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 Why some
Chassis Dynamometer
Systems seem to read High or Low
when compared to others...
There are lots of reasons,
and most of them relate to people comparing different "things" or
measurements.
Very occasionally a dynamometer actually MAY be calibrated badly,
either by accident or because an engine tuner knows his customers
like to see "bigger BHP numbers" than their car really puts out,
especially AFTER tuning...
But usually a "high"
sounding BHP figure, or "low" readings are more a case of not
comparing like with like.
For
example...
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The first thing
you need to compare is if the BHP figures are measure at the
"REAR WHEEL" or calculated at the "FLYWHEEL"
or engine. This is a real "big" difference!
Always compare the actual MEASURED rear wheel BHP figures
only. Flywheel / engine figures are always much
higher, but at best a rough guess or estimate... See
here
Flywheel figures cannot be trusted and are a "rough"
guide at best.
-
The second thing you need to
compare is the actual correction factor used. E.g.
does it say DIN 70020, SAEJ1349, EEC, ECE, ISO, or
some other? Because depending on which
correction factor was used, the results figures vary,
according to conditions. For e.g. DIN 70020 standard
often results in "larger" figures than other commonly used
formulas. Non of them are actually wrong as such
- just different standards. If it says nothing, or
just "corrected" on your graph / data it is useless and if
you are using is as a measure of power, rather than for
comparative purposes, you may as well throw it away.
But for power comparisons you MUST always compare
like with like! See
here for
correction factor page. My own Dyno Systems
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If the dyno has
inadequate airflow, then the first few runs will show good
"normal" consistent power, and all the subsequent ones will
show less, and less, as the vehicle, transmission, engine
oil, engine block, airbox and bodywork heat up (heat soak)
to temperatures that are higher than would be seen on say a
motorway. Partly because the air (and to some
extent the fuel) that the engine receive get warmer! A
few degrees here makes a large difference. See
here.
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Twin roller
braked systems, or single rollers that are "too small"
both cause tyre deformation, and so see losses that
reduce the measured power. This effect increases
with MPH and weight, varies with tyre pressures etc, so
simply cannot be "allowed" for during
"calibration". It can be quite considerable!
Single drum systems with 16 inch diameter drums are
about the minimum that should be used, after a lot of
experimentation! At this diameter or above the
effect is small enough to effectively be ignored.
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