Effect of gearing on dyno figures.

[RobHughes]

I was just wondering how changing the rear axle gear ratio would effect dyno numbers-hp and torque if it would at all?

Thanks in advance for your "answers".

 

[RobHughes]

Would it just move the graph out (or in) on the rpm axis?

 

[enginebob]

The gear sets are torque multipliers.
In the ideal transmission system, with zero losses.
The HP would be the same as at the output of the prime mover. So it all depends on what rpm the engine is running at, and what its power is at that rpm.

 

[dynoguy]

horsepower is work done per time. moving a 33,000 pound weight up 1 ft in 1 minute is one hp.

bringing the dyno up to speed is also work. your engine accelerates the drum of the dyno, with a known mass and a known intertia, to a measured speed in a measured time.

I wouldn't think changing gear ratios would have any effect. changing to a numerically higher ratio would build rpm faster, but also reach redline engine rpm at a lower dyno speed.

 

[Warpspeed]

The problem with a chassis dyno is not the gears or the gear ratios but the tyres.

The big power loss is where the tyre tries to drive the roller. It is a pretty crappy form of friction drive where the tyre must deform to transmit sufficient driving torque. The road is flat, but the dyno roller might be quite small.

As you change dyno roller diameter, wheel diameter, operating RPM, transmitted torque, tyre compound and pressure, and how hard you have to tie the damned thing down, the tyre power losses can vary all over the place.

That is why Bill's dyno always reads higher than Jim's dyno on the same day with the same car. Hub dynos are better, and engine dynos better still.

 

[MaxRaceSoftware]

The big power loss is where the tyre tries to drive the roller. It is a pretty crappy form of friction drive where the tyre must deform to transmit sufficient driving torque. The road is flat, but the dyno roller might be quite small.

As you change dyno roller diameter, wheel diameter, operating RPM, transmitted torque, tyre compound and pressure, and how hard you have to tie the damned thing down, the tyre power losses can vary all over the place.

That is why Bill's dyno always reads higher than Jim's dyno on the same day with the same car. Hub dynos are better, and engine dynos better still.
======================================================

everything i've modeled in my computer program that i had good accurate DragStrip and Engine Dyno data on..the worst i've seen is 84% PerCent total drivetrain efficiency, from Engine Dyno to DragStrip times correlation


but from Chassis Dyno to DragStrip ET Slip correlation,
there seems to be as much as 22 to 26 % PerCent loss
or 74 to 78 % PerCent Total drivetrain efficiency .

a full 10 PerCent more due to Tires on chassis dyno roller effects


Larry Meaux (maxracesoftware@yahoo.com)
Meaux Racing Heads - MaxRace Software
ET_Analyst for DragRacers
Support Israel - Genesis 12:3

 

[hocky70]

I have just had experience with this changing the rear end from a 3.27 to a 4.10 gear ratio in my '02 Mustang GT. My avg HP (3x before and 3x after changing) went down from 260 to 250. I have done a little (very little) research and read that the reason that the measurement goes down is due to the way that HP is taken. HP is measured through an integration function and by installing higher ratio gears, you pull quicker through the gears and therefore lessen the time that the measurement is taken. I have called a few shops that perform dyno runs and 3 of 3 have verified that dyno readings will go down when higher ratio gears are installed. This is also a reason that you want to dyno in say 3rd gear instead of 1st...it gives you a longer duration in which to take more accurate measurements.

Hope this helps.

 

[Craighell]

I see this post is old but...

It depends if you are running an inertia dyno or an absorption dyno and if it is a motorcycle sprocket change or an automotive rear end.

On an inertia dyno, no. Yes the torque at the tire is multiplied, but since the drum RPM is reduced by the same factor as the torque increased, the hp out put remains the same. If you graph hp vs. mph you will see the peak shift to the left. The same type of thing happens when you do an all gear run. Various gear reduction, but same shape curve appearing at different mph.

The only caveat is that some trannys will have lower inertia in higher gears and/or fewer gear interfaces and thus a slight reduction in driveline loss. Likewise, if the new gearing has different tollerances from ring to pinion, that loose or tight fit will either enhance or detract from the peak power.

On an absorption dyno, you will certainly see the rear wheel torque rise (assuming you can read real time torque), although the power should be slightly higher than inertia testing since if you are running step and hold tests you negate the acceleration losses. Like the inertia dyno however, the gearing change will simply show a similar peak hp figure. However the WHEEL rpm (mph) at which it occurs will be different.