wrote:

> On Sat, 22 Jan 2005 22:38:54 -0500, "James C. Reeves"
> > wrote:
>
>
>>Couldn't in my 1967 GTO. The engine torque far overpowered any brake
>>pressure I could place on the brake pedal. Now the car wouldn't actually
>>move (the front brakes kept it in place)...but it would sure billow plenty
>>of smoke from the spinning rear tires!
>>
>
> And if it had been front wheel drive?
> That's the rub with many of todays high powered vehicles. You have
> antilock brakes that are made as small as they can get away with to
> keep the weight down (and since they have antilock, it is hard to
> overwork them anyway) and now we have cars with more horsepower than
> the old muscle cars. The power brakes are engine vacuum operated, and
> the vacuum goes for a dump when the engine is under load.

Horsepower doesn't matter much in this case, it is torque that matters
and only a few cars today have torque ratings above the muscle cars of
the 60s.


> So, yes, there are MANY cars on the road today that would have a hard
> time restraining the engine with the brakes even well below full
> throttle.

I guess it depends on how you define many. I don't think any four
cylinders and probably precious few V-6s can do this. Sure, the large
V-8s probably can generate enough torque to overcome the brakes on the
drive wheels, but I'd have to try it to be sure.

The logic that suggests that few cars can do this is simple. Look at
how long it takes (in time, not distance) to accelerate a car to 60 MPH.
That tells you how fast energy is being put into the motion of the
car. Most cars take 6 or more seconds. Now look at how long it takes
to stop the same car from 60 MPH. It will often be half this time or
less. This tells you that you can remove that same amount of energy
with the braks about twice as fast (or more in most cars) as you can put
it in with the engine. This gives you a rough suggestion that the
brakes are substantially more powerful than the engine.

Now, of course, you have to factor in that the engine is working on
typically only two wheels and thus may be wheel spin limited initially,
but that only applies to cars that are fairly high performance. The
brakes are working on all four wheels, however, mostly on the front due
to weight transfer. Even so, I'll bet that only a few vehicles have
engines with sufficient torque to overcome the brakes on even two
wheels, and certainly won't overcome all four as the Audi proponents
originally claimed.

Keep in mind that most torque convertors stall at less than 2,000 RPM so
you can't consider the engines peak torque, but must look at the
torque available at whatever the stall RPM is for that car's TC. This
will typically be much less than the peak torque.



Matt