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More boost duty causing boost to lower after 35psi

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Tuning an R32 GTR. 2.8 stroker, 7675mm turbo and has a 4 port solenoid.

1.1bar springs apparently but runs 24.5psi on gate pressure making 599kw.

Increasing the boost to 34psi needs around 20.5% boost duty and makes 783kw.

If i then increase the duty to 21.5% the boost and power ramps up then falls off hard. Logs in the link G4+ are showing everything ecu side is ok. Boost duty is steady, timing and E throttle all steady etc

I have never seen this symptom before so am trying to think what it could be.

Feels mechanical like valve float or wastegate blowing open but i have never experienced this.

Any tips or thoughts?

Green run is with more boost duty. On a hub dyno so no wheel spin and no clutch slipping either.

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Could be back pressure opening up the waste gate.

Its one of my thoughts atm. Was going to try taking the exhaust off after the dump pipe and try a run to see if the rest of the exhaust is causing a restriction. Has 3 mufflers but also has twin 38mm wastegates with screamers so thats taking some flow out of the exhaust.

You mentioned it was a bit like valve float, this may indeed be the case, especially if it's occuring at a similar rpm. I suspect there's a simpler explanation, but I'll comment on that first.

Have you replaced the valve springs and/or checked the seat force of the spring on the valve?

The stock R32 sizes seem to be 35 and 29.5 mm, as 'boost' AND exhaust back pressure in the ports increase, this will effectively reduce the seat pressures as the pressure increases on the back of the valves - how important this is will depend on the specific engine build.

A 35mm valve is ~1.37" or ~1.49 square inches, the 29.6mm exhaust is ~1.16" or 1.06 square inches. On the intake side, there was ~36.5 lbf and, with the higher boost, it was increased to ~50.7 lbf - if it was marginal, this could make the difference. On the exhaust side, as the pressure will be much higher from not only the increased boost, but an increasing pressure ratio, there may be a much higher force on the exhaust valves, despite them being of lower area.

What may be more likely is the ignition is unable to ignite the fuel under the higher combustion pressures - you may see this as a lean condition as there will be unused oxygen in the exhaust. If you haven't done so, try reducing the spark plug gap to 25, or even 20 thou' (0.6 or 0.5mm) and see if that makes a difference - it's a easy check one way or the other.

Thankyou Gord :)

this is a brand new engine and the engine builder has told me it has 112 seat pressure.

there is no miss-fire currently either.

Literally just feels like it builds boost then drops off boost. I will send a pic of the log tomorrow

Forgot to ask, if you use a little less throttle, does it pull cleanly to higher rpm?

Did closing up the plug gaps, removing part of the exhaust, and reducing the total back pressure a little make a difference?

Something else that may be making a significant difference is also the A/R of the turbine housing - lower is good for lower power but can cause excessive back pressure with higher exhaust gas flows - can you beg/borrow/ steal the larger options to try, if you aren't already using it/them? The pressure in the exhaust can be several times that of the boost, as I mentioned earlier, so if it's too high...