dynoing with 3rd gear?

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68corvette
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dynoing with 3rd gear?

Post by 68corvette » Mon Jan 30, 2006 1:24 pm

I have problem that my diesel (1.9L TDi) is slipping with 4th (straight) gear.. i would like to dyno it to know how much there is actually torque.

I have dynoed it earlier in same dyno 143hp/320nm and after that i installed a new clutch that should hold up to about 420nm.

can i compare to my earlier dyno if i get slip with 4th and had to dyno it with 3rd?
how much there could epproximately be error?

it's inertia dyno.

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Post by jacksoni » Mon Jan 30, 2006 9:53 pm

As usual posting here I have to preface with " I know nothing" and have to avoid sounding like I do. In this case will quote someone else and see if that helps.
Per Ben Strader in his book "building and tuning high performance electronic fuel injection."
Bottom line, makes no difference what gear or tire size (not counting rolling resistance/gear efficiency etc)

He says(and is addressing tire size and/or gear ratio):
"Since torque is the amount of work we can do and power is the amount of work in a period of time ot ,it makes sense that gear rations will cancel out any effects of power changes to the wheels because as we change the amount of torque multiplication to the tires we also change the tire speed by teh same amount.
For example if we can produce 100 ft-lbs of torque in one rotation of the engine and we have a 4:1 gear ratio, then the tire would have tuned 1/4 of a turn and produced 400 ft lbs.
Now if we have the same scenario but with a 3:1 gear ration we would only have generated 300 ft lbs at the wheel but the tires would only have traveled 1/3 of a rotation. So if we had a tire with a 100 inch circumference we would have traveled 25 inche with the first tire(gear) and 33 inches with the second.
In each case the amount of force available form the engine was the same so over the period of time we have (1 revolution) the same amount of actual work was done. A taller tire will take more effort to move but will move farther in on revolution than the same effort applied to a smaller tire. Thus the tire size acts as another gear ratio.
Bottom line is this: Tire sizes and gear ratios will not affect horespower readings at the wheels."

Sorry for my typing.

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Post by backpurge » Sat Feb 04, 2006 10:49 am

That is untrue. We have a Mustang Chassis Dyno (MD250-P) and have done multiple runs on the same car in different gears. It does make a difference. Keep this in mind...

HP = torque*distance/time

So if in a given gear you can accelerate over a given distance faster in one gear versus another, the dyno will show that it has more horsepower in one gear versus another. Usually it's not proportional (1st gear 100ft/3sec, 2nd gear 200ft/6sec) because of the rotational output of power (not linear). At least that's what we've seen on our dyno.
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Post by craig » Sat Feb 04, 2006 6:07 pm

The text quoted on gearing is correct. There is a small difference on an inertia dyno.

Yes, the effect of flywheel/moment of drive train inertia get slightly lower in higher gears, but mostly the change in number of gear interfaces will affect it to a larger degree. Expect 2% loss per gear interface. So if 3rd uses more directional changes in power transmission or more interfaces you will see that loss as lower power at the tires.

At steady state on an eddy current (or water brake, or AC) dyno, the losses due to inertia are so minimal as to be almost non existant. In such a case the only thing that will change the torque/hp reading from gear to gear is the gear interface and any additional drag brought on by the additional rotation of a shaft and bearings.

In summary the delta between 3rd and 4th gear testing will be smaller on a braked dyno than an inertia dyno. I have verified this with my Stuska, Dynojet and Dynostar dynos.

Please excuse my typos - my 3 year old is helping me!

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Post by rskrause » Wed Feb 08, 2006 6:53 am

craig wrote:The text quoted on gearing is correct. There is a small difference on an inertia dyno.

Yes, the effect of flywheel/moment of drive train inertia get slightly lower in higher gears, but mostly the change in number of gear interfaces will affect it to a larger degree. Expect 2% loss per gear interface. So if 3rd uses more directional changes in power transmission or more interfaces you will see that loss as lower power at the tires.

At steady state on an eddy current (or water brake, or AC) dyno, the losses due to inertia are so minimal as to be almost non existant. In such a case the only thing that will change the torque/hp reading from gear to gear is the gear interface and any additional drag brought on by the additional rotation of a shaft and bearings.

In summary the delta between 3rd and 4th gear testing will be smaller on a braked dyno than an inertia dyno. I have verified this with my Stuska, Dynojet and Dynostar dynos.

Please excuse my typos - my 3 year old is helping me!
I think this is exactly correct.

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Post by BillyShope » Sat Feb 25, 2006 12:48 pm

The inertia dyno doesn't know if it's a 4th gear car or a 3rd gear car with a lower numerical axle ratio. So, the only difference in readings will be due to a transmission efficiency difference.

There is an inherent error in inertia dynos since the engine is accelerating its own rotating inertia AND the dyno flywheel rotating inertia, but, so long as the dyno inertia is very large compared to the engine inertia, it's something we can live with. I would assume the dyno software uses a "typical" value of engine inertia in its calculations.

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Post by MadBill » Sun Feb 26, 2006 10:58 pm

The above posts cover the inertia effects, but if you mean your clutch was slipping before in 4th gear, the HP results will be low, due to the power lost to heat in the clutch during slip.
Unless the new one operates without slippage, there is no point going on the dyno.
(I can't imagine the tires slipping on a 1.9 L., but if they were, 3rd gear would of course be worse.)
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