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Working out the percentage loss of my RR session on Saturday, Surrey Rolling Road was giving an 18.5% drop between wheels and estimated flywheel figures. Charlie seemed to imply that this was more realistic than other rolling roads, but I have no idea whether it was professional pride or the truth . I know that James' recent figures from Powerstation were a lot different to the ones he got on Saturday .

All I can say is that a lot of top tuning companies and individuals seem to go to him, so presumably he knows what he is talking about

Phil

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Yes, there was a slight difference between Surrey and Powerstation

Powerstation: 207bhp wheels = 288 at flywheel = 28%
Surrey: 214bhp wheels = 262 at flywheel = 18%

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Tyres will account for as much as half your transmission losses. I ran mine too soft at the last PowerStation RR day and lost about 30% (the dyno operator mentioned it as soon as I put the car on the rollers)

Normal losses are about 20% - 25% - anything less than this is either very good, or the result of an inaccurate RR

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Powerstation: 207bhp wheels = 288 at flywheel = 28%
Surrey: 214bhp wheels = 262 at flywheel = 18%

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Guess which one the max power boys will go to

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My BIG bugbear though is with so called transmission losses measured on the
overrun. Most dyno firms here now insist on giving "flywheel" bhp printouts and
often the measured wheel figure is never even shown. IMHO it is IMPOSSIBLE to
measure trans losses on a rolling road dyno.

What happens is this - you run the car under load until just after peak power
then the operator drops the clutch and lets the car freewheel down against the
roller which supposedly measures the "negative" power absorbed and treats this
as a trans loss.

Now one thing is for certain - trans loss is a percentage of power input to the
system. If the car is in neutral there is no power being fed in. Also the gears
are now backlashed against the wrong face of the gear teeth as the roller is
driving the car not the other way round. Under these circumstances I see no way
in which the trans loss shown can bear any relationship to actual losses when
the car is under power.

Tweaking the electronics to generate big overrun losses or just using a touch
of brake pedal while the car is winding down makes for big trans losses and a
nice fat flywheel power curve to keep the punters happy. I have a number of
rules of thumb about transmission losses which I have established over the
years from known engines.

On most Front Wheel drive cars trans losses are between 15% and 17% of the
flywheel figure. VW themselves quote 15% as being an average transmission loss
for their cars. (by trans loss I mean all losses between the flywheel and the
road so it includes gearbox, final drive and tyre losses). Low powered cars
tend to have a higher % trans loss because tyre losses are more of a constant
than a % of power input and so represent a bigger proportion of the engine
power than they do for powerful cars.

Rear Wheel drive cars can have 2% or so higher losses due to turning the drive
through 90 degrees before it gets to the wheels but they also often have direct
drive in 4th gear which cancels this out.

Trans losses will vary with the gear in which the car is tested. This is mainly
due to the higher wheel speed in a higher gear leading to greater tyre losses.
Always use the same gear and same tyre pressures to make comparisons meaningful
for power runs taken at different times.

ALWAYS use the measured WHEEL bhp figures and if you are desperate to know a
flywheel figure then add a notional trans loss yourself. The ONLY WAY to
accurately know flywheel bhp is to take out the engine and put it on an engine
dyno.

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I have a nice little side business repairing SuperFlow dynomometers, the
overwhelmingly dominant dyno in the US. Every magazine article I've
ever read used a SuperFlow. The standard SuperFlow is rated at 1000 HP,
10,000 rpm and 800 ft-lbs of torque. The RPM signal is converted to a
voltage by a tach chip before being submitted to an A/D converter. The
torque signal is derived from a strain gauge attached to the absorber.
This signal is also applied to the same A/D converter through an analog
mux. Horsepower before SAE correction is the simple calculation:

(torque (ft-lb) * RPM ) / 5252

This computation is done in an analog multiplier for the analog readout
and by the CPU for the digital readout. So good, so far. But here's
the kicker. The A/D converter is an 8 bit unit. That is, it digitizes
the incoming signal into one of 256 binary values. For torque, that is
800 ft-lbs / 256 = 3.13 ft-lbs per bit. For RPM, 10,000/256 = 39 rpm
per bit. At a constant 6000 RPM, the best HP resolution is 3.5 hp. At
a constant 500 ft-lbs of torque, the best HP resolution is 3.7 HP. This
lack of precision results in the best theoretical HP measurement at 6000
RPM being +- 3.5 hp. Worst case is 3.5 + 3.7 = 7.2 hp. The
root-sum-square (much more representative of the real world) is 5.0 hp.
The precision varies, of course, with RPM. The important point is any
horsepower variation less than about 5 hp is meaningless and is more
likely attributable to quantitizing error in the electronics. Understand
that this does NOT include other systematic error terms such as the
errors associated with the analog electronics or the torque sensor
calibration. I personally attribute no credibility to differences
less than 10 hp.

The other thing to keep in mind when viewing published figures is that
the most frequently published numbers are corrected to SAE Net. This
correction for ambient temperature, humidity and barometric pressure
is only approximate and is really suitable for generating numbers for
ad copy where they are legally required. We have conclusively proved
that the correction is only approximate using a client's dyno cell
that is equipped to control temperature, humidity and baro pressure.

To illustrate the problems involved, I've spent considerable time with a
client because his dyno isn't "producing the numbers he wants". His
engines, which he sells to racers who make buying decisions largely on
dyno sheets, are considerably down on power compared to what his
competition claims. His dyno is spot-on calibrated. He has carried an
engine around to two other shops, one of which is Bill Elliot's shop in
Dawsonville, GA. The span of readings on this engine among the three
dynos is over 80 HP on a 500 hp engine! I have personally checked two
of the dynos and know them to be properly calibrated. The difference is
in the buildup of error terms in this inherently inprecise measurement
system and in the SAE net compensation between Florida at sea level and
here in Atlanta at about 1000 ft elevation.

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As regards the Flywheel figs, we do not use the 'coast down' method, since its so unrelaliable. The Dyno takes into acount the drivetrain type, the tractive effort and other factors to formulate the estimates from the wheel figs it measures. The system was built by taking cars and running the engines on Engine dynos and then in the cars. Your typical front wheel drive car looses 17% to 20% of the flywheel power available through the transmission, It is extremely rare for FWD cars to loose more than this. I have seen 40% losses 'measured' apparently from other rollers and its simply not possible that the car looses this much power through the tyres and transmission!

Hope this helps, if you have any questions, please don't hesitate to contact me.

You will be pleased to know we have a kettle and coffee now!

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