×
×
• Create New...

# Engine Related Formulas

## Recommended Posts

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.)

Engine Displacement

Formula: Engine Displacement =  #Cylinders x pi/4 x Bore(2) x Stroke
Cylinder Volume = pi/4 x Bore(2) x Stroke
Example: 6 cylinder engine with 86mm Bore and 79mm Stroke
6 x .7853982 x 8.6cm(2) x 7.9cm = 2753cc

Static Compression Ratio
Formula: Compression Ratio = Cylinder volume + Chamber Volume/Chamber Volume
Cylinder Volume = pi/4 x Bore(2) x Stroke
Chamber Volume = Cylinder head cc + gasket cc + dish of piston cc (or) - dome of piston cc
Example: Cylinder Volume = 458.9cc (From above)
Chamber Volume =  44.6cc(Cyl. Head Vol) + 7.6cc(Gasket Vol) + 10.9cc(Piston Dish cc) = 63.1cc
Compression Ratio = 458.9cc + 63.1cc/63.1cc = 8.3:1

Piston Speed
Formula: Piston Speed in fpm (Feet per Minute) = stroke (in.) x rpm/6
Example: 3.11" x 6000rpm/6 = 3110 fpm

HP & Torque
Formula:   Horsepower = rpm x torque/5252 or Torque = 5252 x hp/rpm
Example: 140 ft.lbs @5600rpm = 140 x 5600/5252 = 149 hp
Example: 149 hp @5600rpm = 5252 x 149/5600 = 140 ft.lbs

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 formula's  (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.

Other formulas.

Calculate tire diameter:  circumference divided by 3.14159 (pi)

Torque =      Brake HP x 5,252                                                                                     RPM

Brake HP =  RPM x Torque                                                                                      5,252

MPH =        RPM x Tire Diameter in Inches                                                Gear Ratio (final) x 336

RPM =        MPH x Gear Ratio x 336                                                 Tire Diameter

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