Tech Tips

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 than 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 and 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 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, or 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 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.


The octane quality of a gasoline is its ability to resist detonation. Two laboratory octane numbers determine the overall octane quality of a gasoline are 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 keep 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.<.br> 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 are usually above .600 .

Inertia dynamometer

Inertia dynamometers measure how quickly an engine can accelerate a known rotational inertia from one rpm to another.
Torque, which is what we are measuring, equals rotational inertia; which is a function of the wheel you are accelerating times angular acceleration, which 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


Barometric pressure

Dew point


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

From Birgitta / från Birgitta

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