Drive Train Losses - Same At Any Power?

[PHATR32]

Hey,

a mate of mine read on his wrx forums, that some "engineer" worked out that the more power you have at the engine, the more you loose at the wheels. the power loss is a fixed percetage

e.g

if youget 200kw at the fly, and 150kw at the wheels, that would equal a 25% loss, so this dude reckons on the same box, if went up to 1000kw at the fly, ull have 750kw at the wheels. (of course a stock box wont handle 1000kw its just an example)

I personally dont beleve it, but i need proof cause my mate is convinced that the power loss is a static percentige.

anyone have any technical proof??

a search didnt help

steve

Edited September 9, 2005 by PHATR32

[WazR32GTSt]

well its not a fixed number (eg its not 20kw at the wheels, regardless of the power)

as to whether or not its a fixed percentage? i doubt it.

There's too many variables to say that one component will lose 25% of its kinetic energy at one energy level and then act the same at twice or 3 times the energy level...

my bet is that its a varying percentage loss and that with increased power, the percentage loss at the wheels increases slightly... but i have no technical proof obviously as i'm not an engineer/mechanic

Warren.

[Big Rizza]

A major source of power loss on a chassis dyno is through the tyres. I'm guessing that if you are putting out more power, there is more heat being generated by the tyres on the rollers. This heat is energy being dissipated, and that energy is not going towards turning the rollers - i.e the energy is lost. Hence the dyno sees less power.

By that, it would make sense that a more powerful car not only has greater power loss on a chassis dyno in raw numbers (eg 20kw vs. 30kw loss), but would also experience a greater % loss (eg 20% vs 30%)

But I'm no dyno technician. Just someone who got an A in high school physics. So I could be wrong...

(etided for spleling)

Edited September 9, 2005 by Big Rizza

[PHATR32]

ok, take the tyres out of the equation, and say they used a hub dyno, where the dyno roller thingys and bolted directly up to the hub on a car.

[Munna]

about 26% loss for a manual and 30% loss for an auto is the rule of thumb not an exact science but it gives you a good idea.

eg

Man 33 GTST187 at the fly 138rwkw

Man VTclubby 250 at the fly 185rwkw

man VTGTS 300 at the fly 222rwkw

I have seen many dyno figures from these 3 cars and taking 26% off the flywheel figure ends up about right.

[Sydneykid]

Oh dear, the same old question with the same old responses, one of my previous posts on this subject;

22 Dec 2003, 07:50 PM Post #18 

Hi guys, as Steve has already posted, we have an engine dyno and a chassis (roller) dyno (2wd and 4wd). I have also tested a hub dyno (2wd and 4wd) a couple of times, they keep trying to sell us one. Based on this experience I favour the fixed loss methodology.

It is pretty simple really, Newton's law says energy can neither be gained or lost, it can be transformed into other types of energy. Now, I know Einstein disproved this with E=MC2, but there ain't no atomic reactions going on in my Skyline.

So let's say I have a gearbox that transforms 20 kw of power (torque X rpm), or kinetic energy if you like, into heat. Why would it suddenly transform 40 kw if I double the power output of the engine? Secondly where the hell is the extra 20 kw of heat going? Noting how much heat a 1 kw radiator puts out, with 40 kw I could fry eggs on the gearbox case in a couple of minutes of running.

The percentage loss methods just doesn't work for me. Especially when you start tripling or quadrupling the power output, the heat would melt the aluminium gearbox case on the dyno.

Some examples;

I know that an R32 RB20DET is rated at 215 bhp by Nissan

When I put one on our usual dyno they make 140 rwhp

That's a loss of 75 hp through the drivetrain, tyres rollers etc

I know that an R33 RB25DET is rated at 250 bhp by Nissan

When I put one on our usual dyno they make 170 rwhp

That's a loss of 80 hp through the drivetrain, tyres, rollers etc

The above is based on the real world results for over 20 Skylines on our usual Dyno Dynamics roller dyno. Dynos are different and different operators get different results. This is our results, but I don't believe that you can simply transfer them to someone else’s dyno. BTW hub dynos show less power loss, due to their lack of tyres, friction and rollers.

Hope that helps

Merry Xmas to all

cheers