Nissan 370Z Forum - View Single Post - Dyno pull in 4th any equation to tell how much it would've made in the correct gear?

ChipsWithDips · 07-09-2013, 02:06 PM

In general higher gear will show a bit higher horsepower number, but I don't think there's any equation you can plug in to convert. The dyno doesn't really care what gear ration you are in in terms of the power it calculates. All it does is calculates the instantaneous acceleration of the drum, and since it knows the mass of the drum and its rotational inertia, it can calcuate a power number based on that.

The reason there are discrepancies between power output levels in different gears is not that the dyno requires 1:1 gear ratio(it really doesn't give a ****), but because of the rotational inertia in your drivetrain.

Think about your flywheel spinning. If you go to redline in first gear, that flywheel is going from say 1k RPM to 7.5k rpm in a couple seconds. Accelerating this flywheel mass at a particular rate requires a certain amount of power. This is power that doesn't make it to the wheels, it instead gets put into the flywheels kinetic energy.

Now think about doing the same thing in 5th gear. The flywheel goes again from 1k to 7.5k rpm, but this time it takes a lot longer, maybe 5x as long(just a random guess as an example). The amount of parasitic loss from your flywheel in this case is much less because its rotational acceleration is now much lower, 1/5th what it was in first gear.

This is where the difference in power comes in. If you could accurately model all the interial losses in your drivetrain (you would need to know the rotational inertia of ever spinning piece along the way), then you could probably predict the difference fairly accurately, but knowing all that stuff is hard so its not really practical and varies with every individual vehicle configuration.

07-09-2013, 02:06 PM	#3 (permalink)
ChipsWithDips Enthusiast Member Join Date: Mar 2011 Location: Austin, TX Posts: 279 Drives: '11 Gunmetal 370Z MT Rep Power: 14	In general higher gear will show a bit higher horsepower number, but I don't think there's any equation you can plug in to convert. The dyno doesn't really care what gear ration you are in in terms of the power it calculates. All it does is calculates the instantaneous acceleration of the drum, and since it knows the mass of the drum and its rotational inertia, it can calcuate a power number based on that. The reason there are discrepancies between power output levels in different gears is not that the dyno requires 1:1 gear ratio(it really doesn't give a ****), but because of the rotational inertia in your drivetrain. Think about your flywheel spinning. If you go to redline in first gear, that flywheel is going from say 1k RPM to 7.5k rpm in a couple seconds. Accelerating this flywheel mass at a particular rate requires a certain amount of power. This is power that doesn't make it to the wheels, it instead gets put into the flywheels kinetic energy. Now think about doing the same thing in 5th gear. The flywheel goes again from 1k to 7.5k rpm, but this time it takes a lot longer, maybe 5x as long(just a random guess as an example). The amount of parasitic loss from your flywheel in this case is much less because its rotational acceleration is now much lower, 1/5th what it was in first gear. This is where the difference in power comes in. If you could accurately model all the interial losses in your drivetrain (you would need to know the rotational inertia of ever spinning piece along the way), then you could probably predict the difference fairly accurately, but knowing all that stuff is hard so its not really practical and varies with every individual vehicle configuration. __________________ PPE Headers SS/Coated/Wrapped \| Ebay y-pipe \| Custom 3" single exhaust w/ Magnaflows \| UpRev \| Racingbrake 2pc Rotors \| 40th calipers \| Eibach sways \| SPL endlinks, diff mounts \| RJM pedal \| Stillen Intakes, UDP, Flywheel \| M370 Manifold \| Z1 engine mounts, CSC-EK, Clutch \| Forgestar F14 18x10 \| Conti DWs \| Quaife diff/4.08 gears \| Nismo diff cover