jovette

What is the relationship of differential gear ratio to dyno hp and torque results? With ALL other factors remaining the same, what is the difference between a C5, A4 with a 2.73 versus a 3.42 gear set, if the hp and torque #s were, for example 300hp and 330 torque with a 3.15 gear?

Thanks!

Mike04

My tuner said that he sees a drop of about 10rwhp going from 2.73s to 3.42s

vettenuts

I have wondered this as well, I am going from 3.42's to 3.90's on a MN6.

NEPTUNEBILL

It would seem that about 10rwhp and 5tq is the average reported on rearend changes--I still think that since dynos work with a final drive number of a 1 to 1 ratio in forth gear that it has to do with screwing up that ratio, you can find many discussions on this over the years that have very tech answers from some of our engineers--

BarneyZ

Do you mean that RWHP is understated on the dyno when changing to a numerically higher R/E or is it really really decreased? Former makes some sense to me, the latter does not.

Darkness

Shorter gears i.e. 4.10's - 3.90's show less power on the dyno not because its screws up the reading or anything, its just that a bigger ring takes more power to spin due to greater friction and mass. Therefore taller gears, i.e. 3.15's 'rob' less power, although the mechanical advantage shorter gears provide, highly overcome any losses in wheel power.

BarneyZ

Never heard of that but it would explain why my car pulled less than I thought it should. I added Callaway, Dynatechs, Corsas, dyno tune and DTE 3.90's to my '04 coupe and pulled 352 RWHP and 364 RWT. I thought it should have been around 360/370. Mod and learn.

Dan_the_C5_Man

The assumption that a numerically higher ratio has additional parasitic loss to dyno 15HP lower (as compared to a stock, 3.42 ratio car) is completely and utterly false. I’m not saying that the energy required to spin a 3.90 gear set vs. a 3.42 is identical (how can they be?), but it’s a fraction of the HP differences we see when compared to a 3.42 car.

It has to do with the way an inertial dyno (i.e. DynoJet) measure HP (accelerating a known mass in X-amount of time).

Dyno's have no clue as to what gear ratios are involved during a run, and no compensation factors are involved.

The real "issue" here is everyone compares their HP and TQ output to a stock C5 with a 3.42 in 4th gear; ANY other combination (dynoing in different gears, different rear end ratios, etc.) will display an "offset" as compared to a stock C5.

This is just one of the reasons why you here dyno owners repeat the mantra "don't worry about anything except your car dyno'd on my dyno at "X", you made modifications, and now your car dyno'd at "Y".

6Speeder

Shorter gears do not get a bigger ring, they get a smaller pinion gear. There is no room on our Getrag case for a bigger ring gear. The shorter you go in gearing, the smaller and weaker the pinion gear gets. That is why Getrag doesn't recommend a gear shorter than a 3.73:1.

BarneyZ

corvettebob1

Not sure but I think he means a thicker ring gear, thus increasing the mass of the ring gear. However I don't go along with his theory of loss through increased mass and friction.
I might be off base but doesn't the dyno also factor in the engine RPM?If it does this would explain the "implied" loss of HP & torque.
For example if you dyno in 3rd you should show a lower HP & TQ then in 4th and 5th would show higher numbers.
In other words ratio is ratio no matter where you put it!
The real truth is in the time slip!