What gear to dyno in?
 

I'm seeing different people dyno'ing their cars in different gears, and was wondering what the "best" gear to test was. My thinking was this... in a higher gear, there is a different reduction ratio (gear ratio). That reduction changes torque characteristics, so I would think that cars would make different torque/power, but have similar shapes in all gears. Does the dyno back out actual numbers given gear ratios, or does it actually put the same amount of torque at 4k in 2nd as it does at 4k in 5th? My mechanical background is just fucking with my mind right now, so please step in and let me know.

4th gear is what is standard because it is a 1:1 gear..

-Zach

that makes sense to me, that way the torque applied is same as the driveline should see... (ie. TorquexGear=Torquex1) If it's anything else, then torque should technically change, since it's Fxd and the distance is changing (size of gear), a bigger reduction in speed would make an increase in torque (why cars pick up at lower RPM in low gear compared to high gear). But like I said, I'm kind of confused, so the 4th gear makes sense, unless you plug in 3rd gear ratio as x.xxx:1 and it backs out the 4th gear, direct-drive torque from that

You'd have to do that with a VW bug transaxle since their 4th is an overdrive, but third is still underdrive (I think).

alfa romeo has the same thing with underdriven 4th and overdriven 5th. a guy I work with says alot of cars use under 4th and over 5ths, but I've generally seen a direct drive for 5th. For some reason this is an issue that's bothered the hell out of me for a while. I'll be sure to find out the real answer though

I'm just glad that Mazda decided to use a 1.0 geared 4th.

chassis dyno measures acceleration of the drum of known inertia. from that, the applied torque at any surface speed of the drum is calculated (T=I x a). since rpm is also recorded, the the applied torque vs eng rpm is found, which allows the wheel hp curve to be generated.

don't need any gear input, but 3rd vs 4th curves will differ due to extra gear meshes, higher traction heat losses, etc.

I got 4 more horses out of my fc in 3rd

Said and done, if you have a 1:1 gear, you should use it. That is the gear to dyno in... whether its 3rd or 4th... its its 1:1, use it for accurate readings.

You will still get readings in different gears but they will not be the same, which has been pointed out by everyone so far, so just use your 1:1 gear.

Last time I dynoed, i had to use 3rd gear because I was having wastegate problems and I was getting over 15 psi of boost at 5000 rpm. Next run, I put it in 3rd to see how high I could take it up... I was only able to take it to 6800, but made 344 rwhp and 259 rwtq. Wish I could figure the math out to tell me what it would have been in a 1:1 gear....