Rolling road dyno, worth doing?

Francis, are you sure about that figure? Given the standard 15% deduction for powertrain loss, that means your car was making c. 280hp at the flywheel.....seems like a lot for an engine that's had nothing more done to it than a compression bump. Was there maybe some head work, different cams, etc. involved as well?
- Dave

 

Could also just be Dyno differences. We would constantly get comments from customers saying something like, "your dyno shows 400 HP but the shop that built the engine showed 420HP!" The type of dyno, the settings for the dyno (which can be "adjusted" to produce the results you WANT to produce!), and the atmospheric conditions on any given day can change the engine's HP readings. Inertial dynos produce different results than "real" dynos.

A rant follows:
A chassis dyno can produce different figures depending on the gearing/gear selected for the run, the weather, and the settings on the dyno. YES, if a shop wanted to sell you something, they could do the dyno run/produce a chart, tell you that if you add their "whatever" upgrade you will get more power and, even if no additional power is produced, a change in the dyno settings could produce a chart that shows it is! Chassis dyno info is only "accurate" if viewed as a relative thing and only when the SAME dyno/same settings is used. IOW, if the same dyno (same settings) shows 300 HP before and 350HP after, it's reasonable to say there is around 50 more HP. But it is NOT ACCURATE to say that the engine is producing 350HP because chassis dynos base their output on a series of settings/assumptions, including things like drivetrain loss. That loss varies based on the drivetrain configuration and the losses also change in different gears. Engine dynos are the way to go but, of course, most people are not going to pull an engine to do a dyno run!! Engine dynos are usually found in dedicated engine-building shops.

IOW, the only way a chassis dyno is really useful in a one-time test is if several of the same car models are tested that day. If two 328's for example, are tested in succession on the same dyno and one shows 237HP and the other 225, I'd say that the first one IS producing about 12 more HP. If the same two cars had been tested in two different cities with the same HP results I wouldn't be comfortable using the figures to compare the cars.