Another thread got me thinking: How many in here have a thorough understanding of the concept of horsepower and torque, and how they affect performance? We talk about hp and torque everyday but what do you really know about it? Which one rules for ultimate performance?

This article (which I posted in the other thread) is the best I've encountered on the subject:

http://www.nrhsperformance.com/tech_power.shtml

What's your thought's?

 

Can't imagine running my V-Twin at the high RPM's that the rockets run. I prefer my torque and power all at lower RPM's. I don't want to cruise at 100mph but I sure want to get up to65 as fast as possible.

 

Actually that link is more about gearing than it is HP and torque. Low torque motors (such as the hi-revving sport bikes) MUST have lower gears (higher numerically) to compensate. Torque from an engine is constant once parasitic losses are factored in.

High HP numbers are an indication of a fast motor, but unless coupled w/proper torque (and gearing), they will lose in shorter race distances. And of course the weight ration comes into play.

But basically identically geared, similar weight objects will have different results depending on distance AND the discrepancies between their torque and HP ratings.

 

Well, first off they are the same thing. You can't have horsepower without torque and vise versa. Horsepower doesn't really exist. It is a made up number based on the amount of given torque and was used to compare an internal combustion engine's power to the "horse" powerin the day of horse drawn buggy's. Thebore andstroke will determineif an engine needs to rev high or low to make that torque. A large bore and short stroke motor will make what you refer to as torque down low in the powerband and doesn't need to rev up. A small bore and a long stroke has to rev up before you feel it.Both engine types have advantages and disadvantages depending on the application.

 

Yes, of course gearing is important. In these discussions one has to assume that both engines is mated with a set of gears suited for the task. That means that a Harley will never be geared the same as a high reving sport bike for a specific task. HP and torque can't be explained without understanding gearing. The whole concept is that high rpms can be used to make massive torque at the rear wheel. The rear wheel won't feel the difference between torque coming from a torquey engine and torque coming from high revs and short gears.

For example, 100 lb-ft @ 5000rpm will yield the same performance as 50 lb-ft @ 10'000rpm. Both will have the same power. The second engine will have half as long gears. This means it will put exactly the same torque to the rear wheel at all speeds (assuming the curves look similar in shape) and still maintaining the same top speed in all gears.

We could play with the thought that we had a 500 lb-ft engine in a Harley. Some would say "Great, it will out accelerate anything". Well, if those 500 lb-ft was made at a whopping 500rpm redline we would have to use extremely long gears just in order to make the bike reach 60mph. By using so long gears all usable torque will be gone before it reaches the rear wheel. The performance out of that kind of 500 lb-ft engine would actually be worse than a stock TC96. The power of that engine (the 500 lb-ft one) is also no more than 47 hp.

 

I have a very good understanding of torque vs HP. One thing that should be realized is that anywhere (meaning at any RPM value) that torque is increased, HP is also increased. Torque and HP are mathematically relalated and contrained to each otheras described by the equation: HP = Torque (in lb.ft) x RPM/5252. If you look at some dyno runs, you will notice that HP and torque are always equal to each other at 5252 RPM.

 

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We could play with the thought that we had a 500 lb-ft engine in a Harley. Some would say "Great, it will out accelerate anything". Well, if those 500 lb-ft was made at a whopping 500rpm redline we would have to use extremely long gears just in order to make the bike reach 60mph. By using so long gears all usable torque will be gone before it reaches the rear wheel. The performance out of that kind of 500 lb-ft engine would actually be worse than a stock TC96. The power of that engine is also no more than 47 hp.
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You mean that's why semis can't get great times in the 1/4??
I guess I can give up racing the Hondas with the big mufflers and high spoilers with the Peterbuilt. Thanks for shattering my dreams I figured the torque would be victorious.

 

Horsepower is how hard you hit a wall....


Torque is how far the wall move's after you've impacted with it...

 

Exactly!

Put another way: A 3.0 liter, 250 lb-ft Formula 1 engine would put more torque to the ground than a 2000 lb-ft Peterbilt truck. This can easily be seen by this reasoning:

The F1 engine redlines at 20'000 rpm and the Peterbilt at 2'000 rpm. This allows for 10 times the gear reduction with the F1 engine while still maintaining the same speed in gear. The engine torque difference is "just" 8 times in favour of the Peterbilt. The net result is that the F1 engine will put 25% more torque to the ground. The torque of the Peterbilt is not high enough to make up for the gearing advantage of the F1 engine.

So why not use F1 engines in trucks then? The answer is simply that it is not practical or economic. They would wear out in an instant, require constant servicing and would not be very relaxing to drive...

 

Nah, those simplifications never work. I've just shown above how a 250 lb-ft engine puts more torque to the wheels than a 2000 lb-ft engine. This is why you can never look at torque as a performance indicator. Torque without specified RPM is not a useful number, because it can mean anything. It's like saying "My bike is quick because it revs to 15'000rpm". That is not a true statement, beacuse RPM is just one ingredient in the equation for horsepower, just like torque. Neither mean anything without the other. 1 lb-ft @ 15'000 rpm is useless, just like 15'000 lb-ft @ 1 rpm is useless. Both cases equals 2.86 hp.