dynamometer readings

[johnny yuma]

For power readings"at the back wheels" is it correct to assume final drive ratio changes will change the power readings? If not--why not.Similarly if first gear was used the power would be way up as the torque is multiplied.

[tristancliffe]

The gearing won't change the power readings, as power is not multiplied by gearing. And whilst dynos measure torque, the machine also knows the rpm of the rollers, and can work it out as it goes.

The ratios will change the torque output at the wheels, but they deduce the gearing from the engine revs (which they measure) compared to the rollers, and correct it automatically.

Also never believe the rolling road figures for flywheel torque or power, as the estimates used for 'working out' transmission losses are big guesses, and not based on any facts whatsoever. Best to stick with with wheel power/torque in these cases, as ultimately that is what matters. All commercial rolling roads overread, mainly to impress the punters and make them come back. No one likes to see low power figures, so they try to avoid it.

[Dave Brand]

Originally Posted by johnny yuma

For power readings"at the back wheels" is it correct to assume final drive ratio changes will change the power readings? If not--why not.

Final drive ratio doesn't affect power. On a chassis dyno you are, in effect, reading the force produced at the interface between the driven wheels & the rollers; for convenience, in a rotating machine, this is expressed as torque, which is a product of force & wheel/roller diameter; to obtain a power figure, this is multiplied by rotational speed in rev/min.

If you change the final drive ratio you will, for a given engine speed, see a higher or lower roller speed; you will, however, see a proportionate change in torque (more speed, less torque & vice-versa). These two changes cancel each other out - what you do after the engine has no effect on power.

Similarly if first gear was used the power would be way up as the torque is multiplied.

No, power will remain the same - in first gear you'll see a lot more torque at the rollers, but at a much lower speed.

Edit: I must learn to type faster! Tristan's answer was posted while I was still typing mine!

[zefarelly]

I have a few engines in road cars, owners come back from commercial RR sessions over the moon with more BHP at the wheels than I get on a competition rolling road from my race car.

[Dave Brand]

Originally Posted by zefarelly

I have a few engines in road cars, owners come back from commercial RR sessions over the moon with more BHP at the wheels than I get on a competition rolling road from my race car.

Which is why the only valid comparison is between runs on the same dyno.

'How many BHP do you want?'

When a mate of mine was racing bikes, his tuner's quoted BHP figures were consistently 5-10% lower than his rivals - but his bikes won more races.

[SidewaysFeltham]

A good road has no excuse for lack of reasonable accuracy.

Mine, which admitedly was one of the best (Heenan & Froude Water Brake) was simply calibrated, regularly, by using a lever of specific length and a very accurate spring balance.

Same basic technique as calibrating a torque wrench!

After all, it's only measuring the effective resistance of the brake rotating in its cradle against a large spring resistance and then translating this relative to a rotary potentiometer and the gate apperture!

Which is precisely the same as an engine dyno.

In any case, it's all rather subjective: we used our road mainly for final setting up of jetting, chokes and ignition etc.

[AU N EGL]

Dyno tests should be done in a 1:1 gear ratio - 4th or 5th depending on the gear box. The only reason to run the car up thought the gears is to get to that 1:1 ratio ( most times it is 4th gear)

Plus correction factors as per SAE J1349 JUN90
http://wahiduddin.net/calc/cf.htm

Two differnt dynos can give two differnt readings on the same day. What really matters is that dynos are tuning tools.

[SidewaysFeltham]

Of course they can: if one takes into account ambient temp, atmospheric pressure, workshop temp, exhaust evacuation and a whole series of variables.

All back to adiabatic efficiency.

In the same way a known engine will behave totally differently at sea level on a cold day and (e.g.) in Mexico City on a warm one.

Perhaps the most critical is the "Drift" of the electronic components used to express the measured reading on the digital or analogue display.

That said, as I stated previously, (and I concur with what you said), one takes a number of runs and fine tunes the engine against earlier readings in order to optimise not only absolute measured power output, but power at specific RPM/Gate Loads and acceleration between RPM points on a static loading over time.

All depends what one is tuning for in terms of specific performance.

[dtype38]

Originally Posted by tristancliffe

The gearing won't change the power readings, as power is not multiplied by gearing. And whilst dynos measure torque, the machine also knows the rpm of the rollers, and can work it out as it goes.

The ratios will change the torque output at the wheels, but they deduce the gearing from the engine revs (which they measure) compared to the rollers, and correct it automatically..

Agree with all of that, but according to the guy who runs the rolling road I use, in practice the rate at which the dyno is loaded has a big effect on the results. His rolling road measures the torque on the rollers by using a beam with a load transducer attached to measure deflection in the beam. This same information is used as feedback to ramp up the resistance on the dyno to "load" the engine. Apparently, if the dyno is loaded up too quickly (by putting the car in a low gear for instance) then the feedback loop doesn't compensate quickly enough and and the beam deflections don't represent the output at the wheels. He also mentioned that less honest rolling road operators can deliberately use this effect to give virtually any result they want.

Originally Posted by tristancliffe

Also never believe the rolling road figures for flywheel torque or power, as the estimates used for 'working out' transmission losses are big guesses, and not based on any facts whatsoever. Best to stick with with wheel power/torque in these cases, as ultimately that is what matters.

Don't completely agree with that. I do agree that its wheel power that counts, but the rolling road I use don't "estimate" transmission losses, they measure them directly using the rolling road by doing a "run down" test. This is done by putting the clutch in at the end of a power run, then allowing the rolling road to measure the drag on the rollers all the way down through the rev range. This can only come from the transmission and tyres, so I would have thought was pretty accurate. Then the computer simply adds the wheel torque to the transmission loss torque to give the flywheel value.

Originally Posted by tristancliffe

All commercial rolling roads overread, mainly to impress the punters and make them come back. No one likes to see low power figures, so they try to avoid it.

My only advice would be to never use the same company to do your rolling road tests as the one you use for your engine tuning parts. Then they won't have any vested interest in showing that your new bits are making more power. Then tell the rolling road operator that you are in a race series which imposes power limits on each class and that its important that they don't "over-read" the power you're making or you might be disqualified. BUT, that its important they don't "under-read" your power either, because you don't want to think you've got room to increase you're power if you're already at the allowed limit. It sort of makes it clear that your only interest is in finding out the actual power of your car and sets the guy a challenge to see how accurately he can use his equipment.

[tristancliffe]

Originally Posted by dtype38

Agree with all of that, but according to the guy who runs the rolling road I use, in practice the rate at which the dyno is loaded has a big effect on the results. His rolling road measures the torque on the rollers by using a beam with a load transducer attached to measure deflection in the beam. This same information is used as feedback to ramp up the resistance on the dyno to "load" the engine. Apparently, if the dyno is loaded up too quickly (by putting the car in a low gear for instance) then the feedback loop doesn't compensate quickly enough and and the beam deflections don't represent the output at the wheels. He also mentioned that less honest rolling road operators can deliberately use this effect to give virtually any result they want.

Yeah, I'd agree with that too. Good point, well made. I was referring to water brake style dynos, which are still the most common. My bad.

Originally Posted by dtype38

Don't completely agree with that. I do agree that its wheel power that counts, but the rolling road I use don't "estimate" transmission losses, they measure them directly using the rolling road by doing a "run down" test. This is done by putting the clutch in at the end of a power run, then allowing the rolling road to measure the drag on the rollers all the way down through the rev range. This can only come from the transmission and tyres, so I would have thought was pretty accurate. Then the computer simply adds the wheel torque to the transmission loss torque to give the flywheel value.

The 'run down' tests are the ridiculous estimates I'm referring to. Not only are they taking into account rolling resistance, but also engine braking, and engine friction, none of which should be factored into 'transmission losses'. That run down test is, at best, 30% accurate. It's FAR better to use standard values for longitudinal, transverse or chain/shaft driven transmissions, and helical or straight gears, than to rely on the 'run down' test.

Originally Posted by dtype38

My only advice would be to never use the same company to do your rolling road tests as the one you use for your engine tuning parts. Then they won't have any vested interest in showing that your new bits are making more power. Then tell the rolling road operator that you are in a race series which imposes power limits on each class and that its important that they don't "over-read" the power you're making or you might be disqualified. BUT, that its important they don't "under-read" your power either, because you don't want to think you've got room to increase you're power if you're already at the allowed limit. It sort of makes it clear that your only interest is in finding out the actual power of your car and sets the guy a challenge to see how accurately he can use his equipment.

I'll never let a company do my tuning/development work for me (unless I get lucky and buy GroupC cars or something!) - I prefer to get greasy and take a bit longer over everything for nothing (my time, in the evenings, whilst precious, is free [to me]). So rolling roads don't have a vested interest in me at all. In fact, they probably want to get rid of me so they can get a boy racer muggins through the door and empty his wallet properly.

[johnny yuma]

Thanks I have learned a lot,BUT as some have stated it's the torque at the tyre that counts.The dyno operators who attempt to extrapolate flywheel power output by measuring wheel torque are muddying the water it seems.However lets say your engine produces maximum torque at 4000 rpm and maximum power at 6000 rpm,at which rpm would the dyno show it's greatest reading? Obviously a lower gearing will give more acceleration on the road due to torque increase,but on a dyno it will EITHER indicate the engine is doing the most work that it can at 6000 rpm,OR it is producing it's maximum "twist" at any given MOMENT the throttle is opened full at 4000 RPM. The torque then falls away as the revs rise to 6000,but work done (power) INCREASES until 6000 as more FUEL/AIR mixture is successfully ignited -until more revs reveal the engine's limitations.

SO just exactly what is being measured...? If you ask for just a simple torque reading at the tyre/roller interface is this available ?

[tristancliffe]

They measure the torque at the wheels. The power reading is a simple conversion factoring in rpm and a few constants. So max torque at the wheels will register as max torque, and max power will register at max power, at whatever revs they occur at.

Some roads can probably give you an uncorrected wheel torque, others are limited to inbuilt transmission factoring, so you can only have their flywheel guesses. The clever ones (which are few and far between - go to 100 rolling roads and you might one or two that actually know what is going on, and why) will probably be able to give better readings than the popular ones.

[SidewaysFeltham]

This thread has become a tad convoluted.

It is worth remembering that Brake Horse Power is a coeficient of Torque and Speed; in this case rotational speed or RPM.

All any dyno does is to measure torque: notional BHP is thus extrapolated from the rotational speed of (I) The Carden Shaft/Flywheel if it's an Engine Dyno or (ii) The rollers/wheels if it's a vehicle dyno or rolling road.

It matters not at all what specific technology is employed for power resistance: Hydraulic Water Brake; Eddy Current Brake; Mechanical Friction Brake; Air Brake. The working principle is precisely the same: the resistance to planned movement against an external load such as a spring (Deflection), measured by a Weight Balance, Strain Gauge or other transducer.

Any "Figures" given for frictional losses are crass approximations.

Such frictional losses could only be properly measured by applying known value of effort to one end of the drive chain and measuring the outcome at the end.

The value of a brake is in comparing readings at a specific load against further readings at the same load but with tunable elements at different settings.

Thus, it is - or should be! - simply a comparitive process.

[tristancliffe]

Which is sort of what I said - the estimates are useless unless you are comparing the same car on the same day (and even then remain subject to change). There is no need for a 'run down' test, because if you are comparing the same car (before and after 'tuning') then frictional losses are a constant that can be ignored. If you are lowering frictional losses, then that will simply show up as more power/torque at the wheels.

I'm pretty pleased that my "layman's language" explanation isn't too far from the technical explanation of someone that [appears to] knows what they're talking about.

[SidewaysFeltham]

I went into all this very thoroughly years ago, when I was studying Automotive Engineering quite deeply Tristan; much of my knowledge is rusty, now, but the core maths don't change!

My own choice of rolling road was simple: I wanted ultimate accuracy in so far as the then state-of-the-art could offer, which translated into a Heenan-Froude (as they were, now Froude Hofman), Crypton-Triangle Water Brake, with a massive cooling tower and fans to avoid error due to water temp. rise etc.

Thanks to the accuracy, very high power absorbtion (nearly 300BHP at wheels) and hopefully our developing skill, such as it was, we attracted a loyal following of serious race cars from all over Southern England, as well as keen enthusiasts tweaked road cars.

Much I read and hear concerning power outputs is about as accurate as cruiser's ambitious claims for their mobile disco's power outputs!

Cojones!

We also used Titan's engine dyno facilities in Huttingford at times, particularly for the Atlantic engine.

Whilst the IC engine has changed quite a bit since then, with significant advances in the adiabatic efficiency promoted by different head design, more advanced injection, engine mapping, variable valve timing etc, the core realities remain unchanged.

[Dave Brand]

Originally Posted by johnny yuma

Obviously a lower gearing will give more acceleration on the road due to torque increase,

The better acceleration is the result of more power, not more torque. Torque is the product of force & distance; as such it is time-independent. Acceleration can only be measured relative to time, so the torque figure must also be related to time - in other words, it's BHP you need. It's a common fallacy that torque produces acceleration, BHP produces speed. Look at, for example, the 0-60 acceleration times of cars which are available with petrol & diesel engines. For a given BHP the diesel will produce much more torque, but the 0-60 times will be similar. In-gear acceleration times may well favour the diesel, but that's a function of the shape of the torque curve, not the maximum figure.

SO just exactly what is being measured...?

BHP at the wheels! Dynos measure a force & a rotational speed; these figures are then used to calculate a BHP figure.

If you ask for just a simple torque reading at the tyre/roller interface is this available ?

Torque at the roller is irrelevant to what your engine's doing. The dyno printout will give you a plot of your engine's torque & power over the speed range at which the run is done.

[SidewaysFeltham]

Originally Posted by SidewaysFeltham

We also used Titan's engine dyno facilities in Huttingford at times, particularly for the Atlantic engine.

Apols. Edit facility has transpired.

Should, of course, have read "Huntingdon".

Ah me, what age and infirmity does to brain cells and hand-eye co-ordination.........................

[SidewaysFeltham]

Originally Posted by Dave Brand

The BHP at the wheels! Dynos measure a force & a rotational speed; these figures are then used to calculate a BHP figure.
--------------------
Torque at the roller is irrelevant to what your engine's doing.

Circumlocution once again..........

Sigh.

BHP is a Coefficient. The answer (product) is Brake Horse Power, or DIN, or whatever convenient expression is desired.

BHP is expressed as:-

BHP = Torque (ft/lbs) x rpm
5252

More complex calculations are in fact needed to take into account variables of the specific set-up used for measurement.

Therefore a Chassis Dynamometer (Rolling Road) or an Engine Dynamometer measure TORQUE!

The read out whether Analogue or Digital, quantifies the Product of Torque and Rotational Speed and expresses it as BHP.

Torque at the roller is thus foundational to the calculation of BHP.

Quad Erat Demonstrandum

[AU N EGL]

Lots of details here.

I dyno my car / engine, early FEB for Feburay early March races.( cold weather races) Then in Mid Mid march for March - May races, again in JUNE for June to Sept races. ( VERY HIGH TEMPS AND HIGH Humdiity.)

Are there differences? YES, Not much but tuning the car to run in optimal weather is very important to me.

[tristancliffe]

What sort of changes do you notice at different times of year? Fuelling? Advance? Are we talking tiny changes, or pretty large adjustments to stuff?

Personally, I have cars dyno'd for the track whenever (April last year), and leave it, preferring to concentrate on gearing, suspension, downforce and other time finding tweaks. But Mono isn't exactly a great series with regards engine development, so all the cars are pretty standard and thus tolerant of these things.

[AU N EGL]

Originally Posted by tristancliffe

What sort of changes do you notice at different times of year? Fuelling? Advance? Are we talking tiny changes, or pretty large adjustments to stuff?

Winter settings ( Winter in North Carolina that is) 40-50* F Ideal for the engine, but lousy traction do to cold pavement to Summer Settings ( 85-100*F and 80%+ humidity ) lousy performance

I see a 7 RWHP & 8 ft lbs of RWTQ differences between summer and winter setting. In summer run a bit richer fuel ( 100 Octane - US measurment) then winter. When I get a new engine built I may rebiuld this one and use it for E85 race fuel testing.

So small changes but there are changes. and keeping up with them helps. Engine has 20,000 race / track miles. I do pull the heads off each winter to check / replace springs and valves.

Every Oil change ( 500 miles ) I send the oil to get tested for ware.

Glad I dont have too much difference in altitude.

IMO tuning the car once a year is not enough. Things change too much.

I think my engine has lasted so long as I do the matainance and dont beat the living heck out of it.

I have EFI Live tuning and my tuner uses EFI Live as well.

[dtype38]

Originally Posted by tristancliffe

The 'run down' tests are the ridiculous estimates I'm referring to. Not only are they taking into account rolling resistance, but also engine braking, and engine friction, none of which should be factored into 'transmission losses'. That run down test is, at best, 30% accurate. It's FAR better to use standard values for longitudinal, transverse or chain/shaft driven transmissions, and helical or straight gears, than to rely on the 'run down' test.

Just to clarify on that, I did say that the run down test was done clutch in so engine resistance and braking aren't being measured. As far as I can tell the only difference from the power run would be windage and bearing resistance of the rollers themselves and, of course, that everything is running backwards (how that affects an LSD I have no idea). That said, I've been back over my rolling road readouts and while the run down results are reasonably consistant during each session, there is a significant variation between tests on different dates. This is slightly muddied by my having changed gearboxes and adding friction reducer in the box at various times, but I take your point and won't place too much reliance on that bit of info in future.

[Dave Brand]

Originally Posted by SidewaysFeltham

BHP is a Coefficient. The answer (product) is Brake Horse Power, or DIN, or whatever convenient expression is desired.

Horse power is a derived unit, defined by James Watt as the amount of energy required to lift a weight of 550lb through a distance of one foot in one second. Brake horse power is horse power as measured on a brake.

The DIN unit of power, the PS (Pferde Staerke) is a metric derived unit which is almost, but not exactly, the same numerically as Imperial horse power.

Coefficients have only numerical value, they are not defined in terms of any units; their value remains the same irrespective of the unitary system being used. For example, coefficient of friction, coefficient of drag.

[QUOTE]Therefore a Chassis Dynamometer (Rolling Road) or an Engine Dynamometer measure TORQUE!

It's a long time since I studied thermodynamics, but I have a vague recollection of taking readings from a big spring balance on an engine dyno. I stand to be corrected on this, but I suspect that at the heart of many modern dynos there is some form of force measuring device, strain gauge, load cell, call it what you will, from which the torque figure is derived by reference to the distance from the centreline at which the reading is taken. Arguably, other forms of torque measurement, such as torque-sensing couplings, are not measuring the torque directly, but measuring its effect.

I will now firmly replace the lid on that can of worms!

[SidewaysFeltham]

Dave:

Quote: Definition in engineering terms of reference.

In physics and engineering, a coefficient is a quantitative expression of a specific property of matter, or of a phenomenon. Consider an electronic component whose value changes with temperature. It is tested and found to have a resistance of 100 ohms at a temperature of +20 degrees Celsius (°C), and a resistance of 101 ohms at a temperature of +70 °C. This is the equivalent of a change in resistance of +1 ohm for a temperature change of +50 °C, or +0.02 ohm per degree Celsius. Between temperatures of +20 °C and +70 °C, therefore, this component has a temperature coefficient of +0.02 ohm per degree Celsius, assuming the resistance-versus-temperature function is linear over that range of temperatures.
Un-Quote.

You'll agree, I hope that BHP is an Expression of two values being Torque and Speed. Therefore, its value changes as a direct function of two variables.

Yes, modern dynos use a Strain Gauge or similar which measure electrical variation (EMF e.g.) on disturbance of the quiescent state of the core: normally Piezo Electric Crystal or similar.

Thus the output can be translated electronically into some form of visual representation.

Brake horse power is horse power as measured on a brake.

Sure: measurement of what? Precisely?

However, since BHP is quantified as the result of Torque and Speed, ergo any brake measures Torque, not BHP, which as you say, is a derived unit representative of work done, rather than potentiality to do work.

[AU N EGL]

"
Brake horsepower (bhp)

Brake horsepower (bhp) is the measure of an engine's horsepower without the loss in power caused by the gearbox, generator, differential, water pump, and other auxiliary components such as alternator, power steering, and AC compressor. Thus the prefix "brake" refers to where the power is measured: at the engine's output shaft, as on an engine dynamometer. The actual horsepower delivered to the driving wheels is less. An engine would have to be retested to obtain a rating in another system. The term "brake" refers to the original use of a band brake to measure torque during the test (which is multiplied by the engine RPM and a scaling constant to give horsepower)."



SAE-certified horsepower

In 2005, the Society of Automotive Engineers introduced a new test procedure for engine horsepower and torque.[7] The procedure eliminates some of the areas of flexibility in power measurement, and requires an independent observer present when engines are measured. The test is voluntary, but engines completing it can be advertised as "SAE-certified".
Many manufacturers began switching to the new rating immediately, often with surprising results. The rated output of Cadillac's supercharged Northstar V8 jumped from 440 hp (328 kW) to 469 hp (350 kW) under the new tests, while the rating for Toyota's Camry 3.0 L 1MZ-FE V6 fell from 210 hp (157 kW) to 190 hp (142 kW). The first engine certified under the new program was the 7.0 L LS7 used in the 2006 Chevrolet Corvette Z06. Certified power rose slightly from 500 hp (373 kW) to 505 hp (377 kW).



Relationship with torque

For a given torque, the equivalent power may be calculated. The standard equation relating torque in foot-pounds, rotational speed in RPM and horsepower is:
Outside the United States, most countries use the newton meter as the unit of torque. Most automobile specifications worldwide have torque listed in newton meters. The standard equation relating torque in newton meters, rotational speed in RPM and power in kilowatts is:
These are based on Watt's definition of the mechanical horsepower. The constants 5252 and 9549 are rounded.
5252 comes from 33,000 (ft.lbf/min) / 2π (radians/revolution),
and 9549 comes from 60 (s/min) x 1000 (W/kW) / 2π (radians/revolution)."

Source: http://en.wikipedia.org/wiki/Horsepower


sorry for the long post but seams we needed this and not links for better disscussion.