Is it common to have turbo surge with t34.48. Last spring i tried everything to overpass this trouble ,but nothing helps.

Surge occurs in motorway speed cruising ,when engine not need much boost. That t34.48 Charges very early ,almost like stock t3. It's like engine can't swallow all that air.

Car spec: Sierra Cosworth 1988 2wd ,ported head ,4wd inlet manifold,803 greens ,MSD evolution chip, GReddy PRofec B-spec Boost Controller, rs500 style IC , 044 bosch and 2.5" exhaust.

i tried adjustable cam pulleys ,fuel pressure changes ,AFR changes ,Other 803 green chips ,Two different ECUs ,MAP sensors ,new pf09 TPS and other sensors ,but nothing helps. Bad turbo surge and that's it.

And it's really a turbo surge. Not air striking back to compressor trz,trz,zzz noise ,when release gas pedal Dump valve works very well.

Stock T3 is now in place ,but i want little bigger turbo. Is t34.55 or .64 good choice. I think it can't surge that easily like earlier .48

Some friends in our own Cosworth&rs club forum in finland says that 3" downpipe may help? but i'dont think so ,because no surge in full throttle.