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Various Automotive Formulas

There are probably many different ways to calculate for any of the desired results. These are one I use.

Various online calculator tools can be found under the Tools menu.

Cubic Inch Displacement:  Bore x Bore x Stroke x 0.7854 x number of cylinders
Example:  4 x 4 x 3.480 x 0.7854 x 8 = 349.849 cubic inches

Cubic Inches to Liters:  Cubic Inches x 0.01639
Example: 350 x 0.01639 = 5.734 liters

Brake Horsepower:  Torque x RPM / 5252
Example:  400 x 5000 = 380.8 horsepower

Carburetor CFM Requirement:  (RPM x Cubic Inch Displacement x .8x) / 3654
(Note .80 to .85 is considered good volumetric efficiency of engine.)
Example:  (5000 x 350 x .85) / 3654 = 430.4 CFM

Miles Per Hour:  Two methods depending on what variables are known.
#1 (Tire Diameter x RPM) / (Final Gear Ratio x 336)
#2 (Tire Diameter x RPM x 0.002975) / Final Gear Ratio
Examples
#1 (27.0 x 3000) / (3.31 x 336) = 72.831 MPH

Final Gear Ratio:  Two methods depending on what variables are known.
#1 ((Tire Diameter x RPM) / MPH) / 336
#2 (Tire Diameter x RPM x 0.002975) / MPH
Example:
#1 ((27.0 x 3000) / 73) / 336 = 3.31

RPM:  Two methods depending on what variables are known.
#1 (Final Gear Ratio x 336) / (Tire Diameter / MPH)
#2 (MPH x Final Gear Ratio) / (Tire Diameter x 0.002975)
Example:
(3.31 x 336) / (27.0 / 73) = 3000 RPM

Rear Wheel Horsepower:
Determine base power by using a dynamometer or the manufacturer's rated horsepower at the crankshaft. Multiply the base horsepower by the generally accepted loss of the transmission. For example, an automatic transmission will cause a loss of about 20% while a manual transmission's loss is about 15%. Naturally these are only guidelines. For example, an engine rated at 300 horsepower running through an automatic transmission will result in (300 x .80) 240 horsepower at the rear wheels and (300 x .85) 255 horsepower with a manual transmission. GM rated engine horsepower by its gross (without accessories) up to net (with all accessories attached) in 1972. GM was also very, shall we say 'conservative' in their horsepower ratings over the early years for several reasons; racing was one where cars were classed based on advertised horsepower and shipping weight. Safety lobbies and insurance rates were other reasons for understated horsepower ratings. Often horsepower ratings were pure political fiction. When asked why the RPO L71 427cid/435hp Corvette engine was rated as such, the reply was, "it sounded good."

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