SUNDAY DRIVER

Guys/Gals,
Can anyone tell me why should such inherently similar cars suffer such different transmission losses on RR days? I ask this because,although my car {slightly modded MY96 WRX)is putting out a healthy 288.6bhp @6859rpm the power at the wheels is 178.8bhp i.e. a loss of 109.8bhp.When comparing this to the rr results on the dyno section of this board,this loss seems excessive i.e. a lot of the cars have with less bhp at flywheel than mine have significantly more at the wheels where it counts!
Is there anything I can do to cut down this loss?
Thanx in anticipation,SD.

[This message has been edited by SUNDAY DRIVER (edited 05 May 2001).]

SUNDAY DRIVER

Nobody know then ?

SUNDAY DRIVER

Jeez,somebody's bound to have an opinion on this!! Isn't PAW the only figure worth considering at RR days???

WREXY

Hi Sunday Driver, its not your car at fault here. All 4wd or AWD cars lose more power to the wheels than front or rear wheel drive cars do. They lose around 40% to the drivetrain due to the power to all 4 wheels. Theres not much you can do to change this I'm afraid

WREXY

Are you saying that the other cars are losing less power at the wheels than yours is as an overall figure or as a percentage? You must be looking at it as an overall figure and getting confused cause as a percentage its around 40%.

WREXY

Sorry m8 half asleep here just had another thought ,when your car got dynoed at 288.6bhp was it a cold day ? Now when you got dynoed at 178.8 at the wheels was it a warmer day? I'm assuming the 288.6 was done on an engine dyno and the 178.8 on an RR , cause an RR cannot deduct more of a percentage loss from flywheel to wheels from your car to another cars . Temperature, humidity, barometric pressure all play a roll in HP figures.

EvilBevel

Hmmm...

OK, what I know about it: first of all, the drivetrain loss is not a percentage, but a fixed number. Why would the drivedrain give more losses if you tweak your engine ? Think about it.

Second, PAW is the force that is actually measured. The rest is calculated via some formula's, according to different specs (DIN, etc), taking account of ambient temp and some tea leaves

In other words: RR's are not comparable.

But you do indeed get more drivetrain loss from a AWD. 40 % seems a bit much - that would equate to about 100 BHP on a standard car ... mhhm, the thing would probably explode when 100 BHP is turned into heat.

Oh, and another thing I recently realised: doing a run on a cold day is actually unfair towards turbo charged cars ... due to the minimal cooling, and the fact that the induction is forced (thus heats the intake air to about 60 degrees on most RR's), so bye bye advantage of cold air. To make it worse: your BHP at the wheel will we corrected (wrongly) to a lower number because of the cold ambient temp.

That correction would only be valid for normally aspirated cars.

So, there

Theo

WREXY

Standard cars do lose around 100 hp to the wheels ,they have 218 hp flywheel and around 121 to the wheels. Figures obtained from CROYDON AUTOSPORTS SYDNEY , however no 2 dynos are the same so figures will vary slightly. You're right about the drivetrain loss not being a percentage EVILBEVEL my mistake ,in fact the higher the flywheel horsepower the less you lose to the wheels. I just had a chat to Croydon and they confirmed what you said, so again my appologies. If however you were to look at the figures as a percentage loss, a standard car does lose around 40% but the higher you go in flywheel hp, the percentage does drop to hp at the wheels.

EvilBevel

Don't really want to argue, but then why ? :

. do I get between 48 and 57 BHP drivetrainloss (3 different runs on three different dates) to get to the guestimated 255 BHP at our local RR (FLA Bosch)

. do I get 82 BHP "drag output" on Powerstations RR 3 weeks later to come up to a total of 260 BHP at the flywheel ? No mods were done in the mean time... PS use a SUN RR

So I would drop from 208 BHP at the wheels to 178 BHP just by crossing the channel ? And still come up with more power at the flywheel ?

Don't think so.

This must have something to do with the load that is applied to the rollers & the formulas used to get to the flywheel power

WREXY

I would say your right because I'm going off factory quoted figures for flywheel hp and workshops rear wheel hp figures. The dyno I'm going off is probably a hard marker on the cars, however APS in OZ quote 169 hp at the wheels for an OZI spec STI VI and the factory quote 280hp. Tests from tuners however found the STI to only have approx 260hp due to 95 ron juice. Now Chiptorqe say rear wheel hp for a 218 hp turbo is 142 hp and Croydon say 121hp. Im not argeuing here, I agree with you. All im saying is different dynos, different figures. In OZ the dynos only quote rear wheel hp, they dont convert to flywheel as its confusing . The best way is to start off with one dyno and stick to it. Only look at rear wheel hp to see if there are gains in hp due to any mods done on the car.

SUNDAY DRIVER

Thanks for the replies guys,
My post mat have been confusing.I have only had the car dynoed once,with the results being paw288.6bhp , paw 178.8bhp at 20deg ambient temp and 1013mb barometric pressure.I just thought that the transmission losses seemed pretty high compared with other rr results i,ve seen.?

DIGGY

Guys this may be of interest to you....found whilst doing some research........

ROLLING ROAD DYNAMOMETERS
Also called chassis dynamometers, these are used to measure power at the driving wheels. This avoids the inconvenience of having to remove the engine to test it if a tuning modification has been made. However, it means that the power figures obtained will be lower than the flywheel power because of the frictional losses in the drivetrain and tyres. This leads to one of the biggest sources of confusion, error and plain misinformation in the tuning industry. You see, as discussed above, all major manufacturers quote flywheel power so it is understandable that people want to know if the hard earned cash they spent on tuning mods increased the power of their engine and by how much. To know this for certain means knowing how much the transmission losses are. There is enormous pressure on rolling road operators to be able to quote flywheel bhp rather than wheel bhp and most operators now run proprietary software systems which "supposedly" print out flywheel power.

PROBLEM !! - THESE SOFTWARE SYSTEMS DO NOT AND CANNOT WORK !!

Yes - I know - the whole chassis dyno tuning industry quotes flywheel figures and here's me saying none of it works. So I'd better explain some more and then you can make your own mind up.

First, let's look at how a chassis dyno works. The car is driven onto a rig so that the driving tyres are resting between two steel rollers. The torque is measured at different speeds in exactly the same way as an engine dyno works except that it is torque at the rollers rather than torque at the flywheel. The braking load is applied to one of the rollers by either a hydraulic (water brake) or electrical system again in just the same way as the engine dyno would apply a torque to the crankshaft of the engine. The same universal equation at the top of the page can then be used to calculate bhp at the rollers by knowing the torque and the rpm of the rollers (NOT the rpm of the engine at this stage) - but if the engine rpm is measured simultaneously then we can know roller bhp at a particular engine rpm. The BIG problem with all this is if any tyre slip is taking place. Remember these are smooth steel rollers which over time get quite polished. How much grip do you think you would get if roads were made of polished steel rather than tarmac? The effects of tyre slip are complex (i.e. I don't pretend to fully understand them myself!) but what I do know is that you can get some really strange bhp figures from highly tuned engines on narrow tyres and the readings are invariably too high not too low.

What is a transmission loss ? Well all mechanical systems suffer from friction and a proportion of the power fed into a system will get dissipated by friction and turn into heat and noise. Note the key phrase there - "power fed into a system". For there to be a loss there must be an input - simple and obvious yes but we'll see the relevance in a minute. When your car is parked overnight with the engine switched off, the transmission losses are obviously zero. When the car is running then some proportion of the flywheel power will be lost in the gearbox, final drive, drive shaft bearings, wheel bearings and tyres. For a given mechanical system these losses will usually stay close to a particular fixed %, let's say 10% for arguments sake, of the input power. So if the car is cruising and developing 20 bhp then 2 bhp will get absorbed as friction - under full power, say 100 bhp, then maybe 10 bhp will get absorbed. Now it is true that not every component in a transmission system absorbs a fixed % of the input power. Some components like oil seals and non driven meshed gears (as in a normal car multi speed gearbox) have frictional losses which are not affected by the input torque. These losses do increase with speed of course but at a given rpm can be taken to remain constant even if the engine is tuned to give more power. We'll look at real world transmission loss percentages later. Finally, the biggest source of loss in the entire transmission system of a car is in the tyres - they account for half or more of the total losses between the flywheel and the rollers. Each set of driven gears, i.e. the final drive gear or the particular gearbox ratio that you happen to be testing the car in, only absorbs about 1% to 2% of the engine's power.

Ok - so how do these software systems that supposedly measure transmission losses so as to "predict" back to the flywheel bhp work. The power curve at the wheels is taken in the usual way as explained above. Then, at peak rpm, the operator puts the car into neutral and lets the rollers slow down under the drag of the tyres and transmission. The software then measures this drag (or "coast down loss") as "negative" power and adds it to the wheel power to get back to the supposed flywheel power. BUT - and hopefully you've all spotted the problem now - the engine is not feeding any power into the drivetrain while the car is in neutral - in fact it isn't even connected to the drivetrain any more!! Whatever drag this is that's being measured it has nothing at all to do with the proportion of the flywheel power that gets lost as friction when the engine is powering the car in the normal way. The engine could now be an 800 bhp F1 engine or a 30 bhp mini engine for all it matters because it isn't connected to the gearbox or feeding any power into it. Obviously this "coast down loss" is something to do with the transmission and tyres but it is not the true transmission loss - in fact this coast down loss should never be expected to change for a given car at a particular rpm regardless of how much you tune the engine whereas a true transmission loss will increase as the engine power increases because it is dependent to a large extent on the amount of power being fed into the transmission. I've seen a car that over time was tuned from 90 bhp at the wheels to 125 bhp at the wheels and the "coast down loss" stayed the same for every power run to within a fraction of a horsepower - exactly as you would have predicted. As the engine was tuned to give more power the "true" transmission losses must have also increased to some extent but these chassis dyno systems don't, and can't, show this happening.

So is there any way of really measuring the true transmission loss of a car? Yes - only one - by measuring the flywheel power on an accurate engine dyno, the wheel power on an accurate chassis dyno and taking one away from the other. There is no way on God's green earth of finding out the true transmission loss just by measuring the power at the wheels.

So hopefully that's got you all thinking a bit more now instead of just taking for granted the "flywheel" figure you were given last time you took your car to the rollers. Even worse is the fact that some of these software systems allow the operator to just programme in the % transmission loss he wants the system to add to the wheel figures. So if that isn't a nice easy way to show some big fat flywheel bhp then I don't know of a better one. It's certainly a lot easier than actually doing some proper development work to make the engine perform better - just dial in a bigger transmission loss and bingo - the same wheel bhp now turns into a bigger flywheel bhp - happy customer, happy dyno man - just a shame it was all sleight of hand. See the end of this article if you doubt that this sort of thing really happens.


COASTDOWN LOSSES
To round off the articles on power and torque, here is a real example of how the coastdown losses from an actual car were measured on a rolling road dyno. Some time ago I asked a colleague to run a series of tests for me on their rollers. What we needed was a car with a reasonably well quantifiable flywheel bhp and one that we could run in any gear without getting wheelspin on the rollers. This ruled out modified cars that had not been on an engine dyno and anything with too much power. Some time later a completely standard cvh engined Fiesta XR2i came into their workshop and this seemed as good a choice as any. The engine was in good condition and absolutely unmodified according to the owner - a good chance therefore of it producing close to the quoted horsepower.

The aim of the test was to see how wheel bhp and coastdown losses change depending on which gear you run the test in. The rolling road in question is a Bosch flywheel system, which means it has a heavy flywheel attached to the rollers and the system works out power according to how quickly the car can accelerate this large mass. It can't take "steady state" power figures which can be a hindrance when setting up fuel and ignition systems but on the other hand there is nothing for the operator to tinker with and distort the readings - you just sit in the car and floor the throttle and wait for the run to reach maximum rpm. At this point you can put the car in neutral while it "coasts back down" and the system measures these coastdown losses. Some dyno systems then add these losses back to the wheel bhp and call the result "flywheel horsepower". Proponents of this method claim that the "flywheel horsepower" figures so produced are more consistent and repeatable than wheel bhp figures. Hopefully this article will show the pitfalls in relying on coastdown losses by means of this real example - anyway on with the plot.

Ford quote 110PS (i.e. about 108.5 bhp) as the standard flywheel power for the car in question. Obviously every individual engine will differ slightly and this quoted figure can only be a guide to the spread of power outputs that a selection of engines would produce. To restate my own rules for estimating wheel bhp from flywheel bhp - about 15% transmission losses for front wheel drive cars and 17% for rear wheel drive is a rule of thumb. This tends to overstate the losses for high powered engines and understate them for smaller ones. A more sophisticated guide is to deduct 10% of the flywheel power plus another 10 bhp for FWD and 12% plus another 10bhp for RWD cars. The XR2i is FWD of course so if we apply those two rules to 108.5 flywheel bhp we get either 92 or 88 bhp at the wheels respectively. So that's the sort of level of wheel bhp that one would be expecting if the quoted flywheel bhp is correct.

To run the test, the car was warmed up and given a couple of runs on the rollers to stabilize the temperature of the tyres, gearbox oil and engine. A power run and a coastdown were then done in each of 3rd, 4th and 5th gear with a few minutes for the car to cool down between each run to keep the figures consistent. So first let's look at the how the wheel bhp changed in each gear. The figures are as follows:

3rd gear - 95 bhp at the wheels
4th gear - 92 bhp at the wheels
5th gear - 88 bhp at the wheels

So why do the figures show a drop in power as a higher gear is used? The engine of course is producing exactly the same flywheel power regardless of which gear the car is in - what is changing here is the real transmission and tyre losses. A higher gear means that the tyre speed on the rollers goes up too - this leads to more power being absorbed as heat and friction - the measured wheel bhp therefore goes down a bit. There are other factors at work here too but it is not the aim of this particular article to go into all of these in depth. The key thing is that the figures show a reasonable and predictable trend and are in the estimated bhp range calculated above.

It makes the point though that there is no such thing as just one true wheel bhp for a given car on a given set of rollers - it depends on tyre pressure, gear ratio and a host of other things that have already been covered in previous articles. Now devotees of the "coastdown loss" system would say that it should compensate for this - it should reflect the larger losses in higher gears by showing a larger coastdown loss which when added back to the wheel bhp ought to give a flywheel bhp that stays the same in each gear. So let's now look at the coastdown losses that were measured on each of those runs and see if they actually do what is claimed. The coastdown losses were as follows:

3rd gear - 17 bhp coastdown loss
4th gear - 27 bhp coastdown loss
5th gear - 44 bhp coastdown loss

We can add those losses back to the wheel bhp to get the estimated flywheel bhp that so many rolling roads these days quote you.

3rd gear - 95 + 17 = 112 bhp
4th gear - 92 + 27 = 119 bhp
5th gear - 88 + 44 = 132 bhp

Well clearly something isn't working here. The coastdown losses (whatever it is that they are actually measuring) are rising much more in a higher gear than the actual transmission losses are, leading to larger "flywheel" bhp figures in the higher gears. The engine is producing the same power all the time and although we can never know for certain exactly how much power this particular engine had, we can be fairly certain it isn't far away from the factory quoted power. Even the 3rd gear "flywheel" figure is a tad on the high side but it is within the realms of possibility - the figures in the other two gears are obviously not.

The wheel bhp data show a consistent and understandable pattern. Adding back the coastdown losses leads to power figures which vary much more and make less sense. The point to remember is this - if the coastdown losses really were an accurate measurement of the true transmission losses then we would expect to end up with the same estimated flywheel bhp in all 3 gears. The fact that this does not happen means by definition that the coastdown losses are measuring something other than true transmission losses - in turn this means that adding them back to wheel bhp cannot result in true flywheel bhp. The fact that they result in horsepower numbers much larger than the 108.5 bhp claimed for this engine only go to reinforce the message.

So the moral, for the last time hopefully, is to look at the wheel bhp as well as (or preferably instead of) the estimated flywheel bhp. It won't be a figure you can take for gospel and it will change from day to day and from rolling road to rolling road. With a modicum of common sense in keeping the test conditions the same and applying reasonable amounts for transmission losses it will get you "in the ball park" of what the true flywheel figure might be. The flywheel figure generated from coastdown losses though, can vary from the sublime to the ridiculous. Every now and then it might come up with a realistic bhp number but it might equally well be a country mile out.

To estimate true flywheel power from wheel power just apply the rules given at the start of this articles in reverse. The simple formula is therefore:

FWD cars - divide wheel bhp by 0.85
RWD cars - divide wheel bhp by 0.83

The more sophisticated formula is:

FWD cars - add 10 to the wheel bhp and then divide the result by 0.9
RWD cars - add 10 to the wheel bhp and then divide the result by 0.88

Remember these are estimates. The only way of knowing true flywheel bhp for a particular engine is to run that engine on an engine dyno.

long winded but i think it makes sense.....

[This message has been edited by DIGGY (edited 05 May 2001).]

WREXY

Excellent stuff there Diggy, A superb explanation. Learned alot from that m8.