Just running some quick #’s, if at 6psi the compressor was at a relatively decent 70% adiabatic efficiency, at 3psi it would have to be at less then 38% adiabatic efficiency for the outlet air temp to be higher.

OTOH, it is possible for that to happen if you’re using the same turbo on both setups, since HP is about airflow, and to maintain the same air flow at a lower pressure ratio you need a larger compressor…

WRT to the durability question, to give a real answer you’d have to give more information. Making some assumptions, to produce the same 450 hp at a lower boost you either increased VE of the engine after the turbo or displacement, in both cases you might have done something to hurt durability.

Lets assume that both engines actually have the same torque curve:
- then if the displacement is the same bmep has to be the same, but boosted engines generally show a lower, more constant peak cylinder pressure then a comparable NA engine so the lower boost engine will actually have a higher peak cylinder pressure which will have more potential to break parts and pound bearings.
- If the displacement was increased, then the ve and bmep did not increase and there is a good chance that the new peak pressure is not higher (but it still could be). Increasing displacement often results in durability compromises, such as shorter R/S ratios piston pin locations that impinge on the oil ring grove, or even thinner cylinder walls and less crank throw overlap (the correct term for this escapes me right now)

OTOH, real world, assuming that both are well built engines to start with, you will have to raise the power band to make more hp with the same engine with less boost. This will mean that most likely the lower boost engine’s torque curve would be significantly smaller then the higher boost engines, and it would suffer less from the effects of cylinder pressure but more from the effects of high rpm’s… depending on what the weak points on the engine are, this could go either way, better or worse reliability.

In this example, heat production really isn’t an issue. On a 90* day at 70% adiabatic efficiency, 6psig boost will produce an inlet temp of 168.5*F and 3psig would result in 131.6*F. That would usually imply roughly a similar change in combustion and exhaust temps, and it’s doubtful if either 450hp engine would be close enough to the ragged edge that 44* would make a difference.

But in a nutshell, it could go either way depending on which combination is better matched and built with better parts.