The classic mistake is to conclude that the fastest way down, let's say, a 1/4 mile drag strip is to keep the engine RPM at the torque peak (or as close as possible). The technique is usually stated as "shift just after the torque peak", or "shift N RPM above the torque peak so you are N RPM below the torque peak in the next gear when you finish the shift".

Unfortunately, *engine* torque does not tell you the full story. What matters is the torque *delivered to the tires*, including the effects of the transmission. We all know a car does not accelerate as hard in second gear at peak torque RPM as it does in first gear. The transmission amplifies or multiplies the torque coming from the engine by a factor equal to the gear ratio. So to determine how much the car is accelerating at a particular instant, you have to know both the torque output of the engine as well as the gear ratio.

To figure out your shift points knowing only torque, generate tables of transmission output torque vs. RPM for each gear. To get transmission output torque, multiply the engine torque by the gear ratio. You are simply comparing gear to gear, so the final drive ratio can be ignored. You may also need to know the relationship between RPM in one gear and RPM in another gear (which is RPM * (gear2ratio/gear1ratio) at any particular vehicle speed.) Then it's easy to see what shift points to choose to maximize your transmission output torque at all times.

** Shift at the redline, not at the torque peak!**
Walk through an example. You're hammering down the track in 1st gear. Engine RPM is 6000, just past the engine's torque peak. Do you shift? Well, if you do, the engine will be pulled down to 3600 RPM, and 2nd gear will send 246 ft-lb of torque to the wheels (actually, to the differential first, which amplifies the torque by a constant factor and sends it to the wheels). Don't you think it would be better to hold it in first gear? Torque is dropping off, but it's still 389 ft-lb at 7000 RPM, right before the 7200 RPM redline. So, for this powertrain, first gear is *always* the best deal for acceleration, at any speed, except that you can't accelerate past the redline.

The 1-2 shift at 7200 RPM pulls the engine down to 4400 RPM, where 2nd will deliver 265 ft-lb of torque. Not only did you win by maintaining the high torque of 1st all the way to 7200 RPM, you are now better off in second gear.

Same thing goes for the 2-3 shift. 2nd gear output torque at the redline is still greater than 3rd gear output torque at any engine speed, so you wind her out as far as she'll go before you shift to 3rd. Same for the 3-4, same for the 4-5.

But, you ask, isn't your acceleration greatest at the torque peak? Yes, it is! But only within that gear. The next gear down will give you even greater acceleration at the same speed, unless the vehicle speed is too high for that gear.

To use engine torque to understand how your car performs, you MUST include the effects of the transmission.

**Maximum Acceleration VS. Power**
OK, so what about power? As has been noted by a previous contributor, Power (hp) = Torque (ft-lb) * RPM / 5252. Note that power is also force * velocity, specifically:

Power (hp) = Force (lb) * Velocity (MPH) / 374

That's net horsepower, which is engine power minus losses like transmission and tire friction. The force is the sum of the longitudinal forces at the contact patches of the two driven tires.

Hmmm... P = F * V ...rearrange to get F = P / V ...

that means that you get the maximum force pushing the car if you maximize your *Power* at any given velocity. This gives us another useful rule:

**Shift to maximize engine POWER, not engine torque!**
The tires don't see quite these numbers due to [friction and aerodynamic] losses, but I'm going to assume that the losses are comparable from gear to gear and that the overall shape of the power curve remains the same.

**Exceptions**
There are no exceptions; a car running at its (net) power peak can accelerate no harder at that same vehicle speed. There is no better gear to choose, even if another gear would place the engine closer to its torque peak. You'll find that a car running at peak power at a given vehicle speed is delivering the maximum possible torque to the tires (although the engine may not be spinning at its torque peak). This derives immediately from first principles in physics.

However, note the following: - Transmission losses are not shown on engine power curves. The net power curve (power delivered to the ground) may have a different shape or even a different peak RPM as a result. This would result in different shift point. Best results are obtained from a power curve measured by a chassis dynamometer. - The discussion above assumes negligible tire slip. If you exceed the maximum traction available from the tires, then additional power doesn't help. That's why it's sometimes no loss at all to shift early when the tires break loose, and in fact it can be a benefit.

**To the Point**
Torque and power are (almost) flip sides of the same coin. Increasing the torque of an engine at a particular RPM is the same as increasing the power output at the same RPM.

Power is just as useful and relevant in determining vehicle performance as is torque. In some situations it's more useful, because you may not have to play with gear ratios and a calculator to understand what's going on.

A car accelerates hardest with gearing selected to stay as close as possible to the engine *power* peak, subject to the traction capability of the tires.

**Not all cars should be shifted at the redline for maximum performance.** But it's true for many cars. You can determine optimal shift points by graphing horsepower vs. velocity or transmission torque vs. RPM. Engine torque alone will not determine shift points.

I found this on the net. There is no way I would type all that shit up myself.

Also, I will never let my BABY redline. Love her way too much for that.

[ February 28, 2003: Message edited by: TF242 ]