Dyno ratio differences?

[VX L67 Getrag]

Does anyone know what the difference is if doing dyno tests in lower or higher ratio gears other than something thats 1 to 1?

I know in the late model 6 speed autos(or at least pretty sure) there isn't a 1 to 1 ratio so you either have to choose slightly overdrive or slightly under drive!

I'm just wondering why it's so important to do it as close the straight through, I'm thinking if all the dyno'ing is done in the same gear each time anything is done at least's it's all comparative!

Cheers VX L67 Getrag

[Holden202T]

its funny you should mention that, i was talking to a local dyno guy the other day and mentioned changing diff ratios. It changes the hp readings but doesn't change the torque readings so he reckons.

i think the main reason everyone tries to do it in 1:1 ratio is so you can compare two cars etc

at the end of the day, if you want to tune a car it can be done in any gear you want, aslong as the before and after runs are done in the same gear then you have comparable information.

[VL400]

A dyno measures torque, you can also prob see the linear motive force value (or tractive effort). This is the torque at the point of contact that the tyre touches the roller so accounts for the roller diameter - can directly compare motive force from one dyno to another regardless of its roller dia, if you compare the actual torque the load arm sees it will vary depending on the roller and load cell setup.

Now, the motive force figure you see has gearbox ratio, dif ratio and tyre diameter at play. Torque at the engine is not the same as at the edge of the tyre. So yes changing the gear (either gearbox ratio, diff ratio or tyre diameter) you will see a different torque figure.

However, the power reading will not (in theory) change as it is calculated using ... Power = (Torque x RPM)/5252 . Lower the gear and the RPM increase, and conversly a higher gear gets a lower RPM - this is roller RPM not engine. If you plot power and torque on the same graph, using the same Y axis for torque (in ft/lbs) and power (hp) they will always cross at 5252RPM. There will be some difference in HP as the frictional losses will likely change slightly from gear to gear but you should see the same power regardless of gear - the curve will just be in a different spot if you have the X axis as KPH.

[darcy]

In theory there should be very minimal difference.

In practice, you want the tallest gear you can use, with out overspeeding the dyno.
The lower gear you use the more torque you apply to the rollers and thus is harder to transmit effectively.

[VX L67 Getrag]

Jeepers I think I need to go back to school just to understand all that! LOL

But thanks for the clarification, but was as I expected!