What makes the stock frame turbos hit a wall?
If someone mentions trying to reach over 600whp with a MAP manifold and a Garrett (mitsu flange) turbo, what would the choke point be?
I noticed that FP 5 degree clips the TD06H turbine to increase top end flow, so obviously there is a choking point in the STOCK turbine housing. I understand that.
If I remember right the A/R for the stock turbine housing is like 0.89, so that makes sense.

however, usually the comments I see are that "stock frame" turbos just can't make big power. Does that apply to say a modern GTX3576R gen 2 turbo?
I'm staring at a T4 flange and the Mitsu flange and the sizes are comparable.

seeking to understand

I have found ONE guy claiming 650 whp on an Evo X with the GTX3576R gen 2. No dyno plot provided. Just a claim.

 

Since no one answered my question, I believe I had found my answers.
The stock turbine housing has a very small (compared to T4) volute size. This is a choking point. period. To try and decrease the backpressure when using that OEM MHI housing, FP does a 5 degree turbine clip, and they recommend the housing is ported for RED + BLACK. The actual MHI twin scroll flange/port size is NOT A LIMIT. It is nearly the same as a T4.
The Garrett turbine housing offerings (GT30 and GT35) are capable of pretty amazing flows.
Here is a GT35 wheel with Evo X turbine housing making 680hp to the wheels in an MR. DYNO

 

What you find is that the full frame turbos make power more easily compared to the turbos you are talking about. If you notice, that turbo is having to run 36psi to make 680. At 30 psi it's making more than 100 whp less than my 5858 did at 29psi.

I've noticed this in every full frame turbo I have ever tuned, they make more power with less boost compared to the "OEM style" housing internal wastegate turbos.

 

So is that because twin scroll versus open, or the volute size? (or both) ?
Because the wheels are not that different sized....

 

It's the housing size, and more optimal manifold design.

.89 A/R is small. Minimum A/R we run on T4 twin scroll is 1.15. Biiiigggg power, max effort stuff (like a 6466 or 6870 going for all of it) we run the 1.32, or just go to vband. Vband housings are either in the .8-.85 range, or around 1.0

 

Not sure about dyno numbers but I picked up 6-7mph in the 1/4 mile switching from a stock frame FP Black (which made 475-480whp on a DJ) to a T4 6266. Same stock block, pump gas car. Went from trapping 120-121 to 127. Built block might be different but the stock block shines with a big frame turbo.