john banks

"What you get from all this is a power figure at the wheels across the rev range - totally meaningless! The rolling resistance of the tyres depends on road speed. If I run the car up in third gear adn the road speed reaches maybe 90mph. If I run it in fourth gear the rollers might reach 120mph. In third I have less power loss through the wheels than I have in fourth gear, which means that in third (at for example, 6000rpm) I have 80bhp at the wheels. At 6000rpm in fourth I have only 70bhp at the wheels. That missing 10bhp is being lost due to the road wheels rotating at 120mph instead of 90mph. The clever bit is measuring the rolling losses on the run down and adding these to the "at the wheels" figure. I can run in any gear and get the same power curve - but only after the rolling losses have been measured and added to the "at the wheels" figure. If I look at the "at the wheels" power in different gears at the same rpm I get a totally different reading."
....
"To state that you get a given percentage (often quoted as as something like 15% or 25%) only shows a total ignorance of how the whole thing works. No shame in that. I used to happily measure power at the wheels and quote it - because I was ignorant of exactly what I was measuring."

Dave Walker, Engine Management, Technical Editor Car & Car Conversions.

He goes on a bit about tyre drag, condition and pressure, gear oil type and temperature and uses this to support his argument for flywheel figures.

On our local rollers, my car from flywheel power (note these are the only ones quoted to a DIN standard) has varied from 257 to 406 BHP all run in 4th gear, the losses have slowly reduced in PERCENTAGE but INCREASED in absolute terms. Just my experience.

Personally I think rolling roads are almost a complete waste of time for shootouts. They are supposed to be for mapping by holding a car in a zone and adjusting fuelling, timing and boost to see which gives the best torque.

[Edited by john banks - 11/2/2003 9:46:36 PM]

john banks

Who is to say which engine dyno is right? I have heard of results from two different engine dynos with the same engine with approx 20% differences in output! Presumably engine dynos are then left to being relative rather than absolute devices? What about realistic exhausts, ancillaries, loading and airflows?

With regard to chassic dynos, does the G-Force chassis dyno have a susceptibility to wheelbase adjustment giving rather different results on the same car in the same session depending on what wheelbase is set? How can this be quality controlled?

Are we not left with inconsistencies with any method? Arguably are there no more confounding variables using an accelerometer?

I would trust acceleration figures between speeds on a average of two direction which are repeatable at least as much as any set of rolling road figures.

[Edited by john banks - 11/8/2003 10:22:20 AM]

WREXY

To back JB, that two engine dynos will give different readings, I had a 351cubic inch or 5.8litre Ford Cleveland V8 engine run on two engine dynos in the space of two days back in OZ, in 1998. One was at Sam's Performance in Syd OZ and the other was at Baker's also in Syd OZ. On Sam's dyno I got 620hp and on Baker's I got 560hp. Both in controlled rooms with temperature and humidity the same. Two different brand dynos though. Nothing was changed on the engine.

Cheers,

George

[Edited by WREXY - 11/8/2003 8:03:41 PM]

aus73

Sorry Andy, it was said a bit tongue in cheek.

cheers,

chris

edited because its too early and my spelling is very average

[Edited by aus73 - 11/10/2003 8:56:23 AM]

Loon

Just a quick question.

For easy calculation, we'll say a standard scoob has 300bhp @ flywheel, and it loses 1/3 of that in transmission losses 33.3%

So wheel power is 200bhp and loss is 100bhp.

Now, say you tuned the car up to 400bhp. Would the car still lose 33.3%(now 133bhp loss) or would it lose the 100bhp we've just discussed above?

Is the transmission loss fixed or variable? So in real life, if my car loses 70bhp just now, it would lose 70bhp if tuned way much higher?

scoobyslut

Transmission loss is going to remain the same if you think about it. As you have done nothing to improve drive drag coefficient within the drive system have you? The only difference is having a larger torque output, which will improve the power set down. The 4 wheel drive system allows the power to be put down to the tarmac better as you are driving 4 wheels, thus giving a better and larger "footprint" on the road. As you appreciate, 2 wheel drive put half this down.
You could then go onto tyre sizes and weather conditions as to which is best for what weather, wet, dry, snow, all parameters then fall into play.

Loon

So if someone says the transmission losses are 30%, its only referring to the standard setup? If modding that 30% figure is just going to get smaller and smaller.
Do you know by any chance the at the wheel power on a standard classic turbo 2000??

215 @ flywheel, so around 150 at wheels, only giving an overall loss of 65bhp. Does that sound about right??

Surely a scoob with aircon or climate control would have higher losses?

john banks

It does get a bit smaller as a percentage as you increase power, but it doesn't stay as a fixed value no matter what the power IYSWIM.

Loon

IYSWIM??

rroberrto

if you see what I mean?

Loon

1 says its variable to a certain degree and the other says it should not sap any more power.

Need more info. Any scoob techies out there that can solve this one??

Loon

Just had a look at some graphs on site and some things really dont add up. For instance ive compared 2 MY97 cars.

1st car has 245bhp @ flywheel and 148 @ wheels.

2nd car has 245bhp @ flywheel but 162 @ wheels.

How does this 1 work out, they are exactly the same car?? Torque is fairly similar +5 in favour of higher output car.

Fuzz

My UK car has twice put a figure of 89-90bhp loss through the transmission

First time was with 221 BHP
Second was with 255 BHP

Results seem to vary wildly between cars..

The position of the rollers also has a bearing on the figure
as does tyre choice..

Andy

RB5_245

For a start drag co-efficient and tyre sizes have absolutely no effect on transmission losses. Thats overall losses.

The losses are expressed as a percentage, but by my thinking the more power you put through a transmission the less efficient it should become, therefor the percentage loss should increase with power.

I don't see how it could go the other way, but if thats what the real world figures say.....

Also oil choice/temperature and tooth wear will have a small effect on it but nothing much.

Cord

Rolling roads only give an "at the wheels figure" this figure is only repeatable if the car is re-run in identical conditions (tyre pressure, amount of torque applied to ratchet straps etc...etc...) Flywheel figures are only estimates that are worked out by adding a certain percentage to your at the wheels figure, different rollers, different operators and different systems use a different percentage. That is why flywheel figure are always different.

that probably doesn't help any at all, does it????

Cord

"Personally I think rolling roads are almost a complete waste of time for shootouts. They are supposed to be for mapping by holding a car in a zone and adjusting fuelling, timing and boost to see which gives the best torque."

Mr. nail meet Mr. hammer he is about to hit you squarely on the head.
At last somebody has realised that it is a tool to aid mapping, setting up, and measuring power differnces (oh, and mustn't forget bragging down the boozer about my 900brake weapon).But I think mapping on the road (where possible) is better. It is after all where the car is going to be used so a more accurate re-production of the conditions it will be used under.

john banks

Top for talizman.

Agree re mapping on the road, but it does get tricky doing the 6800+ RPM zone in 5th gear

Bob Rawle

But at Bruntingthorpe its easy, if you have the nerve.
FWIW my losses have been fairly consistent at 90-95 bhp across the whole range of power that my STi5 has enjoyed, the '9 dropped 66 bhp only when I ran it at PE, once its been run again we will see if that is as consistant.

bob

aus73

A few notes that may be of interest.....

The ONLY way to get an accurate flywheel bhp figure is on an engine dyno - end of story!!

A chassis dyno is a tuning tool as pointed out earlier. The most important aspect of chassis dynos is repeatable results. Its primary function is to determine the effect of changes made - whether its one degree of ignition timing or a twin turbo/nitrous/supercharging Group B rally engine rebuild.

15 years ago while starting to get involved in Drag racing we found a large variation between car's that produced 400bhp. The cars with engines measured on an engine dyno would always flog cars that made the same bhp on a rolling road. Over a period of time it was deemed more accurate to concentrate on wheel bhp on a chassis dyno, and flywheel numbers on an engine dyno. For racing purposes bhp figures don't mean anything to anyone. What is the point in thinking you have 500bhp while you're getting pumped by people with 400bhp? Its a shame the Subaru community doesn't have a racing series

At the end of the day a car with more bhp at the wheels will ALWAYS beat a car with less. This is where wheel bhp measurement started.

The average owner doesn't understand or care about wheel bhp, they want to know how their car compares to the standard version - which is where these transmission loses come in.

Transmission losses are not dependent upon road speed - this is a common mis-conception. Transmission losses are dependent upon the power or torque being applied through the system. The reason there are changes in wheel bhp and transmission loss on some rollers is that they are inertia dynos, and are affected by acceleration, not the force applied. A decent rolling road will produce identical figures in 2 different gears.

I wouldn't take anything that Dave Walker, the Technical Editor of Car and Car Conversions says about chassis dynos as gospel. If you read the article regarding chassis dyno calibration you would know why.

The best thing to do is find a REPEATABLE engine dyno and you can see what the effect of engine modifications are and quote flywheel bhp figures. If you can't afford to pull the engine out every time you change the air filter then find a REPEATABLE rolling road, take the numbers with a grain of salt, and run down the 1/4 mile. The trap speed at the end of a standing 1/4 mile is the best barometer of engine performance. If cars run less than 110-115mph at the end they've got less than 400bhp.

Fuzz

Powerstation have a good system whereby they have set out marks on the ground for the different cars wheelbases, they can move one set of rollers to within a few mm of these marks. I take this as being why I have had consistent results from them.

At the end of the day though, it's not about the overall number produced. (although a good pub talking point. )
It's the comparison between past sessions that are the most use to me.

saving them to disc also helps when overlaying previous graphs.

Andy
Andy

Andy.F

I disagree. Part of the equation is a fixed load. The transmission takes 'x' lb-ft of torque to rotate. If you increase the speed the power consumption increases in line with x*n2/n1

Increase the transmission load (engine torque) and a percentage of this torque will be absorbed within the transmission, this is dependant on the transmissions efficiency. The power absorbed within the transmission will therefore vary with wheel speed.

I also do not accept that two different gear ratios will give exactly the same efficiency, the tooth contact pressures will differ and this influences efficiency.

IMO, any rolling road that gives an identical PAW in 4th gear at say 120mph as 3rd gear has at 90 mph has a 'fiddle factor' applied.

Andy

Loon

By the sounds of that there is never going to be an accurate way of telling an engines output then.

How do car manufacturers manage it??

WREXY

Probably all use the same engine dyno, at a guess.

aus73

John, we use the same process as used in power testing the British GT cars every time. We should see two runs within 1bhp, with an inlet temperature within 5 degrees of the room temperature. The wheelbase is set by ensuring the front and rear wheel centres are in the centre of the front and rear rollers. There is a bar running under the rollers which must be lined up with the wheel centre caps. To make this process quicker we also have marks on the dyno floor.

Andy, which equation are you refering to when you say power consumption increases in line with x*n2/n1 - it looks more like a momentum equation rather than a power consumption variable.

At its most basic level 1/2 of the transmission loss is through the tyres, and 1/4-1/3 is through the gears. The rest is related to fluid coupling and rubber deformation in the drivetrain.

Now the fundamental friction equation is

Fr = (mu) * N ; where (mu) is the co-efficient of friction, and N is the load applied normal to the direction of motion.

If the tooth profiles ar identical, the teeth pressure ratio will remain the same - which means the friction produced by the teeth contact will also remain the same. The number of teeth on each gear will change between gears, but the overall effect on transmission loss with an extra 2 or 3 teeth is minimal.

Now the greater the force through the transmission system the greater the force on the tyres, and the greater the tyre deformation will be. With little or no load applied to the rollers, you can drive at very light throttle at 100-120mph at 3500rpm. At 3,500rpm at the same throttle position the engine is producing 50-60bhp - at full throttle it might produce 200bhp. If transmission losses were directly related to road speed - and a Subaru loses 80bhp through the transmission (for illustration purposes), then you've got 20bhp from somewhere else - any ideas?

Perhaps if transmission losses are related to power applied, and the tyres deform less at lower power throughputs, you have a lower transmission loss at the same road speed. I think this makes a little more sense.

Sadly, I don't think enough people have been involved in tyre development programs to understand tyre characteristics under load, and the effect on the powertrain.

I don't anticipate everyone will agree with me - after all I don't have a couple of thousand posts on Scoobynet so what do I know. As JB mentioned, the only real verification of improvements is vehicle performance testing - as I said we just need a racing series.

I specified our dyno as it is the only drive-on dyno available that will show an improvement of as little as 2bhp in engine changes. I like to deal in BHP at the wheels as there are no fudge factors involved. This system measures the force at the wheels via a load cell - so there are no fudge factors or guesswork involved. We have software for flywheel bhp extrapolation, but like flywheel figures on any rolling road this is a guide only.

Back to the original post though. We've found Subaru transmission losses to be between 24% and 25% for the most part. Some people like to say it 80 or 90bhp, but I don't see how this can be so - especially if people are led to believe its based on road speed.

Chris

Andy.F

LOL, Chris did you get out the wrong side of bed this morning If I just agree with everything people say, how will I learn ?

I said >>> So if you increase wheel speed, all else being equal, power consumption will increase.
Of course all else is not equal during a power run as the power produced will vary. I agree that the power produced will affect the transmission losses, I said >>>It follows that tyre transmission loss will increase with speed and load as you said >>> I don't think we disagree over this

You said >>>And this is the statement I disagree with (or don't understand if thats more palatable ). I am saying that road (roller) speed will influence the power losses. Otherwise in the example you give of light load running, the car would need the same 50-60 bhp to spin the rollers at 20 mph as 120mph.

Can you expand on this Chris ? Particularly the remaining 1/6-1/4 losses related to "fluid coupling and rubber deformation " Assuming a manual tranny and equal axle speeds, where does this go ?

cheers

Andy

aus73

Tyre friction does increase with temperature. The optimum surface temperature for most tyres is 85-95 degrees, the actual carcass temperature is usually 10 degrees less than this.

A run on the rollers at full power up to 80-90mph in most 300+ cars will get the tyre temps into the right range. If you do a run up to 120mph, the tyre surface may increase in temp slightly - but this will not produce more friction. The increase in temperature may cause the tyres to blister if they are too hot, or damage the sidewall construction if the carcass is overheated. This will obviously make the tyre losses go through the roof - but it doesn't happen on the road where there is a huge amount of cooling air to keep the temps down.

What I'm trying to get at is the engine doesn't know what gear its running in - so the engine power is always the same. The actual drivetrain losses do not vary a huge amount between gears - not half as much as people would think (certainly not as much as inertia dyno graphs would have you believe). The main problem with testing in lower gears (read 1st or 2nd) is that the acceleration rate increases. This leads to wheel-slip on rolling roads. Hence most dynos here have been run in 4th gear - 5th in some cases.

We have a target roller speed of 80-110mph (at peak power) - which is the main reason we can see similar power figures in two different gears. Outside these parameters the dyno figures are innacurate. Lower gears is too fast to get an accurate reading - and as turbo cars need load to generate boost pressure, the boost drops and the power figures are well down. Hgher gears take longer to do the power testing, and lead to excessive heat. Which means in some cases you get a correct power figure - but it cannot be repeated within a short time period because of excessive heat soak - or you get good torque figures and low relative bhp numbers for the engine design as the engine's performance is affected by heat as the power test is performed.

The coast-down loss method of determining drivetrain loss is flawed as the dyno is measuring acceleration (affected by inertia) - not force application. Andy, in this case you are absolutely right, the drivetrain losses are affected by speed - as the mass system has a greater inertia to slow down at a higher speed. What I am saying is that its not quite accurate.

When I reffered to fluid coupling I meant the fluids within the engine - not a torque convertor (appologies). If you change the gearbox fluid for auto trans fluid you will make more power at the wheels. It has a lower viscosity, and therefore less shear-strength than normal gearbox fluid. This means the ATF has less 'friction' than gear oil, and therefore less losses under power. Sadly, if you run ATF in the gearbox you will have to change it all the time, and risk metal parts welding together if it gets too hot - but some racing applications swear by it - for a start. Engine oil, coolant through the water pump, etc.. all effect the fluid losses in the system.

When I referred to 'rubber deformation' I couldn't remember the correct terms. What I meant was every component's behaviour under applications of stress and strain. One of the most important principles in Engineering (aside from Bernouli's equation for the aerodynamicists among you) is Young's modulus of elasticity. Basically all objects, whether metallic or not, will deform slightly when subjected to load. The greater the load, the greater the deformation. Material deformation takes a certain amount of energy - which is where transmission losses really come from. If you apply a small amount of force to a component - 3rd gear for example - it will deform slightly and reduce the amount of power it transmits. If you apply more force, it will deform further, and transmit less force.

This is the principle behind what JB was referring to when he said the actual losses increase, but the percentage gets smaller. The material's behavior in the stress-strain curves will determine its resistance to deformation, and the percentage increases in energy absorption before it eventually deforms and can't return to its original shape - read stripped gear, or bent con-rod.

For the most part I've seen tyre deformation increase as the power goes up to a much larger extent than the rest of the transmission system, so the % losses seem to stay about the same figures. More time spent playing with drivetrains and I should have some more answers.

On that note anyone interested in Carbon Fibre prop-shafts and driveshafts?

Hope this helps.

Chris

Andy.F

Thanks for that Chris, I hadn't considered the elastic deformation of metal components such as gear teeth absorbing power

To probe your area of expertise deeper do you run your rollers at a set engine rpm acceleration rate or roller rpm acceleration rate ? (and what is that rate ?)

I want to test my Subarus output at the acceleration rate most frequently used on track ie circa 500rpm/sec. Our local rollers are set to control engine acceleration to approx 170rpm/sec. Currently a 2000-7000rpm run duration is 30 seconds. (It used to be much longer but we convinced the operator that this was killing the subarus with top mount intercoolers and he reduced it)
This acceleration rate causes me a problem as I don't map the engine for such a 'long haul' and invariably I need to pull some timing and/or add fuel to keep det under control on the rollers. This obviously reduces the power output and renders the exercise futile.

Will your rollers accurately control speed and measure output at 500rpm/sec at say 60-120mph on a 450bhp 1200kg 4wd turbocharged car ?

cheers

Andy

aus73

Andy, we do run the cars at a pre-set acceleration rate. This acceleration rate can be changed depending upon the circumstances for mapping purposes, but the acceleration rate must be identical if two vehicle's power figures are to be compared. With the EVO and Subarus the acceleration rate is pre-set and cannot be altered when doing a power test. The runs in 3rd gear take around 7 seconds - from 2000-7000rpm. The runs in 4th gear take around 10-11 seconds.

The pre-set acceleration rate is part of the SHOOTOUT software which locks the operator out of adjusting dyno parameters. Every power test done here is in shootout mode, so we cannot make the numbers bigger or smaller than they really are. A lot of testing and development has gone into ensuring the numbers produced on the dyno co-incide with numbers produced on accurate engine dynos. Who determines what is accurate and what is not I do not know - but we do a lot of testing with a large oil company who use us to verify lab testing results. We get involved in BMEP (Brake Mean Effective Pressure) and IMEP (Indicated Mean Effective Pressure) and heat release characteristics of fuel and the effect on power output. The results to-date have been representative, so I have no problem in saying our figures are fairly accurate. We're also the scrutineering dyno for the British GT - its my job to go to court over how accurate the figures are.

The inlet temperature, room or relative temperature, humidity and pressure are all printed on the bottom portion of the graph, along with the acceleration rate of the test and 3 dyno check values which co-incide with the dyno set-up and correction factors relating to the room conditions. The dyno check values can be checked by the manufacturer in Australia to ensure they are right, and the corrected bp figure is accurate.

The load cell also does a self calibration every morning when started up, as it is a piezo-electric cell and re-zeros itself to account for small changes over time. We also have a full-scale calibration kit.

Andy, why don't you come down for a free run on the rollers. I'll show you how the system works and see what you think. We do upgrade parts of the cell every 6 months, so I'd like your ideas on how to possibly improve the set-up and make it perfect for Subaru applications.

Let me know what you think.

Chris

Andy.F

Chris

I'd like to take you up on your kind offer it's a pity I'm at the other end of the country (Edinburgh)
I will not rule it out just yet though as I may be travelling south to collect an engine shortly.

The run timing you operate with sounds much more appropriate to me 10-11 secs from 2000-7000 in 4th is much closer to my cars on the road time.

The shorter run time will allow a more representative power figure from the Subarus with top mount intercoolers. It must also be good for identifying laggy turbos. It's one thing to consider an early spooling turbo as one which has been loaded up at fixed rpm on an engine dyno. It's how they respond to acceleration that really matters. Your rollers should capture this.

cheers

Andy