Here's the skinny as I understand it.pitchblack23 said:
Dynoing in 4th gear is the preferred method, as 4th gear is the closest to a 1:1 ratio. Now with that said here are the plusses and minus to each as far as I know..
4th gear
- Should also provide HIGHER HP numbers than running in a lower gear.
- Better engine VE due to going through the RPM's slower, which means the valves stay open longer.
- Less parasitic losses from gearing due to going through the RPM's slower
- More heat generated, great chance for engine damange with sustained load.
- More time to conduct the run (if your doing a dyno day, 4th just takes longer)
3rd gear
- Worse engine VE due to going through the RPM's quicker.
- Greater parasitic losses from gearing due to going through RPM quicker
- Better Gearing multiplication for torque (but could still be lower thanks to the losses mentioned above)
- Less Heat built up during shorter runs, less danger.
- Dyno runs can be done quicker
The end result is, it doesn't matter as long as it's the same for each subsequent run, and that it's documented when you share your dyno so others are aware of the conditions.