Mark Young said:

Holy crap, in the time it took me to type my post the levee broke on this thread.

To highlight:

A dyno measures TORQUE, not hp -

Changing drive ratio, overall tire diameter, etc, changes the Torque reading at the wheels, and thus the dyno gives a different reading.

Its simple physics people.

Mark, hate to disagree with you but you're wrong on a number of counts here. JPH was correct that the inertial dyno (like a Dynojet) measures the rate of acceleration (change in drum speed between discreet time samplings) of the drum and back-calculates torque from it. Torque is a

**static** property that cannot be measured directly from a rotating device like a dyno drum. So although Hp seems

*unreal* to many of us and we dismiss it as nothing more than a derived entity, in actual fact the dyno

**only** measures power and reports torque as the *mathematical* afterthought.

Changing the drive ratio and tire diameter does have an affect on the

**force** that your tire applies to the ground and hence to the dyno drum but I will reiterate that the dyno

**does** factor this out in it's calculations of flywheel Hp and Tq. You said earlier that it couldn't be doing this because there's no way to enter drive ratio and tire diameter into the dyno program. My response is that you don't

*need* to enter that crap because the dyno has a much more simple and elegant way of factoring out the torque multiplication due to gearing and tire and it does so using the ratio of engine rpm to drum rpm. If your crank were hooked directly to the drum axle with no gearing induced mechanical advantage they would rotate at the same speed, right? If your motor rotates the drum at half the engine's rpm then there's a mechanical advantage (torque multiplier) of 2:1 at work

**somewhere** in the driveline, correct? The dyno doesn't give a shit whether that multiplier is due to tranny, diff and/or tire size. It simply knows to divide it's measured power/Tq numbers by 1/2 to get the flywheel values ... it's really a beautifully elegant system when you realize how it works.

As far as why we put the tranny in 4th (1:1) to run dyno pulls? It's not because 1:1 is necessary for the calculation to work, it's because in direct drive (1:1) you get less power loss through the clusters than you do when it's in some other gear. You could perfectly well do a pull in first gear or in 6th but the "rule of thumb" loss figures that we all apply would be wrong.