Gearboxes, Diffs and Dynos...

[MPO]

For peace of mind, I need to clarify if different Gearboxes and Diffs have an impact on Dyno Reads….

1. Would changing form an LT77 to a T5 (or Vic Versa) change the BHP/Torque output?
2. Would changing from a GKN to BTR (or Vic Versa) change BHP/Torque output?

I see most Dyno Runs are run in 3rd Gear and the output calculated back to fly...

Gearbox
LT77 – 1.40:1 (3rd gear)
T5 – 1.34:1 (3rd gear)

Diff
GKN – 3.31 (Early)
BTR – 3.77

Thoughts or experience anyone?

Cheers

MPO

Edited by MPO on Monday 5th September 12:07

[phazed]

It is all a bit of an average really.

Dynos are useful for seeing improvements, therefore a reading taken on one dyno can be compared to a second reading on the same dyno after any changes in the engines specs.

As for performance figures of the same engine through different gearboxes and differentials, I think the difference is too small for anyone to differentiate.

[phillpot]

MPO said:

I see most Dyno Runs are run in 3rd Gear and the output calculated back to fly...

Mine was done in fourth, in effect "straight through" the gearbox, minimum drag/power loss?

[MPO]

phillpot said:

Mine was done in fourth, in effect "straight through" the gearbox, minimum drag/power loss?

1:1

That would make sense and give an even gearbox Ratio playing field, but if you look at the dyno reads that are posted they show 3rd Gear, that would surely make a huge difference running them in 3rd!


MPO

[phillpot]

Mine's just a little S3, only about 100mph at max revs in fourth, perhaps third is used with faster cars to keep roller speed down?

[MPO]

phillpot said:

Mine's just a little S3, only about 100mph at max revs in fourth, perhaps third is used with faster cars to keep roller speed down?

Fair point...

I can see that there would be some extra transmission loss using 3rd over a 1:1 in forth.

So, perhaps it helps balance things out a little.


A run in 4th and 3rd on the same Dyno session should be revealing.

[phillpot]

I'm far from an expert on these things, in fact anything, but as I understand it the dyno can only read power at rear wheels and a correction factor is fed into the thing to "calculate" an engine power figure?
maybe a different calculation is done for third gear runs?

Think I read somewhere that some machines can kind off calculate power losses through transmission etc by how quickly, or slowly, the rollers slow down after engine power is cut off?

[Engineer1949]

any good quality dyno and there are not to many around do a run down after the power run to measure the transmission drag so the flywheel output can be accurately measured most good rolling roads only have one large roller per wheel thus eliminating as much as possible climb out associated with twin rollers so a good rolling road should be reasonably accurate


john

[phillpot]

Engineer1949 said:

any good quality dyno and there are not to many around do a run down after the power run to measure the transmission drag

So I wasn't making it up

[ianwayne]

Power at the whhels is measured on a rolling road, correct.

When I had mine run at Hi-Tech Motorsport, the trace given to me shows the transmission losses as well as the power / torque. (40 bhp lost at 5k rpm!). I noticed a lot of the traces displayed on here don't show this.

[AW111]

Engineer1949 said:

any good quality dyno and there are not to many around do a run down after the power run to measure the transmission drag so the flywheel output can be accurately measured most good rolling roads only have one large roller per wheel thus eliminating as much as possible climb out associated with twin rollers so a good rolling road should be reasonably accurate


john

Unfortunately transmission and tyre losses vary significantly depending on the power being transmitted, and which way the drive is being transmitted.

Power at the wheels is what a chassis dyno is measuring - flywheel power is an estimate only, and only really relevant for pub boasting.

[Engineer1949]

sorry aw11 but the whole point is that a good set of rollers measure the drag required to turn the drive train after a power run or at any other time and it is then added to the original figure to give a flywheel figure

john

[AW111]

Engineer1949 said:

sorry aw11 but the whole point is that a good set of rollers measure the drag required to turn the drive train after a power run or at any other time and it is then added to the original figure to give a flywheel figure

john

I understand what you're saying, but my point was that the losses measured coasting down are not the same as the losses incurred during a power run.

The majority of the power loss is in the tyres, and varies with the amount of torque being transmitted, mainly due to the tyre deformation.
Geabox losses also vary with the transmitted torque, and some differentials have significant differences in drag when driven in overrun (imagine a worm-drive).

Hub dynos eliminate the tyre/roller losses, but you still have driveline losses to contend with.

Incidentally, most of the world is happy with "power at the wheels" as a yardstick, but the UK in particular is fixated on flywheel power.

This is not meant as an attack on you, or anyone in particular , but there is a world of difference between a certified engine dyno in a temperature controlled room, testing to eg SAE J1995, and a chassis dyno.

[MPO]

Adrian, it’s interesting what you say about fixation on BHP at the Fly… I agree with you...

I think it’s good to know what’s at the fly but we also need to know what we are really putting down at the wheels if we are going to understand our engines and drive train efficiency. This then helps us identify where drive train improvements can be made….

After all, we wouldn’t go for a 20 Mile Run in flip flops no matter how supper fit our bodies are!

So, back to my opening questions…

Should we be looking to maximise efficiencies and power at the wheels by looking at various Gearbox and Diff combinations?

I suppose we should also start to consider tyre inflation, tyre grip, wheel/tyre weight, Diff Oil and Gearbox oil quality and viscosity, flywheel weights, alternator weight (had to throw that one in Mathew )…….…

Has anyone drilled down into these areas and found at the wheel improvements beyond the engine?????

This subject should get interesting

MPO

Edited by MPO on Wednesday 7th September 08:48

[AW111]

MPO said:

Adrian, it’s interesting what you say about fixation on BHP at the Fly… I agree with you...

I think it’s good to know what’s at the fly but we also need to know what we are really putting down at the wheels if we are going to understand our engines and drive train efficiency. This then helps us identify where drive train improvements can be made….

After all, we wouldn’t go for a 20 Mile Run in flip flops no matter how supper fit our bodies are!

So, back to my opening questions…

Should we be looking to maximise efficiencies and power at the wheels by looking at various Gearbox and Diff combinations?

I suppose we should also start to consider tyre inflation, tyre grip, wheel/tyre weight, Diff Oil and Gearbox oil quality and viscosity, flywheel weights, alternator weight (had to throw that one in Mathew )…….…

Has anyone drilled down into these areas and found at the wheel improvements beyond the engine?????

This subject should get interesting

MPO

Edited by MPO on Wednesday 7th September 08:48

Many years ago I was involved with an OEM who were chasing top speed on a production model - lighter oil in the gearbox gave them a couple of extra kph IIRC.

Under-gearing the alternator by fitting a smaller pulley is a common mod in rally circles.

Skinny, hard, high-pressure tyres reduce tyre losses, but what's the point of more power if you can't get traction?