I gotta ask when was that pic taken and I feel sorry for you Jim if your still dealing with that Cobra sure looks like it hanging in the background

 

dyno day, it was a saleen, and quite a bit of work was done to it.

I'll just leave it at that most of the saleen equipment was replaced with our spec.

 

Gawd, I love black socks!

 

Here is a quiz for you Dyno experts. Would there be a difference in HP and TQ doing a pull in OD vs High. Or 2nd vs 3rd.? Etc.. Afterall, the Dyno is actually an accelerometer.

I was told a long time ago, the only true HP measurement is with a water brake dyno. I'm not sure if the Eddie Current Dyno would be the same or not. Anyone know?

 

Yes, there is. You run in the gear that works the engine to produce the highest power readings without overworking the engine. This equates to the 1:1 gear 95% of the time. I am looking at an A4 C5 graph right here, and it breaks down, in peak HP numbers, like this:

1st: 210hp
2nd: 270hp
3rd: 303hp

Air to fuel ratios also vary per gear, but more so from 2nd to third.

This does not mean the inertia numbers are way off, but rather, the inertia dyno numbers only indicate the leftover power and torque from the tested weight of the dyno, not the actual weight and aero of the vehicle.

The inertia dyno just essentially tests everyone's motor in the same, lightweight car, with no aero drag. If I put my 14.5 (1/4 mile) Powerstroke in a Camaro, it will run 10's, and that is the point made in the difference of the power numbers the different dynos provide.

The only true measurement in an installed chassis is the road dyno, but the loaded dyno provides the closest to the real world loading condition.

The eddy current is more precise and controllable, but not as feasible for high HP vehicles in an engine dyno configuration. I've seen quite a bit of variables from one of the same model water brake engine dyno to another, and the operator, to some degree, is part of that equation.

 

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.

 

I would have expected that last paragraph from a loaded dyno owner.

 

I told that to a customer one day, and he couldnt stop laughing, so I've been using it ever since. Plus it's not far from the truth.

 

You do know that far more of individuals and peers consider the dynojet industry standard, and get offended by the egojet comments.

The road comparision, when shown in person, ends it all.

 

Just because it's been around longer, and cheaper, doesn't make it the standard, that just means there's more of them.

As for the road comparision, not everybody has an LC1 in their car to show them, but I've seen it time and time again.

 

<In Hank Hill voice.> Yep.