A Comparison in Dyno Readings for One Bike on Two Different Dynamometers
 
 
Generally I do all of my tuning on one Dynojet 250i dynamometer at American Cycle Performance in Glen Burnie, MD.  It provides repeatable and reliable information for optimizing the tune of the XR (and of course other bikes).  But, since riders from around the world like to use dyno data as the ultimate comparison in the "mine is (are) larger" quest, and must compare using data from different dynos, this is a look at how data from different dynos compares. 
 
As a qualifier though, this is only a comparison of data collected from Dynojet dynos, and does not compare Dynojet data to runs from other brand dynamometers.
 
 
 
 
What is the Same
 
For both dyno runs:
  • the XR is 1200 cc with Zippers 575XR cams and CNC ported heads.
  • Valve sizes are nominally stock.
  • Compression is nominally 10.25.
  • The intake is a modified stock airbox with an oversized K&N air filter (but without the internal velocity stack mod I've been playing with).
  • The exhaust is the V&H 2-1-2 Race pipe.
  • The ECM is the TMax.
  • Gearing is set at 38-57 primary and 30-68 drive belt.
  • The rear tire is a somewhat worn stock 180 Dunlop set at 40 psi.
While the two runs were made a few days apart, the weather was very similar - about 60 deg F with low humidity.  Nice days for dyno'ing.

 
What is Different
 
Well the Dynos are obviously different, both were Dynojet. 
  • The operators were different - myself at ACP, the HD tech at Susquehanna. 
  • I hit the run button and snap the throttle wide open.  The HD tech rolled into the throttle a bit slower. 
  • I back off the throttle at 7,200 rpm.  Although I mentioned to him to run to 7,200, he crashed into my rev limiter at 7,400 rpm. 
  • I stand over the bike and put some weight on the rear tire.  The HD tech did not. 
  • And, I run in 5th gear to take advantage of the the 1:1 transmission ratio and lower drivetrain losses, while the HD tech felt he would get better TQ readings by running the bikes in 4th gear.
There were other difference in AFR collection equipment and dyno room set up, but these would not have affected the TQ/HP readings.
 
 
The Graphs
 
The below graph shows TQ and HP readings for the two different dyno pulls.
 
 

Dynamometer Comparison
DynamometerComparison.jpg
Same bike and configuration - Different Dynos
 
 
The shapes of the curves are essentially the same.  The peak HP figures are very similar at about 97 HP.  The TQ curves are a touch higher on the ACP dyno - which could be due to the lower drivetrain loss from the 5th gear pull or my quicker throttle snap.  With the worn rear tire I get a little tire slip on the ACP dyno at the beginning of the run. 
 
The power out of the bike should have been as close to the same as one could manage, so for peak HP number purposes these two dynos do correlate rather well. 
 
Likewise, I have had good correlation between the ACP dyno and a Dynojet 150 at Zippers, and with a second trailer-mounted Dynojet 250 dyno that ACP used to own.  In the past, I have also had good correlation between dyno data from the ACP dyno and the Dynojet dyno at MMI in Orlando, FL.

 
Conclusion

The graphs are similar.  The peak TQ and HP numbers are comparable.  And the results on each dyno are repeatable.  One could get a feel for a comparison of peak power made by bikes run on these different dynos.  But one would not want to compare the effects of different tweaks or engine mods using the dyno figures from these different dynos.  More difference in the power curves is induced by the operator and dyno itself, than subtle modifications to the engine builds.
 
The key is that the graphs are repeatable on each dyno, which makes the dynos very useful tuning tools.  During the day watching dyno runs of other bikes at Susquehanna HD, it amazed many owners to see how bad their AFR tuning was at Wide Open Throttle.  But the key is that the fueling was not only way off at WOT but also throughout the whole riding range - hence the benefit of using the dyno for complete fuel mapping or carb adjustment on the overall ridability of a bike.