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So your car will
read 100BHP at the top of a high mountain, on a hot day when
the pressure is low, as it would at sea level in winter on a
high pressure day! At least that's the plan!
This pressure and
temperature data has to be measured, and entered as figures,
or sensed automatically depending on which dynamometer
system we are using directly just before each "run" is made.
If this is not done the data or graph will not be accurate!
Now this is the
bit that's confusing people. It had to come!
There is more than
one correction factor! None are perfect. All are a
simplistic compromise. But its the best we have!
In my own software, I have a choice of four different ones
that are commonly used, and uncorrected for raw data or
electric vehicles:
-
SAE-J1349
the one that seems to work best! And is used by lots of
dyno companies as default.
-
DIN 70020
Also popular, and used by people who like bigger
numbers! In a lot of conditions it gives figures a few
percent higher than the rest.
-
EEC 80/1269
-
ISO 1585
-
Uncorrected
If you choose a
run and display a power curve in my own
dyno software
you can choose from these 5 options to see how it
effects the measured power. They are all different, but all
are correct! This is where a lot of the
"disagreements" about power figures arise!
See its not "that"
painful!
A word of warning!
If you have a print out, that just says "corrected" throw it
away. Its meaningless and most likely came from a cheap
dodgy dyno, that was built by someone with no idea! I
have seen lots of these, be warned! That was only the
FIRST problem with the data!
If you call
your dyno company up, and ask them "to what standard" their data
is corrected to, and they do not know, HANG UP! Go
somewhere else. If you are thinking of BUYING a dyno,
or dynamometer
the same thing applies, but this time run!
If they don't use
any correction factor at all be really afraid! You are
surrounded by a) IDIOTS or b) people who only test electric
vehicles!
YOU DON'T NEED
TO READ THIS BIT!!!
Here is the ACTUAL
formula, not that you really need to understand it for
SAE J1349:
SAE J1349 JUN90, converted to
pressure in mb, is:
where: cf = the dyno correction factor
Pd = the pressure of
the dry air, mb
Tc = ambient
temperature, deg C
No humidity correction is used for good reason.
Temp and pressure have a direct and predictable effect on
all engines pretty equally. And it is quite a marked effect
that requires correction to allow comparrison from one run
to the next.Here correction for temperature and pressure is
both accurate and simple.
Humidity is a little different. On a highly tuned motor that
may be turbocharged or have lower octane fuel than the
compression really requires the added humidity allows the
thing to make more power as it reduces detonation
possibilities and improves combustion.
On the majority of vehicles it causes a reduction in power
as it displaces some of the air that would otherwise have
been inducted meaning that there is less oxygen which
results is a richer mixture on some engines and just less
power on others that use a closed loop system to control
mixture. Unless they were running a little lean (for
economy/environmental reasons??) and then the less available
oxygen may actually help correct the mixture and either no
power will be lost or some gained...
So its not really possible or even desirable to try and add
a correction for humidity. It doesn't help. |
