How can a chasis dyno measure RWTQ? I see postings everywhere with 380 RWTQ, 400 RWTQ, 376 RWTQ. I can see how the dyno can "guess" at RWHP, but Torque? My GTO makes about 550 H.P. at the crank and 600 Lbs. of torque at the crank. But at the Rear wheels the torque is around 15,000 lbs.! Why are the dyno torque numbers so low? Is the dyno doing reverse math from the wheels up towards the crank, and actually "guessing" at crankshaft torque, and if that is the case, is the RWHP number also a "guess" at an actual crankshaft figure versus true RWHP. For you sceptics about axle torque, here are the numbers for my GTO, 612 Lbs. tq at the crank, multiplied by 2.5 at the converter (thanks to munsinger),then an additional 2.48 (TH400 1st gear ratio), then an additional 4.11 (rear gear ratio) that comes out to 15,595 lbs. of REAR WHEEL TORQUE! Why are the chassis dynos so screwed up? The GTO's race weight is 3990 lbs, runs 11.70's at 114.

 

yeah i have heard that too-the gears are torque multipliers. all i know is about the dynos is that they measure how fast water is being moved by the car. since they now home much force is needed to move the water. they get torque by (HP *5252)/rpm=torque. i know power =force * velcity. i know this doesn't help you too much but hey its a thought.

 

Did that 248rwhp thread make you write this?

 

Ya, I'm with you, Horse power is an effiency rating of torque, it doesn't really exist, hence the 5252 math conversion. My question sounds like I'm being an ***, but I would really like to understand more where these dyno shops are getting their numbers from, I think it is all a bunk ploy by the tuner shops.

 

LOL, ya it got me thinking.

 

yeah its a bunch of bull. there are only two things an engine produces rpms and torque period. hp is bull

 

dont really care so long as i got a lot of both! lol

 

Depends on how you look at it, the more I think about it, the more I want to say its a measure of acceleration. I have nothing hard to support that, but when you look a graphs from different cars you can kinda see where im comming from. Look at the graph of a diesel, then a supra, then a N/A V8. Deisels accelerate slow compared to everything else, and in the dyno graph they produce wayy more tq than hp. Supras start off slow (boost lag) and accelerate hard in the high RPM's. The V8 motors just accelerate all over, lol. Not as fast as some points in the powerband, but you get what im saying.

Maybe its just too early for me to think about things like this, who knows.

 

You'll have to go back to some Physics here. Horsepower isn't really an effiecency rating of torque. One horsepower can be equated to 745.699 watts Watts which we all know is a measurement of energy being produced (or used). Horsepower is not a force, it is simply a unit of energy. By defiinition one horsepower = 33000 lbft/min, or to put another way, if you were to lift 33,000 pounds one foot over a period of one minute, you would have been working at the rate of one horsepower.

Torque on the other hand is a rotational force. Torque is a measure of how much a force acting on an object causes that object to rotate. Torque = radius x force. Well on a dyno the force is being measured and the radius is also known for the spinning drum, so Torque can be calculated.

I'm sorry, I'm a 4th year Mechanical Engineer at Michigan Tech and I just couldn't help but to try to apply myself here.

 

Couldn't have said it better myself

I love physics!

 

Don't forget that it doesn't have to be 33,000 lbs. moved 1 foot in 1 minute.

It could also be 1 lb. moved 33,000 feet in a minute, or 181.659 lbs. moved 181.659 feet in one minute, or 550 lbs. 1 foot in 1 second, 6600 lbs. 1 inch in 1 second, etc.

-4th Year Mathematics Major

 

I've had a lot of success explaining it to my friends this way...

Torque at the wheels is the only thing that matters in making the car go faster. At the wheels, horsepower can be disregarded. The question that statement brings up is this - If a car can get an enormous amount of torque at 3k RPMs, then why not shift early to keep the torque up? The answer - gears. If you're making lots of torque at a low RPM in 1st gear, that's good. As you increase the RPMs the torque falls off, so you're not accelerating as quickly. However, if you were to shift to 2nd to get back to your peak torque, your torque is higher yet your power is lower. In 1st gear with the higher HP, the gears can make more torque at the wheels. In 2nd gear with the higher flywheel torque, your actual torque at the wheels will be lower due to the lack of horsepower. It's better to put down a little less torque in 1st at a high RPM than to shift to 2nd and produce a lot of torque at a lower RPM.

At the wheels you want torque to move the car. That's all that matters. At the flywheel, however, you want power. Power can be converted to torque by the transmission. So, power at the flywheel is fuel for the transmission to make torque, which it sends to the wheels. And, as we all know, torque at the wheels makes wheel stands, 1/4-mile times, and ricer excuses.

 

I believe that on a dyno torque can be derived by using RPM and the rate of acceleration of a know mass of water in the dyno drum. By measuring the rate of acceleration and the rpm at which that acceleration is occuring, they can use mathematics to work backwards and figure your rear wheel torque. No need to convert gears, or torque converter or transmissions.

I could be totally wrong, but in my head it works out ok

 

Don't forget its not quite 15,595 since you must factor in the diameter of the tires.

 

answer:

because its measuring the RPMs of the motor.. not the RPMs of the axle.

 

Yep thats why they put that sensor on your #1 plug wire

 

yada, yada, blah, blah, blah....just take it to the track and run the bitch.

j/k Cool posts.

 

The above is true.



This is not true. Horspower (or any unit of power) is the rate of "using" energy.

What is confusing is that in Imperial measure energy and torque have the same units. Torque is usually defined as force times distance (lb.ft) while energy is measured in (ft.lb). They are used interchangeably so people think they are the same thing.

If you have 1000 lb.ft of torque at the wheels (engine torque multiplied by gearing) and you sweep one full wheel rotation (360 degrees is 2*pi radians) you have performed 6283 ft.lb of work (you have used 6283 ft.lb of energy). If the wheel is rotating at 1000 RPM (or 16.7 rev/s) (this would be ~80 mph) then you are expending 6283 ft.lb of work with every revolution, therefore this is 16.7 rev/sec*6283 ft.lb (per revolution) = 104720 ft.lb/s Since 550 ft.lb/sec is 1 hp , this is 190 hp.

I am not implying you need 190 hp to trvel at 80 mph but if you are accelerating at that moment you can be putting 1000 ft.lb to the wheels.

 

Why is this not being addressed in the dyno section?

 

when something is in the wrong forum and needs to be moved, theres no need to post in the thread... just take the first post and report it... then one of the mods will know to move it.

posting like that in the forum doesnt do anything besides start fights.