First of all...Final Gear = Differential Gear , correct?

Speaking about Chassis Dynos:
]
I take a pull in 1:1 gear with a 3.73 final gear and 26" tire,
driveline losses, slippage, phase of the moon, etc. are all
accounted for.

If the only thing I change is my final gear from 3.73:1 to 4.11:1,
will the dyno show any *SIGNIFICANT* change in power peaks
for torque/horsepower? May it also change the peak RPM of
which the power peaks occur?

Or...

Does the chassis dyno use engine RPM, computer software and
chassis drum rotation to eliminate the gears and tires to seem
like a brake dyno???

Thanks for any explanation.

 

I would like to know this also as I have recently put 4.10's in and had the car dyno tuned with 3.42's

Thx Mike

 

A chassis dyno does not eliminate the driveline from the equation. If it did there would not be a loss from an engine dyno to the chassis dyno.

The chassis dyno measures the rate of acceleration of the drum and calculates the amount of power it is taking to accelerate it at a given rate. From this and engine rpm it calculates torque.

Im sorry but I cant give a more detailed explanation of the way it measures power.

As far as gear changes when you change your final drive to a numerically higher ratio you will show a loss of power to the rear wheels due to the inefficiency of the deeper gears and the inertial losses of accelerating the engine at a faster rate. For the wheels to accelerate the drum through a given range of speed in a given time with deeper gears the engine will have to gain rpm's faster and attain a higher overall rpm. The faster the rate of acceleration of the engine the less power available at the crankshaft because more of the power produced by the cylinders is used to accelerate the rotating assembly.

 

I could be wrong about the way the chassis dyno operates but I think that is correct.

 

A dynojet chasis dyno is an accelerometer. A dynojet measures a gear ratio in rpm/mph and performs all calculations based on that. That is why they don't need to know your ratio or tire size.

The reason pulls are made in 4th in a manual is because it is 1:1 and shows the smallest losses. The 1:1 also usually makes the pull take an appropriate amount of time and gives the computer sufficient data points without keeping the motor working too long. I was told a story once about a ricer with about 100 hp whos shreiking hampsters took a long long time to spin the drum up and his motor was actually overheating by the end, haha.

So to answer the question, no it will not SIGNIFICANTLY affect your numbers. There will be small differences or offset as stated above depending on the losses in your gears and how fast the motor is spinning up, but nothing major. Your peak in terms of rpm should not be effected at all.


Brent

 

Brent said it right. No significant difference when changing rear end gears.

Rich Krause

 

Yep. Right on.

At least one chassis dyno, the Superflow AutoDyn, can control the acceleration of the inertia drum with it's eddy current brakes which are piggy-backed onto the drum. It would be interesting to do back-to-back runs on a healthy (800+) hp engine in the pure inertia mode, equivalent to a Dynojet, and then at about half the acceleration rate and compare curves.

The only times I saw that kind of power on the AutoDyn, the car owner insisted on "inertia only" runs which took about 9 seconds in his mid 9 second Nova. He wasn't too keen about stretching the run out to 15-20 seconds. It certainly wasn't a "hamster-mobile", but neither was the engine built to run that long in anger at one time.

 

thanks for the info guys. I might head back to the dyno in a week or so I will post my before and after gear change #'s just for s--t's and giggles.

mike