When I bought my SuperFlow Cycledyn eddy current roller bike dyno about 10 years ago, they told me to expect lower HP numbers than what I was used to on my DynoJet roller that I had just sold to a friend. SuperFlow technician Brent Williamson told me that DynoJet overestimated the mass value of their drum, leading to overstated HP. That can be easily proven by dyno testing a bike, steady state using the SuperFlow eddy current absorber and strain gauge to read torque/HP in steps, which takes the inertia on the roller and absorber out of the picture. Since a steady-state step test and acceleration only test on the SuperFlow roller dyno match so closely, we have to assume their argument is accurate.
But my argument to Brett was [at that time] there were probably 10,000 DynoJet dynos and one Superflow Cycledyne (mine would be number three). So whatever numbers DynoJet has been offering tuners and magazines are unfortunately “the standard”. So I had to spend the first few years dyno tuning bikes on the Cycledyn, trying to explain why my HP numbers were “lower” than what they made on so and so’s DynoJet roller dyno. The last software upgrade I got from SuperFlow a few years ago now has a “happier” “DJHP” channel so our dyno sheets can show whatever the bike owner wants. The DJHP number is about 10% higher than Superflow’s SAEHP. Good move. Sean Ray used the new HP channel to make 138 DJHP while tuning boost and EFI fuel here on his Garrett turbocharged but otherwise stock 88 CI Softail Custom. A few weeks ago, he took it to a DynoJet dyno shootout at Stan’s H-D here in Batavia and made 138 there, as well (plus runs low 11’s and 120+).
Now with Justin Fuller coming to dyno his R1 turbo on my 901 shaft dyno we have more good data to compare HP at various locations on the bike. When he used my Cycledyne a few weeks ago, he made 200DJHP (182 SAEHP, SuperFlow’s wheel number on the same run) at 5.5psi boost and 12.5/1 A/F ratio. And it made 195 DJHP (177 SAEHP) at a more conservative 11.5/1.
Yesterday, with my 901 engine dyno connected directly to the R1 transmission output shaft, with the same boost setting and fuel very slightly richer (10.7/1) the bike made 189 SAE HP. That is exactly accurate, strictly torque vs RPM plus exact correction for baro, temp and humidity– no fudging for tire slip/ drum mass estimation.
That 189 actual output shaft SAEHP, according to out two SuperFlow dynos, is reduced by 6% from chain loss and tire friction to 177 SAEHP at the rear wheel (tire). That makes sense. But there’s no way the HP increases from 189 SAE HP at the output shaft sprocket to 195 at the tire (DJHP).
So when Justin turned up the boost on his R1 to 9psi, his output shaft true SAEHP was 230, which would equal rear wheel 216 SAE HP (SuperFlow) or 239 DJHP.
I’m hoping this isn’t too confusing, and is just meant to be informative. Mark Dobeck’s invention and subsequent proliferation of the DynoJet roller dyno is surely worthy of a Nobel Peace Prize. 20 years ago, BDJ (Before DynoJet), there were few options for motorcycle performance people to get meaningful dyno numbers. Jerry Branch had (and still has) a SuperFlow engine dyno that connects to the rear axle of the bike. There was another one in CA that Cycle Magazine used to use that had an eddy current absorber, but not much instrumentation. Then there was my place in WNY. I was lucky to have racers like Colin Edwards’ have their bikes tested/ tuned here with Kevin Cameron as consultant. Colin’s team would travel here from Colorado with truckloads of TZ250’s and related stuff to obtain the meaningful information they needed.
But now every bike performance aficionado surely has a DynoJet or SuperFlow (or other brand like Huff or Land and Sea) roller dyno within 30 miles to test and tune with, thanks to Mark Dobeck’s vision.
Regardless of pure accuracy, any dyno can be a useful tool, if it’s repeatable and you make HP progress going from A to B (or Z). Most magazines use DynoJet rear wheel numbers to publish stock bike’s performance, so there’s nothing wrong with comparing modded bikes with different but stock bikes on the same brand of dyno.
Just be aware that every HP is not necessarily created equally.