I've always been under the impression (at least for an M6) that a numerically higher gear ratio will increase the rwtq and a numerically lower ratio will decrease the rwtq for a given rpm. The idea is based on this forumla:

(engine torque * transmission ratio * rear-end ratio) - rotational loss = rwtq

We just dynoed my brothers car, which is an A4 and had a problem dynoing in 3rd because we hit the speed limiter at about 116. Instead, we tried dynoing in 2nd. What suprised me is that 3rd gear dynoed about 20 rwtq higher than 2nd!!

This makes no sense to me - the whole point of having a higher gear ratio is to make more tq than a lower ratio at the same rpm. What is it about an automatic that causes the opposite to be true??

FYI - The car has a 2300 stall and was running with converter unlocked. Can anyone explain this to me?

 

Well, I've still never seen rock-solid "proof" of this, but lots of guys claim that lower gear ratios "kill" dyno #'s. This may be true, maybe not? But assuming that it IS true, that lower gear ratios kill dyno #'s, then by running in 2nd, you're in a lower gear ratio than 3rd, thus, "killed" ~20 rwtq from your first dyno run??

Here's something interesting to consider too from my car's runs on the dyno:

A4 w/ 4.10 gears - 289 rwhp, 307 rwtq
M6 w/ 3.23 gears - 317 rwhp, 335 rwtq

That's an increase of 28 rwhp!! Now since I have a lid and a cat-back, let's "assume" my motor is making ~360 HP (instead of stock 350 HP).

317 / 360 = 12% drivetrain loss.
289 / 360 = 20% drivetrain loss.

Now was it just the automatic tranny that soaked up that extra ~8% power? Or did the low 4.10 gears have anything to do with it??

I dunno, maybe the low ratio = low dyno #'s is true, maybe not??

 

Higher (numerically) gear ratios take more power.
Autos take more power. Can't explain why 2nd would be different, but perhaps the timing was different due to intake/ coolant temp variations.

 

It dynos a bit higher because of the load on the motor. 3rd gear has a higher load on the motor so it produces a bit more power. Doesn't matter because 2nd will always pull harder then 3rd.