To round off the articles on power and torque, here is a real
example of how the coastdown losses from an actual car were measured on a
rolling road dyno. Some time ago I asked a colleague to run a series of
tests for me on their rollers. What we needed was a car with a reasonably well
quantifiable flywheel bhp and one that we could run in any gear without getting
wheelspin on the rollers. This ruled out modified cars that had not been on an
engine dyno and anything with too much power. Some time later a completely
standard cvh engined Fiesta XR2i came into their workshop and this seemed as
good a choice as any. The engine was in good condition and absolutely
unmodified according to the owner - a good chance therefore of it producing
close to the quoted horsepower.
The aim of the test was to see how wheel bhp and coastdown losses
change depending on which gear you run the test in. The rolling road in
question is a Bosch flywheel system, which means it has a heavy flywheel
attached to the rollers and the system works out power according to how quickly
the car can accelerate this large mass. It can't take "steady state"
power figures which can be a hindrance when setting up fuel and ignition systems
but on the other hand there is nothing for the operator to tinker with and
distort the readings - you just sit in the car and floor the throttle and wait
for the run to reach maximum rpm. At this point you can put the car in neutral
while it "coasts back down" and the system measures these coastdown
losses. Some dyno systems then add these losses back to the wheel bhp and call
the result "flywheel horsepower". Proponents of this method claim
that the "flywheel horsepower" figures so produced are more
consistent and repeatable than wheel bhp figures. Hopefully this article will
show the pitfalls in relying on coastdown losses by means of this real example
- anyway on with the plot.
Ford quote 110PS (i.e. about 108.5 bhp) as the standard flywheel
power for the car in question. Obviously every individual engine will differ
slightly and this quoted figure can only be a guide to the spread of power
outputs that a selection of engines would produce. To restate my own rules for
estimating wheel bhp from flywheel bhp - about 15% transmission losses for
front wheel drive cars and 17% for rear wheel drive is a rule of thumb. This
tends to overstate the losses for high powered engines and understate them for
smaller ones. A more sophisticated guide is to deduct 10% of the flywheel power
plus another 10 bhp for FWD and 12% plus another 10bhp for RWD cars. The XR2i
is FWD of course so if we apply those two rules to 108.5 flywheel bhp we get
either 92 or 88 bhp at the wheels respectively. So that's the sort of level of
wheel bhp that one would be expecting if the quoted flywheel bhp is correct.
To run the test, the car was warmed up and given a couple of runs
on the rollers to stabilize the temperature of the tyres, gearbox oil and
engine. A power run and a coastdown were then done in each of 3rd, 4th and 5th
gear with a few minutes for the car to cool down between each run to keep the
figures consistent. So first let's look at the how the wheel bhp changed in
each gear. The figures are as follows:
3rd gear - 95 bhp at the wheels
4th gear - 92 bhp at the wheels
5th gear - 88 bhp at the wheels
So why do the figures show a drop in power as a higher gear is
used? The engine of course is producing exactly the same flywheel power
regardless of which gear the car is in - what is changing here is the real
transmission and tyre losses. A higher gear means that the tyre speed on the
rollers goes up too - this leads to more power being absorbed as heat and
friction - the measured wheel bhp therefore goes down a bit. There are other
factors at work here too but it is not the aim of this particular article to go
into all of these in depth. The key thing is that the figures show a reasonable
and predictable trend and are in the estimated bhp range calculated above.
It makes the point though that there is no such thing as just one
true wheel bhp for a given car on a given set of rollers - it depends on tyre
pressure, gear ratio and a host of other things that have already been
covered in previous articles. Now devotees of the "coastdown loss" system
would say that it should compensate for this - it should reflect the larger
losses in higher gears by showing a larger coastdown loss which when added back
to the wheel bhp ought to give a flywheel bhp that stays the same in each gear.
So let's now look at the coastdown losses that were measured on each of those
runs and see if they actually do what is claimed. The coastdown losses were as
follows:
3rd gear - 17 bhp coastdown loss
4th gear - 27 bhp coastdown loss
5th gear - 44 bhp coastdown loss
We can add those losses back to the wheel bhp to get the estimated
flywheel bhp that so many rolling roads these days quote you.
3rd gear - 95 + 17 = 112 bhp
4th gear - 92 + 27 = 119 bhp
5th gear - 88 + 44 = 132 bhp
Well clearly something isn't working here. The coastdown losses
(whatever it is that they are actually measuring) are rising much more in a
higher gear than the actual transmission losses are, leading to larger
"flywheel" bhp figures in the higher gears. The engine is producing
the same power all the time and although we can never know for certain exactly
how much power this particular engine had, we can be fairly certain it isn't
far away from the factory quoted power. Even the 3rd gear "flywheel"
figure is a tad on the high side but it is within the realms of possibility -
the figures in the other two gears are obviously not.
The wheel bhp data show a consistent and understandable pattern.
Adding back the coastdown losses leads to power figures which vary much more
and make less sense. The point to remember is this - if the coastdown losses
really were an accurate measurement of the true transmission losses then we
would expect to end up with the same estimated flywheel bhp in all 3 gears. The
fact that this does not happen means by definition that the coastdown losses
are measuring something other than true transmission losses - in turn this
means that adding them back to wheel bhp cannot result in true flywheel bhp.
The fact that they result in horsepower numbers much larger than the 108.5 bhp claimed
for this engine only go to reinforce the message.
So the moral, for the last time hopefully, is to look at the wheel
bhp as well as (or preferably instead of) the estimated flywheel bhp. It won't
be a figure you can take for gospel and it will change from day to day and from
rolling road to rolling road. With a modicum of common sense in keeping the
test conditions the same and applying reasonable amounts for transmission
losses it will get you "in the ball park" of what the true flywheel
figure might be. The flywheel figure generated from coastdown losses though,
can vary from the sublime to the ridiculous. Every now and then it might come
up with a realistic bhp number but it might equally well be a country mile out.
To estimate true flywheel power from wheel power just apply the
rules given at the start of this articles in reverse. The simple formula is
therefore:
FWD cars - divide wheel bhp by 0.85
RWD cars - divide wheel bhp by 0.83
The more sophisticated formula is:
FWD cars - add 10 to the wheel bhp and then divide the result by
0.9
RWD cars - add 10 to the wheel bhp and then divide the result by 0.88
Remember these are estimates. The only way of knowing true
flywheel bhp for a particular engine is to run that engine on an engine dyno.
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