Drivetrain loss.. How much do you really lose?

[kemicalx]

I'm a little confused with this,

Now, I know FWD has less drivetrain loss the a RWD due to RWD having a bigger (generally) gearbox, big tail shaft, dif and driveshafts. I have no experience with 4WD's but because of the inclusion of front driveshafts and a smaller front dif I'm guessing 4WD would lose a bit more again?

Exactly how much loss would be entirely car specific I’d presume? But as a rule of thumb I've read and seen from the old Toymods dyno day results that:

FWD lose about 25% from the crank to the wheels

RWD lose about 35%

And I'm guessing (someone correct if wrong)

4WD would lose about 45%

Now that’s out of the way, what I’m actually wondering is, as you increase your engine power and torque, Does the percentage of DTL stay the same? I would have presumed the DT of a car could only rob a set amount of power, for example.

From the dynoday results I could see that a stock 7mgte was putting down about 130-135rwkw, from factory at the crank they produced a bit under 180kw. So in that instance the DT has sucked up ~50kws If you modified that 7m and had it dyno'd at 220rwkw, I presumed that should mean at the flywheel it’d be putting out about 270kw and not the ~295kw I'd calculate if using the "add 33%" rule.

Although that’s not a huuge difference when talking about 220rwkw, when you see dynoqueens putting out 550rwkw then claiming the power at the crank to be ~735kw I just don't see how that could be correct. If anything, wouldn't having more power and torque reduce the DTL?

Any incite would be appreciated,
Cheers guys,
Ashlin.

[AE86slut]

I generally go by 20% RWD and 40% AWD, but it's only a very rough guide. Some race car drivelines have SFA loss compared to a road car.

Regardless of how much power the engine makes, there will always be some form of parasitic or drivetrain loss, and if the driveline setup of a 400rwkW engine is the same as when it was making 200rwkW has not changed, the same percentage applies if you ask me, and most other people.

Very interesting point though... You've really got me thinking now!

Cheers,

Jeremy.

[river]

Hi,

Interesting question....

I go the other way and say that as you increase the HP of the engine, the % loss for the drivetrain will be less.

If, using sluts argument, a 200kw engine (RWD) loses 40% (that is 200kw at the fly, but 120rwkw) - this is a loss of 80kw. Then a 400kw (at the fly) engine would have 320rwkw, (assuming no changes to the drivetrain) which is a loss of 20%.

My reasoning may be flawed, but I assume the drivetrain would require a certain amount of power to overcome and, as the drivetrain isn't being changed, it would not consume more power merely 'cos the engine is more powerful.

However, I am always willing to be enlightened with logic and fact.

seeyuzz
river

[RONA]

Interesting points but when you also look at it the more power you shove through a drivetrain the hotter it gets and the more energy is wasted to heat.

Wheres the experts when we need them, I generally use 25-30% loss for RWD when calculating power figures.

[kemicalx]

If, using sluts argument, a 200kw engine (RWD) loses 40% (that is 200kw at the fly, but 120rwkw) - this is a loss of 80kw. Then a 400kw (at the fly) engine would have 320rwkw, (assuming no changes to the drivetrain) which is a loss of 20%

Thats the way I see it, but then as 7MA61 said the energy being transformed into heat/friction with higher power would also likely effect things perhaps the only real way of knowing engine power output is to have the engine dynoed, and although this isn't a real world figure it's good for comparing with the factory output of new cars.

[SL666]

i think its all crap

i suspect its a much more complicated equation... something like

gearbox takes 10kw+2% to drive,
diff takes 10kw+2% etc etc

it could also be differences in the basic equations/efficiencies.. for example,

on an engine dyno, torque is metered, then hp is calculated from that and rpm, there would be almost 0% loss in this situation.

on a chassis dyno, hp is measured by the amount of energy recieved by the rollers, and then presumably calculated into hp by some known numbers, of course these known numbers while perhaps accurate at 100kw, might not be at 1000kw..

for example, anyone who's driven a car with bugger all power will know how much effort it takes to smoke even small tires, this is energy converted to heat by friction, now, if a car smokes the tires on a dyno, not from 'slip' on the rollers, but because of the heat generated by the rolling friction etc, they could absorb an awful lot of power..

i know of evo lancers with 200kw at the fly, that make 150kw at the wheels... 'only' a 25% loss, but that 50kw of energy has to go somewhere.. 1kw = 1000 watts, a bar heater runs about 2400w, so think of 20 odd bar heaters worth of heat etc going somewhere.. obviously some is being converted into tyrenoise etc, changed into heat in the gearbox oil etc, working at heating the tires etc.. but its still a lot..

most VU ss commodores tend to make around the 170 rwkw (manual), they have a factory quoted figure of 225kw, but they also run big tires, and a quite beefy diff/gearbox combo..

my hilux though its auto made 170rwkw, i suspect its not making 225kw though, and its an auto.. perhaps less pressure on the rollers/skinnier tires etc? nfi really..

i suppose you could even say that some of the power is being transmitted into wind.. after all a wheel spinning at 200km/h creates a draft equivalent to a hell of a fan..


sorry for the rambling nature of this post.

[SL666]

on that thought, FW hp does not have to account for accelerating any of the mass in the drivetrain.. wheels etc.. and perhaps even the rollers on the dyno? (i assume that the makers account for that though)

[Norbie]

I think you'll find the true answer varies for every vehicle/drivetrain combination, which is why you'll get nothing but wild speculation and rough guesses when you ask a question like this. The only way you could get a real answer is to put your car on a calibrated hub dyno, then pull the engine out and put it on an engine dyno, and compare the results. Unfortunately not too many people want to know the answer that badly!

[RONA]

or you could do the reverse get uber motor built get it engine dynoed, then hub dyno it after installation.

[Adam_Cressi]

I think you'll find the true answer varies for every vehicle/drivetrain combination, which is why you'll get nothing but wild speculation and rough guesses when you ask a question like this. The only way you could get a real answer is to put your car on a calibrated hub dyno, then pull the engine out and put it on an engine dyno, and compare the results. Unfortunately not too many people want to know the answer that badly!

I agree, no 2 cars alike,there are so many variants,in terms of cars, car parts... the only way to tell is to dyno, and even dyno's give out varying results depending on where u go

[whatthe?]

who cares about fly figures anyway? rwkw FTW!

[oldcorollas]

on that thought, FW hp does not have to account for accelerating any of the mass in the drivetrain.. wheels etc.. and perhaps even the rollers on the dyno? (i assume that the makers account for that though)

you 've pretty much hit it on the head.
dyno loss is different to real world loss, because of the rates of acceleration and inertia.
a given box will absorb more power as more power is pushed through it, but it is not a linear %. more power in box means the teeth are trying to compress the oil film more, and the resistance of the oil to compression, and the sliding friction as it is compressed and the teeth move, generate heat.
there is always a friction loss for the gears to turn in the oil. you can turn a box by hand, but to turn it at Xrpm takes a certain amount of power.
there is always always a certain amount of friction from the bearings but i am not sure how much a ball bearing increases in friction with side and axial loads...

so equation is something like
(oil resistance to shafts turning x rpm)+
(bearing resistance to turning x rpm)+
(friction from oil on gears that are not engaged, but still turning x rpm)
(oil film friction and heat generated between gear teeth x torque/rpm)
(drivetrain inertia x accelerating rate)

i think the last two are the biggest factor, but one based on rate of accel, and one based on power.... hmmmm...
just a couple of early morning thoughts

[AE86slut]

or you could do the reverse get uber motor built get it engine dynoed, then hub dyno it after installation.

My thoughts exactly.

Basically, if you want an exact figure, this would be the best way.

If you want a general rule of thumb that seems to work pretty well, use 20% and 40%. In fact, I came across a perfect example a couple of days ago - RB26-powered S13... 506kW on engine dyno, 403kW on chassis dyno.

Cheers,

Jeremy.

[EldarO]

Hi,

Interesting question....

I go the other way and say that as you increase the HP of the engine, the % loss for the drivetrain will be less.

If, using sluts argument, a 200kw engine (RWD) loses 40% (that is 200kw at the fly, but 120rwkw) - this is a loss of 80kw. Then a 400kw (at the fly) engine would have 320rwkw, (assuming no changes to the drivetrain) which is a loss of 20%.

My reasoning may be flawed, but I assume the drivetrain would require a certain amount of power to overcome and, as the drivetrain isn't being changed, it would not consume more power merely 'cos the engine is more powerful.

However, I am always willing to be enlightened with logic and fact.

seeyuzz
river

this sounds like a much more sensible theory...

although it does raise a question, if the 80rwkw loss is consistant...

what happens if you get a car with only 80kw at the flywheel...?

Eldar.O.

[myne]

Yeah, that's the sort of thing I've always wondered.
Surely loss is more or less a fixed base + an exponential RPM factor.
Like a supercharger - at low RPM it's actually fairly efficient, but it loses say 10hp, then as the RPM rises its' efficiency plummets as the resistance increases.

So you end up with an exponential 'torque required at RPM (horsepower)' curve as the RPM goes up.
Sure, there's a little bit of extra thrust bearing loading as the torque increases, but how extreme could it be and not fail REALLY quickly?

I think the % people quote are all BS.