# Automotive Formulas

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)

V4 = combustion chamber size converted to cubic inches   (1 cc = .061 ci)
Note: Be sure to allow for valve relief’s and subtract dome size when computing effective chamber size.

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?
To find out, index the input shaft (yoke) and the rear axel.  Turn the yoke until the axel makes one full turn.  The number of turns made by the yoke including fractions is your ratio.

Calculate tire diameter: circumference divided by 3.14159 (pi)
Note: multiply the diameter by pi to calculate the circumference.
 Screw Size N.C. Pitch Drill Size Decimal Equivalent 1/4″ 20 #7 .2010 5/16″ 18 1/4 .2570 3/8″ 16 5/16 .3125 7/16″ 14 23/64 .3680 1/2″ 13 27/64 .4219 9/16″ 12 31/64 .4844 5/8″ 11 17/32 .5312 3/4″ 10 21/32 .6562
Drill sizes for tapping threads – Fine:
 Screw Size N.C. Pitch Drill Size Decimal Equivalent 1/4″ 28 #3 .2130 5/16″ 24 17/64 .2720 3/8″ 24 21/64 .3320 7/16″ 20 25/64 .3906 1/2″ 20 29/64 .4531 9/16″ 18 33/64 .5156 5/8″ 18 37/64 .5781 3/4″ 16 11/16 .6875
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
3.321(1st gear) x
3.54(diff gear) = 11.75:1

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

IH = installed height (closed height)
OH = Open height

Rod Ratio: RR = L ./. R
RR = Rod ratio

L = center to center length
of rod
R = stroke

Starting Ratio: Manual transmission
SR = AR x FG

SR = starting ratio
AR = axle ratio
FG = first gear ratio of transmission
Automatic transmission
SR = AR x FG x TR
TR = torque converter torque ratio at stall

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
increased weight on wheels = [ distance of cg from wheels / wheelbase x weight ] + weight
Formulas for center of gravity
cj location behind front wheels = rear wheel weights / overall weight x wheelbase
cg location off-center to heavy side = track / 2 – [ weight on light side / overall weight ] x track
cg height = [ level wheelbase x raised wheelbase x added weight on scale / distance raised ] x overall weight duction

[dodgedieseldatabase.com]
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