Discussion on transmission loss

[VladaS2]

We all know that wheel hp is very different to actual hp produced by the engine, and that the difference is called the transmission loss. This is what the gearbox, differentials and other parts of drivetrain have eaten from your engine hp. FWD cars have about 13-15%, RWD have about 16-19% transmission loss. Due to complexity of 4WD transmission, these numbers are whooping 22-25%.

So let's say you have healthy RS2 with about 315 HP. You then invest a lot of money to get it up to let's say 420 hp. But due to transmission losses at the wheel you will have only about 310 hp. SO you invested all that money to cover up the difference that the transmission loss ate you in the first place. This starts to add up particulary in high Hp cars. If you have 600 crank HP, count on it that you are loosing about 150 HP as transmission loss!

So my goal here is, instead of investing all that money in more HP, lets see how can we "steal" some of the HP from the transmission loss beast.

[VladaS2]

So first little research... Internet information:

Source of losses:

Losses in hooks joints and cv joints tend to be small, and pretty much proportional to power.

Losses in gearboxes tend to be rather larger, and tend to be worse in percentage terms at both extremes of the power range.

Losses in diffs can be quite spectacularly large (10%), and again tend to be least at mid load.


Some more information

There are two main causes of power loss in transmissions.

1: The nice simple textbook loss due to sliding contact of the gears. Taken to be constant with speed, typically about 2% of power for spur gears and 3% for hypoid bevel.

2: The complex loss (never found in textbooks) due to oil fling and windage. As the gear starts to rotate it picks up oil, has a large wetted area and the loss follows a normal V^3 drag power law. As it picks up speed it tends to fling the oil and carve a groove in the oil bath reducing drag by entraining air. As it flings oil the oil depth reduces, again reducing drag. It moves to a loss approx proportional to speed regime. I don't know what happens at very high speed when the oil level has been reduced as low as it can go or a larger gear on the same shaft is still flinging oil and a smaller gear runs clear of the oil bath. Auto boxes main loss is due to pumping loss in the oil pump and hydraulic system as the gears are not dipped in oil.

Increased temperature will reduce viscosity and reduce drag.

My LSD diff has an oil cooler, not for the benefit of the gears but to maintain the oil at the working temperature of the Viscous LS unit. It has a warning light to tell me when it is possibly an open diff.

I have never tried this myself, but the type and grade of oil you use may have a significant and measurable effect on transmission losses. A lot depends on what the gears are doing.

In a high Rpm low torque situation, (highway cruise) most of the losses will be windage and pumping losses, so much thinner oil may possibly lower those losses.

In a very high torque situation (diff gears on the dyno) most of the losses might be due to high pressure oil shear. A thicker high pressure lubricant may work better and show a lower temperature rise.

Lowest possible transmission loss might require totally different oil in a mileage miser, to a high horsepower endurance race car. Even if the gearboxes and diffs are otherwise absolutely identical.

But I believe measured oil temperature rise is trying to tell you what the losses are. If different oil lowers the operating temperature in your application, that has to be a step in the right (efficiency) direction.

All this info came from here http://www.eng-tips.com/viewthread.cfm?qid=74391&page=2

[VladaS2]

Then some more scientific research

http://www.swri.edu/3PUBS/BROCHURE/D...ch/trantch.HTM

[VladaS2]

So transmission loss being so big specially on 4WD cars like our Audi S2 has to be measurable.

What happens if you replace the standard 5/6 speed S2 gearbox with a newer one (lets say EDU from 2001). The two boxes should be mechanically different enough (preferring the newr one) and the difference in loss should be easilly measurable on dyno.

Then what happens if you replace the rear diff with a V8 Torsen one? This is simpler to test, again difference should be visible on dyno.

Even differences using different gearbox and diff oil. This is easiest to do.

All we need now is to measure and gather this data.

[Greg_S]

Quite an interesting read.

[Sephiroth401]

This is a good subject to have soild information on. As part of a "running the car in a cleaner way" or even "putting less unnecessary stress on components" sort of thing.
I was told - by a chap at our local garage who knows/understands more than most garages in my area about what the're working on - the loss of power was even greater for these old permanent 4WD, up to 30-50hp depending on, well lots of things.
The trans-power-loss is something that would eat up the funds to solve, I'm presuming. Though reading on project threads there is alot of work often replacing components around this problem. So is it not already being reduced on some S2's?
Only the difference is not measured, or something along the lines.

[Dpicariello]

I watched an interesting program in the US where they took a compeltely standard Camaro with standard oils. The put it on a dyno and got a baseline figure.

They then replaced all the oils throghout the driveline and repeated the test. I can't remember what fluid it was but each one was a very deep purple (not pink like silkolene).

They managed to increase power at the wheels by 10bhp. I thought that was quite impressive for something so simple.

Intresting reading the above.

[mcandmar]

Dont forget sticky brakes, i did a dyno run with seized rear calipers and had a massive 66hp loss from drag. ....with a 7A that only left about 100hp at the wheels

[VladaS2]

Tyre pressure also plays a major role (higher the pressure lesser the loss).

I am hoping that we can get some measured information about our drive train components put that in one table and then have really useful information. So we need someone with dyno access and various gearboxes, diffs, oils and other components

I would have done it but I upgraded diff (V8) and gearbox (EDU) at the same time I did engine and turbo so I can not have comperable data.

[markrs2]

An interesting article on this and what you would expect as the transmission losses in cars is here:

http://www.pumaracing.co.uk/POWER3.htm

To summarise in a 4WD car the transmission losses can be calculated as:

4WD cars - add 10bhp to the wheel figure and divide the result by 0.84

so for for a measured 150bhp at the wheels the flywheel figure would be:

(150+10)/0.84 = 190BHP The 40BHP loss is representing 21%

at 200BPH WHP the loss would be 20%
at 300BHP WHP the loss would be 18.7%
at 400BHP WHP the loss would be 18%

This makes sense as the losses in the transmission would not increase in a linear fashion with power and you would expect lower % losses in more powerful cars.

Now this is good for everyone, except those who want to boast about the power their cars produce and tuners who want to show big gains!

[mcandmar]

Sums dont seem to add up for me, or did i do it wrong?

When my cq20v was stock it had 106bhp at wheels + 10bhp=116bhp /0.84=138bhp, so does that mean 32bhp loss?

From the dyno results it shows 66hp loss, almost twice the above figure?

[VladaS2]

Dyno can not tell you accuratly the loss, at best they are only guessing it. Some dynos even have a field for the operator to enter the loss in % which it later displays in your report.

[markrs2]

Sums dont seem to add up for me, or did i do it wrong?

When my cq20v was stock it had 106bhp at wheels + 10bhp=116bhp /0.84=138bhp, so does that mean 32bhp loss?

From the dyno results it shows 66hp loss, almost twice the above figure?

66bhp loss from 172bhp represents a 38% loss in the transmission. This seems to be a little excessive.

If you look at the “transmission loss curve” on your dyno plot it bears no relationship to the power or torque plots. It appears to be a set item dialled in by the operator or software.

Now a 7A CQ is quoted at 170bhp when in tip top condition, and we would expect this figure to drop with mileage as the engine wears. What mileage did yours have at the time?

[mcandmar]

I read that as being proportional to the road speed, as speed increses so does the loss?

Mileage is anywhere between 250-300k miles.

Mark.

66bhp loss from 172bhp represents a 38% loss in the transmission. This seems to be a little excessive.

If you look at the “transmission loss curve” on your dyno plot it bears no relationship to the power or torque plots. It appears to be a set item dialled in by the operator or software.

Now a 7A CQ is quoted at 170bhp when in tip top condition, and we would expect this figure to drop with mileage as the engine wears. What mileage did yours have at the time?

[markrs2]

I read that as being proportional to the road speed, as speed increses so does the loss?

Mileage is anywhere between 250-300k miles.

Mark.

At that sort of mileage then I think the car is doing pretty well to be putting out 140bhp which makes calculation look about right. I do not think that the 172bhp shown is realistic and that as with a lot of other dyno figures the transmission losses and other correction factors are used to manipulate the measured figures.

If a transmission system was soaking up 40% of the power output then on a high powered car (say 500bph at the flywheel) the gearbox and diff would have to be soaking up and then getting rid of 200bhp or 150KW of heat.