Why do you only dyno in 3rd/4th gear?

KBCobra95 · Oct 21, 2005

Why are there only dynos is 3rd/4th gear? What would happen if you dyno'd in first gear? Wouldn't first gear be your main gear in drag racing? I think it would be pretty cool to know what you would dyno in all of your gears to see which one make the most power. Just had a thought, and thought I would share.

WhiteDevil · Oct 21, 2005

KBCobra95 said:
Why are there only dynos is 3rd/4th gear? What would happen if you dyno'd in first gear? Wouldn't first gear be your main gear in drag racing? I think it would be pretty cool to know what you would dyno in all of your gears to see which one make the most power. Just had a thought, and thought I would share.

Different gears dont make different power. 1st gear feels fast because on your T5 its a 3.35 ratio which is very steep to get the car rolling. The reason that they dyno cars in 4th gear is because 4th gear is a 1:1 gear ratio, its for hp calculation reasons.

bimmertech · Oct 21, 2005

really the only time it will make a difference is in a turbo car due to the way a taller gear will accel the engine slower(loading) allowing boost to be achieved. as far as an n/a motor goes, it should not matter. same principle if you go dyno then put a diff gear in the rear end, the numbers should not change.

mo_dingo · Oct 21, 2005

They have to calculate your torque to the rear wheels, and because 4th gear is 1:1, they don't need to calculate the trans gear into the mix.

All they have to do is take into account the rear end gears and tire size, all of which effect rear wheel torque.

Essentially, the dyno reads rear wheel torque, which goes [engine torque * trans gear ratio * rear end ratio * tire ratio = rear wheel torque].

So you will get something in the thousands of ft-lbs of torque with that calc. Then you plug in the known variables (trans, tire, rear wheel torque) which will isolate engine torque, which is not known. So once you have engine torque, you multiply that by .85 or .80, depending if you have a stick or auto, respectively, and that is the torque that the engine alone applies to the rear wheels.
Scott

KBCobra95 · Oct 21, 2005

It'd still be cool to know what you dyno'd in first. I dunno.

bimmertech · Oct 21, 2005

mo_dingo said:
They have to calculate your torque to the rear wheels, and because 4th gear is 1:1, they don't need to calculate the trans gear into the mix.

All they have to do is take into account the rear end gears and tire size, all of which effect rear wheel torque.

not really bustin your balls or anything, but in all my experiences with a chassis dyno the operator has never entered rear end ratio. what about trannies where 4th gear is not 1:1 like an M3?

94-302-vert · Oct 22, 2005

I think it is doen to slow down the accleration of the tires so the dyno has time to take more data.

zenboy99 · Oct 22, 2005

FYI, I have 4 dyno graphs of 3rd and 4th gear pulls. There was a 20rwhp difference between the two. These pulls were done back to back with all variables being equal. If you ever look inside a T-5 you can see that in 4th gear there's less gear surface area then in 3rd, less friction equals more power.

final5-0 · Oct 22, 2005

You could not make a pull in first gear as the tires would not hold.

Grady

Rootus · Oct 22, 2005

Dynos do not measure torque -- they measure horsepower, and calculate torque from that. Trying to measure torque would require more knowledge about the transmission and final drive ratios.

On cars with ratios of exactly 1:1, like Mustangs, that will be the gear that makes the most power because it doesn't transfer power through the countershaft, it just connects the input and main shaft together.

I also expect that the other reason for using a taller gear is to add more time to the pull, to increase the number of samples taken. This really only applies to an accelerometer dyno like a Dynojet, a Mustang dyno controls the rate of acceleration itself.

Dave

Why do you only dyno in 3rd/4th gear?

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