Transmission losses
Discussion
I'm sure one of you clever chaps can answer this,
We all know that we get significant losses through the transmission and tyres etc, when comparing flywheel BHP or Nm, with road wheel figure's.
My question is why does the actual value of the losses increase with increased engine power.
For example my car dyno'd at 290bhp wheel's and 360bhp flywheel, therefore losses of 70bhp.
A friends car with much more power (FI), demonstrated losses of approx 120bhp between the flywheel and road wheels, so 50bhp more than mine, on the same rolling road.
Given the cars similarity of transmission etc, shouldn't the systems losses for all our cars be similar irrespective of power output? within a tolerance or course.
We all know that we get significant losses through the transmission and tyres etc, when comparing flywheel BHP or Nm, with road wheel figure's.
My question is why does the actual value of the losses increase with increased engine power.
For example my car dyno'd at 290bhp wheel's and 360bhp flywheel, therefore losses of 70bhp.
A friends car with much more power (FI), demonstrated losses of approx 120bhp between the flywheel and road wheels, so 50bhp more than mine, on the same rolling road.
Given the cars similarity of transmission etc, shouldn't the systems losses for all our cars be similar irrespective of power output? within a tolerance or course.
Alot of these more basic types of dyno work on a % loss or gain from wheels to f'wheel. Most work on a % power loss through the drivetrain, depending on your diff ratio & transmission etc. Therefore if your mate has significantly more power at the wheels it will work back and increase the f'wheel hp by a similar percentage & vice versa. eg:
290hp @ tyres (+25%) = 362hp @ f'wheel = 72hp loss
450hp @ tyres (+25%) = 562hp @ f'wheel = 112hp loss
ETC.
Or maybe I'm way off?
290hp @ tyres (+25%) = 362hp @ f'wheel = 72hp loss
450hp @ tyres (+25%) = 562hp @ f'wheel = 112hp loss
ETC.
Or maybe I'm way off?
My ill-educated thoughts...
- Transmission losses are not independent of transmitted power, or else your car wouldn't be able to pull away with a 70 hp engine (it would be slow, but not that slow!).
- Transmission losses are due to things like: viscous drag forces, which are proportional to the square of the speed of the component (e.g. gear through gearbox fluid); sliding friction, related to force between the components (e.g. gear cogs, wheel bearings); deformation of tyre side-walls, related to torque at wheels. So it's complicated.
- Chassis dynos only *estimate* FWHP (by definition, or they'd be bench dynos).
- Engines can't travel very far on the bench, so FWHP is irrelevant except when comparing to stock figures quoted by manufacturers. If only mfrs would quote wheel HP, mankind would be spared millions of man-years* on internet forums.
(*) find me a woman who cares, and I'll use a gender-neutral term.
John
- Transmission losses are not independent of transmitted power, or else your car wouldn't be able to pull away with a 70 hp engine (it would be slow, but not that slow!).
- Transmission losses are due to things like: viscous drag forces, which are proportional to the square of the speed of the component (e.g. gear through gearbox fluid); sliding friction, related to force between the components (e.g. gear cogs, wheel bearings); deformation of tyre side-walls, related to torque at wheels. So it's complicated.
- Chassis dynos only *estimate* FWHP (by definition, or they'd be bench dynos).
- Engines can't travel very far on the bench, so FWHP is irrelevant except when comparing to stock figures quoted by manufacturers. If only mfrs would quote wheel HP, mankind would be spared millions of man-years* on internet forums.
(*) find me a woman who cares, and I'll use a gender-neutral term.
John
nomercy said:
Alot of these more basic types of dyno work on a % loss or gain from wheels to f'wheel. Most work on a % power loss through the drivetrain, depending on your diff ratio & transmission etc. Therefore if your mate has significantly more power at the wheels it will work back and increase the f'wheel hp by a similar percentage & vice versa. eg:
290hp @ tyres (+25%) = 362hp @ f'wheel = 72hp loss
450hp @ tyres (+25%) = 562hp @ f'wheel = 112hp loss
ETC.
Or maybe I'm way off?
290hp @ tyres (+25%) = 362hp @ f'wheel = 72hp loss
450hp @ tyres (+25%) = 562hp @ f'wheel = 112hp loss
ETC.
Or maybe I'm way off?
Well I think your probably spot on, but this must be hugely inaccurate and therefore make flywheel figures even more irrelavant than we ever thought!!
LathamJohnP said:
My ill-educated thoughts...
- Transmission losses are not independent of transmitted power, or else your car wouldn't be able to pull away with a 70 hp engine (it would be slow, but not that slow!).
- Transmission losses are due to things like: viscous drag forces, which are proportional to the square of the speed of the component (e.g. gear through gearbox fluid); sliding friction, related to force between the components (e.g. gear cogs, wheel bearings); deformation of tyre side-walls, related to torque at wheels. So it's complicated.
- Chassis dynos only *estimate* FWHP (by definition, or they'd be bench dynos).
- Engines can't travel very far on the bench, so FWHP is irrelevant except when comparing to stock figures quoted by manufacturers. If only mfrs would quote wheel HP, mankind would be spared millions of man-years* on internet forums.
(*) find me a woman who cares, and I'll use a gender-neutral term.
John
- Transmission losses are not independent of transmitted power, or else your car wouldn't be able to pull away with a 70 hp engine (it would be slow, but not that slow!).
- Transmission losses are due to things like: viscous drag forces, which are proportional to the square of the speed of the component (e.g. gear through gearbox fluid); sliding friction, related to force between the components (e.g. gear cogs, wheel bearings); deformation of tyre side-walls, related to torque at wheels. So it's complicated.
- Chassis dynos only *estimate* FWHP (by definition, or they'd be bench dynos).
- Engines can't travel very far on the bench, so FWHP is irrelevant except when comparing to stock figures quoted by manufacturers. If only mfrs would quote wheel HP, mankind would be spared millions of man-years* on internet forums.
(*) find me a woman who cares, and I'll use a gender-neutral term.
John
To your first point, certainly thats just what the figures suggest that if my car had just 70bhp it indeed wouldn't move.
Also your right again, that as speed increases viscous drag does increase, but generally our cars are dyno'd at the same speed/rpm in the same gear, so viscous drag would be similar as would dynamic friction in the gears bearings and shafts be very similar.
So as above I guess the rolling road use a percentage calculation which IMHO is rubbbish!!
d getting the straight jacket on 
Yup, if you want flywheel hp use a bench dyno....you may as well piss in the wind than use a chasis dyno to determine what your engine is making at the crank. We all know it's what it's making at the back axle that proves a powerfull car. I still think the best proving ground is on the strip, what MPH your making through the trap (not necessarily your ET) is a real indicator of how much grunt you really have!!
nomercy said:
Yup, if you want flywheel hp use a bench dyno....you may as well piss in the wind than use a chasis dyno to determine what your engine is making at the crank. We all know it's what it's making at the back axle that proves a powerfull car. I still think the best proving ground is on the strip, what MPH your making through the trap (not necessarily your ET) is a real indicator of how much grunt you really have!!
Your argument has some merit with the exception of one quite important factor you are introducing,
Traction.
Or lack of it.
A properly sorted car will thrash a high horsepower car because if it (or lack of it)
Yeah, that's fairly obvious as far as ET's go but traction doesn't have much impact on final MPH over the 1/4. Traction will affect your time but not so much your terminal speed. My SS has gone a 11.9 on street tyres @ 127MPH and on the same night run a 12.3 @ 126MPH (torching most of the track I might add), so you can see the terminal speed is very similar but the times are quite different
Edited by nomercy on Monday 2nd July 22:11
Guy's, appreciate your input, but doesn't really answer the original question. Whilst we all agree the only power figure worth considering for a rolling road is the figure at the road wheel. I'd still like to know why the apparent transmission losses increase so significantly with engine power increases, given the same testing conditions/parameters and speeds etc,
Boyce said:
Guy's, appreciate your input, but doesn't really answer the original question. Whilst we all agree the only power figure worth considering for a rolling road is the figure at the road wheel. I'd still like to know why the apparent transmission losses increase so significantly with engine power increases, given the same testing conditions/parameters and speeds etc,
It really is due to work done.
Whilst the results use the same rev range a more powerful engine will push harder to reach that maximum sooner. (work done=power)
For example,
In a swimming pool you hold your arms out to the side and then slowly bring them together in front of you.
Frictional forces are low so effort required is also low.
Then try moving your hands together as fast as possible.
Although you are still operating within the same `rev range` the increase in effort will be many times greater than before, and certainly far more than the increase in speed.
As well as fluid dynamic`s (which is responsible for the higher losses in an auto box) there are mechanical losses, heat losses and acoustic losses that all follow the same basic principle.
This all adds up to a shed load of wasted flywheel power.
Boyce said:
Guy's, appreciate your input, but doesn't really answer the original question. Whilst we all agree the only power figure worth considering for a rolling road is the figure at the road wheel. I'd still like to know why the apparent transmission losses increase so significantly with engine power increases, given the same testing conditions/parameters and speeds etc,
The reality is they dont increase that much. The dyno is 'estimating' flywheel somehow (run-down method perhaps) and this is skewing the losses. The dyno figures that you are comparing should be rwhp not calc fly.
Interestingly enough at the corvette nats a dude from mobil said oils can make a good few percent difference so getting good oils into the trans, engine and diff will reduce the losses and raise rwhp... but not fwhp.. God knows how a dyno can measure losses between a car with a decelleration fuel cut off and one without etc.
Rearwheel or Trap speeds are the go.
Rearwheel or Trap speeds are the go.
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