- Finding your Volumetric Efficiency -

   Volumetric efficiency is a measurement of theoretical volumetric flow. Your engine has a defined volume, but unfortunately no engine is 100% efficient and is capable of using all of this available volume. You lose efficiency from friction on bearings and cylinder walls as well as leaks within the cylinder. The engine modifications you perform to your engine from porting to intake and exhaust all increase your volumetric efficiency. FLOW = HP.

Datalog Current Specs:

   In order to keep your efficiency calculations as accurate as possible you will need to get some sort of Tool to measure Mass Air Flow rate that your engine currently consumes and at what RPM you got that reading as well as the intake air temperature. When you have something that can datalog the flow of the MAFS, IAT and RPM. Go onto a flat straight road in the middle of nowhere and run your car for a 3rd and 4th gear pull and get some data. Although, I do not reccommend you do this on public streets as it can endanger other peoples lives.

Finding Air Density:

   When you find your Mass Air Flow value at a given RPM and Temp. you're not finding your volumetric air flow. What you have found is basically "Weight Flow" now we need to convert that value to a volume. Since the density of air changes throughout the temperature range we'll need to use a formula to convert your mass air flow number into a volumetric air flow number. In this formula you must also convert degrees F into degrees Rankine by adding 459.67°F to your existing measurement. You may not be familiar with the Rankine system but to keep it short it is the 'absolute' temperature measurement. Depending on where your temperature sensor is mounted, it may effect your true intake air temperature. Also dependant on your intercooler system or if you have metanol injection. You may want to leave the IAT reading alone or add or subtract from the value based on where it is in relation to your setups accessories.

Original Equation:



t₁ = Known density air temp.(°R)
t₂ = IAT Sensor Reading(°R)
d₂ = Density of known Air(lb./ft.³)
d₁ = Density of unknown Air(lb./ft.³)

Solving for d₂:

True Volumetric Flow

Now that we have the density of the air that is flowing through the engine we can now define the volume of the flow.

Original Equation:



AVF = True Volumetric Flow(CFM)
MF_k = Mass flow rate observed(lb./min.)
d₂ = Density of True Air(lb./ft.³)

Solving for AVF:

Finding Theoretical Air-Flow

The theoretical air flow value is the amount of air flow that in perfect conditions that would flow through the engine at the given RPM.

Original Equation:



RPM = Max. Engine RPM
TAF = Theoretical Air Flow(ft.³/min.)
VE = Volumetric Efficiency(%)
ED = Engine Displacement(C.I.D.)
ES = Engine Stroke(2 for Four Stroke)
C = Conversion Factor for in.³ to ft.³(1728 in.³/ft.³)

Solving for TAF:

Calculating Volumetric Efficiency

   You're just now one step away from being able to figure out your own fairly accurate calculation of volumetric efficiency for your engine. This value will allow you to be able to know where you need to improve the flow dynamics of your engine or allow you to more accurately size a turbocharger for your engine, power goals and needs.

Original Equation:



VE = Volumetric Efficiency(%)
AVF = True Volumetric Flow(CFM)
TAF = Theoretical Air Flow(ft.³/min.)

Solving for VE:



   Now you have a baseline for what your volumetric efficiency is. You might want to play around with different mods and recalculate your VE with every progressive mod or different style of intake or exhaust. If you can repeat the test conditions and acheive better flow and volumetric numbers than you're going to become a better tuner and get a better grasp on making horsepower efficiently. You can never be too efficient.