Andy and Kevin are right on. The key concept is that torque is
_not_ conserved through the drive train. Torque at the wheel of
the car is all that matters, and this is equal to torque at the
crankshaft multiplied by the gear ratio. The gear ratio consists
of two parts: the tranny gear ratio multiplied by the
differential gear ratio.
However, a much simpler way to think of the gear ratio is the the
wheel spin rate versus the engine spin rate. When the engine is
spinning a lot faster than the wheel, you are getting a bigger
torque multiplier. So an engine that has low torque at the crank
can still get lots of torque to the wheel, if it can rev high
enough.
So engine RPM and engine torque are equally important factors in
accelerating the car. Double either one of them and you can
double your acceleration. And Power is equal to the product of
the two.
That is why the optimum shift point for straight line
acceleration is always somewhere between the peak power and
redline. This is true for every engine, whether a monster torque
V10 or a low torque S2000 or rotary, or even an electric motor.
Another factoid is that _all_ engines have torque in ft. lbs. and
power in horsepower equal at 5,252 RPM (if they can run at that
speed).
A completely separate issue that is often confused with the above
is the shape of the torque curve. Smaller, high revving low
torque engines that produce high power (like the S2000 or a
Wankel) often have a non-flat torque curve that increases with
increasing RPM. And big, low revving high torque engines often
have a flat torque curve -- they produce pretty much the same
torque at all RPM. But there are plenty of exceptions to both of
these general rules.
It is the shape of the torque curve, _not_ how much torque, that
determines how the car launches. A car with a flat torque curve
will launch well, regardless of how much or how little torque it
has. That's because the gear ratio (torque multiplier) is
constant at all RPM, even if the torque curve itself is not flat.
Big high torque low revving engines usually launch better, _not_
because they have more torque, but because they have a flatter
torque curve.
James Creasy wrote:
>
> http://www.vettenet.org/torquehp.html
>
> i found this to be an excellent description of torque and horsepower in
> cars.
>
> what i took away from the article is that torque is what works if you cant
> be in the right gear for high revs. since we dont shift much in autocross
> and rarely hit the power peak, mid range torque should rule the roost.
>
> so how did a torqueless S2000 win A-stock at nationals this year? i imagine
> because the redline is so high that it doesnt have to shift and lower the
> effective torque. it doesnt pull harder, but it pulls LONGER. so it ran
> the course in 1st, vs. andy's MR-2 in 2nd and so had more effective torque?
>
> -james c
--
Michael R. Clements
mrc01@flash.net
It doesn't matter who votes, it only matters who counts the
votes.
-- Joesph Stalin
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