S4 Eric

A friend asked me this yesterday and I couldn't answer him.

TommyS401`

izza tue toobose! yes yes... the toobose. theyza smalls and spools fasts... (too much episode 1)

CactuSavant

The flat torque curve is somewhat artificial. Audi makes it that way for the sake of drivability.

EdP

Torque manager responsible for that,

donp

Two small turbos which spool up very quickly are able to produce full boost at low rpm.

TipSter

All other things being equal, it's easier to completely fill a cylinder at lower engine speeds than at higher ones. In a turbocharged engine, you also have to consider how much energy there is in the exhaust to help the compressor get more air/fuel in. So they tend to need higher engine speeds to generate enough exhaust energy. However, with small turbos, like we have, you can get better boost, and thus cylinder filling, at lower rpms.

A supercharger on the other hand can be geared to produce lots of boost at low rpms if you want it to. These cars typically have lots of low end torque.

In either case, the designer has to make some trade off between low end torque which feels good but doesn't generate high HP numbers, and high end torque which gives you the big HP. Variable valve timing/lift, 5 valve cylinder heads, and variable intake geometry are ways to help make the trade off less severe.

The S4 is 'tuned' to be torquey, thus small turbos and less high end HP than you might otherwise be able to get (like with KO4s)

Guest

LOL, bought the DVD yesterday... watching currently