If you shift at the top of your torque band, you will drop to the bottom of your power band each time. By shifting at peak hp (which is keeping you moving after the torque got you there), you will fall somewhere around peak torque(which will continue your acceleration). So you want to start at peak torque and shift at peak horsepower which brings you pack to peak torque again, repeat this cycle until you finish 1/4 of a mile or the straight if you're a road course guy like me!

 

Horsepower and Torque Explained- from canadiandriver.com


I get asked a thousand times a year - how can I get more power from my car/truck/SUV/motorhome - you name it! What many people really want is better acceleration, or improved fuel economy, or to be able to pull a bigger load. Before putting a pile of money into the engine, we should take a look at what power really is, and how it moves our vehicles.

Look at almost any automobile review and you will find horsepower and torque listed in the specs. For example, my "Quickmobile" has 200 horsepower at 9000 rpm and 57 ft lbs. torque at 2000 rpm. Is this vehicle really quick, or do numbers sometimes play tricks on us? Well, it has 200 horsepower. What does that mean?

Horsepower is related to torque, and torque is turning force. Place a lever on the engine's crankshaft and measure the twisting force produced, and you have torque. Note that time is not a factor - torque can be constant over time. Horsepower, on the other hand, is the rate at which the torque is produced.

Horsepower is a measurement of the engine's ability to do work. One horsepower can lift 33,000 pounds up one foot in one minute. Horsepower is measured over time. The more horsepower a vehicle has, the more mass it can move in the same amount of time, or it can move a fairly constant mass (the weight of the car) in less time. In simple terms, to get a vehicle accelerating quicker, we need to produce torque faster.

There are many different ways of measuring the power of an engine. Brake horsepower is a common term and refers to the power measured at the crankshaft using a "prony" brake. A prony brake was a simple lever that was connected to the crankshaft with a brake. As the brake was applied, the lever would deflect and power was indicated. Nowadays, computers and strain gauges are used to measure the power.

Rear wheel horsepower is just what it implies - horsepower applied by the rear wheels. A chassis dynamometer is used to measure the power transferred by the drive wheels onto the dynamometer's rollers. Brake horsepower is much lower than the horsepower specs provided by the manufacturers, because of all the power used by the transmission, final drive, and accessories.

Gross horsepower is the power measured at the crankshaft without any engine accessories. This includes removing the air cleaner and ductwork, exhaust system, water pump drive, and any other devices driven by the engine. Prior to 1973, horsepower specifications were listed by the manufacturers as gross horsepower. After 1972, manufacturers provided net horsepower specifications.

Net horsepower is also measured at the crankshaft before it goes into the transmission, but this time the engine is operated as it is installed in the vehicle, complete with accessories and ductwork.

Occasionally, usually when reading overseas vehicle information, I find reference to taxable horsepower. Taxable horsepower is a simple mathematics calculation used to determine the taxes to be paid on the vehicle. It has very little to do with actual horsepower because it doesn't factor in key elements like compression ratios, cylinder head design, and camshaft specifications. To calculate taxable horsepower multiply the cylinder bore x cylinder bore x number of cylinders x 0.4, and don't forget to pay the tax!

Many engineering documents and European manufacturers list engine power in Kilowatts. We more commonly use kilowatts to measure electrical power, but it is easy to convert from kilowatts to horsepower. One kilowatt is equal to 1.341 horsepower. While you are converting, you can also change torque measured in Newton metres (Nm) to ft lbs. by multiplying the Nm by 0.737561.

There are other measurements of horsepower. It can be converted to British thermal units (BTU) or it can be listed in PS. PS is short for Pferdestarke, the term for metric horsepower. Don't ask - I haven't seen it used anywhere either, but if you want to dazzle your friends, one horsepower is equal to 1.0139 PS.

So back to my Quickmobile. It really isn't very quick. With only 57 ft lbs. of torque, there isn't a lot of twist to turn the wheels. Sure, it has 200 hp, but it comes at a high 9000 rpm so it isn't very useful for normal driving or accelerating from a stop.

To achieve good performance, operate the engine at its peak torque, and the lower the rpm this occurs, the stronger it will pull from a stop. I see too many "hot-rodded" engines where all they go after is maximum horsepower at high rpm. If you have a light vehicle, steep axle gearing, and are willing to constantly rev the engine high, then high horsepower numbers will make you go fast. Great for racing.

For most of use however, it is better to have an engine with higher torque ratings and the lower or broader the rpm range it achieves it the better. Torque is what gets your vehicle moving. For everyday driving, I would pick a vehicle with high torque ratings at low rpm over a high horsepower, high rpm one any time.

Want the best of both? Then consider vehicle with a supercharged or turbocharged engine. By packing more air into the engine, torque is increased dramatically, but the engine can still be built for high rpm operation. Who says you can't have your cake and eat it too. Unfortunately, cake costs money, and so do superchargers and turbo's. There is always a catch.
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great article! thanks!

 

the actual formula for HP is:

Horsepower = RPM * Torque/5252

and Torque and horsepower are always equal at 5252 RPM

My two cents worth

 

if you also noticed:

big block v8 with peak torque at high RPM torque and HP are almost the same. what happens though is that you have all this torque and power at high revs and none at bottom end. You therefore have to tune your torque and power according to your preference.

for example: our evo VIII is tuned for rally therefore we get peak torque from 4000 RPM. the oly drawback with that is we have average horsepower. But for rallies you need the torque to come out of the corners fast.

 

I know it to be completely the opposite on the V8's... the torque is always low... around 2k or so... as long as it doesn't fade off then your peak horsepower is around 5,000 rpm's...

the reason you get peak torque in an evo up where you do is because it is a 4 cyl and they by nature place all of the torque band above 3 grand for the most part. Mitsu engines are a little different than most.. and have more low torque... but in general most is up high. Also a v configuration versus an inline will generate a considerably different torque characteristic.

 

and here are some dyno graphs from some cougars and t-birds that have around a 4.6L engine as proof.. note the massive torque early on and then most of the peaks are around 3.5K and then fall off... very different from a 4 cyl graph.

 

I think this should be a sticky. Very Informative. Good work everyone.