Hello all,
I'll start with a little history, in 2023 I took my car to a dyno in Ohio for a tune. The clutch started slipping so we turned the boost down to about 250whp where the clutch was stable. I will attach the graph from that session, you will notice a run with a weird fuel ratio which was later found to be a bad fuel pressure regulator.
I then spent the next year putting in a kpower ZF5 kit. So after installing a getreg 3.23 diff, v8roadsters axles, kpower driveshaft (I'm making another thread about that STUPID driveshaft), ZF5 trans, and ACT clutch, I was ready for more boost.
Fast forward to fall of 2024 I went back to the dyno and got up to 330whp before turning the boost up yielded no increase in HP. The tuner suspected some sort of restriction (obviously), I do not believe I am out of the compressor map of the EFR6258, I also am running Kraken low mount manifold and their 3" exhaust, 3" intercooler piping, and Skunk2 throttle body.

Is the BP05 really keeping my engine from receiving any more airflow?
Attached is the dyno run at 23psi and 330whp.

I've been tossing the idea around of where do I go from here, BP4W? The +1mm valves I already have new in box?

The head is currently stock with the valve seats and deck surface being touched before the last assembly.
E85 in case that matters.

 

That is tapped out on a 6258 in my parts (Denver)

 

Just looking at the dyno graphs, with no tune or datalog, I'd say you're approaching the limit of turbo and head. You had a healthy gain of 80hp for 5psi, but if it's not going up further, you're on the right track looking for a restriction. I'd be concerned if I put a BP4W head on I'd reach the limit of the turbo shortly after. But the heads going to be your biggest headache (pun intended), so I'd start there with the valves and turbo you already have.

Part of your restriction is probably also your intake manifold, but that's solved with a flattop or S2 manifold on the NB head, so that's worth mentioning as well. You could try a S2 manifold on the BP05 head, but we all know they changed so much with the BP4W, I don't know if it's worth it. It's transferable to the NB head if it doesn't fix anything, but IMO it's such an ugly and PITA to install aftermarket piece that'd I'd much rather have an OE flat top on my build.

 

Borg Warner has a handy EFR calculator that explains this: https://www.borgwarner.com/go/SR8IFY

The EFR 6258 is running out of chooch around 22psi / 330ish whp on a stock-ish flowing BP.
At that flow rate you're rapidly dropping off the compressor efficiency island above 6000rpm. Your peak power is 6300-6500rpm. Pressure ratio is still fine at lower altitudes.
To make more power you need to move the dots to the right. And your efficiency is dropping the further right you go. This will generate higher charge temps and also higher exhaust backpressure. Both degrade the ability to make power.
#5 = 6000 / #6 = 7000rpm


Your limit is also the turbine flow rate. At 7000rpm / 22psi MAP you are at -2psi engine delta. Meaning your exhaust manifold pressure is 2psi higher than your intake. Now this can be mitigated. It looks like there is a 0.85 A/R 58mm exhaust that still fits the T25 flange. I'm not sure if the housing fits with the Kraken low mount manifold.


You can also consider that peak compressor efficiency is 2% higher on the 6258 (76%) vs 6758(74%), which is significant, and reduces the benefit of automatically going to the larger compressor wheel.
I'd need to play around more to see if a 6758 T25 0.85 A/R turbine housing would make sense. It'll definitely negatively impact boost response for higher peak flow. The 6258 is a well match turboski (comp vs turbine) and shows B/W knew what they were doing. You're basically at the point of going to a larger turbo to increase the peak flow rate efficiently and at that point you need to seriously look at engine volumetric efficiency and potentially reving higher to get more power.