Originally Posted by LinearX

The biggest difference between a Dynojet and a Mustang dyno are how they load the engine.

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As much as anything, they're a tuning tool. If you start doing mods, it's best to pick a dyno and do your before and after on the same one.

Originally Posted by vincentjames501

Mustang reads actual torque then calculates HP from it and a dyno jet is vise versa. Mustangs are better but will give you lower numbers. IE a buddy dynod 300HP on a dynojet and 280 on a mustang dyno.

Originally Posted by Uncle John

Horsepower cannot be measured. It is simply a calculation based on torque and rpm and a constant. The formula is HP = (torque x rpm) / 5252. Ever wonder why HP and torque are always equal at 5252 rpm. Heres your answer.

Originally Posted by Tuner@Straightline

HEX has been all over this one, and for the most part I agree with his answers. The basic reasons for the difference is in part on how the power is calculated, and partially due to the amount of "load" the vehicle "sees" during the pull.

DynoJets measure horsepower and derive torque from that. Loaded Dyno's have a load sensor, and actually measure the torque being applied to the drums (it's a little 5volt reference load sensor). Horsepower is then derived from that information. Torque is an actual force (like gravity), where as Horsepower by definition is a derivited of torque (work over time).

The load sensor data is then sent to the computer where it takes two constants, vehicle weight and Hp@50mph (aerodynamic coefficient used by the EPA) where the computer then calculates the amount of resistance to be applied to the vehicle (PAU force).

For an example of why there's a difference due to load, imagine this. It takes less power (however you measure it) to accelerate the mass of a 2500lb roller from 300rpm to 400rpm in five seconds than accelerating the same roller (2500lbs) with 550lbs of resistance via load cell from 300rpm to 400rpm in the same amount of time (5 seconds). That's why if there's a car that made 425rwhp on our Mustang MD-1100SE dyno, it will be faster than a car that made 425rwhp on a DynoJet.

As far as the timing example that somebody brought up, I've found that to be true as well. Sometimes the cars are faster with 1 to 2 degrees less timing than they see on the Dyno. Best was without being at the track is to check the plugs. That's why, no matter how good the dyno is, the opperator, or the tuner is, it will really only get you about 95% there. The last 5% is going to be needed to be done at the track.

When it comes to manipulating dyno numbers, if there's ever any questions, all you need to do is ask these simple questions if it's a Mustang Dyno. What Parasitic Loss Multiplier are you using (easiest one to change), and ask to see what the weather station is reporting. All you have to do that is go under "calibrations" and weather station peramitors. I can't remember what the SAE values should be off the top of my head, but it's not too difficult to find. If the box check "User defined" is checked, then they might be playing around with the numbers.

Because of these reasons, it's like trying to compare apples to oranges. As far as the "10%-15%" difference, we've found that to be untrue. In our tests it doesn't seem to be a constant at all. At 390rwhp on our dyno, the same car makes about 405rwhp on a local DynoJet. At 422rwhp on our dyno, the same car on the same DynoJet made 458rwhp. I've witnessed cars seeing close to a 80rwhp difference between a Mustang Dyno and a DynoJet at 600rwhp (Mustang MD-1750SE), although I cannot verifiy that with our dyno.

We're fairly conservative, and hence tell our customers that the difference is closer to 6-7%, but as you make more power, the difference increases as well. You must remember, Dyno's regardless of the type are tuning tools, and are in no means ment to tell people how fast their car is. Now which one is more "real world" is a totally different question. I like to explain it like this..... If you drive your car in a situation in which you have no mass and you're in a vacuum, so basically if you do intergalatic racing in space, use a DynoJet. If your car sees gravity, and has an aerodynamic coeffecient, and you race on a planet called Earth, then use a Mustang Dyno.

Originally Posted by TheRadioFlyer

okay. if this is the case, then why does Horse power matter at all. A graph of torque over the powerband should be a much more accurate gauge of a vehicle's power.

Originally Posted by Tuner@Straightline

Then everybody would be driving a DuraMaxx.

Part of it is tradition, plus horsepower is still a way to basically measure energy. Since they are dependant on eachother, together they are going to show how much force (torque) the motor can apply, as well as how much power the motor can make at any given rpm. You really can't get a good idea of the power of a car without both. If you use just the torque curve, then you'd never be reving past 4800rpm or so (usual rpm in which torque starts to decline with and LSx based motor), yet the motor is able to continue to accelerate the vehicle well beyond the peak torque band. This is why horsepower is still imporant.