Horsepower vs. Torque: Which one wins?

SeriousEric said:

Clint Sever said:

By the way - what does the dyno measure then? I know Vipers dyno in fourth cause the ratio is 1:1 - does that cancel the torque multiplication factor? In what gear would the RWTQ be 2456?

In my mind, if the ratio is 1:1 and the rear is a 3.07 then the dyno is measuring power at the 3.07 ratio - why then is the RWTQ 1000 ft/lbs less when measured on a dyno?

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Good question. The # that the dyno reports should actually be called flywheel torque as measured at the rear wheels. You are correct in that a stock viper making 500 lb.ft torque (at the flywheel) is actually putting down 1500 lb.ft. at the rear wheels (minus losses).

So how does the dyno *know* how to factor out such things as gearing (trans & diff) and wheel/tire size? It does that by a ratio of drum speed to known engine speed. They're measuring your rpm, right? And the dyno knows at any instant how fast the drum is turning and that allows them to factor out the mechanical advantage (torque multiplier) due to gearing etc.

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Actually, the dyno *doesn't* factor it out. If there was a way to input your car's drive ratio, then I'd say yes, but there isn't. The reason we use 4th gear is because the effect of gearing is lessened due to the 1:1 gear - but you're still going through the final drive ratio of 3.07-1. This is why folks who do gear swaps see different dyno results.

I've never understood why dyno software didn't include an entry for final drive ratio - because then it would seem it should be able to tell you your driveline loss, measured values, and engine values. As I think about it, you may need the flywheel weight as well, so maybe this is a pipe dream.

The bottom line is that an engine makes torque on an output shaft to make you go. HP is a mathematical derivation of that force and is pretty meaningless to the overall task at hand. TorqueM is right in that its the final RW measure that is most important - and he's right - because that includes the gearing.

Small liter motors have to spin at ultra-high revs in order to make... TORQUE! They then translate those high output shaft rpms to the wheels through gearing, which translates a relatively small amount of motor torque into RW torque (with the help of a flywheel).

The question is flawed, and Tito had it right. Moving something down the track as fast as possible is about Torque - and the area under the curve is where its at. In fact, moving a locomotive and a train full of cars is about torque too, same with a Semi, or MarkO's tractor.

The reason we still talk about Horsepower at all - is because as TorqueM pointed out, it is a measurement over time. A Semi has 600+ lb/ft of torque, but my lowly stock Viper with its 500 lb/ft will beat it in the 1/4 every time. This is not exactly a HP issue - though it looks that way because my Viper has twice as much horsepower as the Semi. It is largely a gearing issue - which translates my torque over time much better than the semi's.

And finally, there are other factors, such as vehicle weight and flywheel mass. In my Semi example - imagine gearing it such that the semi was designed only to move its own weight (without any trailer). You'd then have a pretty speedy 1/4 vehicle (and I think I've seen this on Neckvision). The problem is that it is not ideal, because the low motor RPM is not ideally suited to drive the gear ratio.

I hope I've made some sense. I believe we'll start to see this play out with more events like the V10 Nats. We're getting to the point where we have several cars making huge power and torque, but I expect them to have interesting 1/4 results. As John pointed out- turbo cars have lag, blower cars have power right off the line. The torque exerted over the duration of the run looks very different for both cars - but the overall "area under the curve" will be similar.

Torquemonster said:

Jerome said:

SeriousEric said:

I'm not sayin' you're wrong Jerome, I just don't think you're right.

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I really wish I could prove this, but it is beyond my limited use of the English language. /images/graemlins/laughing.gif

Try this, think about the amount of tire sidewall flex. When you wind up that engine on the chassis dyno think about all the flex. The "wind-up" if you will.

And why do we use drag slicks with wrinkle walls ?

That is just one of the factors that make this scenario possible.

Edit:

I see TM is on to it. I knew with time he would come up with some ideas. :thumb: I need to learn to type faster. /images/graemlins/laughing.gif

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Actually your word picture was a lot better than mine - what you say makes sense. What we need now is wind up tires - /images/graemlins/supergrin.gif

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Like this?

Not the best photo

[image]http://www.kalittaracing.com/photos/2003/03_indylc_004.jpg[/image]

Jerome said:

Torquemonster said:

Ok - here's a wild stab in the dark

maybe the bigger sidewalls twist more and this twisting when it reaches its limits provides a counter force that actually helps the power - kinda like a pendulum effect... kind of...

maybe not

:help:

:screwy:

:leaving:

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Sidewall flex = stored energy :nod: Think wrinkle walls

Keep goin' Barry, you are on a roll. :thumb:

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I feel like the constipated mathematician who worked it out with a pencil...

I've got a firm grip of the turles head - but the bugger won't come out...

I think the roll has come to a stop - I can't go beyond the stored energy released bit - but on that basis - a drag slick must really store some energy!

Coffee now - head hurts /images/graemlins/freak /images/graemlins/surrender

Clint Sever said:

BTW - why is 5252 the magic number in the relationship between horsepower and torque?

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Math formula. Exactly why that number is beyond me.

Here, knock yourself out. /images/graemlins/laughing.gif

If you would like more math to rack your brain just say the word. :nod:

************************

Formulas for brake horsepower

horsepower = rpm x torque / 5252

torque = 5252 x horsepower / rpm

brake specific fuel consumption = fuel pounds per hour / brake horsepower

bhp loss = elevation in feet / 1000 x 0.03 x bhp at sea level

Formulas for indicated horsepower & torque

horsepower = mep x displcement x rpm / 792,00

torque = mep x displacement / 150.8

mep = hp x 792,000 / displacement x rpm

mep = hp x 792,000 / displacement x rpm

mechanical efficiency = brake output / indocated output x 100

friction output = indicated output - brake output

taxable horsepower = bore2 x cylinders / 2.5

Torquemonster said:

Jerome said:

Torquemonster said:

Ok - here's a wild stab in the dark

maybe the bigger sidewalls twist more and this twisting when it reaches its limits provides a counter force that actually helps the power - kinda like a pendulum effect... kind of...

maybe not

:help:

:screwy:

:leaving:

Click to expand...

Sidewall flex = stored energy :nod: Think wrinkle walls

Keep goin' Barry, you are on a roll. :thumb:

Click to expand...

I feel like the constipated mathematician who worked it out with a pencil...

I've got a firm grip of the turles head - but the bugger won't come out...

I think the roll has come to a stop - I can't go beyond the stored energy released bit - but on that basis - a drag slick must really store some energy!

Coffee now - head hurts /images/graemlins/freak /images/graemlins/surrender

Click to expand...

Like you would not believe. :nod:

Have you seen the pics of slicks on Fuel cars when they go "flat" coming up the backside?

I will see if I can find a pic of this too.

Did you happen to see the pic I posted above?

I got it!!!

Jerome - pick me pick me

please pick me

ok - I think that if we look at the drag slick pic then we see the story.

It takes a lot of power to twist those babies - that will suck up a lot of energy and power...

BUT

once the limits of that tire are reached - that energy has not been wasted or lost - a lot of it is stored and then released.

If that happens on a dyno - that will definitely add measurable hp at the wheels during that process - I'd think that a Top Fuel slick twisted up to the max would unleash enough torque to move a small car.

Am I right?

Or do i get the bozo prize?

Mark - I beg to differ - now the picture is becoming clearer - I can understand why I've heard of people getting more hp on tires that twist more...

there's no mystery about it - try twisting a tire with your hands then let it go. If you've ever had a toy powered by a rubber band - it's the same principle - wind it up and let it go. There's losses - but the stored energy left is real and does work when released.

I have no problem with this idea now - thanks Jerome! /images/graemlins/laughing.gif

That shows that a good chassis dyno could be accurate afterall.

Torquemonster said:

I got it!!!

Jerome - pick me pick me

please pick me

ok - I think that if we look at the drag slick pic then we see the story.

It takes a lot of power to twist those babies - that will suck up a lot of energy and power...

BUT

once the limits of that tire are reached - that energy has not been wasted or lost - a lot of it is stored and then released.

If that happens on a dyno - that will definitely add measurable hp at the wheels during that process - I'd think that a Top Fuel slick twisted up to the max would unleash enough torque to move a small car.

Am I right?

Or do i get the bozo prize?

Click to expand...

I knew you would come up with some answers. :thumb:

We are getting there. :cheers:

You can carry on, my head does hurt bad. Not from this......I don't think. /images/graemlins/laughing.gif

There are so many variables with these things that we could be here forever talking about them.

Jerome,

The severe wrinkle-wall phemomenon on a drag slick helps increase net torque at the tire by shortening the rolling radius (effective lever arm) between the axle and the ground. After a few revolutions, the tire begins to grow circumferentially. This raises the axle center from the ground and lowers torque, but the tire is covering more distance in one revolution than at rest, or low rpms which accounts for the huge mph gain. A shorter tire will accelerate the dyno rolls more quickly than a tall tire. I believe that should raise the indicated power levels.

Steel belted street tires exhibit very little "growth" and maintain circumference. HP street tires (Pilots, not E/Ts) have very stiff sidewalls and there's not much energy wind-up in them at recommended inflation pressures. That's why we have to run such low pressures at the strip to make them work well. Flexing stiff sidewalls creates heat and wastes energy. That's why you should run plenty of air in them on the chassis dyno. Some of the "cheater" boys' cars were moving around laterally a good bit - wasting power. Probably should cross the tiedown straps to minimize that.