Quote:

Gears multiply torque. Torque x RPM / 5252 = Horsepower.

The dyno measures torque. The gear, as well as multiplying torque, is dividing RPM by the same amount. So horsepower will remain the same no matter what the gear.

I've always assumed that since the dyno measures the torque and it knows the rpm of the tires it just takes the torque measurement and then reduced it to a 1:1 ratio.

Remember that even in direct drive the differential is multiplying torque and the height of the tires is dividing the torque. I'd be interested to hear from someone who knows for sure how the dyno is figuring it out.



Chassis dynos are good for comparing changes while tuning. The horsepower numbers they give are usually not very accurate,but are useful if you have a good dyno and operator to keep things repeatable.
Different dyno manufacturers have different ways of "figuring" what the numbers are. I have some pretty good paperwork comparing some of them. There is one brand of chassis dyno (would have to look up which one) that came up with a formula by back figuring based on what a vehichles manufacturer claimed the flywheel horsepower was.
If you ask some of them how they calculate things,some will tell you and others will tell you it is proprietary.
There are some that use a generic percentage for drivetrain losses too. We know some vehichles are more efficient then others though.
Back to the OP's questions though,tire and gear changes can change the numbers the dyno shows. The engine makes the same power. You can change tire pressure,or tighten the tie downs down more and see a difference due to effective gear ratio.
Get a good operator,use it for tuning changes,and you will be fine.
If you want real flywheel numbers,stick to an engine dyno.

Keith

www.dunnuckracing.com