FORMULAS

                                                            Useful Formulas


                                                   Carburetor Cfm Calculator:

(Cubic Inches/3456) X Volumetric Efficiency X Maximum RPM = Ideal Carb size

For instance, if you had a 440 you wanted to spin a maximum of 6000 rpm, and assuming 90% efficiency, you could use 687.5 CFM, I usually use the idea that you move one step up from that, so in this instance, a 700-750 cfm carb might be in order.


(440/3456) x 0.90 x 6000 = 687.5

Formula to figure approximate volumetric efficiency

Theoretical CFM = RPM X Displacement / 3456

Volumetric Efficiency = Actual CFM / Theoretical CFM X 100
 

                                                Potential Horsepower with heads

All flow values are for 10in-H20 because you can estimate potential Horsepower using these numbers.

Horsepower (per Cylinder @ air flow) = 0.43 º (Head flow CFM @ 10in-h20)
or 3.44 º CFM for an 8-cylinder engine.

For Instance, A stock "906" head flows 126 cfm at 0.500" lift. The potential HP is 126 º 3.44 = 433 HP

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This formula shows at what RPM peak HP should occur based on engine displacement.

This lets you select the proper camshaft duration to match to the heads airflow and engine size.

Formulas: Peak HP RPM = [2000 / Engine CID (per Cylinder) ] º Head flow CFM @ 10in-h20
Or HP RPM = [16000 / CID (for 8 cylinders) ] º CFM


For instance, the head same as above on a 440 engine. HP RPM = [16000 / 440] º 126 = 4,582 RPM


                                            CALCULATING PERFORMANCE

HP = Torque º RPM / 5252

HP = (MPH / 234) ^3) º Weight

torque = 5252 x horsepower / rpm brake specific fuel consumption = fuel pounds per hour / brake horsepower

bhp loss = elevation in feet / 1000 x 0.03 x bhp at sea level


                                      FORMULAS FOR SPEED AND GEARING

mph = (rpm x tire diameter) / (gear ratio x 336)

rpm = (mph x gear ratio x 336) / tire diameter

gear ratio = (rpm x tire diameter) / (mph x 336)

tire diameter = (mph x gear ratio x 336) / rpm


                               FORMULAS FOR ENGINE STROKE AND VOLUME

cylinder volume = pi (3.1414) / 4 x bore ^2 x stroke

stroke = displacement / (pi / 4 x bore ^2 x number of cylinders)


                                   FORMULAS FOR COMPRESSION RATIO

cylinder volume = pi / 4 x bore ^2 x stroke

chamber volume = cylinder volume / compression ratio - 1.0

displacement ratio = cylinder volume / chamber volume

amount to mill = (new disp. ratio - old disp. ratio / new disp. ratio x old disp. ratio) x stroke swept vol (cc) = bore ^2 º stroke º .0031416 / #cyl

Deck height (cc) = bore ^2 º deck height (inch) º .01996

Compression Ratio = (Cyl Volume + Chamber Volume) / Chamber Volume

Compression Ratio = [swept vol + deck height + head vol] / [deck height + head vol] (all in cc's)



                                        FORMULAS FOR PISTON SPEED

piston speed in fpm = stroke in inches x rpm / 6 rpm = piston speed in fpm x 6 / stroke in inches


                                       FORMULAS FOR AIR INTAKE FLOW

theoretical cfm = rpm x displacement / 3456 volumetric efficiency = actual cfm / theoretical cfm x 100


street carb cfm = rpm x displacement / 3456 x 0.85

racing carb cfm = rpm x displacement / 3456 x 1.1