kw at the wheels & on the fly?

[Nrage33]

ok, I'm perplexed as to how the two are calculated. If a car can do 200kw at the wheels, how do u figure out how many kw it makes on the fly? In most car mags, is the figure written down "rwkw" or "kw on the fly"?

[strutto]

Most new car mags (eg MOTOR, Wheels) will quote flywheel figures. These are usually obtained from the manufacturer.

Eg: A brand new Nissan 33GTS25t made 185kw at the engine. A brand new HSV GTS makes 300kw at the engine.

Accurate flywheel figures can only be obtained when the engine is out of the car and set up on an engine dyno.

Guesstimates about the flywheel figures can be made from "at the wheels" readings by factoring in some amount of drivetrain loss.

Eg: If I make 138rwkw in a stock 33GTS25t, and I use a drivetrain loss of 35%, I should be making about 186kw at the engine...

138 x 135% = 186.3

Different amounts of drivetrain loss are found with different types of drivetrains.

Eg: RWD might be 30% loss. 4WD might be 40% loss. Etc...

Hope this helps...

[Nrage33]

oh ok...I get it. Thanks for that strutto.

looks like my car ain't that bad after all..

[benm]

Also FYI:

100kw = 134hp

100hp = 74.6kw

So a "brand new" stock R33 GTS-T makes approx ~248hp at the flywheel.

[skylineakt]

Then there is also the difference between an auto box and a manual box. Auto loss tends to loss more kws at the rear.

[Steve]

There are a couple of trains of thought on this one.

Some favour the % loss theory, some the fixed loss theory.

Fixed loss works on the principal that a certian type of car will experience a certian amount of driveline loss. And that although you may increase Flywheel power, the (driveline) loss wont vary at the same rate as the power increase. a manual R33 GTS for example may have a 45-50kw driveline loss, if you increase the power from stock by 100kw, the resultant gain in driveline loss will be very minor. So, stock 140rwkw, 185kw, with 200rwkw, 245kw fly (plus maybe a few kw in extra heat, but nothing major)

Remeber the power has to go somewhere, if not to the wheels, then what does that leave? Heat? Say if the same above car had 400rwkw (not impossible), using the percentage theory, that may indicate a flywheel figure of 540kw, but using the fixed value, a flywheel figure of closer to 450rwkw

With quoting power, if someone says xxxkw, that would indicate at the flywheel (to me anyway) generally the rwkw figure is stated either as rwkw or 'at the rears' (or at all four if 4wd etc), also commonly used is bhp (which stands for break horse power) this is also a flywheel figure.

Dont be too concerned what power figure your car is making, unless of course it is well down on what 'should' be expected for the car you have dynoed. Dynos are a great tuning tool, and good to be able to somewhat quantify gains between mods - but should never be taken as the be all and end all.

[strutto]

An interesting theory Steve. A fixed loss makes more sense than a % loss doesn't it. Has the fixed loss principle been your real world experience?

On another point, are there any useful ways to reduce driveline loss?

[benm]

Swap to a FWD

[Cubes]

Not sure, it still gets me thinking after all this time.

It tend to lean away from both of those theory's now after thinking a little about it.

I don't think the equation is as simple as 300rwkw * 35% or 300rwkw + 105kw drive train loss.

When you create more power you put more load on bearings etc. A little like a heavy car having problems spinning its wheels compared to a light car. More load = more friction/resistance.

So.. The only answer I think is, yes there maybe a fixed drivetrain loss but the equation doesn't stop there with a percentage that increments in relation to the amount of power loss through the extra load in turn friction on bearings etc.

The incremental power loss is probably so small that the power levels we are talking about the variance is stuff all SO.... FIXED DRIVE TRAIN LOSS

The problem is it wouldn't work to have a set incremental power loss as every car and even the same car over time will vary. Depends on if we need to really know every little bit of power loss.

If we had an equation we will know if we require a fixed or percentage drive train loss in there some where. Anyone??

[strutto]

engine power = (rwkw + fixed loss) + (rwkw x 5%)

[Steve]

I havent had any personal exerience with engine/rear wheel back to back dynos, so it is purely theory to me as well.

Sydneykid though has, and it appears from his posts that the fixed loss method is the go.

[rb25]

Ive answered this before i think...... and i shall again.

A pretty solid rule for RWD skylikes ONLY:

say you have 200rwkw. that equals 267rwhp. now stick "rwkw" behind the "rwhp" figure (267) and thats how many fwkw you have in your GTST - 267fwkw

simply put: 200rwkw = 267rwhp, so 267fwkw

for example a stock R33: most get 140rwkw = 187rwhp, so 187fwkw. pretty close to the factory output claimed....

i dont know how well it works when up and abouve 400rwkw, the fixed loss method may be a better way to calculate it at that point, but i rekon my method is alright to get a rough idea on lower powerd motors.

-rb25

[browny]

So all your really saying is that 25% is a good guess for losses.......

[funkymonkey]

with most things in physics like the drag coefficient etc, you have

(x + major constant) * coefficient

so maybe its (rwkw + drivetrain loss) * drivetrain loss coefficient

??

[funkymonkey]

so if the drivetrain loss coefficent for a stock car is 1

then (140rwkw + 46kw) * 1 = 186kw @ fly

wheras if you had a highly modified car that heats up its headers and other parts (energy loss to heat conversion outside of combustion area), you could mark the coefficient at about 1.08 or sumfin, giving a net result of

(380rwkw + 46kw) * 1.08 = 460kw @ fly

mind you tht last coefficent was hypothetical, dunno what the coefficient would be in real life. Only way you could find that out is if you got all the energy disspated from friction and heat as these are the two major outputs in kilojoules, and then convert to find the coefficent.

Good thing about coefficents is that they usually work on a linear scale (usually not always) so there should be a direct relationship between the cofficient of the stock car and that of the modified car, which can be derived from the rwkw.

Lol... who likes maths?

[R31Nismoid]

So all your really saying is that 25% is a good guess for losses.......

yeah... it was either 20% or 30% was a good number to go buy.

anything in that range would be acceptable.

While giving different results, it gives you an indication only.

[Duncan]

has anyone every put their engine on an engine dyno?

[Sydneykid]

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

[PTR33]

...

Now, I know Einstein disproved this with E=MC2, but there ain't no atomic reactions going on in my Skyline.

One point twenty-one gigawatts!?!?!?!?? What was I thinking?!?!?!? :zap:

Oh hang on... That's in the deLorean, not the Skyline... heh heh as you were people.

[Cubes]

Gary, is my theory off track? or is it that the power loss due to increased power output (higher load on bearings = higher friction) is negligible?