Originally Posted by Skeeterbop

Yeah, I could see how doing before and after dynos would skewer the results and make for bad comparisons. I guess i just assumed people would be smart enough to use the same gear for before and after. But yeah, I think the easiest thing to do would be to just post which gear the car was dyno'd in. I do have one assumption I figure you would be able to clear up since you have a lot of info on this. I'm assuming that for any given tranny, that the dynos done in the lower gear are producing more torque due to transmission ratios correct? Maybe not a huge difference but still noticeable right?

Originally Posted by DarkZide

I could be totally wrong here, but i think the 1:1 rule was started for a few reasons.

First, cars have all sorts of gear ratios, but what we are after is engine output. It's not feasible to put an engine on an engine dyno so we go after the reading that is closest to that, and that would be a 1:1 ratio output from the engine to the gearbox. Final drive is mostly irrelevant, as it won't necessarily affect peak power, but where you make peak power.

Second, disregarding overdrive, the 1:1 gear is usually towards the top of the gearseat without being in overdrive, so it takes a lot longer to go through that gear, which equals a longer amount of time for the dyno to take readings. Imagine trying to count a stack of money thats being put down bill by bill every second, then imagine trying to count a stack of money that someone just throws down all at once.

Originally Posted by Hot Rod Mag

Will different final drive ratios affect the dyno readings?

This one's tricky. First, there are potential discrepancies because different gears have different inertia values, generate more friction, and change the amount of tire slip. Higher numerical gears tend to be more inefficient, so as gear ratios increase numerically, power levels tend to slightly drop, particularly on an inertia dyno. When torque is multiplied by steeper gears, tire slippage also tends to increase.

However, there's another, often overlooked, factor in the brew: rpm and torque are inversely related to calculating horsepower, so changing the rear axle ratio or testing in other than a 1:1 transmission gear seemingly shouldn't change the horsepower numbers. But this doesn't take into consideration the fact that changing gear ratios changes the engine's rate of acceleration. For example: We know that on an engine dyno, if you change a sweep test's acceleration rate from, say, 300 rpm/second to 600 rpm/second, the flywheel power number (bhp) drops due to the faster rate of acceleration. As an engine accelerates at a higher rate, the power required to accelerate the engine increases, and a greater portion is consumed before it gets to the flywheel. Going to numerically higher gear ratios-whether in the trans (testing in a lower gear) or in the rearend-is like increasing the rate of acceleration in a sweep test. Whether this actually changes a given chassis dyno's reported results depends on how the specific dyno manufacturer does its math. For the most consistent results, always test in the same trans gear (generally 1:1) and rebaseline the vehicle after a rear-axle ratio change.

Originally Posted by C&D"

the 1:1 gear ratio with the least friction, [is] preferred by dyno testers