I'm told that the difference between Torque and Horsepower is a basic fundamental thing that any gear-head should understand.

I have had it explained to me before, but I'm dense or something, and to me, power is power. There's either enough energy to move an object or there isn't. And changing the length of the "lever arm" by altering the gear ratio doesn't change the basic limitation of X energy versus Y mass, it just means you distribute the force over a longer time period. The same amount of net energy is still required.

Can anyone put it into proper layman's terms so that I can understand it?

And while I'm at it...

I've seen the term BHP used frequently, but I've seen conflicting opinions on what it stands for. Does it stand for "Base Horsepower" or "Brake Horsepower"?

Horsepower is a result of torque * velocity... umm... how to explain it. Torque is the measure of force it takes to get something rotating ... like force to get something moving straight. Screw it... i just found this page. It explains it better than i can =]

http://www.off-road.com/hummer/tech/power.html

I've been wondering about BHP too....

here's another REALLY good explaination.

Okay, from those pages, it seems to me that the Horsepower number can be derived from the measured Torque. This makes sense to me.

What I never understood was how an engine could have "a lot of torque" as opposed to having "a lot of horsepower" and that the two numbers were somehow unrelated and derived separately.

Knowing that one number is derived from the other sounds much more logical. So the horsepower will change depending on the RPM. Makes sense.

Excellent explanation, indeed, but with one (common) error:

If you have a one pound weight bolted to the floor, and try to lift it with one pound of force (or 10, or 50 pounds), you will have applied force and exerted energy, but no work will have been done.

One will have 'exerted' no energy. Energy is work (or ability to perform work), not force. Only when the weight is actually lifted against the force of gravity, work is performed and energy spent (or, more precisely, transferred to weight, which can spend it by, say, smashing something upon fall).

Sorry for nitpicking .

Don't forget, Tony, that both torque and power vary with engine revs. If you're buying an engine for more than just driving around, you'll get a plot of torque against speed (and probably also horsepower against speed, though that can be derived).

You'll find that diesel engines have lots of torque at low revs, compared to petrol engines (which have lower peak torque but maintain it into higher revs). This is why diesels are better for towing and hill-climbing, but less good for high-speed flat racing - when the load increases, a small drop in engine revs gives the required torque increase, maintaining power output.

I don't know about other fuels, and there must be people around here who can explain this stuff better than I can. The above is just what I picked up from driving farm tractors in my teens.

Here's my understanding of BHP:

When testing an engine on a dynomometer, a device (called a brake) applies a load to the engine to keep it at a particular RPM (typically at full throttle). A load cell/strain gauge measures the amount of force applied by the brake. By taking the RPM and force applied by the brake, horsepower is derived. That is where Brake HorsePower comes from.

And don't forget that both can be measured at either the crankshaft or at the wheel, which are often much different. So make sure to compare apples to apples, if that's what you're doing.

I grew up with BHP meaning British HorsePower, but I've often seen reference to it lately as standing for Brake HorsePower, apparently referring to the fact that the denotative way to measure it is to figure the amount of pressure applied to a friction brake required to stop the crankshaft. I don't think I've ever seen Base HorsePower.

I've never come across BHP meaning anything else but Brake Horsepower. The explanation put forward by philp69 is the one I've always understood to be correct. In fact I've never heard any other explanation either.

I've been wrenching for a while and I still don't feel I have a grip on this subject. But here's how I rationalize it for myself, what relationship it has to real physics is beyond me. Torque is directly related to the stroke of your crankshaft which somewhat relates to the definition of torque(the lever) but it's more complicated. The longer the stroke the more torque you can make. My crankshaft has a 3.75" stroke, the size of the motor is 379CI or 6+ liters or there about. If I enter a corner with another car that makes the same amount of HP but using a smaller motor(hence shorter stroke) and we're both running at the same RPM I can put a couple of car lenghts between us. If it's a short straight away and I have to start braking again then I'm getting ahead. If it's a long straight and he has more HP but less torque then he will start catching up to me because I'm going to run out of steam.

-Mark
12 Gig Blue
http://www.geocities.com/legoland0/

Torque and Horsepower are pretty darn related. In fact, if you look very carefully on *ANY* Dyno Chart, you will notice that the Torque curve and the Horsepower curve will *ALWAYS* cross at 5252 rpm.

Torque is a measure of the amount of rotating force. Horsepower is an indication of how much work can be done. For example, if you look at electric motors, some of them are capable of a huge amount of torque, but have an upper limit on the rotating speed. Some types of electric motors interestingly enough will increase torque if you try to oppose the rotation. So if there was a motor spinning a wheel, and you put a brake on the wheel to try to stop it, the motor will begin boosting torque, the more you try to stop it, the more torque it will produce regardless of rpm. It does this at the expense of heat, and eventually the motor will burn up.

Yeah I know that doesn't answer the question but it's just another point. :)

Calvin

Yes, there is a mathematic relationship between torque and horsepower. But remember that the critical point is physical limitations. If you had a motor that redlines at 4000rpm with X amount of torque, versus a motor that redlines at 12,000rpm with the same amount of torque, the horsepower curves work out differently. When people talk about having a lot of torque they usually refer to usable torque, like off the line. There are often times when a motor has very little torque, but because it's designed to rotate to a higher rpm value, it will do more work (horsepower). So you can say that a motor has very little torque but can do a lot of work. If everything was flat surfaces with no wind forces and whatnot to slow things down, like say a track on the moon, you can use a single gear and a motor that can spin to any speed without blowing apart-- that motor can get to any speed or horsepower it felt like going with almost no torque. It just takes a while. The real world does impose physical limitations... like, if you had a motor with very little torque, and you tried to make it do a lot of work, eventually an opposing force will build up to cancel the torque (acceleration). (say an incline or wind drag or whatever).. so eventually your horsepower will top off. So being that we live in the real world, very often you need tremendous amounts of torque to get tremendous amounts of horsepower.

Hmm.. I'm kind of thinking about that electric motor example I gave before. Unlike gas motors, electric motors have the interesting property that if you tried to *oppose* torque it is producing, the motor will *increase* torque output. But if you didn't try to oppose the torque they have a flat curve all the way to redline. Theoretically, if the motor is made to not burn up, and you applied a brake to an electric motor it will give you infinite torque at near 0 rpm. I've heard stories of electric cars wasting Vipers off the line, well, for a little while anyway. :-)

Calvin

Brake Horsepower can not be measured at the wheels. A lot of times a brake dyno destroys the engine (or damages it severely) in the process of measurement. A brake dyno is considered to be the most accurate. A wheel dyno doesn't factor in fluidic and other efficiency losses between the crankshaft and the wheel. I suppose it might be possible to build a brake dyno for a wheel, but nobody would be willing to destroy their engine and drivetrain to try it.

Calvin

"British" and "Base" Horsepower do not exist. BHP has always been Brake Horse Power due to the method of measurement of peak developed power (read on).

The horse power is an imperial (i.e., British) unit of measurement anyway, derived from a mythical horse's ability to pull a hundredweight of coal a known vertical distance up an incline in a measured time. This allowed a calculation of the rate of work, and the peak power developed by the horse doing the pulling. (The alternate way to measure power is to brake the rotating object to (almost) stalling point. Given the measured radius of the rotator, and the force applied, the torque could be calculated, and then the power developed.)

I joke not, in the early 1800's during the Industrial revolution, this was the way people could assess the capability of one of the 20 million (no spelling mistake or shifted decimal point) equine inhabitants of the British Isles at this time. Prior to the introduction of the static or mobile steam engine, the horse was the staple motive power, with the population of horse almost equalling that of human on that small island (side note: this is how the Brits have established such a strong relationship with the horse, and will no doubt explain countless jokes about the British, their gardening habits, Roses and the best way to grow them, Shakespere's references to horse flesh, and many other curiosities of that small island). The assay of a horse's worth inevitably depended on a health assessment of teeth and hoof by a vet, followed by a pulling power test at the local assay yard. These inclined planes still exist around the country. Various large engineering structures around the UK take the form of inclined planes set at a standard angle to allow horses to perform under measured conditions, and allow (for example), mine owners to assess the maximum possible load a given pair or four would be able to tow up or down an incline to a canal or railway basin.

After steam became the prime motive power (then later IC engines) the unit of power measurement stuck. The population of horses dwindled extremely rapidly to a fraction of their previous count, but their legacy is recorded in buildings and structures all over the country in coaching houses, large stables and livery yards, horse markets and donkey mills. Even up to the middle of the Stalingrad offensive, they were still used as the prime mover of choice for armies and artillery pieces (on all sides; the Polish army retained a mounted cavalry which was used to charge German tanks and machine guns during the invasion of Poland). One of the primary reasons for the inability of the beseiging German 6th Army's to break out of the Russian Kessel offensive (an encirclement) was that 120,000 horses used to tow artillery pieces into the battle zone at the start of the campaign were moved out of the forward area to reduce the logistics overhead of providing fodder and stabling materials, and to free army stable hands to fight the German offensive on the city. It was done by the Wehrmacht bureacracy as a cost cutting measure!

Brake Horsepower can not be measured at the wheels

Errr... then why use the rolling road or Dynamometer?

A lot of times a brake dyno destroys the engine (or damages it severely) in the process of measurement

That's because you need to bring the engine to almost stalling point from maximum power, almost instantaneously.This inevitably means the engine and transmission being forced to decelerate to a near stall whilst trying to develop max power (Blimey, just caught myself about to type Mexx Paaaaahhhh ), which will inevitably do naughties to it to due the mechanical stress and (inevitable) overheating. This is rarely done these days as there are less destructive and more accurate methods. As for abuse on a rolling road, well, it's an un-natural operating situation for a car anyway, and overheating while tuning for power is inevitable. Yer pays yer money....

A wheel dyno doesn't factor in fluidic and other efficiency losses between the crankshaft and the wheel.

I think you're being a bit dramatic here; you are measuring the power developed by the engine, and although the camshaft does have direct influence on the power an engine design can produce, it has very little to do with loss at the road wheel. You only measure engine power output at the flywheel, or the driving wheel. Differences between these two measured values are caused by mechanical losses incurred in the gearbox, differential, and other transmission components (including the tyres due to contact patch sizes). Camshafts do not affect this loss. However, internal engine power losses can be caused by bearing friction, pumping efficiency, pumping loss, cam timing and lift, etc. etc. etc. These all result in a figure measured at the flywheel though - which is where we came in.

I suppose it might be possible to build a brake dyno for a wheel, but nobody would be willing to destroy their engine and drivetrain to try it.

... then you have just wiped out the entire dyno based tuning market at a stroke . The driven wheel drums of a dyno are fluidic brakes; one techinique is to run a car at constant road speed on a dyno with a given braking load applied at the wheels. If you time the run, you could measure the temperature rise in the hydraulic brakes to calculate the total energy dissipated during the run. You can then evaluate the instantaneous energy generation, and hence the power.

Tony, why do you keep changing colour all the time?

Tony, why do you keep changing colour all the time?

Trying to be egalitarian.

Ahh..! That was how BHP was explained to me, and I didn't know that rolling dynos are fluidic brakes. That makes sense. What I meant is the horsepower given at the wheels is not the same thing as BHP which is traditionally a measure at the flywheel. The difference between what the dyno tells you versus the measured horsepower at the flywheel is the amount of inefficiency from crank to the wheel. That's what I meant by dynos not being able to give you a true BHP measurement because every car is a little different, and the actual losses are not known.

Calvin

There are 2 primary types of dyno's: a chassis dyno and an engine dyno. They both use a form of a "brake". They are generally not friction brakes like you would find on a car, but usually hydrodynamic brakes or eddy current brakes. Hydrodynamic brakes are basically pumps that have their output restricted by a servo valve of some sort. Eddy current brakes are essentially electric generators that have an electrical load placed on the output.

Chassis dyno's measure (to use the vernacular, for those of you with front wheel drive) "rear wheel horsepower". This is effectively what the car puts to ground. The car's drive wheels are placed on large rollers, which in turn drive the brake. It takes into account losses from the transmission and axle, rolling friction of the tires, etc. They are generally abusive to tires (don't bring you car to a chassis dyno unless you don't mind beating on the tires) and require fans, etc. to make sure you get really good airflow through the engine compartment. It's very easy to overheat on a chassis dyno, especially if you are doing a lot of tweaking.

Engine dyno's measure just the engine, not taking into account losses from the rest of the drivetrain. The engine is mounted on a cart or stand of some sort and the brake is coupled directly to the engine. I worked at a place that had an engine dyno (SuperFlow 7100 w/2000 ft/lb brake) to do performace testing for race cars. It was a hydrodynamic brake that used water as the pumped medium.

Running an engine on a dyno almost never (at least it's not supposed to!) destroy the engine. In fact most of the engines we tested/tuned went straight from the dyno to the race car. In some respects it's actually easier on the engine that smashing your foot down on a straightaway. The runs are generally over within 10 seconds, and there are no impact loads (such as shifting gears) on the engine. I ran my own engine on the dyno for 3 days straight, at least 10-15 dyno runs per day to work out exhaust, ignition, and fuel tuning. It went right from the dyno to my street car with no ill effects. And the best part was the engine was already broken in!

Cheers!

Brake Horsepower can not be measured at the wheels.

No, but the horsepower at the wheels can be calculated from torque at the wheels. I've seen numbers quoted at both places. It's usually based on someone trying to be underhanded about proving whose car is more powerful, which is why I cautioned Tony on it.

There's a difference between horsepower measured at the wheels and measured at the crank. I always thought of a true BHP value (at least as given by a car manufacturer as a spec) is measured at the crank. That is why a measurement at the wheel will always be a smaller value than specified for the car. The loss is more for automatics and less for manuals.

What happens if you have an all wheel drive vehicle? If the power is distributed to all four wheels in some fashion, then how do you get a total horsepower that is realistic?

Calvin

Exactly.