Originally Posted by ITAC Ops Manual
Known Horsepower
In this method, the horsepower number published by the manufacturer is ignored, and instead, the expected horsepower comes from another source. These days it is pretty common to have dynamometer results, usually from a chassis dyno. For chassis dynos, the ITAC standard is that RWD cars have an 18% drivetrain loss (from crank to wheels) and FWD cars have a 15% loss. Thus, for example, if a RWD car is “known” to make 150 horsepower on a chassis dyno, then the “expected horsepower” for power-to-weight calculation should be 150/.82, which is 183. Care should be taken that the highest dyno numbers seen are used, and that cars classed by this method should be at the limit of the IT rules.
In order to use this method, an ITAC member should put together a set of documentation that proves what the maximum known horsepower is. Then each member should say how confident he/she is in the documentation, considering the source, how well-prepared the measured cars are, etc. If the average confidence is 75% or greater, then the “known horsepower” method may be used to derive the expected horsepower.
A 75% confidence level is a pretty high level. It is expected that the committee would bring with them a healthy amount of skepticism when it comes to reviewing dyno sheets. One should take into account the source of the info, the shop that ran the dyno, information about how well-prepared the car and the engine car, who did the build, and any hidden motives of anyone involved in providing the data. In the best of circumstances, dyno results are hard to trust as they can be extremely variable even when all conditions are the same. It is reasonable to look at on-track results of the cars in question to see if the perceived acceleration matches what the dyno sheets appear to say.
All dyno sheets should be posted to the thread on the letter-tracking site that is associated with the letter under discussion. That way all information associated with the decision is saved and findable. The letter number is known by the author and also published in Fastrack. Any CRB or ITAC member can then find that letter number in the system and find all dyno sheets associated with its weight decision.