was talking to Jurgen about this thread when I was over XO/JM on Saturday and found some info from the church automotive testing site who run 2 sets of dynapack:
here is a extract :
As power increases for a given combo, the difference will grow due to the change in acceleration rate on the Dynojet (assuming the same gear is used for all tests). As the acceleration rate goes up, the total torque required to accelerate the wheels and tires will go up (remember, the torque calculation is MoI * acceleration - twice the acceleration rate requires twice the torque). But on the Dynapack the acceleration rate is kept the same (or at least should be), so losses remain largely the same. Furthermore, both dynos are subject to inertial losses accelerating the flywheel, transmission, etc. and the faster acceleration rate as hp climbs will show increasing losses on the fixed load Dynojet.
This is why we recommend people use a rough percentage adjustment to estimate flywheel hp on the Dynojet versus a rough fixed adjustment on the Dynapack. In our experience, a manual transmission FWD car will lose 20-25 hp to the hubs on the Dynapack. A RWD car will lose 25-30 hp and an AWD car about 35-40 hp (the FWD case has been verified on an engine dyno). In contrast, losses on the Dynojet will be in the 12-14% range for FWD and 14-16% for RWD (opinions vary).
http://home.earthlink.net/~spchurch/index.html