Rolling roads and gearing

[crankshaft]

Lets say the power output of a car is measured on a dyno and produces say, 100bhp.

You then change the final drive to a lower (numerically higher) ratio, and measure the power again.

By doing this, surely you will increase the torque at the wheels and would see a higher power reading ? Even though the actual power output has not increased.

Or, you do the dyno run in 3rd gear instead of 4th.

Any thoughts ?

[HarryW]

My understanding is the gear that gives you 1:1 final ratio or near-as is the one to use as it removes any torque multiplying effect of the gears. For most cars 1:1 is 4th gear.
However some types of rolling road have a limited top speed and 4th gear could over speed the rolling road or take it out of its calibrated speed range, so a lower gear is selected. The more complex (read expensive) rolling roads can hold a car at any given Revs at full throttle at any given speed. Whislt others may just measure as as the car spins up (its over in a few seconds).

The only true way to measure power is to have the engine out of the car and on a bench dyno. Which for obvious reasons is not always practical . Next best imho is to go by the wheels figures and lastly the corrected figures.

[crankshaft]

When my car was on a rolling road, the reading was taken in 4th gear (which is 1:1).

My final drive ratio is 3.1:1. If it was changed to 5:1, and the reading was taken again in 4th gear, would the reading change ?

[stevieturbo]

It does change slightly, but in reality it shouldnt.

The dyno doesnt know or care what gear you are in, aside from its own mechanical rpm limit on the rollers/bearings etc

It knows how fast the rollers are spinning, and the torque being applied to them. It calculates everything from there.

So if you ran it in first gear, the rollers may see more torque from the car, but as the rollers are spinning so slowly, it can calculate the correct figures.
If it was in 5th gear, it may see less torque, but the rpm speed and change of the rollers will be huge, so again, it knows whats going on, and how much actual work/effort has been done.

[HarryW]

The 1:1 is for the gearbox, the final drive is normally taken care of when 'calibrating' the RR to the car i.e. holding set speed/revs in calibration mode (two points I think). So assuming the RR has been properly calibrated on each of the differing runs (or is indeed capable of being calibrated) then in theory there should be little change. It does depend on the RR and or the operator but I would not be surprised to see it change a bit.

If you have had no other changes to the car except for a diff ratio change I would just take the last set of figures as the new datum and use that for any further improvements as opposed to thinking the diff has change the power output somehow .

[GreenV8S]

crankshaft said:

My final drive ratio is 3.1:1. If it was changed to 5:1, and the reading was taken again in 4th gear, would the reading change ?

Almost certainly. The measuring system takes account of the basic torque multiplier effects of the gearing i.e. the power is determined by torque times speed which in an ideal world is the same regardless of the gearing. But changing the gearing affects some of the errors that occur during the measurement. For example, the amount of wheel slip can be significant so your back wheels may be doing 70 mph but the rollers may only see 69 mph. These effects are dependent on torque at the back wheels and may change as the gearing changes.

[bertelli_1]

But surely if you change the final drive, at 100mph you'll be revving the engine a lot higher, therefore the torque should be the same as it was at the 'old' rpm (ie 70mph), no?

[rev-erend]

I think the double contact patch of the tyre can make a big difference due to the friction.

In theory - if you increase the pressure to say 30 psi
up from a more normal running pressure of 22psi .. then you reduce the friction and will increase the indicated bhp.

Mind you the operator is supposed to enter the psi figure into the RR software but I would guess is not done often.

[//j17]

I can't help smiling to my self - for once you get a rolling road thread that doesn't talk foolishly about BHP and it's the one that probably should

Changing the gearing will change the torque delivered at the wheels for a given engine RPM but, as already noted the rolling road should be calibrated to the car by flooring the accelerator and the tech. using the rolling road to hold the engine at X RPM. A couple of these and the RR can work out and gearing factors it needs.

If you change the gearing then either more or less torque will be available at X RPM. When you re-calibrate the car/RR you get a different set of numbers for the RR to play with.

Of course if you work in BHP you don't really have to worry about the gearing - BHP being the product of RPM and torque. Change the gearing just moves you to a different point on the power curve (as the gearing will have increased/decreased the engine to X RPM to get to the speed site but the engine still produces Y N/m torque at X RPM).

[LeoSayer]

//j17 said:

Changing the gearing will change the torque delivered at the wheels for a given engine RPM

Sorry, that makes no sense to me at all, can you explain that?

In my mind, an engine that produces 100lb ft of torque at 3000rpm will do so regardless of the gearing.

[Marf]

LeoSayer said:

//j17 said:

Changing the gearing will change the torque delivered at the wheels for a given engine RPM

Sorry, that makes no sense to me at all, can you explain that?

In my mind, an engine that produces 100lb ft of torque at 3000rpm will do so regardless of the gearing.

Yes an engine will produce a given torque figure at a certain RPM point in any gear, but the torque being transmitted through the gearbox will be multiplied by the gearing and this affects the reading at the wheels.

[stevieturbo]

Marf said:

this affects the reading at the wheels.

And as the rollers know what speed the wheels are rotating.....gearing should, and almost does become irrelevant again.

[Vixpy1]

I should really write a long and detailed reply to this thread, but i'm having a week off and can't be arsed. Sorry

Ps, not following you around the forums Stevie, honest

[stevesingo]

Here is a comparison of two dyno runs on the same engine, same rollers only 12 mins apart...


3rd gear
http://e30m3performance.com/installs/2.5_rebuild/dyno_charts/dyno3d.htm 

4th gear
http://e30m3performance.com/installs/2.5_rebuild/dyno_charts/dyno4th.htm 

Both graphs shoe power at the wheels

Hope this helps

Steve

Edited by stevesingo on Friday 28th December 18:44

[stevieturbo]

4th gear should slow slightly higher if its a 1:1 ratio. Although with a BMW box...good chance it isnt a 1:1. Oddly they like making top gear a 1:1

[stevesingo]

This is a US M3 which have a different 'box to the euro cars. The euro cars are 1:1 in 5th the US are 1:1 in 4th. The difference is probably within the tolereance of the rollers, so effectivly there is little or no difference in the outputs.

Steve