I've heard opinions that Mustang Chassis dyno's typically read slightly low. Anyone have experience with them?

At my college we have 2 Mustang dyno's that are in an ex-emissions testing station. In one of my classes we have started using them for senior thesis work. It appears that their HP readings are extremely low. As an example, what HP would you think a low mileage, manual '03 SVT Cobra with an intake and cat-back make (may have had other mods as well that I didn’t see)? According to the Mustang program, it made around 300-310 whp in 4th. I don't see how this can be correct. However, the 1/4 mile time and torque numbers both look reasonable (11.69 sec / 430 lb/ft in 4th). We also had an automatic '78 Camaro with a built 383ci that posted 12.6 sec in the 1/4 and made over 400 lb/ft torque. But again, the HP seemed way off at around 165 whp in all gears.

The instructor claims that these figures are normal. He says that the best powertrain loss he's seen on these dyno's is around 50 hp on a manual fwd car.


Is it possible that there can be an error in the program's conversion calculations? I know the conversion calculation, but I can't use them for my car. I'm testing an MSD ignition and there is no way to get a true rpm signal with an inductive coil (that I know of). So, my graphs will be of HP and torque vs. speed.

 

I was told last week by a local Mustang dyno operator that you should add 13.5% to the Mustang generated RWHP numbers to get equivalent DynoJet RWHP numbers. At the same time, I found a few Miata forum posts where others had posted that the conversion factor should be +15%. Further, based on 5 different Miatas run on the Mustang vs. what similar cars have generated on Dynojets, I believe the 13.5% adjustment factor to be reasonably accurate.

 

13.5% percent would make the Cobra's whp more believable at 341 - 352.