Re: Turbos and low-end torque

[tdonohue@xnet.com (Terry Donohue)]

"Arun Rao" <rao@frisbee> wrote:
>
>        Last week, when I read on this list that the 20V Turbo I-5 had
>        peak torque at 1,950 rpm, I was quite curious. As someone pointed
>        out, it's pretty hard to have the turbo spooled up at this (almost
>        off-idle) engine speed, so maybe it's really a red herring.

This always bothered me too, because the low end torque of our turbos is
generally useless.  However, the specifications are correct, but only in
the dyno lab.  As only a slight exaggeration, what the engineers do is load
up the engine at full throttle at low RPM, go out to lunch and come back
and see what the torque is.  (Needless to say, large fans are directed to
both the radiators and intercooler.)  Whether it takes 1 sec or several
minutes to achieve full boost is irrelevant to SAE (or DIN) specifications.
So these output numbers, while technically correct, are truly misleading
in the real world.  If you want to confirm full boost at low RPM in your
car, put it in 5th (or 4th) at 2000, give it full throttle, and also apply
enough brake to keep it at that constant speed-you will slowly see the
boost rise (and I mean slowly) before either your brakes or your
determination gives out.

Terry Donohue
tdonohue@xnet.com
'71 BMW 2002Ti
'200TQ (1.8 bar)
'95 BMW M3