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We are frequently asked to explain the meaning of statements
used within this industry such as "torque"
"detonation" and so on.
Here are a glossary for some of the FAQ.
What is:
Torque
The work that is being done. The moment of inertia
around an axis. (Force is the moment of inertia on a straight
line) This is what is being measured during dynamometer testing. Horsepower is
calculated from torque and shaft rpm. The formula is
torque x RPM / 5252 = Horsepower
Where does the 5252 come from? This is the constant that combines the
definition of one horsepower, the conversion of minutes to seconds and the
translation of circular motion into linear motion. Therefore when rpm is less
then 5252 power is less then torque and when rpm is greater then 5252 power is
higher then torque.
550 lb ft/sec* = 1 horsepower
1 Minute = 60 seconds
1 Revolution = 3.1416 (Pi) x 2
550 x 60 / 3.1416 x 2 = 5252
*This value was adopted by Scottish engineer James Watt
after experimenting with horses and found they could do work equivalent
to lift 550 lbs 1 foot in 1 second and sustain this work load for one day = 1
horsepower
Horsepower
The rate at which work is done. Horsepower measured at the output shaft of
an engine is termed brake horsepower or shaft horsepower.
Horsepower determined from the pressure in the cylinders is termed indicated
horsepower. Brake or shaft horsepower is less then indicated horsepower by the
amount of power lost to friction within the engine itself.
A weather correction and sometimes an inertia correction based on engine size
and flywheel weight etc. is added to the actual shaft horsepower reading.
The electrical equivalent to 1 horsepower is 746 Watts and the heat equivalent
is 2545 BTU (British Thermal Units)
Thermal efficiency
Thermal efficiency measures how well an engine
converts the chemical energy in the fuel into mechanical work.
The thermal efficiency can be calculated if you know the BTU of the fuel used
and your engines fuel flow requirement in pounds per hour.
1 horsepower equals 2545 BTU per hour.
Thermal efficiency in production engines is around 25% and in well developed
race engines around 40%
Several factors affect the thermal efficiency including the speed of the flame
front, cam timing and the homogeneity of the air-fuel mixture. Gasoline droplets
do not burn efficiently so it is important that the fuel mixture is totally
atomized for best combustion. Combustion chamber designs and port configurations
that produce high swirl or tumble tend to separate the air fuel mixture at high
speeds and is not desirable in high rpm race engines. A portion of the
fuels energy is lost trough the exhaust and if excessive amounts of unburned
gases are going out the exhaust ports the engine has not successfully converted
that energy into useful work.
Volumetric efficiency
Volumetric efficiency is expressed as a ratio of actual mass of air induced
into the cylinder to the theoretical mass of air that can fill the
cylinder under atmospheric conditions.
Typical values for volumetric efficiency for a naturally aspirated engine range
from 80 to 90% This is due to the fact that air is a mass and obeys the law of
physics and does not instantly fill the cylinder when the intake valve opens.
Values greater than 100% are attainable due to inertia and pulsation effects in
the intake and exhaust systems. Inertia effects can be modified with changes to
the intake runner lengths and cross sectional areas along with engine
speed.
Detonation
Detonation is the phenomenon in which, instead of the
mixture burning progressively as the flame spreads from the spark plug a portion of
the unburned air/fuel mixture gets raised to a temperature and pressure it can
not tolerate and ignites before the flame front gets to it.
Detonation occurs when the octane number is to low for the engines operating
conditions this includes to much timing, inefficient intake runners and combustion chambers that allows
for fuel separation, excessive lean or rich fuel mixture, high coolant
temperature, oil in the combustion.
Detonation causes the maximum pressure in the combustion chamber to be reached
before the piston reaches top dead center. The extreme temperature and pressure developed can cause broken
rings and / or pistons. An early sign can be found on the
spark plugs with aluminum specs on the porcelain and when it gets more severe
cracked porcelain and/or broken off side straps.
Pre Ignition
Pre ignition is started by a hot spot in the combustion chamber, witch
causes the fuel mixture to ignite before the plug fires. It is important to
smooth out any sharp edges in the combustion chambers and pistons tops to avoid
this phenomenon. Excessive soot build up caused by over rich fuel mixture or
inadequate oil control can also start pre ignition. Pre ignition will
destroy pistons in seconds under wide open throttle conditions.
Octane
The octane quality of a gasoline is its ability to resist detonation. Two
laboratory octane numbers determine the overall octane quality of a gasoline RON
(Research Octane Number) and MON (Motor Octane Number) The octane number
displayed at gas stations is an average of RON and MON (Ron + Mon / 2)
RON is measured under mild conditions and is more important in controlling
part throttle knock. If an engine is pinging at part throttle it needs more RON
MON is measured under more severe conditions and as a result the number is lower
than for the mildly tested Ron. If an engine is detonating at wide open throttle
a higher MON is required. MON is the most important number for race engines.
Higher octane does not mean more power unless the engine is experiencing
detonation.
Specific gravity
Specific gravity is a measure of how heavy (its density) the gasoline is
compared to water. If a gasoline has a SG of .690 this means that it is 69% of
the weight of water.
The higher the SG number is the higher the float sits in the gasoline. When the
float "rides" high on the fuel it will shut the fuel flow off earlier
at the needle and seat causing a lower fuel level in the float bowl. With a
lower fuel level there is not as much head pressure acting on the jets to help
get the fuel moving. The height of the fuel level should be maintained the same
for every fuel used.
Also when moving from a heavier (higher SG) gasoline to lighter (lower SG) the
fuel mixture needs to be richened up and consequently leaned out if moving
in the other direction. Change 1 jet size for every .010 change in SG assuming
the carburetor was correctly jetted with the old gas.
BSFC
BSFC (Brake Specific Fuel Consumption) is an indicator of fuel efficiency.
It tells us how much fuel the engine is consuming to make one horsepower
for one hour. Calculated in pounds (weight) since varying density fuels have
different volume. If we have a BSFC of .500 it means the engine is consuming ½
pound of fuel per hour for every 1 horsepower it makes.
For example if you make a valve lash change and the power increases 10
horsepower and the engine is consuming the same amount of fuel
it did before the change the engine
has become more efficient and will
have a lower BSFC
The lower the BSFC number you have while making the same power the more
efficient the engine.
Sometimes people wrongfully think of BSFC as a mixture indicator and tend to
look at what BSFC value an engine
should have for maximum efficiency.
For example if an optimized engine with a 390 rule carburetor has a BSFC of .500
and you change to a 750 carb and BSFC changes to .450 a change in optimum
BSFC has occurred because of the pumping losses created by the smaller carb not
because it should have had a lower BSFC with the smaller carb.
All out normally aspirated race engines on gasoline have BSFC around .350
Low compression street engines are in the .450 range and super charged engines
is usually above .600
Inertia dynamometer
Inertia dynamometers measures how quickly an engine
can accelerate a known rotational inertia from one rpm to another.
Torque witch is what we are measuring equals rotational inertia; witch is a
function of the wheel you are accelerating times angular acceleration, witch you
calculate from how quickly the rpm is changing.
Angular acceleration is the rate of increase in rotational velocity. For example
if you go from 5000rpm to 6000rpm in one second the angular acceleration is
1000rpm per second.
If you know the rotational inertia of the inertia wheel and you know it goes
from one rpm to another in a certain time you know the torque it took to
accelerate it at that rate.
Rotating inertia is mass combined with the geometry of the rotating part.
To find the rotational inertia of the inertia wheel you need to know the density
of the material it is made of, or simply just weigh it and measure its
dimensions. The rotational inertia can now be calculated.
Lamda (air fuel mixture)
Heat range (spark plug)
Ambient air temperature
Humidity
Barometric pressure
Dew point
Balancing
Conversion formulas
Mercury (Hg) x 13.596 = Water (column flow test pressure)
Water x .07355 = Mercury (column flow test pressure)
Stroke x rpm / 6 = Piston speed ft/min
Rpm x tire diameter inches / gear ratio x 336 = Speed mph (calculate tire growth for drag slicks)
Foot pond (lbs) x 1.355 = Newton meter
Square centimeter x .155 = Square Inch
Gallon x 3.785 = Liter
Liter x .2642 = Gallon
Kilogram (Kg) x 2.204 = Pound (lb)
Gram x .03527 = Ounces (Oz)
Psi x .069 = Bar
Bar x 14.5 = Psi
Mph x 1.609 = Kilometer/hour (Kph)
Rpm x Torque / 5252 = Horsepower
Cubic Inches x 16.387 = Cubic centimeters
Cubic centimeters / 1000 = Liter
Liter x 61.025 = Cubic Inches
8 cylinder cubic inch = 6.283 x stroke x bore x bore