P31 DYNO
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E63
A true equation but that is not how a Dynojet measures horsepower.
Here's another explanation, from Bristol Dyno that backs up the first data point I made.
Here's another explanation, from Bristol Dyno that backs up the first data point I made.
Force = Mass x Acceleration
That's one of Newton's laws. It's not important which one it is, because I don't remember. Force is one of the things that we're looking for. Force in the automotive world is called - you're supposed to shout it out..... torque. Torque is rotational force, and its most common unit for us is foot-pounds. Plain and simple, if you have a 12-inch wrench and you lean on the very end of the handle with 10 pounds, you're applying a force of 10 ft-lb. to the nut you're trying to turn.
The mass in our case gets a bit complicated. Mass in most cases is easy - how much does the object weigh that you're accelerating. In the case of the dyno drums, however, it's not that simple because we are not "moving" the drum, we're spinning it. We are not creating a "translational" motion on the drum, we are creating a "rotational" motion. To understand the difference, think of the actual dyno drums. Each one weighs 2700 pounds. It would take a pretty impressive force to push a 2700 lb. object across the floor. Now imagine just spinning those drums. The shaft going through the center of the drums rests on two hugely expensive bearings that I'm probably going to have to replace a lot more often than I want to. The drums spin with the slightest touch. To calculate away this difference, physicists came up with the "mass equivalent" of a rotating body, which is very similar to the "moment of inertia." I don't know the actual numbers, but let's just pretend the mass equivalent of the dyno drum is 50 pounds. That means that spinning the 2700 lb. drum is like pushing a 50 lb. weight across the floor (forget about the friction - you nerd.) Now I've heard some people say that this is not the correct way for calculating horsepower, but in this case - it is. Many of you are familiar with the hp = rpm x torque / 5252 formula. In many situations, that formula is the way horsepower is calculated. In the case of the Dynojet dynamometer, it is not. The Dynojet calculates horsepower even if there is no torque reading, and it does this in the manner that I am in the process of describing. Those of you that have had your vehicles tested at my shop and have watched the computer screen closely have seen that the computer plots out only the horsepower after each run, and I have to click the mouse a few times to get your torque. It does this because sometimes there is no torque reading, or the torque reading is faulty. This can happen because inductive and optical rpm pickups may not be perfect, and/or your ignition system may make things go a little haywire. If the rpm readings are not perfect, and the computer used the rpm x torque / 5252 formula, the hp reading would be inaccurate.
The mass in our case gets a bit complicated. Mass in most cases is easy - how much does the object weigh that you're accelerating. In the case of the dyno drums, however, it's not that simple because we are not "moving" the drum, we're spinning it. We are not creating a "translational" motion on the drum, we are creating a "rotational" motion. To understand the difference, think of the actual dyno drums. Each one weighs 2700 pounds. It would take a pretty impressive force to push a 2700 lb. object across the floor. Now imagine just spinning those drums. The shaft going through the center of the drums rests on two hugely expensive bearings that I'm probably going to have to replace a lot more often than I want to. The drums spin with the slightest touch. To calculate away this difference, physicists came up with the "mass equivalent" of a rotating body, which is very similar to the "moment of inertia." I don't know the actual numbers, but let's just pretend the mass equivalent of the dyno drum is 50 pounds. That means that spinning the 2700 lb. drum is like pushing a 50 lb. weight across the floor (forget about the friction - you nerd.) Now I've heard some people say that this is not the correct way for calculating horsepower, but in this case - it is. Many of you are familiar with the hp = rpm x torque / 5252 formula. In many situations, that formula is the way horsepower is calculated. In the case of the Dynojet dynamometer, it is not. The Dynojet calculates horsepower even if there is no torque reading, and it does this in the manner that I am in the process of describing. Those of you that have had your vehicles tested at my shop and have watched the computer screen closely have seen that the computer plots out only the horsepower after each run, and I have to click the mouse a few times to get your torque. It does this because sometimes there is no torque reading, or the torque reading is faulty. This can happen because inductive and optical rpm pickups may not be perfect, and/or your ignition system may make things go a little haywire. If the rpm readings are not perfect, and the computer used the rpm x torque / 5252 formula, the hp reading would be inaccurate.
Last edited by RStevens63; Jul 18, 2010 at 05:51 PM.
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E63
To add another layer of confusion DJ switched from measuring via inertia to eddy current aka e brake, so all newer DJs calculate HP from TQ like mustang, superflow, DD, apparently unlike the older versions.
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2011 GLK 350, 2013 GT-R, & 2013 RAM 1500
The newer models (like the 224xLC we use up here) can switch between inertia and eddy current. Inertia is still used for dyno runs. It is nice that they have the option to load test now as it is great for tuning purposes.
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E63
No confusion. DJ did not make a permanent switch, they simply added the option for load testing.
The newer models (like the 224xLC we use up here) can switch between inertia and eddy current. Inertia is still used for dyno runs. It is nice that they have the option to load test now as it is great for tuning purposes.
The newer models (like the 224xLC we use up here) can switch between inertia and eddy current. Inertia is still used for dyno runs. It is nice that they have the option to load test now as it is great for tuning purposes.
I would think in eddy current mode HP is still calculated via TQ? What I don't like about DJ's and the newer eddy current option is that it's simply a percentage, ie 30% load. Well, 30% of what? TY
