Spac
For reasons, I have been researching a couple of different vehicles lately. One thing that I have looked at is power outputs.
I have quickly concluded that the vast majority of people don't have more than the barest understanding of how a dyno works, and that many of the remainder are deliberately confusing the issue to make their business and/or their car look better.
Here is my rant. I was fairly sober when I started typing, but less so now. Apologies if it makes less sense than I hoped.
None of this is to pretend that I am an expert, but shit always stinks like shit.
Part 1.
There are three basic places that power can be measured:
1. At the flywheel, on an engine dyno.
2. At the hubs (wheels removed) on a hub dyno.
3. At the tyre tread on a rolling road dyno.
The raw output changes in each circumstance. Obviously an engine dyno does not worry about losses through the gearbox, diff or anything else - they are the ONLY accurate way to get a true power figure at the engine. They can't even try to guess at the losses through the driveline so they don't even bother trying.
At the other end of the scale, a rolling road dyno has to deal with the mass of the driveline and wheels, the losses through the gearbox, tailshaft, diff, half shafts and tyres.
This can create considerable differences in measured output from two identical motors - changing to big heavy wheels will reduce the measured power at the wheels. So will rally tyres compared to road tyres. Or fitting a 9" diff in place of a little stock diff. Etc.
But the bright side is that they measure what is actually happening at the wheels. Having a motor that makes 400hp at the flywheel means little if the driveline soaks up half of it.
A hub dyno avoids the last step - anyone who hears a car on a rolling road dyno will know that there must be some power loss at the tyres.
So... the raw numbers from each type of dyno will be different. If you want to be pedantic, there are even differences between the diameter of the roller, or if it is a single roller per wheel vs two rollers.
Part 2.
There are two main types of dynos in use in 2018.
The first type is an Inertia Dyno. This uses a large rotational mass and calculates a power output based on how quickly the mass can be accelerated.
This is the most direct way of measuring power, but is less useful for holding an engine at a particular RPM/load for tuning.
The newer type is an eddy current or electric brake dyno. These are basically a big alternator that convert mechanical energy into electricity.
They have the distinct advantage that they can hold the motor at a set number of RPM, or at a set load - this is obviously a big help when tuning a motor.
An electric brake dyno actually measures torque and then calculates power.
Part 3.
Here's where it gets messy: Compensation Factors.
All of the above is straightforward enough but dumb, egotistical humans have to turn it into a shit fight.
The first complication is the only non-offensive one, but also where it all begins to unravel: we want consistency in the readings. Calibration.
In the first instance, this means that dyno operators can compensate for changes in air temperature and density. This makes sense - no matter how healthy the motor is, it will always make less power at the top of a mountain on a 40* day, compared to sea level on a 12* day. And it is unfair to make the tuner take responsibility for those variables.
So the dyno makers have "calibration factors", which is good for everyone.
However, dyno operators quickly discover that they can manipulate the calibration factors to give a far better looking read out...
Imagine how tempting it is to bring a customer's car in, run a baseline figure and then change the calibration factor and get a dyno readout 25% higher without even having to touch the car. Your customer will love you and it costs very little of your time - it is easy money...
Of course this quickly spirals out of control, with BS figures and lots of snake oil.
Part 4.
Because people are dumb and egotistical, we ignore most of the above, and demand consistency between dyno types while despite the technical difficulties involved and the human element (dumb and egotistical).
This leads to hub and inertia dyno people trying to make their equipment give engine power figures.
The inertia dyno can do this with reasonable accuracy: after the power run, disengage the clutch and let the whole lot coast back down. This allows the dyno software to measure the inertia of the driveline and then calculate roughly how much power was lost in accelerating the driveline during the power run.
It cannot accurately account for mechanical losses.
Electric brake dynos do a terrible job of estimating driveline losses. They suffer the same inability to measure mechanical losses AND have no ability to measure driveline inertia.
This leads to useless, misinforming garbage like "Shootout mode" which demands a bunch of crude inputs about engine/driveline type, and then makes a bunch of equally crude assumptions to derive a horrible guesstimate of flywheel power.
In defence of shitout mode, it does lock the dyno into a set compensation factors which stop the operator from playing games between cars at dyno days.
What does it all mean?
1. If the dyno measures power at the tyres or at the hubs, but corrects for driveline losses, then you cannot say that the reading is "at the wheels".
In reality it is "someone's guess of flywheel power, loosely based on reality".
2. You can (still) manipulate dyno figures through a myriad of ways.
3. If we were brave and knowledgeable enough to understand and accept that different dyno types give different outputs, we would have better and more useful information from them.
4. Contrary to popular opinion, the power reading from the same car in different gears barely changes.
5. People suck because their egos and stupidity interfere with basic science.
6. If your car is going on a rolling road dyno, ask for the raw figures because they are the most honest indication of how fast it will be.
7. Our frineds in the USA have a particularly poor grasp of the above. Take anything from them with a grain of salt.
Rant over, time for another beer.
Happy to discuss further with anyone with experience. Happy to ridicule internet experts.
Spac
jamesinc;135468 wroteMost rolling dynos have adjustable rolling resistance don't they? Would that affect the readings?
Yes, the electric brake ones do.
The adjustment is used to hold the motor at a constant speed for tuning.
It is also used for significantly different motors during power runs. Obviously a load suitable for a 500kW drag car is going to be pretty useless on a 75kW car... And vice-versa.
The dyno software should give accurate results regardless. I am not aware of people manipulating this in modern dynos.
Vee_Que
A drag strip is a very good dyno overall, you can flywheel dyno an engine, then dyno tune it in car, and the track will tell you how accurate the car dyno is. A moresco(spelling?) scale is great for this.