Dyno pull in 4th any equation to tell how much it would've made in the correct gear?

DEpointfive0

So, since my 7AT car has no tune, my 5th gear run was cut short.
BUT, in 4th gear my car made 321.01WHP on a DynoJet, now, I don't know how, but is it possible to mathematically convert that using the gear ratios or whatnot into a number that would show what the car WOULD have made in 5th, the 1:1 gear ratio?


Thanks!

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Rob@TSM

There is not formula that can predict what you would've made. Dyno measure horsepower with respect to the calculated gear ratio. Meaning you make virtually the same power if run in 3rd, 4th, or 5th. However you will make slightly more in the 1:1 gear since it is a much more accurate measure of rwhp since the pull takes longer, it effectively creates more resolution in the graph.

You probably would've made 4-5rwhp more in 5th, assuming the tune was safe. Alot of competitors will tune in a lower gear, and when the car is later dyno'ed in the 1:1 it will knock because of the extra load/time spent under load.

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ChipsWithDips

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.

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ChipsWithDips

Another situation one would see even bigger power difference in different gears is with turbocharged vehicles, but this is because in lower gears there is not always enough load to get the turbo spinning fast enough to get to peak boost. In higher gears it has plenty of time to get there and stabilize.

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DEpointfive0

Awesome responses guys, thanks for the wealth of info!

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DEpointfive0

The original reason I asked is because I hit the speed limiter.
And often times when it's someone's first time to a dyno and they come back with a TERRIBLE number like 228WHP the first thing asked is "what gear did you run on the dyno?" And the answer will be "3rd" and there's the general consensus that running in 3rd was the ONLY culprit in why the numbers were so low

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