Quote:
Originally Posted by RugbyBro
Boost threshold = the RPM at which an engine needs to be at to generate the desired boost pressure from the turbo
Turbo lag = the time in which the turbos themselves need to spool to reach optimal boost pressure
S58 has both a higher boost threshold and more turbo lag than S55 because of its lower compression ratio and bigger turbos. The "linear" powerband is just a byproduct of the classic big turbo formula. If you're driving below say 3.5k (as the above poster suggests) you will be out of the boost threshold whereas S55 is within or closer to its boost threshold. When out of boost, S58 will take slightly longer than S55 to spool - which is usually represented by throttle response which I think you are describing above. S55 throttle response is greatly aided by its high compression ratio and smaller turbos (theoretically S55 is producing more natural power relative to its total output due to the higher ratio and therefore needs less help from the turbos - i.e smaller turbos), the expense of which is less objective top end power and tuning potential (which is why S58 is such a monster). Simplistically, that's really all there is to it - I've driven X3M and noticed both the higher threshold and slightly more lag in "normal" driving, but both of those aren't a problem if you're driving in a performance context as the engine is always under load and you're always in boost and above the threshold.
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I agree. And still don't understand any comparison with a NA engine, yes the S58 has a nice and strong top end, but the throttle response is far away from any NA engine in the world, especially the M's. I never felt like I was driving an NA engine when I drove the S58, I felt more I was driving a turbo engine with big turbos and therefore an obvious boost threshold, a slight turbolag and a cool top end. But not a NA.