What is a Dynamometer and How Does it Work?

by pumaracing.co.uk

There is in fact no way of directly measuring power – all types of dynamometer measure torque and then power is calculated from the formula we saw in the previous articles – BHP = Torque (ft/lbs) x rpm/5252. This basic equation is the cornerstone of all engine design and development work. 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. We’ll look at both of these separately.

Engine Dynamometers

If we want to know the power of the 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 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 were not so then the engine would either accelerate or decelerate. Let’s say we want to know the engine torque at full throttle at 3,000 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 3,000. The torque being applied is written down and then the operation would be repeated at say 4,000 rpm. Soon we get a complete chart of torque 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 steps – or they can measure a continuous torque curve while the engine accelerates at a pre-set 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 convertor 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 escape 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 formulae 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 on the continent 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. Copyright David Baker and Puma Race Engines

One of the tricks I’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. I saw an engine dyno sheet the other day where the ambient air temperature in February, in England was supposedly 37 degrees C. Now either that test was done with the temperature probe sat right on top of the engine or it’s a part of country I don’t yet know about where I would very much like to live !!

Engine Ancillaries

When installed in the car, the engine has to drive a number of items like the alternator and power steering pump which sap power. Also the exhaust and air filter systems will reduce power to some extent. If the engine is tested without any of these ancillaries fitted then it will show much higher power figures. The Americans used to rate their engines like this back in the fifties and sixties and often the installed power of the engine would only be 2/3 of the claimed figure in the sales blurb.

This used to be called “gross” flywheel power and if the ancillaries were fitted the power was called “net” flywheel power. Nowadays the gross system, which was very misleading, is not used and all modern published data should be “net flywheel” power. Major manufacturers abide by rigorous standards which set out how the engine should be installed on the dyno to simulate closely the “in car” conditions. Net power has nothing to do with it being measured at the wheels as so many people seem to think. That’s wheel bhp which is measured on a rolling road dyno and is discussed below. Copyright David Baker and Puma Race Engines

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.


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. Copyright David Baker and Puma Race Engines

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 %. For example if the loss percentage was 10% (just picking a nice round number for ease of explanation) and the car cruising on a level road was developing 20 bhp at the crankshaft then 2 bhp would get absorbed as friction. Under full power, say 100 bhp, then 10 bhp would 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.

Hub Dynamometers

Hub dynamometers are starting to become more commonplace nowadays. These work by lifting the car off the ground, removing the wheels and bolting individual dyno units to the wheel hubs. The obvious effect of this on power measurements is that tyre losses are removed from the equation. The power now being measured is flywheel power minus anything lost in the gearbox, differential and driveshaft bearings.

Most of these losses are proportional to flywheel power whereas it’s tyre losses that tend to be speed related and less affected by input power. So we would expect, and indeed see, power figures that are close to a constant percentage of flywheel power. The Swedish Rototest Institute has been testing production cars, and a few modified ones, for several years now on a very accurately calibrated and carefully managed hub dyno system so they have a huge database of standardised tests showing how hub power and torque compare to manufacturer’s claimed flywheel figures.

Something over 640 tests last time I looked. They don’t try and measure flywheel power, as indeed you can’t on anything other than an engine dyno, and their technical articles which are well worth reading explain why just as I’m also doing here.

Rototest Web Site

What is clear from their data is that regardless of how much flywheel bhp an engine is producing the hub figures are on average close to a constant percentage less than that. On FWD cars the hub bhp is about 93% of flywheel and RWD cars which lose a little more power because of having to turn the drive through 90 degrees show about 91% of flywheel bhp. When you see data on their site with different losses it’s because the claimed manufacturer’s figures are wrong, usually on the high side of course.

Coast Down Losses

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. Copyright David Baker and Puma Race Engines

True Transmission Losses

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.

So what should you do when you take your car to a rolling road? Firstly, make sure you get printouts that show the wheel bhp and not just the flywheel bhp. Then at least you can see if they look sensible in comparison. If you have a desperate 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 average front wheel drive road car with between 100 and 200 bhp loses about 15% of the engine bhp as transmission losses.

The average rear wheel drive road car with between 100 and 200 bhp loses about 17% of the engine bhp as transmission losses.

The 2% increase in losses over front wheel drive is because the differential has to turn the drive through 90 degrees at the back axle which soaks up a bit more of the engine’s power. Copyright David Baker and Puma Race Engines

4wd cars will have higher losses because of the extra differentials and other power transmission components. The tyre and main gearbox losses will be the same though. Correlating the performance of vehicles with the both 4wd and 2wd options (Audi’s and the Sierra Cosworth are examples) shows 4wd transmission losses to be about 5% higher than rwd. 22% seems to be a good average.

What each individual car loses is an unknown – it will depend on tyre sizes and pressure, suspension angles and other things, but it shouldn’t be far from the figures above. For sure though, no 2wd car in the world, unless it has flat tyres and a gearbox full of sand, loses anything like 30% of the engine’s power in the transmission and tyres as many rolling road operators would try to have you believe. In general though it is fair to say that low powered cars have higher % losses than high powered cars.

This is because some of the frictional losses are independent of engine power and so represent a bigger drain on a small engine. For example, a 60 bhp Fiesta will have around 14 to 15 bhp total transmission and tyre loss (25%) whereas a 90 bhp XR2 will only have about 17 to 18 bhp loss (20%) – a smaller % obviously. By the time you get to RWD cars with engines in the 300 to 500+ bhp range, losses can eventually drop to as little as 12% to 14% or so.

Converting wheel bhp to flywheel bhp and vice versa

To reflect the fact that % losses are high for low powered cars and vice versa I use the following equations which have been found to correlate well with real world transmission losses.

FWD cars – add 10 bhp to the wheel figure and divide the result by 0.9

RWD cars – add 10 bhp to the wheel figure and divide the result by 0.88

4WD cars – add 10bhp to the wheel figure and divide the result by 0.84

To estimate the expected wheel bhp from a known flywheel bhp just reverse the equations

FWD – multiply flywheel power by 0.9 and then deduct a further 10 bhp

RWD – multiply flywheel power by 0.88 and then deduct a further 10 bhp

4WD – multiply flywheel power by 0.84 and then deduct a further 10 bhp

Remember, these percentages are not “gospel” – they are good realistic averages. The measured wheel bhp can change depending on tyre pressure, tyre size, suspension angles and other things which won’t affect flywheel power – so the actual transmission loss % will also change. It pays to try and standardize as many of these things as possible if you intend to do a series of power runs over a period of time. Always use the same tyre pressure because this is a factor which can easily change from day to day and make sure the tracking is correct on a FWD car.

Also please remember that the manufacturer’s claimed power figures for a standard car are not gospel either. Even engines in perfect condition can vary by plus or minus 5% due to manufacturing tolerances. High mileage or poorly maintained engines can be well below the claimed output. It is no proof that a rolling road flywheel bhp estimate is correct just because it comes out as the same figure as the manufacturer’s. Always compare with the measured wheel bhp to see if the transmission losses agree with the data above.

DYNO Comparison

Imagine we take a car with a true 200 flywheel bhp engine to each of the various types of dyno. Assuming accurate dynos which is by no means always the case and calibration standards can be very lax then we would expect to see the following results.


Engine dyno – 200 bhp

Hub Dyno – 200 x 0.93 = 186 bhp

Wheel dyno – (200 x 0.9) – 10 = 170 bhp


Engine dyno – 200 bhp

Hub dyno – 200 x 0.91 = 182 bhp

Wheel dyno – (200 x 0.88) – 10 = 166 bhp

Tyre Pressure

Some time ago I had three almost identical race cars set up together in a group session at a rolling road. The engines were very similar except for minor differences in the camshafts fitted. One showed 118 bhp at the wheels, another showed 124 and the third showed only 98. The operator spent ages I’m told (I wasn’t there) trying to find why the third car was so poor.

It wasn’t till the next day when that particular owner was checking things before the race that he noticed that the tyres only had 7 psi in them – the car had sat unchecked over the winter and no-one had bothered to standardize the pressures before the dyno test. In the race, that car went just as well as the other two and if anything was slightly the fastest of the three. That gives you some idea of how much power a set of flat tyres can absorb.

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 in the 100 to 200 bhp category, I’ve seen as high as 35% and as low as 10%. So take the same car with 100 bhp at the wheels to 2 different rollers and you might get anything from 110 bhp to 140 bhp being “predicted” as the flywheel figure. In reality 100 bhp at the wheels will be no more than about 120 bhp at the flywheel. If being told a bigger figure makes you happy then good for you – the car won’t go any faster and you’ll be no nearer to knowing whether you really got more power out of it than standard.

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. So I don’t get too excited anymore when I see other people quote huge power outputs for their engine mods. If my engines still beat their ones on the track then they can quote whatever power figure they like. As the saying goes – “when the flag drops, the bullshit stops”.

Finally, the best rolling road con I’ve heard of to date is from a friend (before I knew him I hasten to add) who took his VW Passat to a well known VW specialist in the Oxford area for one of their proprietary “air box mods” which they said would give an extra 10 bhp. Sure enough he came away with a lighter wallet and a printout which showed 10 bhp more at the flywheel.

It wasn’t until you examined the printout carefully though, that it became apparent that the power at the wheels had dropped from 125 to 120 but the “coast down losses” had gone up by 15 bhp to give a net 10 bhp extra “predicted power” at the flywheel. The car, he says, felt slightly slower, which of course it was – by 5 bhp and that’s a poor way to spend your hard earned money. Exactly how they fiddled the rollers to show such a hugely increased “coast down loss” I’ll leave you all to speculate on.

The moral of the story is clear – if you don’t know the power at the wheels you don’t know diddly-squat – so as the man in Hill St Blues used to say – “be careful out there folks”.

Here’s a very good article by a Canadian tuning company on dynos and transmission losses – SDS dyno article – their main tech page has lots of other good stuff too and you’ll see a link to it back on my main menu page.


The “coast down loss” which some rolling roads add to the measured wheel bhp is not an accurate estimate of real transmission and tyre losses and will not give a reliable measure of flywheel bhp except by coincidence in some cases.

Average real transmission and tyre losses are about 10% of the flywheel power plus 10bhp for FWD cars and 12% plus 10bhp for RWD cars. This equates to about 15% to 17% for cars of “average” power output.

VW technical also quote their cars as losing, on average, about 15% of the flywheel power in the transmission and tyres.

The chassis dyno division of Bosch UK also suggest 15% as being a realistic estimate of transmission losses.

Hub dynos will show a fairly constant percentage loss of flywheel bhp regardless of engine size or power of about 7% for FWD cars and 9% for RWD ones.

Be wary of “correction” factors for temperature and pressure which are often used to “massage” measured bhp figures in an unrealistic way.

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