The Effect of Dynoing in Different Gears

Jordo! · 02-22-2011, 06:12 PM

Quote:

Originally Posted by 370Z Purist

Transmission gearing and "multipliers" are very complicated; the final gear has little to do mathematically with the rest of the gears. The final drive has more to do with the rotation after it's been through the transmission. If we're in 5th gear, the ratio is 1:1 in the motor/transmission connection... then if the final gear is 5 and the diff is 1, that means that for every one revolution of the engine, that's one revolution of the tranny gear, which is five revolutions of the final gear and the diff gear is 1, so that means the wheel turns a total of five times for every single turn of the engine.

Sorry, just building up my own logic there.

Of course, with the data you've given me, assuming that both the AT and MT cars have the same diff and diameter of tire, we can say that the AT car will produce less torque because it has a longer final drive, if RPM and gear (5th for 1:1).

TL;DR

AT has less torque when RPM, 5th gear, same diff and tires than an MT.

I'm probably wrong but at least we're thinking now.

Then, this would suggest that at least to keep everything scaled similarly we would want to dyno both in 4th.

Likewise, in terms of performance, both cars would be in 4th gear up to 125 MPH. By 5th gear, the differences in final gear ratio make their performance too dissimilar, and in any case due to wind resistance and power limitations, neither will achieve the top speeds possible based on gearing.

Put another way, the MT will have a big mechanical advantage in 5th gear over the AT in either 4th or 5th, whereas they will be more similar mechanicaly in 4th.

Thus, I think 4th gear might be preferable for both cars.

Im curious to see what the AT puts down in 5th gear. Power values won't be affected as much, at least not on a dynojet, as power is computed based on the rate at which the drum is acellerated.

However, a mechanical advantage due to gearing will probably affect that to some extent, even if the greatest difference is in measured peak torque rather than peak power.

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