Hi-flow turbo= retune?

[DX20VT]

I say no.

ECUs are tuned to for any amount of load (=boost, to a limit) at any point in the RPM range.
Changing to a turbo which spools earlier or later will not effect if the engine then runs leaner/richer with the existing tune.
The only time you will need a re-tune is if you start pushing over the boost that the engine has been tuned for in the past.

You also need to think about exhaust back pressure.
If you change the exhaust housing or wheel,
you may end up with less back pressure in the exhaust
which will give more airflow for a set RPM and Boost pressure.

[MWP]

Just to clarify, I'm sure that MWP is talking about AFM'd engines. MAP is different entirely as it doesn't understand that you're flowing more air at the same pressure... while AFM can compensate as it measures air in not pressure.

Nope, it applies to MAP as well.
To the turbo, the engine is a restriction, and is trying to push air through it.
More flow = more pressure. Cant have one without the other without reducing the restriction.

You also need to think about exhaust back pressure.
If you change the exhaust housing or wheel, you may end up with less back pressure in the exhaust which will give more airflow for a set RPM and Boost pressure.

... this however is a good point
A change in exhaust back pressure will change the tune slightly.

[Z2TT]

But doesn't a high flow turbo flow more air for a given pressure, so the highflow turbo would flow more air than a standard at 14 PSI, in that case you would need to tune to give more fuel accordingly for the increased airflow? And since the MAP reads the pressure then you would need to re-tune.

[boxh34d]

Yes, the turbo may be CAPABLE of moving more air for a given pressure, but due to the engine, not really. . . .
From what I understand on this is correct:
The engine is only an air pump capable of moving x amount of air per rev. Without making any improvements in engine efficiency (ability to move air), the turbo will still effectively run at the same boost, moving the same amount of air. If there was no engine in the way, yes, the turbo could move more air for a given pressure rating, but the engines ability to breathe is governed by a couple of things being: Capacity, revs, volumetric efficiency, and atmospheric pressure. If you don't change any of the above, your engine can't move more air. The one exception to this is when there is a big difference in turbo efficiency at the given duty point. If the old turbo was running out of puff, and running at say, 50% effeciency, and the new highflow turbo is more suited to the operating point, its efficiency might be say 70%. This can result in more mass of air for a given volume due to temp difference.
The only thing that will allow more airflow at the same pressure is as mentioned, removing restriction from the exhaust, or removing restriction from the engine (cams, manifolds, head work etc etc).

[Duk]

If the exhaust side (turbine housing or turbine wheel) is flowing better, then the engine may be less likely to suffer from detonation, as the combustion chambers should be a little bit cooler. So the engine could/should tolerate more on boost ignition advance and get a power advantage that way.
If it's improvements on the compressor side, then the same thing should apply. Less heat for a given pressure equals more tolerance to having a bit more ignition timing and a touch more fuel. The thing with the compressor side, is that negative effect of an over worked compressors (heat) can be hidden quite effectively by a decent intercooler.

[Indelible]

when i recently changed from a billetturbochargers S-GT2 tubo ( gt3076r spec) to a garrett gtx3076r my tune definately needed to be fiddled with
ran slightly rich down low, turbo's were different only 50 ish rpm difference in lag but still felt better after a tweak of the tune and returned the nice drivability

obviously it will run and be close but to get it right and get the best out of the new setup get it retuned IN MY OPINION

cheers,