heinrich

I can pull up my dyno runs on my dynojet software and make the car look 100bhp higher or lower by skewing the factors ... so that is software. The data is the same.

hacker-pschorr

Not all dyno's measure the same way. Superflow's are a resistance based dyno that measure absolute torque which is completly different then Louie's dyno which works off inertia. These are not the only two types of dyno's out there.

heinrich

If there are only two types of dynos out there, then there are only two types of data. All of the same type, if they are correctly calibrated, will measure within an acceptable range, and it's up to the operator or reader to input variables when playing with software ... but the dynamometer;s output gives a consistent value for all tests across the board.

The answer is to get the file from the dyno as opposed to walking away with a printout.

hacker-pschorr

Talk about splitting hairs, are you just in the mood to argue today?

You are incorrect, there are more than two ways dynos measure power (unless you mean brake or inertia....):
Eddy current or electromagnetic brake
Magnetic Powder brake
Hysteresis Brake
Electric motor/generator
Fan brake
Hydraulic brake
Mechanical friction brake or Prony brake
Water brake

Just to name a few

So you are trying to tell me the ouput data from every one of these is going to be the same? I have three tire pressure guages that vary up to 5psi from each other. All of them are up to the current standard.

The answer is to stay with the same dyno for before / after results to keep the data consistant.

heinrich

LOL why don't we post the WikiPedia link where you got that data. http://en.wikipedia.org/wiki/Dynamometer

hacker-pschorr

I felt like giving you something to do

heinrich

I thought this conversation was interesting ... seems the consensus is, variations are small and calibration brings results into an acceptable range of accuracy:

Date: Tue Jan 18 10:43:44 1994
Subject: filing down spark plugs
X-Sequence: 3386

I recently read in the latest Hot Rod magazine about a method to
increase the performance and fuel economy. The method was to file away
the top electrode till its edge was concident with the axis of the
central electrode. Any luck with this method.

[Probably not and in any event, none that Hot Rod could measure. When
you read increased horsepower claims such as the 5 hp claim
SplitFire likes to claim for their plugs, consider this.

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.

Bottom line - take any claims of small increases in HP due to "tricks"
with a LARGE grain of salt.

JGD]



--------------------------------------------------------------------------------

From: pumaracing@aol.com (PumaRacing)
Subject: Re: ALUMINUM FLYWHEEL? yay or nay....
Date: 08 Oct 1998
Newsgroups: rec.autos.makers.vw.watercooled

>From: Mike Kohlbrenner
>
>And in the same vein -- have you ever worked with a DynoJet?
>
>Those devices are effected by the rotating masses in the
>vehicle being tested. As a result, they do not give an
>absolutely accurate measure of a car's performance. They
>certainly don't measure real engine horsepower. They also
>do not give an accurate portrayal of acceleration
>performance. For them to be really accurate, the ratio of
>the inertia of the testing drum (in terms of horsepower
>requirements for acceleration) to the rotating inertias of
>the car being tested would have to be exactly equal to the
>ratio of the car's linear inertia to the rotating inertias.

We don't have the Dynojet machine over here as far as I know but there are 3
types of rolling road dyno which I am familiar with.

1) - Clayton type water brake dyno's. 2 rollers in the normal way and an
internal viscous type torque converter to measure the applied force to the
driven roller.

2) Electric eddy current dyno's - Hoffman and Sun are the usual makes.

3) Massive flywheel Bosch system which a good friend of mine has in his race
workshop. A big cast iron "flywheel" is on the end of the driven roller and the
machine's electronics knows the inertia and calculates power from this. You
can't unfortunately hold a car at steady rpm and load as there is no "brake" as
such in the system.

All have their good and bad points but IMO they are all capable of reasonably
accurate results if calibrated and used properly. I have a beef with systems
with small diameter rollers which seem to suffer from tyre slip more often. The
Claytons fall into this category and I have seen weird results with highly
tuned engines. A guy I met once had a race car with a supposed 150 bhp wheel
figure on a Clayton system i.e 175 or so at flywheel. He was not competitive
with this engine and the dyno operator sent him to me. I built a complete new
engine for him which was set up on a Superflow engine dyno at exactly 141
flywheel bhp. He was most disappointed as the power curves kept coming and used
to wander off behind the dyno hut for a ciggie and a sulk. He took the car out
for the first time a few weeks later and knocked 4 seconds off his previous
best lap time and set 4 new UK track records in the next 6 races.

On another eddy current rolling road the car showed 116 bhp at he wheels some
time later which would be spot on for the measured flywheel figure.

My best guess is that his original engine had no more than 130 bhp true
flywheel figure but the engine builders never had it set up on an accurate
engine dyno so he doesn't know. Claytons have always made me suspicious after
this experience.

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.

My chart for trans losses for FWD is as follows:

wheel trans flywheel
bhp loss bhp

80 17 97
100 19 119
125 22 147
150 26 176
175 30 205
200 34 234

If the dyno you use shows much higher losses than this then you are being told
porkies.

As to your original point about rotating inertia - I agree that the dyno system
cannot "know" this about a particular car but some of the systems I have seen
take an input for the vehicle weight and use some compensation on the basis
that a heavy vehicle will have larger rotating bits. If the run is taken in a
high gear then the effect of rotating components should not be large due to the
slowish acceleration rates generated. I think the error in measured engine
power should not be more than 2% and I see bigger variation in power figures on
different rolling roads than this anyway. I tend not to use rolling road power
figures as absolutes as I don't think the machines work to close enough
tolerances. They give a decent guide to power and are of course mainly of use
in setting up fuel and ignition curves.

For accurate work you need an engine dyno where you can control oil and water
temps to get repeatable results.

Dave Baker at Puma Race Engines (London - England) - specialist flow
development and engine work.

heinrich

This is an excellent report on the subject: http://www.mainlineauto.com.au/produ...OWACCURATE.PDF


22 July/August
A U S T R A L I A N W O R K S H O P M A N A G E R
There is much deliberation around the industry about dyno accuracy.
Just how accurate are the dyno power readings? Why does the
same car get different results on different days? Why does the same
car get different results on dynos from the same manufacturer and
from different manufacturers?
One needs only to peruse some of the "forums" on the internet to see
that there can be significant differences in the results produced by
the same vehicle when run on different brand dynos and even on the
same brand of dyno.
But some people say that the accuracy of a dyno is not as important
as its ability to reflect the outcome when modifications are made to
a vehicle, and its repeatability. Others say a dyno is only useful as a
tuning tool and does not need to be relied upon to be accurate. Is it
any wonder people get confused. Read on to get an understanding
of the reasons behind the variations in results.
Once upon a time, there were really only 3 key reasons why a vehicle
could show a different power reading on the same Dyno at different
periods in time, they are
1. The Dyno was not correctly calibrated;
2. The Atmospheric (Weather) Conditions had changed;
3. The Vehicle had a problem that affected its Power Output.
Early chassis dynos were only able to apply load, and subsequent
models were extended to measure some basic outputs like torque,
power and speed. The dyno operator had little influence over the
dyno and had to accept the results that the dyno produced.
Now, would it not follow that if the vehicle was unchanged and
weather conditions were constant, that one vehicle could be
expected to produce at least similar results on several of the same
brand of dyno, or all dynos for that matter?
Is incorrect calibration likely to be a factor? Perhaps, but with
modern dyno hardware, a dyno is not likely to lose calibration
through normal use. Dynos that are available these days have
mostly evolved into state-of-the-art diagnostic tools and keep pace
with the high-tech advances in motor vehicle technology. The better
quality dynos can now generate vehicle specific information and a
level of accuracy that was not conceivable even a few years ago.
Faster computers and the flexibility of the Microsoft Windows
operating system have given dyno programmers a far more powerful
tool than they have ever had before.
So what other factors are involved? We would surely all agree that
any dyno manufacturer worth their salt is interested in producing
accurate and honest results from their dyno. An intrinsic part of
producing accurate, honest and consistent results from a dyno
requires that the dyno dictates the power produced by a vehicle
without undue influence from the dyno operator. Let's look at how
results can be influenced.
THE DYNO OPERATOR
The evolved dyno operator of today has much greater influence over
the results produced on a dyno than ever before, and the dyno
operator has become another reason why a vehicle can show
different power readings on the same dyno at different periods of
time. Is it possible that at least some operators would mislead a
customer about the extra power produced by an aftermarket add-on
or a performance tune if they were able to do that? In a market place
where more power seems to be better, would it be in a dyno
operator's interest to satisfy a vehicle owner's desire for high power
figures?
Many dyno manufacturers have ensured that the "operator factor" is
eliminated as far as possible, meaning that they lock the operator out
of areas of the dyno controller where results can be "influenced" and
follow a policy where all relevant dyno run factors are displayed on
run files and printouts so that any attempt that has been made to
manipulate the results can be easily identified.
But not all dyno manufacturers have taken that approach. At least
one brand of dyno has built in an easily accessible "correction factor"
that the dyno operator can use to increase or decrease the results
produced, and so, for example, in the hands of an unscrupulous
operator, the "correction factor" could be used to mislead a customer
about extra performance when no performance benefit was actually
achieved. It could also be used to convince a vehicle owner that a
vehicle generates far more power than it is capable of.
VEHICLE INERTIA FACTORING:
Another example of why sometimes people get curious dyno results
from different dyno brands is because one dyno manufacturer may
attempt to factor in Vehicle Inertia while another may not. Vehicle
inertia is best explained as this. If you are accelerating a car, then
part of the torque produced is consumed to accelerate all of the
rotating components of the car, ie Wheel/Tyres, Driveshafts, Gears
etc. This is not Driveline Loss, Driveline Loss is the friction generated
from gears, bearings, universal joints and Tyre to Roller contact.
There is nothing wrong with trying to account for vehicle inertia,
provided it is used correctly. The problem with applying vehicle
inertia is that not every car is the same. For example, if you were to
change from 17" Rims to 19" Rims on a car, you have effectively
changed the vehicle inertia, because with inertia the further the mass
is from the centre of a rotating object, the more inertia there is.
So on a dyno that uses vehicle inertia correction, an operator may
specify an inertia value for a vehicle, say for example a Commodore.
But the available Tyre/Rim Combinations from early to late model
Commodores can vary from 14" to 19" Rims. One model might also
have steel rims while another has nice light alloy rims. It is easy to
dismiss this an unimportant, however do not underestimate the
difference in tyre/wheel combinations as the Tyre/Wheel is
responsible for around 80% of the Vehicle's Drive Train Inertia
because these items have a much larger radius than any other
component in the drive train. A difference of 8KW's can be seen on
a Subaru WRX by just changing from one tyre/wheel combination to
another. The chance of over or under applying Vehicle Inertia is
fraught with inaccuracy and can provide misleading results, and
some dyno manufacturers choose to steer well clear of it. They take
the view that at the end of the day, a dyno is measuring how much
power is actually getting to road surface, and the power that gets to
the road surface (Motive Force) will govern just how quick a car really
is.
ATMOSPHERIC CORRECTION
Modern dynos also have the ability to correct power on the basis of
changes to weather conditions. A short explanation is needed here
to fully understand atmospheric correction. Atmospheric correction
is applied to compensate for changes to the combustive properties
of the ambient air (the quantity of oxygen per unit volume) in an
attempt to provide a level playing field between dyno runs.
Atmospheric Correction Standards are defined by organisations
such as SAE, ISO, DIN, ECE etc and each uses a slightly different
way of measuring change. Using the widely accepted SAE J607
standard, on an ideal day when the temperature is 15 degrees
Celsius, and there is 0% humidity, and the Barometric Pressure is
1015mbar, zero power correction is applied. Variations in any of
these three atmospheric factors will either cause positive or negative
power correction to be applied. If the temperature changed to say
19 degrees Celsius, the humidity to 34% and the Barometric
Pressure to 989mbar, the conditions are not as ideal and the vehicle
will not make as much power. By applying the SAE J607
atmospheric correction (in this case +3.89%), the power readings
JUST HOW ACCURATE ARE DYNO RESULTS??
A U S T R A L I A N W O R K S H O P M A N A G E R
are corrected to what the vehicle could be expected to make on an
"ideal" day.
The amount of power correction applied to a vehicle always needs
to be an accurate reflection of how much power is actually lost by or
gained by the vehicle as a result of unfavourable weather conditions.
The SAE J607 standard specifies a maximum ceiling of 10% power
correction, on the basis that any power correction in excess of 10%
will produce a power figure that cannot necessarily be reproduced
by the vehicle under optimum conditions.
So, a dyno that can accurately correct vehicle power according to
changed weather conditions should produce more consistent and
accurate results from run to run. A dyno that cannot correct vehicle
power according to weather conditions or where weather conditions
have not been regularly updated can produce results that vary
considerably from run to run. A 5ºC inaccuracy in air temperature
can lead to a 0.9% change in power figures. A 3 mBar (normal range
900 to 1050 mBar) inaccuracy in barometric pressure give a 1%
change in power.
SOME GOLDEN RULES FOR MORE ACCURATE DYNO RUN
RESULTS
Follow these golden rules to limit the variations in results between
dyno runs and give you a better idea as to whether the results of a
dyno run can be relied upon to be accurate:
• Dyno Printouts should always clearly show which atmospheric
correction standard was used (eg SAE, ISO) AND how much
atmospheric correction has actually been applied to arrive at the
end result.
• Be wary of any dyno that uses uncapped correction, as it can
produce figures that can never be reproduced even under ideal
weather conditions. If the percentage correction that has been
applied is not specified, there is a good chance that uncapped
correction has been applied.
• If the dyno uses the air intake probe temperature to calculate
atmospheric correction, make sure that the probe is not
improperly placed during the dyno run, and check that the
ambient temperature and the air intake temperature on the
printout are not unreasonably different.
• Use a dyno that provides automatic correction from an inbuilt
weather station, or make sure that the atmospheric conditions
have been updated just prior to the dyno run if the dyno cannot
provide automatic atmospheric correction.
• Make sure that the operator uses consistent test parameters
(gear, start speed, end speed, ramp rate) for multiple runs so that
any variations in results are from the vehicle and not the way it is
tested.
Mainline Automotive Equipment is a local manufacturer of high
quality chassis dynamometers, and has a reputation for honesty,
accuracy and repeatability on its dynos no matter where they are.
Mainline follows the tradition that the dyno should determine the
vehicle's performance, not the dyno operator, and Mainline dynos do
not have inbuilt "fudge factors" or other areas where vehicle
performance can be "adjusted" without detection.
Contact Mainline Automotive Equipment on (02) 9759 6661 for
more information, or visit their website at
www.mainlineauto.com.au.

cobalt

Please explain? To be within standard tolerance pressure gages need to be within +/- 5% full scale. I think we are getting off track here.

hacker-pschorr

Two of them go past 100psi (used on road bikes) so that's within 5%. Also, who knows if all of them are still calibrated properly? I'm sure I dropped each one of them more than once.

928ntslow

LOL...All good info. I think Eric (and others) is correct when it comes down to it. It doesn't matter what the number are, it's the difference in numbers on the same unit after any modification is made.

That being said, the question remains...would a greater range of increase be shown on one unit as compared to another? Meaning you dyno a baseline on 2 different units and then dyno again on 2 different units after modifications. Would one unit show a greater increase? Hmmmm.

We can certainly conclude that dyno's that measure rwhp are only good for measuring performance after modifications. The rest/numbers...well, I guess that's for bragging rights accompanied by your favorite alcoholic beverage