Some pre-Christmass basics answers. Some of these should be in the FAQ...
> 1. I have seen graphic analysis of engine performance data and have
> always wondered why neither torque nor horsepower have a strictly linear
> relationship with rpm. That is, at high rpm both either level or drop off.
> Why is that?
They will both drop off at some point, but on the right engine that may
be the rpm at which the engine explodes... For most engines in "real car"
applications, the problem is breathing. The engine starts having to suck
too hard on the intake and blow too hard on the exhaust to keep putting out
more and more power and torque.
> 2. Why is it that torque and horsepower do not always peak at the same
> rpm? Related to this question: how do torque and horsepower relate to each
>other in light of the driver's perception of car/engine performance (not making
> a distinction here between acceleration rates and top speeds).
The "related question" is really the answer: in consistent units,
Power = Torque X rotational velocity.
If the torque flattens out, the power keeps rising because the rotation
rate is rising. As long as the torque isn't falling off as quickly as revs
are rising, you will continue to get more power.
As far as feeling is concerned, torque is what you feel. Torque is what
applies force to the wheels, and force at the drivewheels is what presses
your butt into the seat.
> 3. With torque and horsepower peaks in mind, what is the preferred shift
> point with maximum acceleration intended? In otherwords, isn't it pointless
> and even sort of stupid to red line if torque or horsepower peaks at lower
> engine speeds?
Draw curves of applied torque in each gear. Note the intersects of the
curves. As long as they are a bit shy of redline, those are the best
shiftpoints for acceleration. Here's a sample:
|
Torque |
|
|
|
|
|
| 1st gear
| _____________
| _____/ \ 2nd gear
| ____/ \ ______________
| / \ ______/
| __/ \/
| / _______/ \
| / _______/ \
| / _____/ |
| / ___/ ^ |
| / / |
| |
| shift point
__________________________________________________________________
RPM
--
---
John R. Lupien
lupienj@wal.hp.com
|