Torque and Horsepower?

Question

So torque controls acceleration from a standstill and low rpms right? Horsepower controls top speed and acceleration at high rpms? Is this correct? So when you downshift it is supposed to go to the horsepower peak to get maximum acceleration? Is this correct?

Answer 1

Down shifting does 2 things. First thing is that by going into lower gear, it (the engine) has greater mechanical advantage. The engine has more "power" in lower gear. This is much more important that the next thing.

The second thing is it gets the engine into power band. In another words, if you are cruising at HWY, your engine may be revving too low. Then by downshifting, it can rev the engine into specific max torque / power RPM.

Now I wrote that the first thing (lower gear) is more important. That is the reason why in 1st gear the car can accelerate fast even though at first the engine is revving still too low for max torque and hp.
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Answer 2

Your first two points are generally correct.

The third one is a mixture.

When you downshift the engine performance results depend on your actual speed AND what gear is selected. They are not independent.

An engine does not have two distinct power supplies: they overlap. Broadly speaking, torque is the "grunt" that gets things moving, and horsepower keeps it moving.

The application of torque is limited by the speed range (RPMs) of the engine. When the engine speed increases you will reach a point set by the manufacturer. At that point, you will reach the engine's peak torque output, but it will continue to produce torque above that.

Horsepower will begin before that point, and it will stay into the higher RPMs. At some point in the RPM range of your engine horsepower output will also drop off, sometimes very sharply, so running the engine at its highest RPM level will be counter-productive. In other words, downshifting at high speed may get a lot of engine noise, but little or no acceleration.

Manufacturers plan for average use, and set up the engines acording to this plan. You can modfy the settings for higher horsepower, at higher RPM,s or for more torque.

When you downshift you change the gear ratios, meaning the relative speed of the engine measured against the speed of rotation of final drive gears. This allows you to choose the gear where the most power is availalble from your engine, at a given speed.

Changing gears, whether up or down, is to permit the engine to deliver its maximum power.

Answer 3

Horsepower depends on torque. The formula for figuring horsepower includes the torque in pound feet and the rotational speed in revolutions per minute, divided by a constant, 5252.

This means that at rpm approaching zero, torque is dominant, but since internal combustion engines need some rpms before they develop their maximum torque, you need to keep the rpms up a little, even in engines with lots of torque, to get maximum acceleration.

To get proper shift points for maximum acceleration, you need to see the power and torque curves. A large displacement engine like an 8-liter Viper V10 which develops its maximum 535 lb-ft of torque at 4600 rpm and maximum 510 horsepower at 5,600 will need to be shifted at much lower rpms than say a Ferrari 599 Fiorano, which doesn't get 'on the cam' (develop its maximum torque of 448 lb-ft) until 5,600 rpms, and doesn't develop its maximum 612 horsepower until 7,600.

The reason for frequent downshifts is to avoid dropping too far below either the power or torque peaks between downshifts, so that the engine is 'on the cam' when you are ready to resume acceleration.

For fuel economy, it's almost exactly the opposite. Internal combustion engines are most efficient (produce the greatest power with the least fuel) when 'unthrottled,' meaning (a seeming contradiction) with the throttles wide open, at low rpm. The reason for this is the power cost of pumping air past a partially closed throttle, and the friction losses from higher rotational speeds. For this reason, a car with a large torquey engine and tall gearing can get extraordinary gas mileage if you just keep it in overdrive.

Interesting trivia: because of that constant, if you charted the horsepower and torque curves of an engine with rpm on the bottom and horsepower and torque on the side (using the same scale for hp and torque), the horsepower and torque curves ALWAYS cross at 5252 rpms.

I hope this helps explain it. Good motoring to you, and a safe holiday!

Answer 4

just get a hemi and have the best of both worlds