How To Calculate CFM:  Engine size (CID) x maximum RPM / 3456 = CFM CFM @ 100% volumetric efficiency 

Cylinder volume (displacement):  3.1416 x bore x bore x stroke  
Compression ratio:  Vi ñ V2 (Where Vi is total volume of one cylinder and V2 is volume of space above piston at top of stroke) 

Brake Horsepower (BHP):  RPM x torque (in lbs ft.) = 5252 (Note: Formula is actually 6.28 x RPM x torque by dividing 6.28 into 33,000 we get 5250Piston speed (ft. per mm.) = 2 x RPM x stroke in feet) 

1 mile per hour:  1.467 feet per second  
Engine displacement:  Cylinder volume times number of cylinders  
Piston speed (ft. per mm.):  2 x RPM x stroke in feet 

Torque:  BMEP x Swept volume (in cc) 2473 

MPH: 
(RPM x wheel diameter in inches) = gear ratio x 336
(Note: Wheel diameter is overall diameter of the inflated tire, not the nominal diameter of the wheel.) 

Carburetor size formula:  CID x RPM x volumetric efficiency* /3,456 = CFM Volumetric efficiency is how well your engine can breathe. For a street stock (slight modifications) plugin .80. For a full race engine use .90. 

How to calculate your compression ratio:  Compression ratio = V1+V2+V3+V4 / V2+V3+V4 V1 = calculated swept volume (bore x bore x stroke x .7854) V2 = deck height volume (bore x bore x .7854 x measured deck height) V3 = head gasket volume (bore x bore x .7854 x compressed gasket thickness) 

Bore and stroke formulas (common combinations given):  CID = bore x bore x stroke x .7854 x # of cylinders 350 Chevy Block w/ 400 crank 4.000 bore x 3.750 stroke = 377 CID 4.030 bore x 3.750 stroke = 382 CID 4.060 bore x 3.750 stroke = 388 CID 

How to determine your transmissions gear ratio:  1. Index the input (the part that goes in the engine) and the output (the part that goes to the drive shaft) shafts with a thin line. Make corresponding lines on both ends of the transmission case to use as your pointers. 2. Put the transmission in the gear you want to check. 3. Turn the input shaft until the tail shaft makes one full revolution. The number of turns made by the input shaft, including fractions, is the ratio for that gear. High gear is almost always 1:1, over drive will be more than that but the lower gears will normally produce less than one full revolution of the tail shaft. To make this process even more accurate turn the input shaft until the output shaft makes ten revolutions, then divide the number of revolutions made by the input shaft by ten (duh!). 4. To determine the final ratio for running in that gear multiply the transmission ratio by the rear end ratio. Do you know the rear end ratio? 

Calculate tire diameter:  circumference divided by 3.14159 (pi) Note: multiply the diameter by pi to calculate the circumference. 

Drill sizes for tapping threads: 


Drill sizes for tapping threads – Fine: 


Transmission Gear Spacing Ratios:  Formula: Gear spacing ratio = Gear (new) / Gear (old) Example: Difference between 280Z 5spd 1st and 2nd gear. 2.077 (2nd gear) / 3.321 (1st gear) = 0.625 or 2nd gear is 63% of 1st gear. 

Overall Gear Spacing Ratios:  Formula: Overall Ratio = Transmission gear ratio x Final Drive (Differential Gear ratio)
Example: 280Z 5spd 1st gear with 3.54:1 Differential Ratio 

Maximum MPH in each gear:  Formula: MPH = rpm x tire diameter (in) / gear x final gear ratio (diff) x 336 Example: Maximum mph in 280Z 5spd 1st gear using 6500rpm redline. MPH = 6500rpm x 24.0″(tire dia.)/ 3.321(1st gear) x 3.54(diff gear) x 336 = 39.5mph Note: Top gear is most likely hp limited, and would not be able to reach max rpm. 

RPM @ 65mph (Derived from Max MPH formula):  Formula: RPM = mph x gear x final ratio x 336/tire diameter Example: RPM at 65mph for stock 280Z 5spd 65mph x 0.864(5th gear) x 3.54(diff gear) x 336/24.0″(tire dia.) = 2783 rpm 

Shift Points:  Formula: RPM desired = RPM shifted at x(gear shifted to/gear shifted from) Example: 4000 is desired rpm after shift into 2nd gear from 1st gear. 4000rpm / (2.077/3.321) = 6396rpm is when you shift from 1st to 2nd. 

Valve Spring Rate:  R = (OL – CL) ./. (IH – OH) R = valve spring rate OL = open load CL = closed load IH = installed height (closed height) 

Rod Ratio:  RR = L ./. R RR = Rod ratio L = center to center length 

Starting Ratio:  Manual transmission SR = AR x FG SR = starting ratio 

Mile per hour:  MPH = (RPM x R) ./. (168 x AR) RPM = revolutions per minute R = rolling radius of driving tire (inches) AR = Overall gear re Formulas for weight distribution percent of weight on wheels = weight on wheels / overweight x 100 
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