Board logo

Decent BHP calculator
nathanharris1987 - 9/11/08 at 05:41 PM

Does anybody know of a half good BHP calculator that you can work out how much you loose through your drive train to give BHP at the wheels?

Ive had a scan but all i can find is small calculators that tell you your power to weight per ton answer!?

ta

nath

iscmatt - 9/11/08 at 05:43 PM

does this not depend on each individual cars mechanical resistance?

philw - 9/11/08 at 05:48 PM

I usually budget for a 20% loss.

nathanharris1987 - 9/11/08 at 05:49 PM

well yeh but if youve got a certain ratio diff, certain ratio box, sized wheels, weight, engine power surely you can get a "fairly" accurate idea of what your cars got without getting it dyno'd?

mr henderson - 9/11/08 at 05:49 PM

Rolling roads normally extrapolate the flywheel bhp by measuring the power train losses (including the tyres) on the over-run after doing a power run, so I doubt there is a standard ration available for estimating these losses

John

C10CoryM - 9/11/08 at 05:51 PM

This actually ends up being a fairly complex calculation. You need to know the rotational inertial of all rotating parts which is pretty hard to calculate. At least it was to me .

General rules of thumb is about 10% driveline lose for a manual trans, and about 15-20% for an automatic.

dinosaurjuice - 9/11/08 at 05:55 PM

depends on transmission, oil type, tyre type, tyre temperature, pressure, weight on back axle, list is endless..... a true 'at the wheels' calulation should also take into account the drag of undriven wheels, as this is a constant.

BHP figures are only good for pub talk

is the car fast enough for you? thats all that matters IMO.

dinosaurjuice - 9/11/08 at 06:04 PM

whoah, 4 posts in the time it took me to write 1! you guys type fast!

mr henderson - 9/11/08 at 06:44 PM

quote:
Originally posted by chris mason
I was always lead to beleive that RR's measured the torque at the wheels, it then used this info to calculate the bhp at the wheels and then took a guess at the flywheel figures.

Chris


They don't need to guess they can measure the losses. When the power run finishes the operator lets the road slow down while the car is still in gear. The machinery can then mesure how much power is being lost (including the tyres which is usually where most of the losses occur IIRC

John

v8kid - 9/11/08 at 08:47 PM

% figures seem to be OK mine worked out at 11.1% on the rolling road with a higher horsepower than the transmission was designed for so I'd say it was a % thing rather than a fixed figure.

Cheers

mr henderson - 9/11/08 at 09:12 PM

quote:
Originally posted by v8kid
% figures seem to be OK mine worked out at 11.1% on the rolling road with a higher horsepower than the transmission was designed for so I'd say it was a % thing rather than a fixed figure.

Cheers


Can't see that myself. It must take a fixed amount of power to turn a transmission, set of tyres, joints etc at a certain speed, how would those things be relevant to how much power the engine was m?

John

andyharding - 10/11/08 at 08:53 AM

quote:
Originally posted by mr henderson
They don't need to guess they can measure the losses. When the power run finishes the operator lets the road slow down while the car is still in gear. The machinery can then mesure how much power is being lost (including the tyres which is usually where most of the losses occur IIRC


Can't see how it can be "measured" without disconnecting the engine from the drive train. For example resistance will be created by a closed throttle and subsequent increase in vacuum that would not be present during acceleration.

mr henderson - 10/11/08 at 10:23 AM

quote:
Originally posted by andyharding
quote:
Originally posted by mr henderson
They don't need to guess they can measure the losses. When the power run finishes the operator lets the road slow down while the car is still in gear. The machinery can then mesure how much power is being lost (including the tyres which is usually where most of the losses occur IIRC


Can't see how it can be "measured" without disconnecting the engine from the drive train. For example resistance will be created by a closed throttle and subsequent increase in vacuum that would not be present during acceleration.


Perhaps he uses the clutch? It's a while since I've had a car on the rollers, but thinnking about it now, if he was to finish the power run and put the clutch down then the equipment would be able to measure the power losses up to the flywheel

John

C10CoryM - 10/11/08 at 02:53 PM

Using a percentage is not accurate. It is just a guess. As is any horsepower number you hear unless you have been on a dyno (engine or wheel).
However, 10-20% usually works because if you have a weaker engine, typically the driveline parts are lighter.

On the wheel dynometer (rolling road?) you are only measuring the wheel horsepower and torque. If they are giving you flywheel horsepower as well, is just a guess (probably using 10-20% driveline loss ). If you want to know flywheel hp you need to run it on an engine dyno.

One more thing of note. I've seen as much as 15whp difference on dynometers depending on brand. Meaning one brand will always read higher.

Cheers.

mr henderson - 10/11/08 at 03:44 PM

quote:
Originally posted by C10CoryM


On the wheel dynometer (rolling road?) you are only measuring the wheel horsepower and torque. If they are giving you flywheel horsepower as well, is just a guess (probably using 10-20% driveline loss ). If you want to know flywheel hp you need to run it on an engine dyno.


For the third time on this thread!!!!!
They do not need to guess, they can measure the driveline drag after the power run.

OK, it may not be as accurate as an engine dynamometer, but seeing as engines are usually run in cars rather than on dynos, engine dynos aren't very relevant anyway.

So, let me just repeat, again, flywheel figures from a rolling road are not guessed, or arrived at using a percentage, they are deduced by adding the braking effect of the driveline and tyres MEASURED by the RR, to the power figures obtained during the power runs

John

philw - 10/11/08 at 04:25 PM

quote:
Originally posted by mr henderson
quote:
Originally posted by C10CoryM


On the wheel dynometer (rolling road?) you are only measuring the wheel horsepower and torque. If they are giving you flywheel horsepower as well, is just a guess (probably using 10-20% driveline loss ). If you want to know flywheel hp you need to run it on an engine dyno.


For the third time on this thread!!!!!
They do not need to guess, they can measure the driveline drag after the power run.

OK, it may not be as accurate as an engine dynamometer, but seeing as engines are usually run in cars rather than on dynos, engine dynos aren't very relevant anyway.

So, let me just repeat, again, flywheel figures from a rolling road are not guessed, or arrived at using a percentage, they are deduced by adding the braking effect of the driveline and tyres MEASURED by the RR, to the power figures obtained during the power runs

John


The op's original post was asking for a decent calculator, the only way of an accurate figure is as you say an RR session, i piped up with 20% loss which is a fair guess, my car 185@ fly 150 at wheels, a friends car 210@ fly 170 at wheels, as i said only a guess and it will vary different with set ups but with a 20% guess you wont be too dissapointed.

mr henderson - 10/11/08 at 05:37 PM

quote:
Originally posted by philw
quote:
Originally posted by mr henderson
quote:
Originally posted by C10CoryM


On the wheel dynometer (rolling road?) you are only measuring the wheel horsepower and torque. If they are giving you flywheel horsepower as well, is just a guess (probably using 10-20% driveline loss ). If you want to know flywheel hp you need to run it on an engine dyno.


For the third time on this thread!!!!!
They do not need to guess, they can measure the driveline drag after the power run.

OK, it may not be as accurate as an engine dynamometer, but seeing as engines are usually run in cars rather than on dynos, engine dynos aren't very relevant anyway.

So, let me just repeat, again, flywheel figures from a rolling road are not guessed, or arrived at using a percentage, they are deduced by adding the braking effect of the driveline and tyres MEASURED by the RR, to the power figures obtained during the power runs

John


The op's original post was asking for a decent calculator, the only way of an accurate figure is as you say an RR session, i piped up with 20% loss which is a fair guess, my car 185@ fly 150 at wheels, a friends car 210@ fly 170 at wheels, as i said only a guess and it will vary different with set ups but with a 20% guess you wont be too dissapointed.


I have no argument with that, apart from the clear implication that the losses between flywheel and road are a percentage of the total output. They are not. To provide an eaxample, if an engine's output was to be suddenly doubled, but the rest of the car was to stay the same, why would the losses need to go up? There's just no reason for it.

So, while I will agree that a guess of losses in the region of (in your example) 35bhp might be reasonable, I won't agree that those losses are a proportion of the engine's output

John

C10CoryM - 11/11/08 at 03:15 AM

quote:
Originally posted by mr henderson

So, let me just repeat, again, flywheel figures from a rolling road are not guessed, or arrived at using a percentage, they are deduced by adding the braking effect of the driveline and tyres MEASURED by the RR, to the power figures obtained during the power runs


I guess your dynos are fancier than ours as I have never seen that here. You typically don't get flywheel readings. What dynometer are you using?
It also would not take into account slippage, but on most locosts that shouldnt be an issue.

MikeRJ - 11/11/08 at 08:58 AM

quote:
Originally posted by mr henderson
So, let me just repeat, again, flywheel figures from a rolling road are not guessed, or arrived at using a percentage, they are deduced by adding the braking effect of the driveline and tyres MEASURED by the RR, to the power figures obtained during the power runs


Transmission loss as measured by a rolling road are at best a rough approximation, they are never accurate. You are driving the entire drivetrain backwards for starters, so all the gears will be using the wrong thrust face, and helical gears will be producing thrust in the opposite direction to normal. The tyres play a huge part in the loss on a rolling road as well, much more so than on the road.

Transmission loss is neither a percentage of total power, nor a fixed number, but a combination of both. A higher power engine is going to show more loss through the same transmission than a lower powered one, but it won't be proportional.

[Edited on 11/11/08 by MikeRJ]

mr henderson - 11/11/08 at 09:07 AM

quote:
Originally posted by C10CoryM
quote:
Originally posted by mr henderson

So, let me just repeat, again, flywheel figures from a rolling road are not guessed, or arrived at using a percentage, they are deduced by adding the braking effect of the driveline and tyres MEASURED by the RR, to the power figures obtained during the power runs


I guess your dynos are fancier than ours as I have never seen that here. You typically don't get flywheel readings. What dynometer are you using?
It also would not take into account slippage, but on most locosts that shouldnt be an issue.


Well, power figures are basically used for comparison purposes and, perhaps unfortunately, the comparisons in the UK tend to be with flywheel figures. It's desirable, therefore, that rolling roads provide flywheel figures. I guess the equipment over there could also do that but maybe it just isn't needed, as I understand whp (wheel horsepower) is the standard measure in North America.

There's a good descrition of how a typical UK rolling Road works here (written by Dave Walker)

http://www.emeraldm3d.com/em_r_road.html

John

mr henderson - 11/11/08 at 11:35 AM

quote:
Originally posted by MikeRJ
The tyres play a huge part in the loss on a rolling road as well, much more so than on the road.



Well then it's a good job that the rolling road measures the tyre losses too!

quote:
Originally posted by MikeRJ

Transmission loss is neither a percentage of total power, nor a fixed number, but a combination of both. A higher power engine is going to show more loss through the same transmission than a lower powered one, but it won't be proportional.



Why would a more powerful engine show higher losses throught the same transmission? I'm not saying that it wouldn't, but I see no inherent reason why it would, so would be interested to hear your thoughts about that.

Also, in your quote above, you start by saying that the power losses are partly a percentage of the total power, how can that be? (in effect the same quetion as I asked above)

John

v8kid - 11/11/08 at 12:02 PM

I may be wrong but it is reasonable to suppose that gear losses are in part frictional and frictional liosses are proportional to load. It takes more effort to push a heavy wheelbarrow than an empty one.

Part of the losses will be due to the oil sloshing around in the gearbox getting chewed up by the gears and I think it is reasonable to assume that these losses would be proportional in some way to the speed of the gears. I would hazard a guess that the loss is proportional to the square of the speed - most power things are.

There will be some bearing losses, tyre losses and if you want to get really into the nitty gritty some inertial losses.

How much of each there is I havn't a clue! oil losses can be significant though thats one reason why dry sumps are fitted ans tyre losses are also significant - withess the chunks of rubber that are flung off them on the rolling road.

I bet a third each and would be suprised if I am far wrong.

Remembering that to get more power we usually up the revs the power loss should not be far off proportional to the input power.

If you can be arsed searching there are loads of academic papers on the subject

v8kid - 11/11/08 at 12:02 PM

I may be wrong but it is reasonable to suppose that gear losses are in part frictional and frictional liosses are proportional to load. It takes more effort to push a heavy wheelbarrow than an empty one.

Part of the losses will be due to the oil sloshing around in the gearbox getting chewed up by the gears and I think it is reasonable to assume that these losses would be proportional in some way to the speed of the gears. I would hazard a guess that the loss is proportional to the square of the speed - most power things are.

There will be some bearing losses, tyre losses and if you want to get really into the nitty gritty some inertial losses.

How much of each there is I havn't a clue! oil losses can be significant though thats one reason why dry sumps are fitted ans tyre losses are also significant - withess the chunks of rubber that are flung off them on the rolling road.

I bet a third each and would be suprised if I am far wrong.

Remembering that to get more power we usually up the revs the power loss should not be far off proportional to the input power.

If you can be arsed searching there are loads of academic papers on the subject

mr henderson - 11/11/08 at 12:21 PM

I agree that if equipment is run faster then the losses would increase too, however the rolling road will be measuring power output at specific speeds, so the question of the gears etc running faster doesn't really arise.

I've been thinking about this a bit, and I think a quite decent analogy would be raising water in a well, by use of a winch. Naturally there would be power losses in the winch, but the analogous effect of a more powerful person raising the water would be a bigger bucket rather than a quicker lift, therefore I don't see any particular reason why the power losses should be proportional to the strength of the person.

John

v8kid - 11/11/08 at 03:22 PM

John read the bit where I said the friction losses were proportional to the applied load. Think about pushing heavy versus light loads across a gravel drive.

David

mr henderson - 11/11/08 at 03:51 PM

quote:
Originally posted by v8kid
John read the bit where I said the friction losses were proportional to the applied load. Think about pushing heavy versus light loads across a gravel drive.

David


Where in the rolling road situation would you expect most of the increased losses due to friction to be?


(I'm sure you read my bit where I explained how the tyre losses were already accounted for) (and the other bit where I explained that there were no speed increases to be allowed for)

John

MikeRJ - 11/11/08 at 04:19 PM

quote:
Originally posted by mr henderson
Well then it's a good job that the rolling road measures the tyre losses too!


The problem is that during a power run the tyres will be heavily loaded, hence distorting more and absorbing more power. During the coast down run where transmission losses are measured, the tyre has far less load and will look like a lower loss.

There are so many factors that a coast down measurement of loss can only be an approximation.


quote:
Originally posted by mr henderson
Why would a more powerful engine show higher losses throught the same transmission? I'm not saying that it wouldn't, but I see no inherent reason why it would, so would be interested to hear your thoughts about that.


Think about the measurement of power; Torque x RPM. A more powerful engine is going to be producing more of either or both. High RPM will always give more frictional losses, especial viscous friction loss through churning the oil. More torque gives higher loads on both gear teeth and bearings, again increasing frictional losses.

For any given RPM, it will take a certain amount of power just to turn a gearbox/final drive with no load on it's output. Once you start loading the gearbox, the losses will increase as more torque is put through the box. This gives you a fixed and a variable component of the power loss.

v8kid - 11/11/08 at 04:48 PM

John I'm afraid thats just the way it is! More power at the same speed means more torque. More torque means more sideload on the gears and bearings. More load means higher frictional losses.

Can't change the way things are!!

C10CoryM - 11/11/08 at 06:34 PM

Sorry, I'd have to say that the flywheel numbers given from a chassis dyno are no more accurate than using a percentage.
As David says, the harder you load gears the more friction it takes to slide the helical gears over each other. Also applies to tire loading (deformation), and alignment changes. Way too many variables to be accurate.

There is no real standard for measuring HP here. Racers use rwhp because that's all that really matters. The automakers use SAEnet hp which is flywheel power with accesories,exhaust, air cleaner, and emmisions.