[BiturboS4] US vs Euro spec

[Ian McCloghrie]

On May 20, 2002 Andy Turner wrote:
> > I know its about 181mph, equivalent to 290 k per h because I have checked i
t against the rev counter
>
> Is that a reliable way of calculating a top speed? Surely if Audi
> somehow put a 7th gear in, it wouldn't suddenly be able to do 200mph?
> Surely something else gives up first? Presumably it gets to the point
> where the engine can't push the car anymore, no matter what the revs
> are?

Top speed is either:

1 - redline in top gear
2 - the electronic limiter
3 - the speed at which the force generated by the wheels pushing against
the pavement equals the force pushing backwards on the car from (mainly)
air resistance

That force is at-wheel torque times the radius of the wheel+tire combo.
So let's pretend an S4 makes 200 lb-ft after accounting for drivetrain
losses.  With .584:1 6th and 4.111:1 final drive (US figures) and
225/45R17 tires (radius of 12.5 inches) it's being pushed forwards with
200*4.111*.584*(12.5/12) or roughly 500 pounds of force.

Going with higher gears (either through adding another, or by lowering
the final drive ratio) helps in case 1 (obviously).  It can sometimes
also help in case 3 if the engine hits top speed after the torque has
fallen off from the peak.

--Ian