3" with good sweeps got me to 1000 HP or so but start to be a back pressure issue. I got about 3-4 psi on the downpipe.
4" got me to 1200 but the larger 4" downpipe goes back up to 3-4 psi at that power. Most of my BP is at my turbine though (55-60 psi on the hotside) because the turbine is so small. I'm swapping turbos soon and that might move the problem to my downpipe, but for right now it looks like the 4" is OK to about 1200 HP or so in my car before it gets what I consider high.
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89 LX. 363, single turbo, Super Vic EFI, TFS high port heads by TEA, solid roller, glide. Holley HP EFI. (exact combo varies)