Re: tourque versus horsepower

I read that, and I'm not convinced it's completely accurate.

What you want from your _engine_ for maximum acceleration is lots and lots of HP. Torque is irrelevant.

You want as much HP as you can get, and you feed that into the input of your transmission.

Your transmission takes this HP, and alters the RPM through gearing. This trades off RPM for torque at the wheels.

To accelerate fastest, you want to use your engine at max HP, and use your transmission to maximize the torque to the wheels for any given speed.

In an ideal world, you'd have a CV transmission. The engine would remain pegged at the peak HP, and the transmission would accelerate the car by starting with low RPM on the output shaft (high torque), and increase the RPM on the output as fast as the torque could accelerate the car.

The torque on the input of the transmission would be irrelevant.

In our imperfect world, we have a finite number of gears, and best acceleration is made by staying as close to peak HP for as long as we can.

HP is torque applied over time.

Torque is the "Twisting Force"

Apply torque over time, and you get Horsepower.

No torque, no horsepower. Easy.

Equally easy - no HP, no torque.

The hard question is with a lot of torque, how much horsepower?

No! you can have torque with no HP, think of a wrench tightening a nut, no rpm but heaps of torque therefore no HP.

Hi, Like Diesel engine. Tony

>

From how I interpret what he is saying, I think you are both saying the same thing. You will get the most out of the car at the engine's peak horsepower; witness a hard vs soft launch of virtually any car.

However, I don't think torque is really irrelevant. If one looks at electric motors, their peak torque is the moment the thing starts to rotate, while its horespower rating is usally at some rotational speed. Consequently, electric drive cars that use DC motors will accelerate comparably with an internal combustion engine (assuming everything else is equal) initially, but have much lower horsepower ratings and don't go as fast.

I had certainly never thought of horsepower in the way in which the article describes it. But when you think about it, if torque is the rotational force, engines all rotate, so they all make torque. But, if you are making torque at high RPM, according to that article, you should also have high horsepower. Consequently, what you are saying is the same and doesn't really contradict the article's assertions.

Brain fart. That's what I _meant_ to type. I think my keyboard must be busted.

"Backwards" example for 500HP (enormous?) and 0.01lb-ft. (The correct term for torque is lb-ft. - One pound moved over a distance of one foot.)

Let's find the RPM required!!!

Rearranging the equation for HP gives us:

RPM = HP*5252 divided by Torque

Put in the numbers:

RPM = 500*5252 = 2.626 million (divided by) .01

RPM = 262.6 Million RPM!!!!

Let's see why torque is the MASTER of the equation!

What would be the torque REQUIRED at 5000RPM to create our 500HP?

Torque = HP*5252 divided by RPM

Torque = 500*5252 (divided by) 5000

Torque = 525.2lb-ft.

See.... Big torque figure good!!! (Grunt)

Either way you look at it, your HP rating is directly related to HOW MUCH torque produced and at what RPM.

This is why there is now 5 speed gear boxes and soon most cars will have six speed gear boxes. 5 speed gearboxes were originally employed to utilize the torque in motors which have high HP but not so high torque. Most small motored 4 cylinder cars started to come out with 5 speed boxes before the 6's & V8's because the 4's had lower torque and that torque occured at a higher rpm as well. In order to get the small

4 cylinder cars off the line quickly they used low ratioed 5 speed boxes which kept the motor in the max torque/power band. Notice that 6's & V8's are able to have higher 4th or 5th gears for cruising whereas 4's usually have lower ratioed 5th gears to keep it closer to the torque peak whilst cruising. Most 5th gear ratios in these cars are just below or on the peak torque figure whilst at normal highway cruising speeds. Having the max torque figure at its cruising speed will attempt to keep the car at that speed whilst going up hills and fighting against head winds etc and it is also usually where the motor is most fuel efficient (excluding drag of the vehicle shape etc). When you need to overtake, you drop back a cog or two to get the revs up to the max power band (or just below so the motor will accelerate to the max power) and this is what accelerates the car at its maximum rate to get you past quickly.

This is what the torque and power achieves. Torque to keep the motor at a speed and power to accelerate to a higher speed. HP is basically torque x rpm (time). Smaller motors have less torque and therefore the only way for them to be able to work successfully is to increase the rpm in able to increase HP. In order to utilise this extra HP the gearbox has to have many ratios to stay in the max HP band for best acceleration and top speed. Also ever notice that a 4 cylinder (and even some 6's) will usually have a top speed faster in 4th gear instead of 5th gear? It is because the available torque is not enough to overcome wind resistance in 5th gear (and also 5th gear at top speed is out of the max torque band) but in 4th it is in or near the max power band.

A correctly designed gear ratio box is essential to make a car perform at its peak, whether that peak be for cruising, speed off the line or for economy. Most modern cars are designed as a compromise of all of these factors in mind.

That's kind of a meaningless statement since that fact is dependent on the units that are being used (in other words there's nothing magical about 5252 rpm in a physical sense). Use different units, and the magic rpm to make them "equal" changes.

For example - if you talk in terms of watts for power and N·m for torque, is 5252 rpm anything special?

Bill Putney (to reply by e-mail, replace the last letter of the alphabet in my address with "x")

5252 is just a Constant ("k") used in the conversion process. Using different units would yield a different conversion "k", therefore a different number. The "k" is just a combination of all the math that actually converts the units and such so you don't have to perform the entire, whole calculation with all the sub-conversions to get all the units to work out correctly.

It has been eon's since I actually had to do all this in college, so forgive me if it sounds a bit vague.

How can torque be the master of an equation that multiplies A*B?

HP = 5252*RPM*Torque

Doubling RPM has EXACTLY the same effect as doubling torque. Exactly.

But if you substituted 10,000 RPM, then the HP would be two times greater! Big RPM figure good!!! (Grunt)

Exactly. One is NOT more important than the other.

Same here (about the eons since college) - but that was my point: that the 5252 was nothing special since it was purely a coincidence of a certain set of arbitrary man-made units.

Bill Putney (to reply by e-mail, replace the last letter of the alphabet in my address with "x")

But this discussion concerns a street driven vehicle. Personally, I'd like to make my power BELOW

7000RPM where it is REALISTIC. This means:

No Torque = No HP. (realistically speaking)

Yep, the 5252 happens to be the "magic number" that makes all the units work out when you convert torque to HP at a particular RPM.

What do you have against engines that rev faster? Sure revving a high torque desiel past 7000 isn't realistic, but smaller lighter engines have no problem with higher revs.

My motorcycle NEVER gets ridden below 5000 RPM, and usually above 6000. The engine is happiest there, and the gearing reflects it. If it had a high revving turbine of some kind it would be unrealistic to bog it down to a mere 10,000 RPM.

I don't know where the mindset of the high RPM is bad has come from, but it certainly has a grip on the American population.

I suspect it is a holdover from many moons ago when engines were heavier, of poorer quality metal with lubricants that aren't as good as today. Back then, these high revvers had a nasty habit of exploding... at least, they didn't last as long as big ole' pushrod machines with the displacement of a swimming pool.

Of course, the Europeans and Asians caught on years ago that you can get good life out of a high revving engine as long as it is maintained properly.

Only if torque is in foot-lbs. 8^)

Bill Putney (to reply by e-mail, replace the last letter of the alphabet in my address with "x")

Motorcycle? High-revving turbine??

The original post talked about a Subaru, a Chrysler and a Mazda.

I might have been mistaken, but I thought this thread pertained to production automobiles used as daily drivers only. I never said that High RPM was bad. 10k RPM is just not REALISTIC for a daily driven automobile.

Of course bike motors wind up there, but like I said earlier, I would like to make my torque below 7000RPM. This was in context to my daily driver.

(Until the day I get blown off by Granny in her 10k RPM mini-van!)