DQ, Whats the difference between a Mustang dyno and a dynojet?

[jimberg]

How do we know that the Dynojet is not artificially high is some applications?Just because a larger number of them are in service (probably because they are literally half the price of a load bearing unit) does not mean they are more accurate.

We don't, actually. DynoJets may be less expensive since they sell more. That doesn't mean they are of lesser quality.

Turbo cars in particular are incredibly sensitive to loading for their tune. I have seen turbo 5.0L cars spin the hell out of a Dynojet, but for some reason run mediocre at best at the track. There is a reason for this.

Probably because turbo lag is an issue and since the DynoJet can spin up faster into the powerband of the car it will overcome lag sooner. The exhaust gas velocity is probably also higher so it can transfer more energy to the turbines.

I suppose this could even be an issue on NA cars. Hmmm...

This, by the way, is an excellent point in favor of a Mustang Dyno.

...and much like a good torque wrench, one should be able to calibrate the dyne AND verify torque readings vs. a known force.

The weight of the roller is a known force.

Since Mustang dynos measure torque directly with a load cell (strain gauges) that can be calibrated and certified, I have a higher confidence in them than a dynamometer that gets torque by extrapolating HP from roller acceleration and again extrapolating with respect to engine speed.

You're essentially comparing a balance (DynoJet) with a spring scale (Mustang Dyno). I'm saying that the DynoJet is like a balance since the wheels are moving an actual known mass, the roller. A known mass doesn't really need to be calibrated unless you're worried that it will lose a signifcant amount of mass on its own. The electronics that measure the acceleration of the roller would have to be calibrated, of course.

On a Mustang dyno you have two points of potential failure instead of one. The strain gauge which can be off, and which will more likely be off than a known mass, and then the same electronics used to measure acceleration of the drum. This is what makes it like a spring scale. I, as most scientists, would prefer a balance over a scale for making accurate measurements.

Dynojet claim to "measure HP directly", but I have yet to ever see a "horsepower meter" in any instrumentation catalog.

Horsepower isn't really measured directly since it is a measure of work over time. Dynamometers measure torque which has to be factored into an equation to determine horsepower based on the rpms. That's the formula that Unit 5302 used above.

Every other accurate engineering power measurement I have ever seen deals with measuring force directly at a fixed distance (torque) and multiplying by speed to find power.

This IS what a DynoJet does. See above.

Thanks for taking the time to discuss this. I have to do some more thinking about the positives and negatives of both.

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351W 89 Mustang GT Convertible

[This message has been edited by jimberg (edited 08-20-2001).]