Dipstick

If my car had 4.11 gears and dyno'd at 300HP, what would it dyno at with 3.73 gears?

Do dyno operators use correction factors for gears?

I called three places that had dynos and got three different answers. Seems to me that with lower gears, the numbers would be higher without a correction factor.

WS-Sick

I'm not sure what the difference would be, but the steeper the gear the more hp lost.

Derek98z

iTrader: (55)

This is true, even though I can't tell you why

CTSmechanic

iTrader: (2)

fewer teeth on ring and pinion translate into less friction...i.e. a 6.50 is more efficent than a 4.11...

dynocar

Hold it, a 1.0:1 is the most efficient, a 2.0:1 is less, etc, etc, a 6.50 is less efficient then a 4.11, assuming we are discussing mechanical efficiency.

Visceral

iTrader: (1)

Guys, the number of teeth creating friction has little or nothing to do with it. Ive seen about 10hp lost with 4.10s over 3.42s.

When you acelerate on an inertia-type roller chassis dyno, you are accelerating a large drum. The rotational inertia of the drum is a known... so it can be determined how much power is being used to accelerate the dyno drum.

The problem with steeper gears is that the drum isnt the only thing being accelerated when you accelerate the dyno. You have the rotating assembly in the engine, the flywheel, the clutch parts, the driveshaft, etc... all behind the rear gears. They require energy to spin up. You can measure part of this by stepping all the way on the gas with the clutch in. Your engine applies all of its power to accelerate the rotating assembly of the engine and the flywheel. It takes 2 seconds or more for the engine to hit the rev limiter. Again... when you go for a pass on the dyno, not every last amount of energy produced by the engine then goes to the drum. Some of it accelerates just the engine, the transmission, the driveshaft, the heavy wheels, etc.

Higher gears raise this element of "power loss" by requiring that the engine accelerate faster for a given acceleration of the dyno drum.

A dyno racer would have 2.73s, almost no flywheel, a CF driveshaft, and very very light wheels... and would gain 20+ "horsepower" over a car without these "dyno tricks".

verbs

10rwhp loss here going from 3.42s to 4.10s. dyno proven

Orange Peel

iTrader: (20)

But an easy .1-.2 off the ET and 1mph+ gain in the 1/4 So that 10 rwhp is only dyno queen ****

RED98WS6

I can see how there would be a loss in horsepower, but wouldn't the torque curve be bumped up when going to 4.10's?

prostock_bigblock

iTrader: (11)

u got it man..... thats true

CTSmechanic

iTrader: (2)

Thats the way we "used "to think but were proven wrong in a variety of extremely controled tests none of which were done on a chassis dyno.Look at it this way how many teeth are on the pinion of a 3.42...? then look at how many teeth are on the pinion of a 6.50 the fewer times the ring and pinion contact each other translates into less friction.. were not talking about big numbers were talking 10ths of a percent maybe 2 or 3 in some situations but in my line of work if we find a solid 3 10ths that a big gain...

RedHardSupra

iTrader: (1)

i have two two dynos of near-stock 02 camaro here, done in 3rd and 4th gear. the difference is 7-15rwhp, depending on rpm. same car, dynos done few minutes away, with the only difference being a gear. and please don't tell me 'but that's different gears, not rear gear' as they do the same damn thing: change the rpms of the output shaft to the rpm of the wheels by a given ratio.

BSRMotorsports

The change in gearing shouldn't have any actual change in hp on an inertia dyno, since hp is calculated throught the difference in rpm vs. torque along with the acceleration of the drum (known). The changes in acceleration during testing is where the hp differance is found, the faster the acceleration of the engine the less accurate the readings. So if you make a run in 2nd vs. 4th, the 4th gear run will overall be more accurate, The same is true for engine dyno's, I have seen a differance of 10hp on a 300hp engine in an A-B-A test where the only change was acceleration speed.

Brad

CTSmechanic

iTrader: (2)

If you run any trans in anything but 1 to 1 you lose power ...

verbs

Agreed! I'd rather be able to put an extra buslength on you with the 4.10s than dyno 10rwhp more with 3.42s