**
Dyno Correction Factor and
Relative Horsepower**
**So what's all this correction factor stuff anyway??**
The horsepower and torque available from a normally aspirated
internal combustion engine are dependent upon the density of the
air... higher density means more oxygen molecules and more
power... lower density means less oxygen and less power.
The relative horsepower, and the dyno correction factor,
allow mathematical calculation of the affects of air density on
the wide-open-throttle horsepower and torque. The dyno
correction factor is simply the mathematical reciprocal of the
relative horsepower value.
**What's it good for?**
One common use of the dyno correction factor is to
standardize the horsepower and torque readings, so that the
effects of the ambient temperature and pressure are removed from
the readings. By using the dyno correction factor, power and
torque readings can be directly compared to the readings taken
on some other day, or even taken at some other altitude.
That is, the corrected readings are the same as the result
that you would get by taking the car (or engine) to a certain
temperature controlled, humidity controlled, pressure
controlled dyno shop where they measure "standard" power, based
on the carefully controlled temperature, humidity and pressure.
If you take your car to the dyno on a cold day at low
altitude, it will make a lot of power. And if you take exactly
the same car back to the same dyno on a hot day, it will make
less power. But if you take the exact same car to the "standard"
dyno (where the temperature, humidity and pressure are all
carefully controlled) on those different days, it will always
make exactly the same power.
Sometimes you may want to know how much power you are really
making on that specific day due to the temperature, humidity and
pressure on that day; in that case, you should look at the
uncorrected power readings.
But when you want to see how much more power you have solely
due to the new headers, or the new cam, then you will find that
the corrected power is more useful, since it removes the effects
of the temperature, humidity and atmospheric pressure and just
shows you how much more (or less) power you have than in your
previous tests.
There is no "right" answer... it's simply a matter of how you
want to use the information.
If you want to know whether you are going to burn up the
gearbox with too much power on a cool, humid day, then go to the
dyno and look at uncorrected power to see how exactly much power
you have under these conditions.
But if you want to compare the effects due to modifications,
or you want to compare several different cars at different
times, then the corrected readings of the "standard" dyno will
be more useful.
**How's it calculated?**
The Society of Automotive Engineers (SAE) has created a
standard method for correcting horsepower and torque readings so
that they will seem as if the readings had all been taken at the
same "standard" test cell where the air pressure, humidity and
air temperature are held constant.
The equation for the dyno correction factor given in 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
The pressure of the dry air Pd, is found by subtracting the
vapor pressure Pv from the actual air pressure. The relative
horsepower is simply the mathematical reciprocal of the
correction factor.
**Horsepower and Torque:**
Power is the rate at which work is done. When the engine
torque is turning the crankshaft and power is being delivered,
the resulting horsepower may be expressed as:
which can be simplified as
where: hp = horsepower, hp
t = torque, ft-lbs
rpm = engine speed, revolutions per
minute
This is a great formula. Basically it says that if you can
keep the same amount of torque, then the more rpm you can turn,
the more horsepower you get!
That's why Formula One and CART and IRL engines all turn
incredible rpm. The faster the engine turns, the more power it
can make (when it's properly tuned to operate at that speed).
Consider for example: a normally aspirated internal
combustion engine typically produces about 1 to 1.5 ft-lbs of
torque per cubic inch when it is properly tuned to operate at
any specific rpm. With a 2 litre (1 litre is about 61 cubic
inches) engine, producing 1.5 ft-lbs of torque per cubic inch,
you would expect to get about 180 hp at 5200 rpm... but you will
get a whopping 415 hp if you can get it to run at 12,000 rpm.
The 3.5 liter IRL engine is reported to produce about 650 hp
at 10,700 rpm. That would be about 1.5 ft-lbs per cubic inch.
The Ferrari 3.0 liter Formula One engine is rumored to
produce about 860 hp at 18,500 rpm. That would be about 1.33
ft-lbs per cubic inch.
And at the other end of the rpm spectrum, one model of the
360 cubic inch four cylinder Lycoming IO-360 aircraft engine
produces 180 hp at 2700 rpm, which is 0.97 ft-lbs per cubic
inch.
In general, production automobile engines that have a broad
torque band will produce about 0.9 to 1.1 ft-lbs per cubic inch.
Highly tuned production engines, such as the Honda S2000 or the
Ferrari F50 are in the range of 1.1 to 1.3 ft-lbs per cubic
inch. Highly tuned race engines such as NASCAR, IRL and Formula
One are often in the range of 1.3 to 1.5 ft-lbs per cubic inch.
Artical taken from www.wahiduddin.net/calc/cf.htm |