on the dyno sheet, i noticed that my gear ratio = 50.00:1 (is that 50.00 mph?) and some of the mph/rpm figures were off. the run was in 3rd gear.

the dyno sheet shows 60mph at 2983rpm whereas hptuners (logging run) showed 60mph at 2804rpm.

at 112mph, dyno showed 5592rpm, whereas hptuners showed 5035rpm.

it's almost like the dyno was setup for 3.73 ratio
the dyno sheet showed my car hitting 6214rpm at 124mph, whereas hptuners showed that the operator let off at 5952rpm at 134mph.

what affect would this have on my dyno results?


here's the dyno result (without nitrous)

 

i know someone's gotta have some insight into this.

 

What does JBA say?

Jim

 

i emailed them about it, but it was the weekend, so no response yet.

 

More than likely, the operater was taking RPM from the rear wheels. If he was off by a couple of hundred, it would provide the results you saw. A Dyno Dynamics dyno is similar to a Land & Sea or a Super Flow in the fact that it uses both a polar moment of inertia (the drum), and a load cell for deriving rwhp....so the graphed hp would be inaccurate. Not by much, but useless for comparisons sake.

 

no response from JBA.

waSStock, i'm not sure i understand your explanation of how the inaccuracy applies?

thanks

 

fuckit. i'll run with these numbers until i add some REAL mods and hit some other dyno.

 

yes and no. We use a special harness we made to tap into the injector pulse for rpm as its HIGHLY accurate. Taking wheel speed for tach signal is fine for a front wheel drive car that makes less then 300 RWHP but not rear wheel drive high horse cars The DD dyno is full Eddy current load control and there is no drum for enertia. They also need to set the correction factor of the dyno to 1.100 to put the numbers in check with "internet" DynoJet numbers.


We tried a new ZO6 that pulls 155 +mph wheel speed and the tach numbers where way off! That is one of the cool features of the DD unit to pull injector pulse.

 

The inertia is derived from both the rollers and the absorber (and any rotating mass, like shafts or pulleys). The combined (rollers and absorber) inertia for your dyno is 2.590 kgm2 (61.6 lbft2). Every chassis dyno's rolls has a polar moment of inertia. If it did not, only steady state tests would be accurate.

I agree that the injector, or coil pack tapping is the most consistent RPM source. We do the same thing with the Dynomite Dynos. I would imagine that the original posters dyno operator was taking the rpm from the drum speed though. Its odd that you would see a difference between the ECU and the injector pulse.

 

ECU or injector pulse..........same difference I agree. Just out of curiosity, do you know the inertia for a true enertia dyno like a DynoJet?