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WHY DO THE DYNO NUMBERS CHANGE?
The purpose of a chassis dyno
1. Used as a tuning tool. Primarily measures horsepower and torque at
the wheels/tires of the vehicle. It also
measures other vehicle/engine operations such as rpm, mph, boost/vacuum,
etc.
2. Less important but often used - Bragging rights.
Several variables can change the displayed horsepower
(HP) and torque (TQ) numbers that are produced on the Dynojet graphic
display/sheet.
1. The dyno
a. The roller portion of the dyno:
- Dyno roller bearing temperature (cold bearings = less displayed HP/TQ)
- Car placement on the roller
-- Angled vs. perpendicular to the roller (angled car placement = less
displayed HP/TQ)
-- Tire placement on the roller i.e., forward, top center, or aft (slightly
forward of center = more)
b. The dyno brand, model and type.
- Different brands can and will produce different HP/TQ numbers.
- Dyno companies often manufacture several models and/or types of dyno's.
There can be HP/TQ differences.
c. Initial dyno set up.
- How the manufacture or user sets up and calibrates the initial dyno
set up. This can affect the displayed HP/TQ numbers.
2. Cooling fan:
a. Too little air may increase the engine operating temperature and
reduce the displayed HP/TQ by retarding
the ignition timing due to engine knock.
b. Too much when the outside air temperature is low may delay the proper
operating engine temperature to be
reached and reduce the displayed HP/TQ.
3. Atmospheric Conditions:
a. Absolute pressure. Less pressure = less displayed HP/TQ because there
is less available oxygen in the air.
b. Outside air temperature. High temperatures = less displayed HP/TQ
because there is less available oxygen.
c. Humidity. Higher humidity = less displayed HP/TQ because there is
less available oxygen.
These 3 variables are combined together to produce a Density Altitude
(DA) atmosphere. Your engine utilizes fuel and oxygen, within the atmosphere,
during combustion to produce HP/TQ. If there is less oxygen in the atmosphere
due to a higher DA, less fuel can be burned and the engine will produce
less power.
The displayed HP/TQ numbers can be corrected with a correction factor
(CF). The CF is used to correct the DA and therefore, the displayed
HP/TQ back to a standard sea level (Standard Atmosphere) conditions.
The standard sea level atmosphere reference point for automotive use
is 60 degrees F, 29.92 inches of mercury (14.69 psi) and 0 % humidity.
Bottom line--High DA = less power. The CF is used correct the displayed
HP/TQ as if your car is operating at sea level with a standard atmosphere.
4. The car:
a. Operating fluid temperatures (cold fluids = less displayed HP/TQ)
b. Tire pressures (low tire pressure = less displayed HP/TQ)
c. Tire temperatures (low tire temperatures = less displayed HP/TQ)
d. Tire construction (a stiffer sidewall = more displayed HP/TQ)
e. Gasoline
- Octane (The wrong octane level (higher or lower) = less displayed
HP/TQ)
- Alcohol content (A different alcohol content from the standard 10%
used in most of the US will produce less displayed HP/TQ)
5. The Dynojet Graphic Display Options:
a. Smoothing - Scale of 1 to 5, with 5 being the smoothest. A 1 will
give you a higher displayed HP/TQ number
and 5 will give a lower number. Most tuners utilize 4.
b. The displayed scale i.e., rpm, mph, time. Changing this scale will
change the displayed HP/TQ.
c. The correction factor (CF). There are 6 different correction factors.
Most tuners in the US use STD.
Sometimes a tuner will use uncorrected (No CF) while tuning the car.
That will display the actual HP/TQ for the current atmospheric conditions.
Once tuned, they will add the CF to indicate what the HP/TQ would be
at sea level.
6. Modern ECU's: The computer that controls the combustion
process of your engine has an adaptive learning function. Based on how
you drive it can make adjustments to the tune of the engine. Since we
don't drive around at full throttle all the time, the engine's tune
may change slightly and produce less full throttle power HP/TQ. That
amount is usually very small and will not be felt.
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