Has someone experience with an G-Tech Performance Meter like the PM-423. Is it possible to determine the horsepower or the torque on the wheels ?

 

It has a wheel horsepower measurement mode. You have to enter the weight of the car as-run. It tells you the peak horsepower observed based on speed and acceleration. This should be a reasonably accurate method, though a few things can throw it off:

Inaccurate weight. Weigh the car, with you in it. I've used the scale at the drag strip, my buddy has persuaded the guys at the weigh station for semis to let him use their scale.

Hard shifts. When you shift, energy stored in the flywheel gives you a momentary boost in accleration (this is why the tires chirp, or even break loose). The meter will pick up on that and report the maximum observed power, even if it only lasted a tiny fraction of a second.

Wheelspin. This will make the numbers low. Best way to get an accurate measure in horsepower mode is to take off gently and roll into the throttle.

Weather conditions. To get an accurate reading, you need to correct to standard air density and humidity. This is called SAE corrected horsepower. If you want to get close, you need to carefully note the weather conditions and altitude, then find someone with a density altitude calculator (like a dyno operator) or use one of the calculators on the web.

Air drag. Wind can throw the measurement off, as can high speeds.

The best way to get an accurate measure that fixes all these problems is to pay for a Dynojet chassis dyno run. Should be about $75 for a measurement-only run with no tuning. You also get a nice graph showing the horsepower and torque curves, instead of just a peak number. This graph can help you determine if the car is running right. Also, many shops have a wideband O2 sensor that can let you see the air/fuel ratio across the RPM range. Very helpful for tuning the mixture for maximum power.

The G Tech is pretty much a toy, though admittedly a fun one.

 

A test was refused, do to the method they are using.
I have an TH700R4 (Overdrive) in my car.
They put the car on rolls, starting normal driving to top speed and then shift into neutral position. When doing this hardly there is a certain risk to enter the Rear.
As you explain me the principles and the many factors you had to pay attention, I got the idea to run first a reference car under the same conditions and as the G-Force-Meter is movable, put it in the Corvette and do the same test again. Only difference then the weight of the cars.
Ok, not a very precise measurement. But we have also to accept, that for all test, the conditions are never the same. :steering:

 

Find another dyno shop. There is no reason to shift into neutral.

 

:iagree:

At work (www.thedynoshop.com) we dyno a dozen cars a day or so. Never use Neutral! The laws of physics hold the car in place. The momentum is the DYNO spinning, not the car moving. We block the front wheels, run at 15 miles per hour for a few seconds so the car finds itself centered on the rollers, and then let 'er rip. Once the Dyno is spinning there is not much forward pressure at all. The wheel choks handle it.

We also do a lot of buggys, race cars, and off road vehicles. Some are very light and in that case they are strapped down to the Dyno. On blown cars :D often we need to have two of us sit on the back bumper so there is limited wheel slippage on the machine. Now that's scary!