I've been told that the RRds that use the rundown to calculate trans. losses are more accurate.
The tuning industry thinks otherwise (depending on dyno make/model) as the coastdown method doesnt actually measure a transmission loss. Some measure the loss as a percentage and applies this to guess at a flywheel power.
Take a car with 100hp at the wheels. Dyno measures a loss of 20hp during coastdown and applies this as a percentage so 20%, 120hp at flywheel. Add a turbo and nitrous and it makes 200hp at the wheels and apply the same 20% coastdown losses = 40hp to be added. Thats 20hp for free. Modern coastdown dynos use other factors but older ones use the above as a measurement factor.
Then look at what coastdowns measure, its not actually a transmission loss. The coastdown is often done in neutral, and sometimes in gear, either way its measuring a loss when the engine has no load on it, power must be tested while the engibe is under load, so this also applies to the losses. it cant measure losses through the tyres as during coastdown as the tyres are not having any force applied to them as they do under load during the run.
The day that Europe falls in line with Australia and the USA and uses wheel power will be the end of the dyno lottery.
I have had a car (380hp) on our dyno that has been on 5-6 other dynos and the actual wheel power has been within 10hp on all, and within 1 hp on another dyno dynamics. The estimated flywheel power has been +/- 15hp on other dynos and it was +56hp on the Dastek.
When comparing power figures, whp is the most accurate method.
