kb58
SuperDork
1/25/21 2:00 p.m.
I've had my car tuned on two different types of dynos: the in-floor roller-type, and the external pods that are bolted to the wheel hubs. What was most memorable that they produced hp readings of about 30% difference. In my opinion, the roller type is much closer to reality, and yes, I know the bolt-on type will always read somewhat higher due to the tire/wheel inertia being removed, and yes I know that for tuning purposes, it doesn't matter. Still, in this age of microsecond-timing and 0.1% sensor measurement accuracy... 30% difference, really? What I often hear is "well it depends how the operator adjusts it", which seems odd if the unit has the necessary sensors, but oh well.
The above aside, I prefer the bolt-type dyno because it can vary the load as necessary, which is extremely helpful when developing fuel maps.
In reply to kb58 :
Some of that 30% is in the calibration of the system. Production based dynos are checked daily, and worked on for two solid weeks twice a year. Given how they measure, it's not hard to get them to drift, and stay pretty drifted off reality.
Then again, I don't expect most aftermarket dynos are even calibrated the first time- installed, numbers are gotten, and off they roll. So really, they are delta machines, not absolute machines.
alfadriver (Forum Supporter) said:
Some of that 30% is in the calibration of the system. Production based dynos are checked daily, and worked on for two solid weeks twice a year. Given how they measure, it's not hard to get them to drift, and stay pretty drifted off reality.
It depends a bunch on the dyno and how it works. Dynos based on load cells or eddy currents have lots of components in them in which the calibration can drift. Interial dynos (like a dynojet) work by measuring the rotational speed of a drum of known mass, and that's something that is very simple to do very accurately. This is why a dynojets tend to be much more consistent dyno-to-dyno than other types, and why sanctioning bodies like NASA will use dynojet plots for power-to-weight classes.
In reply to alfadriver (Forum Supporter) :
Yeah, I remember having a dyno conversation back in the day--might have been Ed--about the units of measure. Don't get wrapped up on the actual units; instead focus on the gains made.
My Miata makes 112 horsepower at the wheels--or, rather, it did when we tuned it 20 years ago. Is that a lot? Doesn't sound impressive until you look at our starting point: just 91 horsepower.
