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Old 09-03-2013, 07:45 PM   #11
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Originally Posted by suss6052 View Post
For accuracy you want to minimize the effect of gearing on the power results, and fourth gear is the closest the mtx75 gets to this ratio.
But the dyno measures power, not torque, so gearing should be irrelevant, right?

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Old 09-03-2013, 08:18 PM   #12
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But the dyno measures power, not torque, so gearing should be irrelevant, right?
Different gearing leads to different mechanical advantage. This is why your car pulls harder in 1st than 5th. It's the same reason that you want to maintain as close to a 1:1 ratio for dyno pulls.
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Old 09-03-2013, 08:26 PM   #13
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Quote:
Originally Posted by Laminar View Post
But the dyno measures power, not torque, so gearing should be irrelevant, right?
Quote:
Originally Posted by dyn085 View Post
Different gearing leads to different mechanical advantage. This is why your car pulls harder in 1st than 5th. It's the same reason that you want to maintain as close to a 1:1 ratio for dyno pulls.
This is what we've been trying to explain. You want the least mechanical advantage possible in order to get the most accurate power measurement at the wheels. The car would pull harder and hit redline sooner in the lower gears, but that's not beneficial to getting accurate power figures.

Power = Torque*Engine RPM / 5252, therefore anything that increases the torque multiplication will shift the power curve around without actually making the car more powerful in the real world.

Hence why you want a 1:1 ratio or as close to it as possible, which is easy enough for the MTX-75, but a lot trickier for the DCT and The MMT6 due to their split final drive ratios.

Ideally you'd also dyno the DCT in 4th as well as this is the closest ratio to 1:1, although the actual axle ratio is much shorter than it is in 1,2,5, and 6th gear with the 3.85:1 axle ratio on those gears.

Although with the MMT6 in the ST 4th is on a 4.063:1 axle ratio vs the 2.955:1 axle ratio in 5th, which gives a lower effective gear ratio despite 5th being shorter than 4th gear.
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Old 09-03-2013, 08:29 PM   #14
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Different gearing leads to different mechanical advantage. This is why your car pulls harder in 1st than 5th. It's the same reason that you want to maintain as close to a 1:1 ratio for dyno pulls.
Yes - different gearing leads to torque multiplication, but constant power. Because the dyno measures power, its measurement is independent of the torque multiplication. Or am I wrong?
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Old 09-03-2013, 08:36 PM   #15
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Yes - different gearing leads to torque multiplication, but constant power. Because the dyno measures power, its measurement is independent of the torque multiplication. Or am I wrong?
Greater driveline losses in gears other than direct drive / 1:1 also can skew the results.

It also depends on the dyno calibration. It's not always necessary to dyno at 1:1 or similar, but without the proper wheel speed and engine speed measurements your results would be inaccurate.
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Old 09-03-2013, 08:45 PM   #16
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Greater driveline losses in gears other than direct drive / 1:1 also can skew the results.
Are you using "direct drive" as a synonym for a 1:1 gear ratio? Or was the slash supposed to be an "or"?

This sentiment has been repeated several times in this thread, but I'm not convinced of any reason behind the sentiment. And I'm not sure you guys have agreed on whether dyno-ing in 3rd will show more or less power than 4th.
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Old 09-03-2013, 08:55 PM   #17
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Are you using "direct drive" as a synonym for a 1:1 gear ratio? Or was the slash supposed to be an "or"?

This sentiment has been repeated several times in this thread, but I'm not convinced of any reason behind the sentiment. And I'm not sure you guys have agreed on whether dyno-ing in 3rd will show more or less power than 4th.
Direct drive is a 1:1 ratio.

It's been common to dyno in as close a gear as possible to this ratio.

The raw power might be similar in the end, but depending on how the dyno measures load/ power/ acceleration it's less accurate a representation of the actual engine torque.

Quote:
If you use a gearbox to achieve a different rotational rate at the wheel as compared to the engine, the torque at the wheel will not be the same as the torque at the engine crankshaft, even if there are no frictional losses. The reason that this is so is that the rate at which work is done must be constant throughout the system (excepting only for the loss of energy within the system due to friction), and because power is the product of the torque and the angular velocity.

Analogies between mechanical systems and electrical systems are often insightful, so I'll ask you to indulge me for a moment while I digress and talk about electrical transformers. The voltage transformer is the electrical equivalent of the mechanical gearbox. A voltage transformer (which, by the way, only works for alternating current) takes advantage of the inductive coupling between the primary and secondary windings to transform the voltage. In electricity, power is the product of the voltage and the current. Because the rate at which work is performed must be constant throughout the system, the product of the voltage and the current in the secondary must be the same as the product of the voltage and the current in the primary ? at least in the case of an ideal transformer where there is no loss to heat. If the voltage is reduced or increased by a factor of N, the current will be inversely changed in that same proportion.

Okay, back to mechanics. Because power is the product of the torque and the rotational speed, and because the power must be constant throughout the system, the transmission increases the torque in the same proportion by which it reduces the rotational speed. If you measure the torque at the rear wheel, you have to use the overall reduction ratio to calculate the engine torque. At a given engine speed, the rear-wheel torque will depend on what gear is selected. Any dynamometer chart that shows torque must be engine torque or else it would be applicable only to a specific gear and the chart would have to specify the gear, which would not be especially useful. Note, however, that if the torque is measured at the rear wheel and then the overall reduction ratio is used to calculate the engine torque, the result will not be the same as the result that you would get if you rigged the dynamometer directly to the crankshaft, because the measured rear-wheel torque is subject to power train losses. Nevertheless, any dynamometer chart that shows torque and that does not specify the gear is most definitely the engine torque, albeit adjusted for drive train losses.
http://www.bimmerboost.com/showthrea...earing-matters
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