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.
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 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 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:
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.
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 !!
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.
Also called chassis dynamometers, these are used to measure power
at the driving wheels. This avoids the inconvenience of having to remove the
engine to test it if a tuning modification has been made. However, it means
that the power figures obtained will be lower than the flywheel power because
of the frictional losses in the drivetrain and tyres. This leads to one of the
biggest sources of confusion, error and plain misinformation in the tuning
industry. You see, as discussed above, all major manufacturers quote flywheel
power so it is understandable that people want to know if the hard earned cash
they spent on tuning mods increased the power of their engine and by how much.
To know this for certain means knowing how much the transmission losses are.
There is enormous pressure on rolling road operators to be able to quote
flywheel bhp rather than wheel bhp and most operators now run proprietary
software systems which "supposedly" print out flywheel power.
PROBLEM !! - THESE SOFTWARE SYSTEMS DO NOT AND CANNOT WORK !!
Yes - I know - the whole chassis dyno tuning industry quotes
flywheel figures and here's me saying none of it works. So I'd better explain
some more and then you can make your own mind up.
First, let's look at how a chassis dyno works. The car is driven
onto a rig so that the driving tyres are resting between two steel rollers. The
torque is measured at different speeds in exactly the same way as an engine
dyno works except that it is torque at the rollers rather than torque at the
flywheel. The braking load is applied to one of the rollers by either a
hydraulic (water brake) or electrical system again in just the same way as the
engine dyno would apply a torque to the crankshaft of the engine. The same
universal equation at the top of the page can then be used to calculate bhp at
the rollers by knowing the torque and the rpm of the rollers (NOT the rpm of
the engine at this stage) - but if the engine rpm is measured simultaneously
then we can know roller bhp at a particular engine rpm. The BIG problem with
all this is if any tyre slip is taking place. Remember these are smooth steel
rollers which over time get quite polished. How much grip do you think you
would get if roads were made of polished steel rather than tarmac? The effects
of tyre slip are complex (i.e. I don't pretend to fully understand them
myself!) but what I do know is that you can get some really strange bhp figures
from highly tuned engines on narrow tyres and the readings are invariably too
high not too low.
What is a transmission loss ? Well all mechanical systems suffer
from friction and a proportion of the power fed into a system will get
dissipated by friction and turn into heat and noise. Note the key phrase there
- "power fed into a system". For there to be a loss there must be an
input - simple and obvious yes but we'll see the relevance in a minute. When
your car is parked overnight with the engine switched off, the transmission
losses are obviously zero. When the car is running then some proportion of the
flywheel power will be lost in the gearbox, final drive, drive shaft bearings,
wheel bearings and tyres. For a given mechanical system these losses will
usually stay close to a particular fixed %, let's say 10% for arguments sake,
of the input power. So if the car is cruising and developing 20 bhp then 2 bhp
will get absorbed as friction - under full power, say 100 bhp, then maybe 10
bhp will get absorbed. Now it is true that not every component in a
transmission system absorbs a fixed % of the input power. Some components like
oil seals and non driven meshed gears (as in a normal car multi speed gearbox)
have frictional losses which are not affected by the input torque. These losses
do increase with speed of course but at a given rpm can be taken to remain
constant even if the engine is tuned to give more power. We'll look at real
world transmission loss percentages later. Finally, the biggest source of loss
in the entire transmission system of a car is in the tyres - they account for
half or more of the total losses between the flywheel and the rollers. Each set
of driven gears, i.e. the final drive gear or the particular gearbox ratio that
you happen to be testing the car in, only absorbs about 1% to 2% of the
engine's power.
Ok - so how do these software systems that supposedly measure
transmission losses so as to "predict" back to the flywheel bhp work.
The power curve at the wheels is taken in the usual way as explained above.
Then, at peak rpm, the operator puts the car into neutral and lets the rollers
slow down under the drag of the tyres and transmission. The software then
measures this drag (or "coast down loss") as "negative"
power and adds it to the wheel power to get back to the supposed flywheel
power. BUT - and hopefully you've all spotted the problem now - the engine is
not feeding any power into the drivetrain while the car is in neutral - in
fact it isn't even connected to the drivetrain any more!! Whatever drag this is
that's being measured it has nothing at all to do with the proportion of the
flywheel power that gets lost as friction when the engine is powering the car
in the normal way. The engine could now be an 800 bhp F1 engine or a 30 bhp
mini engine for all it matters because it isn't connected to the gearbox or
feeding any power into it. Obviously this "coast down loss" is
something to do with the transmission and tyres but it is not the true
transmission loss - in fact this coast down loss should never be expected to
change for a given car at a particular rpm regardless of how much you tune
the engine whereas a true transmission loss will increase as the engine power
increases because it is dependent to a large extent on the amount of power
being fed into the transmission. I've seen a car that over time was tuned from
90 bhp at the wheels to 125 bhp at the wheels and the "coast down loss"
stayed the same for every power run to within a fraction of a horsepower -
exactly as you would have predicted. As the engine was tuned to give more power
the "true" transmission losses must have also increased to some
extent but these chassis dyno systems don't, and can't, show this happening.
So is there any way of really measuring the true transmission loss
of a car? Yes - only one - by measuring the flywheel power on an accurate
engine dyno, the wheel power on an accurate chassis dyno and taking one away
from the other. There is no way on God's green earth of finding out the true
transmission loss just by measuring the power at the wheels.
So hopefully that's got you all thinking a bit more now instead of
just taking for granted the "flywheel" figure you were given last
time you took your car to the rollers. Even worse is the fact that some of
these software systems allow the operator to just programme in the %
transmission loss he wants the system to add to the wheel figures. So if that
isn't a nice easy way to show some big fat flywheel bhp then I don't know of a
better one. It's certainly a lot easier than actually doing some proper
development work to make the engine perform better - just dial in a bigger
transmission loss and bingo - the same wheel bhp now turns into a bigger
flywheel bhp - happy customer, happy dyno man - just a shame it was all sleight
of hand. See the end of this article if you doubt that this sort of thing
really happens.
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
corrections you need to make for cars with manual gearboxes are these:
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 increase in %
loss 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.
4wd 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 but it
isn't a subject I have sufficient data on to be definitive.
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. For sure though,
no 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. So take the wheel figure
and divide by 0.85 for FWD or 0.83 for RWD and that will get you as close to
the true engine bhp as you are ever going to know. In general though it is fair
to say that low powered cars have higher % losses than high powered cars. For
example, a 60 bhp Fiesta will have around 14 to 15 bhp total transmission and
tyre loss whereas a 90 bhp XR2 will only have about 17 to 18 bhp loss - 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.
Another rule of thumb I use which is quite accurate is to treat
the losses as being 10% of the flywheel power plus 10 bhp for FWD and 12% 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 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.
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 mainmenu page.
SUMMARY
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 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.
Be wary of "correction" factors for temperature and
pressure which are often used to "massage" measured bhp figures in an
unrealistic way.