it depends on how they place a load on the vehicle. it also depends on how they have the dyno setup/calibrated
typically the Dyno Dynamics and Mustang read lower. the Dynojet & DynaPack usually read higher.
here's some info from a local club member & Mustang dyno sales engineer:
The reason a Mustang and a Dynojet read differently is because they are two totally different types of dynos. The Mustang is a loading dyno and the Dynojet is an inertia dyno.
What is an inertia dyno you ask? Basically, you have the rollers which have a calculated weight called the inertia weight of
the dyno. This is a fixed value from the manufacturer. You speed the car up on the dyno, get it into 4th or 5th gear based on what is your one to one gear ratio. Then you do your power pull. It measures acceleration rates, time it takes to do the test, distance traveled during the test, inertia weight, etc. From these known values, it's plugged into the equation of HP=(mass)*(acceleration)*(distance traveled/time of test). From HP, torque can be equated using RPM from the vehicle. TQ=HP*5252/RPM. This is technically the backwards way to do it because HP is a theoretical number. Whereas Torque is a real/measureable number. A power pull is the only test you can do with an inertia style dyno. For the normal dyno owner, tuning is difficult to do with an inertia style dyno b/c the load applied to the vehicle is not real world loading. It is only the fixed weight of the roller's inertia (~2500-2900 pounds). This is much lower than the weight of most cars out there. Therefore you are tuning you car to run on the dyno and not run on the street. It is especially hard to tune EFI cars b/c their fuel map is a matrix of Load vs. RPM. So all your tuning on the dyno may be in the 50% load column, but when you go out on the street, you are pushing around more weight and now you are in the 70% load column - the column you couldn't tune on the dyno. Another problem that customers like us have is the ability to make full boost on the dyno. Again, this relates back to the not enough load created by the dyno. Turbo spool up is the effect of load on the motor. If there isn't enough load, you aren't going to spool up the turbo - kinda like revving the motor in neutral, you get some boost, but not as much as you would with load on the motor.
How does a Mustang apply a load on to your vehicle? Mustang couples a Eddy Current Power Absorber Unit (PAU) to the rollers of dyno. A PAU is basically a large magnetic frictionless brake also known as an electromagnetic brake. There are large
magnets in the center and large cast iron rotors on the outside. These rotors spin with the dynos rollers. As current is applied to the magnets, they try to "grab" the rotors and stop them. This is what applies the load to the vehicle. One PAU has the ability to make a 1000HP car under full throttle come to a complete stop. Attached to the PAU is a load cell (strain gauge). As you accelerate, the PAU will apply a load to the load cell because the PAU will try to spin with the rollers b/c of the friction
between the rotors and magnets. You also have the distance from the center of the PAU to the strain gauge. From these two measurements, you get torque. Torque=Force*Distance. Then from torque you get HP. HP=TQ*RPM/5252. The PAU
controls are extremely quick and accurate. The dyno can simulate the forces on any car going down the road using it's patented Road Load Simulation. That includes weight of the vehicle, wind friction, tire to road friction, etc. What your car sees on these dynos is exactly what it will see on the road. You can do a power pull, constant acceleration test, constant speed test, constant PAU force test, 0-60mph test, 0-100mph test, quarter mile sprint, 0-200 yard test and speedometer calibration test. EFI tuning is also extremely easy to do because you are in the proper load column during all of your vehicle simulation testing. You can also run the constant speed test which will allow you to tune every cell in the matrix one at a time.
The whole debate of which dyno is right is just opinion. Some people say Mustang is right, some say DynoJet is right, some say Dynapak is right, etc, etc, etc. Read the article in this link: http://www.mustangdyne.com/Articles...-article-01.htm
Basically, Chevy High Performance did a Mustang vs. DynoJet vs. a 5th wheel dyno and the Mustang's HP curve was spot on with what the 5th wheel dyno's HP curve looked like.
As for who is better than who, that's every person's personal opinion and I'm not posting this to change that, just giving real information that isn't word of mouth.
Hopefully this was informative and you learned something. If you have any questions, let me know.