Air Filter Selection:

An average foam filter will flow 4.38 cfm/sq-in.

A good paper filter will flow 4.95 cfm/sq-in.

An oiled cotton gauze (K&N) will flow 6.03 cfm/sq-in.

To get your required filtered surface area for a oiled cotton gauze filter:

A = filtering area (square inches)

A = ( CID * RPM ) / 20839

CID = cubic inch displacement

RPM = rev./min. at max power

Then using the following modifying factors if using an alternative filter media:

A * 1.3767 = required surface area for foam element

A * 1.2181 = required surface area for paper element

Horsepower and Torque:

HP = (TQ * RPM ) / 5252

TQ = ( HP * 5250 ) / RPM

Corrected BHP = BHP * (1 - ((elevation/1000) * .03))

Horsepower, ET, and Weight:

A quick calculation for horsepower based on your 1/4 mile trap speed:

HP = ( TS / 234 )^3 * weight

HP = Horsepower

HP = ( TS * 0.00426 )^3 * weight

TS = Trap Speed (MPH)

This horsepower output is the minimum required for the specified trap speed.

It assumes ideal track conditions, weather conditions, traction, and vehicle aerodynamics.

It will understate horsepower required at speeds exceeding 100 mph.

ET = 5.825 * ( weight / HP )^(1/3)

Weight = ( ET / 5.825 )^3 * HP

HP = weight / ( ET / 5.825 )^3

For a quick idea of ideal ET assuming good street rubber and decent traction....

ET = 1363 / MPH

Horsepower:

Calculation assuming sea level and known Volumetric Efficiency

AP = pressure in psi

CR = compression ratio

HP = ( AP * CR * VE * CID * RPM ) / 792001.6

VE = volumetric efficiency

CID = cubic inch displacement

RPM = revolutions per minute

To convert from Barometric pressure in inches of mercury to psi:

PressurePsi = (pressureHg * 3376.85 ) / 6894.757

Cubic Feet per Minute:

Theoretical Engine CFM = ( CID * RPM ) / 3464

Actual Engine CFM =  ( CID * RPM * VE ) / 3464

Carburator CFM = ( CID * RPM * VE ) / 2820

Volumetric Efficiency:

VE = ( Actual CFM * 1728 ) / ( CID * RPM )

VE = ( Actual CFM *100 ) / ( Theoretical CFM)

VE = ( HP * 792001.6 ) / ( AP * CR * CID * RPM )

Cubic Inch Displacement:

CID = Number of cylinders * 0.7854 * bore * bore * stroke

Note:  All measurements in inches.

Rev Limits:

There are some rough standards for RPM limits.

These are based on piston speed measured in feet per minute.

Cast crank and rods should aim for under 3500 fpm.

Forged crank, rods, and beefed main caps can handle closer to 3800-4000 fpm.

Remember...these are rough....talk with your engine builder or an expert.

Piston speed (fpm) = (stroke * RPM ) / 6

RPM = ( Piston speed * 6 ) / stroke

RPM vs. MPH:

MPH = ( Tire Diameter in inches * RPM ) / ( 336 * Diff Gear Ratio * Trans Gear Ratio )

RPM = ( 336 * Diff Gear Ratio * Trans Gear Ratio ) / ( Tire Diameter in inches * RPM )

Diff Gear Ratio = ( Tire Diameter in inches * RPM ) / ( 336 * MPH * Trans Gear Ratio )

Tire Diameter in inches = ( 336 * Diff Gear Ratio * Trans Gear Ratio * MPH ) / (  * RPM )

Fuel Injectors:

BSFC typically ranges from 0.4-0.6 for gasoline engines.

BSFC = (Pounds of fuel per hour ) / HP

Since are good target duty cycle for fuel injectors is 80%...

Injector Flow Rate (lbs/hr) = ( HP * BSFC ) / (# of Injectors * 0.80 )

HP = ( IFR * # of Injectors * 0.80 ) / BSFC

Increasing fuel pressure can alter an injectors capability…

P1 is the fuel system pressure (psi) the injector is rated for

P2 is the fuel system pressure (psi) you want to use

F1 is the injector's static flow (lbs/hr) at it's rated fuel system pressure (psi)

F2 is the calculated injector static flow (lbs/hr) at the higher pressure

F2 = F1 * ( P2 / P1 )^(1/2)