Twin Scroll turbo).

Fuelling was miles out in particular, i am now getting over 100 miles more to a tank of fuel on a long cruise run, 50 miles more commuting to work.

One issue i cant seem to fix is if i am cruising at circa 3000rpm-3500rpm in top gear, when i got full throttle as the turbo spools, it starts to surge, which is obviously not a good thing, so at the moment to control this i have to blend off the throttle through the zone it surges when full throttle, then i can go WOT again post the surge point.

I've tried slugging the turbo in 6th in this rev range with a custom table build on my Syvecs, where i limit wastegate duty and target boost just in sixth gear and in the problem engine rpm range, but i have to drop the boost so much, it seems nicer to just blend with the throttle and drive it with this issue in mind.

If i am understanding this correctly, the problem is the turbo is trying to force more air into the engine than it can consume/pass through at this window in the engine rpm, is this correct?

If so, are there any tuning techniques i can try to kill this surge? I have variable valve timing, would it help to make a change to cam timing for example?

Any ideas welcome.

Yes, play with AVCS (reduce) to stay more out of surge zone is a good way...

It really points to a mismatch in turbo sizing which is actually pretty unusual to come across these days. Adjusting your cam control as Xavier suggested may help, as will reducing your waste gate duty cycle and boost through the surge zone. Generally this sort of surge behaviour is more prevalent at low rpm in a high gear where the engine is more heavily loaded.

yes, it only happens in 6th gear at low rpm on full throttle.

Turbo compressor wheel size to exhaust housing size is the limiting factor on this setup. We are really pushing the limits of what can be achieved on a genuine twin scroll exhaust housing on this stock position turbo. I used to have no surge on a previous gearbox with lower top gear (JDM STi), but have swapped that out whilst my normal box is refreshed (Have a UK gearing box in with higher 5th/6th), that extra load has introduced this issue to a point i would like to sort it.

I'll have a try with adjusting the valve timing and see what effect that has. Surge is actually worse if i try and slow the turbo through the zone by dropping WG duty and target boost.

regarding turbo sizing, i previously had a 2mm smaller compressor wheel version of this turbo, but found with the current one for the competition environment i could achieve the power i wanted with 0.3BAR less, but it is less suited to road use and if specking for mainly road use i would go back to the previous smaller compressor wheel version. On both i am getting over 520lbft at 3600rpm out of my 2.1, which is going some on a stock position genuine twin scroll, so it has a proper punch for a road car.