Formulas and Tuning Principles

Getting The Basics: General Formulas

100% VE AIRFLOW (cfm) = DISPLACEMENT (ci) x RPM / 3456

Actual Air Ingested / Theoretical Max Air Inj = VE

Mass Airflow (pph) = 12.5 (Pounds-per-pound) x Fuel Flow (pph)

It is generally accepted (and demonstrable) that a given engine (of resonable design) will achieve its best power on a mixture strength of approximately 12.5 parts of air to one part of fuel (gasoline) by weight.

BSFC = Brake Specific Fuel Consumption.

The ratio of fuel consumed (in pounds per hour) to horsepower produced.  simply how efficient the engine is at converting the fuel into power.

Typical BSFC values:

• Lower values = more efficient
• NA = .50 - .55
• Turbo = .55 - .60

Horsepower and Torque

• HP = (Torque x RPM) / 5252
• TQ = (Horsepower x 5252) / RPM

Engine Displacement

Disp = #Cyl x 0.7854 x bore x bore x stroke / 1000

Example: EJ25 2.5 liter engine
4-cylinders, 99.5mm bore and 79mm stroke
Disp = 4 x 0.7854 x 99.5 x 79 / 1000 = 2457.1cc

Cubic Inches of Displacement to Cubic Centimeters

CID = CC/16.39
CC = CID x 16.39

Fuel Injector Equations
Finding injector size required for a certain HP amount (total engine output)

LBS/HR = ([BSFC / (#cyl)] x HP) / Peak Injector Duty Cycle
CID = 1834.5/16.39 = 111.9

Max HP of injectors (lbs/hr) on gasoline

HP = ([injector size (lbs / hr) x duty cycle] / BSFC) x # of injectors)

Fuel Conversions

CC/Minute to LBS / Hour

LBS / Hr = CC / 10.5
CC/Min = LBS x 10.5

LBS / Hour to Gallons / Hour

GPH = LBS / 6
LBS = GPH x 6

CC / Minute to Gallons / Hour

GPH = CC x .01585
CC = GPH / .01585

Fuel Pump Flow Conversion

LBS / HR to Gallons / Minute

GPM = LBS / HR / 360

Miles to Kilometers Conversion

Kilometers = Miles x 1.61

Kilometers to Miles Conversion

Miles = Kilometers / 1.61

Kilowatt to Horsepower Conversion

Horsepower = kilowatt x 1.34102209Hp

Horsepower to Kilowatt Conversion

Kilowatt = horsepower / 1.34102209Kw

Speed Density and VE: What does it mean?

Volumetric Efficiency is defined as the charge into and out of the cylinders. More correctly, volumetric efficiency is a ratio (or percentage) of what volume of fuel and air actually enters the cylinder during induction to the actual capacity of the cylinder under static conditions. Therefore, those engines that can create higher induction manifold pressures - above ambient - will have efficiences greater then 100%.

Volumetric Efficiency can be improved in a number of ways, but most notably the size of the valve opening compared to the volume of the cylinder and streamlining the ports. Engines with higehr volumetric efficiency will generally be able to run at higher RPM and produce more overall power due to less parasitic loss moving air in and out of the engine. This applies to both NA and Turbocharged applications.

One of the most common ways to increase VE, (more air/fuel) is to increase the size of the intake valves along with port work on your head. A turbocharger can increase VE by raising the air pressure above ambient pressure (vacuum to boost). In addition,  more modern technique is variable valve timing, think Honda/Subaru/Mitsubishi. What this does is allow the valves to remain open for a longer period of time in reference to engine RPM, allowing your engine to take full advantage of the specified displacement in every RPM range. At higher RPM, the engine needs the valves to stay open for a greater percentage of the cycle time to move the charge (air/fuel) in and out of the engine.