The Effect of Dynoing in Different Gears

Originally Posted by 1slow370

6MT at 1500rpm in 4th = 319.66 wheel rpm
7AT at 1500rpm in 4th = 316.45 wheel rpm

370z purist, your thinking about final drives and multiplying the numbers all wrong, the final drive is the differential gearing so it's after the driveshaft.

Here is a link to stick to your favorites Top speed calculator

Originally Posted by Jordo!

Well, up to 125ish MPH, both the 6 and 7 speeds are in 4th gear, so I think that makes them more comparable than 5th to 5th.

Your point on the tire size and roll out distance is well taken too, but so long as everyone uses the OEM diameters, the difference will be negligible. Also, even if they are using different sizes, the effect will be further minimized in 4rth than 5th given the closer startng points.

In terms of determining changes to an individual car, it doesn't matter so long as you use the same gear each time, but I've now seen a few dynos where baselines were taken in 5th and post-modification dynos were in 4th, which led to all sorts of confusion.

Also, I was trying to figure out why all the data I saw pointed to a baseline wtq on a dynojet of around 225 whereas some people claimed a higher baseline of about 240 -- the answer, it seems, is that it depends on the gear used on the dyno.

Someone also recently started a thread asking about typical baseline torque, so clearly it's not just me/us who find this issue somewhat important

At minimum, it means people need to indicate the gear they used at the dyno when posting numbers (which some members have been good enough to do) in order to interpret their results.

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