[SEric]

Torquemonster gave a pretty damn good explanation. Bottom line is that the theory is all on the side of HP. That is, if you could do anything you wanted with all the other factors involved then the higher HP car would always win. The perfect drag car would have an F1-like engine spinning at 20,000 rpm and a CVT gearbox to ALWAYS keep it's motor at the HP peak.

What you want to do is compare two cars at the same speed and see which one applies the greatest motive force (torque) to the ground. If you make that comparison (integration) over the range of speed seen during the race then you will see which will be the winner. Since we're just talking about the difference between torque and HP then let's assume gearing is completely optimal for both cars.

Take two identical cars with CVT's and run one of them at the torque peak and the other at it's power peak. Conservatively, the power-peak rpm will be something like 30% higher than the torque-peak rpm. So at the same speed, the HP car is using 30% deeper gearing because of the engine's greater speed ... thus factor in a 30% increase in rear wheel torque due to the multiplication factor of the gears. Of course the engine torque of the HP car is less at it's power peak than that of the torque car but that's typically only about 10% and doesn't make up for the advantage gained by the lower gearing in the HP car. Torque at the rear wheels translates into force and force causes acceleration. Therefore the HP car wins because it's always putting down more torque at the rear wheels at the same speed as the torque car.

Sure now pick me apart with real world gearing and all but my point about HP vs TQ remains true. A good drag racer is going to optimize his gearing for his engine anyway.

 

[SEric]

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?

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 in fourth gear (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.

 

[SEric]

Who wants to tackle this one, TM?

Why can you dyno one car with low profile 19's on it and then change them out to say 16's of the same overall rolling circ. and have the hp and tq readings be different? :idea:

All else being equal and the runs back to back with only the rim/tire change. And then switch back to see the same results the other way.

And who can tell me which will give the better readings?

The readings should be the same. Tire size should have no affect. On the other hand, chassis dynos do not give repeatable results. Figure an error of +- 7%. That's probably what you're seeing.

 

[SEric]

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?

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.

Wow, ok I think I got it :thumb:! So, Eric, does that mean when we ask how many RWHP/TQ a car is making, we are really referring to the amount of power being transferred to the wheels from the crank, rather then the actual amount of power at the wheels?

Correct. The actual torque at the rear wheels is (trans gear ratio) X (rear gear ratio) X flywheel torque. Talking about 4,000 to 5,000 lb.ft in first gear.

 

[SEric]

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

 

[SEric]

Holy crap, in the time it took me to type my post the levee broke on this thread.

To highlight:

A dyno measures TORQUE, not hp -

Changing drive ratio, overall tire diameter, etc, changes the Torque reading at the wheels, and thus the dyno gives a different reading.

Its simple physics people.

Mark, hate to disagree with you but you're wrong on a number of counts here. JPH was correct that the inertial dyno (like a Dynojet) measures the rate of acceleration (change in drum speed between discreet time samplings) of the drum and back-calculates torque from it. Torque is a static property that cannot be measured directly from a rotating device like a dyno drum. So although Hp seems unreal to many of us and we dismiss it as nothing more than a derived entity, in actual fact the dyno only measures power and reports torque as the *mathematical* afterthought.

Changing the drive ratio and tire diameter does have an affect on the force that your tire applies to the ground and hence to the dyno drum but I will reiterate that the dyno does factor this out in it's calculations of flywheel Hp and Tq. You said earlier that it couldn't be doing this because there's no way to enter drive ratio and tire diameter into the dyno program. My response is that you don't need to enter that crap because the dyno has a much more simple and elegant way of factoring out the torque multiplication due to gearing and tire and it does so using the ratio of engine rpm to drum rpm. If your crank were hooked directly to the drum axle with no gearing induced mechanical advantage they would rotate at the same speed, right? If your motor rotates the drum at half the engine's rpm then there's a mechanical advantage (torque multiplier) of 2:1 at work somewhere in the driveline, correct? The dyno doesn't give a shit whether that multiplier is due to tranny, diff and/or tire size. It simply knows to divide it's measured power/Tq numbers by 1/2 to get the flywheel values ... it's really a beautifully elegant system when you realize how it works.

As far as why we put the tranny in 4th (1:1) to run dyno pulls? It's not because 1:1 is necessary for the calculation to work, it's because in direct drive (1:1) you get less power loss through the clusters than you do when it's in some other gear. You could perfectly well do a pull in first gear or in 6th but the "rule of thumb" loss figures that we all apply would be wrong.

 

[SEric]

So I humbly drop trou and offer up my lilly white ass for a public VA caning, and can only add this link with a good writeup of how the whole thing works:
(I'm stickin to my guns on the drag tires though!)

You graciously fold in the face of my undeniable rightness, just like that? Turn in your Alley membership card and get out!

What's the first rule of the Alley? Right ... fight to the bitter end. What's the second rule of the Alley? Right again ... fight to the bitter end and if you're wrong, use more insults.

 

[SEric]

Good info, SE's explanation of the dyno finally made it click for me as well. Something that I still don't understand though is that if torque is a simple mathematical derivative of HP, then given the same HP curve for two different cars, will they also have the same torque curve? Seems like they would have to, but doesn't seem like they necessarily do in practice.

Yep, they would be identical.