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Old 08-31-2013, 08:04 PM   #1
Laminar
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Dyno runs with stock speed limiter

I believe that a stock 5-speed car will hit the speed limiter in 4th gear before running out of revs. When doing dyno runs on a car with the speed limiter still in place, would you run in 3rd gear to ensure data collection from the entire rev range?



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Old 08-31-2013, 09:08 PM   #2
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3rd has been the usual choice.

The only difference is slightly lower whp number due to not being direct drive, and a bit more mechanical advantage also lowering figures slightly.
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Old 09-01-2013, 02:17 PM   #3
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Before my tune all my dyno runs happened in 3rd gear.

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Old 09-01-2013, 08:30 PM   #4
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Quote:
Originally Posted by Slo86GT View Post
3rd has been the usual choice.

The only difference is slightly lower whp number due to not being direct drive
I'm not aware of any FWD transaxles that have a "direct drive." That happens in a RWD transmission when the input shaft goes straight to the output shaft skipping the countershaft, usually in 4th gear. The T5 does this:



The MTX75 has no countershaft, just input and output. No matter which gear you select, power is transferred from through both shafts to the differential.



Quote:
and a bit more mechanical advantage also lowering figures slightly
Assuming that the dyno isn't using a static RPM-to-wheel speed ratio, this isn't true. A Dynojet will measure the power produced, then use RPM and wheel speed to back calculate engine torque. Differences in dyno results between gears can usually be explained by picking static ratios, inertia of the transmission's gears, or heatsoak.
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Old 09-02-2013, 02:17 PM   #5
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Heatsoak would cause higher gear's numbers to be lower. So that works against the known rise in power in lower ratio'd gearing on a dyno.

Inertia of transmission's gears wouldn't cause over 2% difference in readings common to 3rd vs 4th gear pulls.

Picking static ratios? You'll have to explain this one to me. Do you mean in the software configuration for calculations, or selecting which gear to dyno in? Isn't that the whole question here?

I will agree many transmission don't have a direct drive, or a 1:1 ratio, which will always show more power than using mechanical advantage through the system.
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Old 09-02-2013, 08:53 PM   #6
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Quote:
Originally Posted by Slo86GT View Post
Heatsoak would cause higher gear's numbers to be lower. So that works against the known rise in power in lower ratio'd gearing on a dyno.
Not just during the run but between runs.

Quote:
Picking static ratios? You'll have to explain this one to me. Do you mean in the software configuration for calculations, or selecting which gear to dyno in? Isn't that the whole question here?
If a dyno doesn't use a direct RPM pickup, the dyno operator can input a wheel speed to RPM ratio and let the dyno back calculate engine RPM. This introduces error.

Quote:
I will agree many transmission don't have a direct drive, or a 1:1 ratio, which will always show more power than using mechanical advantage through the system.
This post and your previous post confused me. In what gear do you expect more power?
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Old 09-02-2013, 09:31 PM   #7
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The manual should be dynoed in 4th gear as this is a gear ratio of 1.03:1, or the closest ratio to 1:1 on the straight 3.82:1 axle ratio.

The DCT is a bit trickier due to the split final drive, because 4th gear is 1.02:1, but it's on the 4.28:1 axle ratio where as 5th is 0.87:1 on the 3.85:1 axle ratio.

The MMT6 in the ST has a similar dilemma as 1-4 are on the 4.063:1 axle ratio vs 5 and 6 on the 2.955 axle ratio. 4th is 1.029:1 x 4.063:1, but 5th is 1.129:1 x 2.955:1.

So without entering the specific ratios or at least an engine speed vs road speed relationship it's much trickier to dyno a DCT or ST.
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Old 09-03-2013, 06:47 AM   #8
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Quote:
Originally Posted by suss6052 View Post
The manual should be dynoed in 4th gear as this is a gear ratio of 1.03:1, or the closest ratio to 1:1 on the straight 3.82:1 axle ratio.
Why does that matter?
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Old 09-03-2013, 10:29 AM   #9
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Quote:
Originally Posted by Laminar View Post
Why does that matter?
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. There is also a greater frictional loss in gears with a higher torque multiplication such as a shorter gear.
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Old 09-03-2013, 04:36 PM   #10
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Quote:
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. There is also a greater frictional loss in gears with a higher torque multiplication such as a shorter gear.
^^ Which is what I was getting at from the beginning.
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