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Reasonable article on dynometers by KTR Performance.

How is engine power measured?
The fact is there is no way of directly measuring power - all types of dynamometer measure torque and then power is calculated from the formula BHP = Torque (ft/lbs) x rpm/5252. This basic equation is the foundation of all engine design, development, and tuning. Two main methods of measuring power are used in the automotive industry - (1) measurement at the crankshaft of the engine or (2) measurement at the driving wheels. Read on to find out more about these two methods.

(1) Engine Dynamometers
If you want to know the power of your engine alone then an engine dynamometer (or dyno) is used. This is how nearly all manufacturers rate the output of car engines. The engine is bolted into a cradle and connected to the dyno - usually with a prop shaft which bolts onto the back of the crankshaft (or the flywheel). The power figures measured in this way are therefore usually called "flywheel power". The dyno is essentially a "brake" which can apply a known torque (or "load") to the engine. When the engine is holding a steady speed under a given dyno load then the torque being applied by the dyno must be exactly equal to the torque being produced by the engine. If this was not true then the engine would either accelerate or decelerate.

For example, if we want to know the engine torque at full throttle at 4,000 rpm. The throttle is gradually opened and at the same time the load applied by the dyno is increased - eventually by juggling the amount of load applied we get to the situation where the throttle is fully open and the rpm is steady at 4,000. The torque being applied is written down and then the operation is repeated at another interval like 5,000 rpm. By continuing this process we get a complete torque chart at all engine speeds. Of course we could also measure part throttle power if desired. Modern dynos are computer controlled and can generate power and torque curves very rapidly without the operator having to manually adjust throttle and load controls. They can be programmed to measure every so many rpm, say in 250 or 500 rpm increments - or they can measure a continuous torque curve while the engine accelerates at a preset rate. This can be used to simulate how the engine would actually operate in a particular gear when installed in the car.

There are various ways in which the dyno load can be applied. Older dynos use a hydraulic system with a rotor inside a water filled cavity - rather similar to the torque converter in an automatic gearbox. Modern dynos generate the load with large electric motors. Even a simple friction disk or drum brake will work fine and this is where the name "brake" in Brake Horsepower came from. The important thing is that the load is able to be measured accurately and that there are no frictional losses in the system that escapes measurement.

In order for dyno results to be comparable and universally understood there are a number of things that need to be closely controlled during the measurement process: Operating Conditions - air temperature, pressure and humidity affect the amount of power an engine produces. Cold dense air means a greater mass of oxygen per power cycle and thus more power is generated (provided of course that air/fuel mixture is properly calibrated for the conditions prevailing). There are formula that can be used to calculate how much the measured power would change if the test conditions were different. This enables dyno results to be "corrected" back to standard conditions to enable comparison with anyone else's test results. Sadly however there is no one universally accepted set of "standard" conditions because different automotive bodies in different countries use different standards to calibrate to. "SAE" power standards are used in the USA and sometimes in England. "DIN" standards are used in Europe and there are a few other oddball systems just to confuse the issue. So just because your car is rated at 100 bhp and a friends at 110 bhp doesn't necessarily mean that his engine is more powerful - it depends whether both measurements were corrected to the same standard conditions.

One of the tricks we've seen used to get bigger "corrected" bhp numbers is to use a very high ambient temperature reading for the dyno test. If the operator measures the temperature close to the engine rather than well away from it then obviously he will get a reading that is much higher than ambient. When the bhp numbers are corrected back to a lower standard ambient temperature they will increase.

(2)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 tires. 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 have spent on performance upgrades has 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 figures. These software systems are useful for estimating factory performance figures. However, in our experience, as engine performance is increased their "flywheel" power figures generally become unreliable. We know the majority of other rolling road tuners in this country quote "flywheel" figures and here's us saying it is irrelevant. So we'd better explain how a rolling road dynamometer works. The car is driven onto a rig so that the driving tires 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, BHP = Torque (ft/lbs) x rpm/5252, 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 know roller bhp at a particular engine rpm.

The BIG problem with all this is if any tire slip is taking place. Remember most of these other rolling road tuners use 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 tire slip are complex but what we do know is that you can get some really strange bhp figures from highly tuned engines on narrow tires and the readings are invariably too high not too low.

How does an inertia type rolling road work?
The majority of rolling roads used to determine power figures in the US are inertia dynamometers. Inertia dynos do not directly measure the force on the dyno rollers to determine power figures. Instead, these systems calculate the force on the rollers using the formula:
F = ma
Where F is force; m is mass; a is acceleration

The mass and system inertia of the dyno rollers is known. In order to calculate the force applied, inertia dynos measure acceleration of the rollers by measuring the increase in current and voltage production when the dyno's eddy-current retarders are used as a power generator instead of a power absorber (as used to hold the dyno load when mapping engines).

Force on the rollers is therefore the roller mass multiplied by the acceleration determined by the voltage output. This force is multiplied by the radius of the roller itself to give torque at the wheels using the following equation:
T = Fr
Where T is torque; F is force; r is radius of application
Power is determined by using the formula:
BHP = Torque (ft/lbs) x RPM/ 5252

This calculation is then used for the power at the wheels measurement. If an ignition pickup on the engine is used, these power figures can be used to plot a power curve. In order to determine power at the flywheel figures, a coast down procedure is used which measures the deceleration of the rollers, and uses this figure as negative acceleration and the F=ma calculation is used again to obtain the power losses through the transmission.

The major problems with these systems occur when changes are made to any of the rotating masses in the system. This includes items such as the clutch, flywheel, or aftermarket wheels. These items do not change the power of the engine (obviously). However they will change the rate of acceleration of the vehicle. Therefore these changes will change the power output measured on an inertia dyno. This is another reason why a number of dyno manufacturers will not guarantee accuracy greater than 5% for their dyno systems.


How does our Dyno Dynamics load bearing type rolling road work?
To overcome the problems associated with estimating drivetrain losses to determine flywheel horsepower – as performed by inertia rolling road systems – the load bearing system measures power at the wheels. This system itself is unique – as the system uses a load cell (with 99.99% accuracy) to measure wheel power, as opposed to inertia measurement systems which have around 95% accuracy. This means the system is accurate to within 1.5bhp at 1200bhp.

To eliminate the problem of wheel spin the Dyno Dynamics dynamometer has a superior vehicle tie down method.This revolutionary method of securing the vehicle to the rolling road results in very accurate power measurements - first time - every time. This method of securing the vehicle is so effective a twin-turbo Chevrolet V8 powered vehicle was accurately measured with 1072bhp at the rear wheels on an identical rolling road system. Similar rolling roads have great difficulty with performance measurements above 420bhp due to their ineffective tie down methods, and the use of smooth rollers.

In order for dyno results to be comparable and universally ambient air temperature, pressure and humidity are closely monitored during the measurement process. These factors have a big effect upon the amount of power an engine produces. This information enables our dyno results to be "corrected" back to standard conditions to enable comparison with anyone else's test results.

To ensure the safety of our customers’ cars, a Lambda meter is used to monitor air/fuel ratio, in real time, as the car is run on the dyno. This ensures the vehicle’s engine is accurately monitored, so that problems can be identified and a dyno run halted before any damage can occur.

What is a transmission loss?
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. 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 tires. 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. Real world transmission loss percentages are discussed later.

The biggest source of loss in the entire transmission system of a car is in the tires - 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.

How do these systems that measure transmission losses 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 tires 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. However - 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 tires 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.

We've seen a car that over time was tuned from 280 bhp at the wheels to 335 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.

Is it possible to measure 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 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 program in the % transmission loss he wants the system to add to the wheel figures. If you need to know the flywheel bhp then you will have to estimate it - there's no other way short of using an engine dyno. The corrections you need to make for cars with manual gearboxes are these:
The average rear wheel drive road car with between 100 and 200 bhp loses about 17% of the engine bhp as transmission losses. We find Porsche transmission losses are 12% to 18%, depending upon specific model and the age of the vehicle. We use 15% as a rule, and find the estimates of flywheel power compared to factory figures very reliable. 4-wheel-drive cars will have higher losses because of the extra differentials and other power transmission components. A reasonable estimate of an average 4wd car's losses might be 22% to 25% of the flywheel power. As a guide we have found using a transmission loss of 24% for Mitsubishi Lancer EVOs and Subaru Imprezas to be very accurate. What your own specific car loses is anyone's guess - yours is as good as mine - but it shouldn't be far from the figures above.

No car in the world loses anything like 30% of the engine's power in the transmission and tires as many rolling road operators would try to have you believe. So take the wheel figure and divide by 0.85 for RWD and that will get you as close to the true engine bhp as you are ever going to know. A rule of thumb we use which is quite accurate is to treat the losses as being 12% of the flywheel power plus 10 bhp for RWD cars. This equation "loads" low powered cars more than high powered cars which is more closely like what happens in reality. Remember, these percentages are just good realistic averages. The measured wheel bhp can change depending on tire pressure, tire size, suspension angles and other things which won't affect flywheel power - so the actual transmission loss % will also change. For these reasons we try to standardize as many of these things as possible if we do a series of power runs over a period of time. We always use the same tire pressure because this is a factor which can easily change from day to day.

As you tune a particular car, the losses won't increase exactly in proportion to the power because as mentioned above, some components in the transmission have fixed losses which are not dependent on engine power. However, neither you nor the dyno operator will have any real idea of exactly how the losses have changed so you might as well just continue to apply the percentages above to give some sort of realistic guide to the new flywheel bhp.

What sort of % transmission loss do these software systems show?
Well for normal road cars up to 250 bhp, we've seen as high as 35% and as low as 10%. So take the same car with 150 bhp at the wheels to 2 different rollers and you might get anything from 165 bhp to 200 bhp being "predicted" as the flywheel figure. In reality 150 bhp at the wheels will be no more than about 180 bhp at the flywheel.

What other conditions affect power figures?
Another good way of bumping up the power figures on rolling road tests, as mentioned above under engine dynos, is by "playing about" with the air temperature and pressure corrections. If you dial in your own "standard" conditions as being freezing cold with the barometer going off the scale, or you put the temperature probe near the engine, you can get the system to add huge amounts of power to what was actually measured. So make sure you know if such corrections were made or not and to what standards they were made if any. Plenty of rollers still just quote the measured figure because they don't have computer systems to do the calculations. Hopefully it should be apparent that 100 bhp is not just 100 bhp and end of story. It depends how it was measured, where it was measured, what corrections were applied and of course whether the dyno was even accurate in the first place. We don't get too excited when we see other people quote huge power outputs for their engine modifications. If our engines outperform theirs on the track then they can quote whatever power figure they like.[/b]
 

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If our engines outperform theirs on the track then they can quote whatever power figure they like. --

This is why I like the video method. One speed to the next speed on tape, in the real world under real world conditions. Also one of the reasons I prefer measuring with accellerometers and/or videocameras, even if their have their own faults.

Cheers,
Dubbya
 

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Laptop with WiFi ?


I sincerely hope what you meant to say was Powerbook with AirPort

Excellent article, though, thanks for reproducing it.
Good reading for someone like myself who's never been anywhere near a Dyno. Hadn't a clue how they worked, tbh!

I really must get the Saabs on one, if only to satisfy my own curiosity as to how much extra
the nice men at Abbott really gave me.

Cheers

James
 

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Expose: one of those things you didn't know you needed, but now can't live without

Keeping on topic: isn't it dynamometer? Or are they interchangeable?

See where the article talks about inaccuracies in measuring wheel hp due to tyre slip, smooth rollers etc, isn't there a relatively simple solution?

What about a dyno that bolts directly to the hubs, thereby taking away all the uncertanties inherent in frictional transfer between tyre and roller? Kinder to your tyres, too.

Cheers

James
 

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There are quite a few hub dynos about, Abbey Motorsport who tune Skylines use one. Main problem is the length of time required to set a car up on one, compared to a wheel dyno.
 

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I find it amusing that they go very little in trying to explain how their special dyno works. They say it uses a load cell that is much more accurate than other dynos, but they failed to explain what a load cell is and how it works. Sounds like they are just using the article as a tactic to pull in customers to their shop to me.

I also have a few comments about their explanations. The first is that the technician should not put the car in neutral but simply push the clutch pedal and then indeed, the transmission is still getting turned over on the drivetrain loss part of the run. Yes, it is not perfect to get the entire loss, as the losses will be greater with higher thermal load on the power, but it indeed gets all the drivetrain loss after the clutch to the wheels and even tire friction. I would still think in the end that the rundown check is more accurate than their way of guessing the drivetrain loss.

The next point is that they say that thier system is unique that it checks the ower at the wheels. Well, no **** ! They all do, don't they? So how does that make it unique?

Also, ok, a third point, for them to say that correction factors aren't any good is also a line of [expletive deleted]. The entire industry uses them and those are the numbers we all see in the magazines, on websites and such. So this one shop is out to say that the rest of the industry is wrong. Oh, please. I understand that it is possible for a shop to try and fudge the numbers but I would think a good shopo would avoid doing such a thing. Also, that is actually what a baseline run done before a mod is done for. So regardless of correction error and change in conditions, it will indeed be seen the difference in power that a certain mod makes.

In my opinion, they did a great job explaining how dynos work, but their way of trying to say their's is better fails to convince me.
 

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Good article Mark, thanks for posting it.

Some of my own thoughts and comments.......

Interesting statement that all rolling roads measure torque. If so why are there so many that only produce a power curve ?

Measurement on rolling roads. They seem to be suggesting that they use the roller brake to apply load so that power/torque can be measured at a given RPM. I have only ever operated a RR this way when checking for misfires at certain speeds. Otherwise, once the car is up to temperature it's normally a full run from low revs right up to the point where power is dropping off.

Tyre slip. I know all about this one..... usually solved with a couple of lorry tie down straps. They tend to be more consistent than a couple of blokes & drink less tea

Coast down procedures. A little different to what I am used to. Putting the car in neutral eliminates gearbox losses from the equation. All you need to do is dip the clutch and you get a reading which measures full drivechain losses. It's then the rolling road feeding force into the drivechain.

Transmission losses. Hmm, I was having a chat on this very subject with my favourite rolling road guru just a couple of weeks ago. (all to do with a new race car I'm developing). I initially thought that transmission losses would increase as power output from the engine increased. His reply? "Why should it, there is only a given amount of effort required to overcome the frictional losses within the drivechain for a given road speed" I couldn't disagree with him !

Transmission losses II. They quote Porsche transmission losses are between 12-18% so they use an average of 15. I just don't understand their logic, but with two different models have observed losses of 20-30hp on engines developing 210ish hp, so this validates their figures.

Programming transmission losses. No RR I have ever used had a "preprogrammed" transmission loss figure. The Hoffman printout I get from one RR shows the actual run down losses as well as the power (not torque) curve. It's then a matter of adding the two together manually.

Power at the wheels. Yep, gotta agree this is what matters


Calibration. Just about every RR I use has a calibration certificate. Yet every RR gives a different figure, and not just by 1 or 2 kw. I have come the conclusion that RR power checks are about 3/4 physics and 1/4 art
 

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Originally posted by carrera:
[qb]Transmission losses........ I initially thought that transmission losses would increase as power output from the engine increased. His reply? "Why should it, there is only a given amount of effort required to overcome the frictional losses within the drivechain for a given road speed" I couldn't disagree with him ![/qb][/b]
I was having the same conversation (with myself
) when I read the article. But surely the losses would only stay the same a steady road speed? If you have tuned the engine to increase its power through the range, you will have also increased the torque available at each engine speed.

Consequently, if you go to full throttle for max acceleration, you will be putting more torque into the system than before at any given engine speed. If you put more torque into a gearbox, you will put more load on the teeth and bearings. The coefficient of friction will almost certainly be the same - so more load = more friction = more losses. Probably similar increases in bore friction and journal friction, differential etc. Also more tyre distortion losses?
 

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I'd have to agree with sgould ... also as the speed increases the frictional force increases. It's not ideantical, but similar to wind resistance, it's not completely linear. HOWEVER ... here's the problem with coast down: You will never put the kind of load on the system during coast down that you do during hard accelleration. Your engine certainly isn't decellerating with 200 hp and since frictional losses depend greatly on the power into the system (in either direction) why would the coast down be at all accurate? It will only measure coasting friction which will be considerably less, and as sgould mentioned there will be no tire distortion loss (or very little) during coastdown.

There's just no sense in guistimating crank hp. Compare wheel hp, or if you want to make a more "real world" comparison, compare videos or something. Guessing crank hp just seems too non-sequitur. It's usually quoted when the person is embarassed about the wheel hp figure.

Cheers,
Dubbya
 

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Originally posted by Adrian W:
[QBThere's just no sense in guistimating crank hp.  Cheers,
Dubbya [/QB][/b]
...yet this is the measure that just about every single make race championship uses, which is where all my experience comes from, & why it's so damned important to me.

We tried using in car GPS systems this year, with very little success. End of the season the cars were trotted off to the designated rolling road. The GPS system did not seem to give a good correlation between car speed and calculated power and won't be used for 2004. Hub based chassis dyno is a possibility though.

I'm still thinking about the friction issue, I can see merit in both arguments, but lack the engineering background to "know" the answer.
 

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...yet this is the measure that just about every single make race championship uses, which is where all my experience comes from, & why it's so damned important to me. --- Carerra

Indeed. I can see where it would be important, but I would assume if you're racing against a car in a professional class it's going to be similar-ish in design. Probably all RWD. Which means tranny and frictional losses will be similar. So as long as you were in the same region as the other members in regards to wheel hp, you should be the same with crank hp.

What they really should do is change the classes to classify based on wheel hp as it's so much more easily measureable. Oh well, wonders never cease.

Cheers,
dubbya
 

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So if you guys are saying the rundown to get drivetrain loss is [expletive deleted] and the correction to standard conditions is [expletive deleted], you'll just complicate things worse. You know why? Everybody will be bringing up the fact that it was too hot, or it was at a high altitude, bla bla bla. Then, people like yourself, Adrian, will come out and try to be the math whiz and get comparable flywheel figures for everybody anyways.

If I told you guys all the mods I have and then proceeded to give you a power reading at the wheels from the dyno you'd get a really big laugh and think I got ripped off on the mods. Reason being is that Hirsch's shop is in St. gallen Switzerland which just happens to be 779 meters above sea level. Then, in the middle of September, when I was there for the remap, the temperature in the dyno cell was 27 degrees Celsius. So then when I would give these excuses for the low wheel hp numbers I'd sound like Adrian W saying something along the line of, its too hot and dry here in California to get good dyno numbers, so it won't be fair for me.

Therefore, the drivetrain loss rundown, and correction factor, when done in the same fashion by everybody will indeed give us reasonably comparative numbers to discuss from our varied places on the Earth. The power numbers given to you by the factory are corrected flywheel numbers. And the case I want to make about that is that my car started out as a 185 hp model. When I first took it to hirsch for a remap, the only mods it had was the airbox mod and a cat-back exhaust. The base dyno run before tuning began showed 192 corrected hp. Considering the mods, this figure is fairly accurate and was about what was expected. This tells me the correction does work. It may not be 100% accurate but it was pretty close.

Yes, I agree that in certain situations, the corrected figure doesn't mean anything but for what we are doing here, it seems that the correction is the only way a large number of us can give a comparison of our modding efforts. The only way we can use true wheel hp as a group is at a dyno day where we are all there in the same conditions.
 

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Discussion Starter #20
Jeeze guys,

Better add dynos next to Religion, Politics and your mother to the list of things one mustn't talk about.


The article was only posted to give a basic description of how the things worked...
 
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