Keeping in line worh the OT talk here?
The most common dyno in US is Dynojet. Its usually regarded as the standard and what others are compared to. It is also regarded as the most optimistic of them all. As a matter a fact some other brands have "dynojet correction factor" to be able to make them read similar values as the dynojets in the area and to satisfy the fat corvette drivers etc.
Almost all of them have some way the operator can affect the readings. Dynamics with the shootout mode is a good way of keeping the operator out of the results.
Small roller vs big roller and inertia vs braked is another common battle. Both methods have their pros and cons. Inertia is accurate and simple way of measuring the power. You cant tune steady state or part load areas with just inertia so for tuning stand alone ecus they are far from optimal. This is where small roller low inertia braked dynos realy shine. You can select a rpm or speed and the dyno keeps the engine at that point no matter if the go fast pedal is at 10% or 100%.
So much crap being thrown around in this thread, calling different manufacturers of dynamometers good/bad. 95% of accuracy comes from the operator understanding the dyno and how it works... Near enough every time I see a dyno being operated, I near enough cry. The complete lack of understanding of how dynos work by operators is just ridiculous. Go watch some videos on YT of people operating dynos - wheelspin, stupid fast ramp rates, hitting the brakes as soon as the run is over, leaving the engine in gear at the end of the run, cars jumping around on the rollers... Then you see some of the graphs which are put out, which at best have the bit showing a complete cockup by the operator cut out, or some of them with the coastdown left in which has lumps and bumps in - but nobody questions it because "it was calibrated last week" and "it was $30k"...
The amount of times I hear quotes from operators such as "If you're getting traction issues, use a faster ramp rate" and from a reputable dyno installer I even heard this gem: "An inertia only run should be around 15% higher than a braked run"... Just ***.
It appears that VERY few dynamometers have the moment of inertia configured correctly in the software, I've seen this time and again that there's a huge lump in the graph where the absorber cuts in to hold the ramp rate when the engine starts making enough power, but it's just completely covered up by the fact the smoothing has been cranked to 11. In my opinion, a braked vs inertia run on any dyno with braking should yield near identical results, the only case it may not is a heavily turbocharged car on a set of rollers with a very small rotating mass. It should make no difference if the rollers are large or small, heavy or light - the configuration of the moment of inertia in software is what allows it to know how much energy was put into the rollers to accelerate the rollers when the brake was not absorbing the energy, rather the energy was put into accelerating a known mass.
A braked dynamometer is at best a "guess" to calculate the amount of power transferred from the engine via the transmission and tyres into the rollers, minus some of the energy lost in the transfer of that power. You can't measure all of it, it's simply not possible. It is absolutely NOT flywheel horsepower, it's at best a poor estimate of that.
An inertia dyno again is at best a "guess" to calculate the amount of power transferred from the engine, via the transmission and tyres into the rollers, not taking into account any amount of energy lost in that process - since you cannot measure that. It is NOT wheel horsepower, since the pressure your tyres are at make a huge difference, the surface of the dyno, the temperature, the compound...
The only dynos that can measure something approximating WHEEL horsepower are hub dynos, however, they still cannot measure exactly the losses involved in the transmission etc, they, again, can at best provide an approximation.
Unless you're going to rag the engine out and put it on an engine dyno, don't even bother have the argument, just smile and nod - because no amount of people whining xyz dyno is better than abc dyno is going to make any difference.