Formulas

 Predicting
Power

 BHP = PLAN/33,000
 P is brake mean effective pressure, in
PSI
 L is piston stroke, in feet
 A is the area of one piston, in square
inches
 N is the number of power strokes per
minute

 Piston Speed

 Cm = .166 x L x N
 Cm is mean piston speed, in feet per
minute
 L is stroke, in inches
 N is crankshaft speed, in RPM

 Brake Mean Effective Pressure
(BMEP)

 2Stroke BMEP = (HP x 6500)/(L x RPM)
 4Stroke BMEP = (HP x 13000)/(L x RPM)

 L = Displacement in
Liters

 i.e., 80 cc = .08 Liters
 1 ci. = 16.39 cc

 Piston
Acceleration

 Gmax = ((N^2 x L)/2189) x (1 + 1/(2A))
 Gmax is maximum piston acceleration, in feet
per second squared
 N is crankshaft speed, in RPM
 L is stroke, in inches
 A is the ratio of connecting rod length,
between centers, to stroke

 Piston Stroke
Motion
 S = R cos X + L cos Z
 S = the distance piston wrist pin is from
center of crankshaft
 R = the radius of the crankshaft wrist
pin
 L = the length of the connecting rod
 X = the angle of the wrist pin
 Z = the angle of the connecting rod
 or
 sin X = R/L sin Z

 Piston Travel vs. Crank
Rotation

 d = ((S/2) + L)  (S/2 cos X)  L
sin[cos1 (S/2L sin X)]
 S = Stroke (mm)
 L = Connecting Rod Length (mm)
 X = Crank Angle Before or After TDC
(deg)
 Note: (L) Rod Length is usually 2 times the
(S) Stroke
 OR
 For Spreadsheets and some Calculators
 HT = (r + c)  (r cos (a))  SQRT(c^2  (r
sin (a))^2)
 r = s/2
 dtor = PI/180
 a = d x dtor
 HT = The height of piston
 r = The stroke divided by 2
 c = The rod length
 a = The crank angle in radians
 d = The crank angle in degrees
 dtor = Degrees to Radians

 Exhaust Systems Tuned
Length

 Lt = (Eo x Vs) / N
 Lt is the tuned length, in inches
 Eo is the exhaustopen period, in
degrees
 Vs is wave speed in feet per second (1700
ft/sec at sea level)
 N is crankshaft speed, in RPM
 Length of Curved Pipe
 L = R x .01745 x Z
 L is length
 R is radius of the pipe bend
 Z is the angle of the bend
 Diffuser Proportions
 D2 = SQRT( D1^2 x 6.25 )
 D2 is the diffuser outlet diameter
 D1 is the diffuser inlet diameter
 6.25 is the outlet/inlet ratio constant
 Baffle Cones
 Lr = Le/2
 Lr is mean point of the reflection inside
the baffle cone
 Le is the length of the baffle cone

 Port Open Time

 T = ( 60/N ) x ( Z/360 ) or T = Z/( N x
6)
 T is time, in seconds
 N is crankshaft speed, in RPM
 Z is port open duration, in degrees

 Compression
Ratio

 CR = ( V1 + V2 ) / V2
 CR is compression ratio
 V1 is cylinder volume at exhaust
closing
 V2 is combustion chamber volume

 Carburetor Throttle Bore
Diameter

 D = K x SQRT( C x N )
 D is throttle bore diameter, in
millimeters
 K is a constant ( approx. 0.65 to 0.9,
derive from existing carburetor bore)
 C is cylinder displacement, in liters
 N is RPM at peak power

 Crankcase
Volume

 Primary compression ratio =
 Case Volume @ TDC / Case Volume at BDC
 or
 CRp = V1 + V2 / V1
 CRp is the primary compression ratio
 V1 is crankcase volume @ BDC
 V2 is piston displacement

 Resonance
Effects

 F = Vs / 2¼ * the square root of A / Vc (L +
1/2 the square root of ¼ A
 Vs is the sonic speed Uusually about 1100
ft/sec)
 A is the crosssectional area of the
inlet
 L is the inlet pipe length
 Vc is the flask (crankcase) volume

 Average Exhaust
Temperature

 Determine the exhaust gas temperature in
Kelvin
 (k = C + 273.15). This is usually a function
of the engine's BMEP.

 Torque

 1.00 lbft = 0.138 kgm = 1.35 Nm
 1.00 kgm = 7.23 lbft. = 9.81 Nm
 1.00 Nm = 0.102 kgm = 0.737 lbft

 Mass

 1.00 lb = 0.454 kg = 4.45 N
 1.00 kg = 2.20 lbs = 9.81 N
 1.00 N = 0.102 kg = 0.220 lb

 Distance

 1 in = 2.54 cm = 0.0000158 mi = 0.0000254
km
 1 cm = 0.394 in = 0.00000621 mi = 0.00001
km
 1 ft = 30.5 cm = 0.000189 mi = .000305
km
 1 mi = 63,360 in = 160,934.4 cm = 1.609
km
 1 km = 0.621 mi = 100,000 cm = 3281 ft

 Pressure

 1.00 bar = 14.5 psi = 1.02 kg/sqcm = 100
kPa
 1.00 psi = 0.069 bar = 0.070 kg/sqcm = 6.89
kPa
 1.00 kg/sqcm = 0.980 bar = 14.2 psi = 98.1
kPa
 1.00 kPa = 0.010 bar = 0.145 psi = 0.010
kg/sqcm

 Temperature

 F = 9 / 5 x C + 32
 C = 5/9 (F  32)
 K = C + 273.4

 Area / Volume

 1.00 sqin = 6.452 sqcm
 1.00 sqcm= 0.155 sqin
 1.00 cuin = 16.387 cc
 1.00 cc = 0.0610 cuin

 Power

 1.00 HP = 746 W
 torque (lbft) = 5252 x hp / rpm
 hp = rpm x torque (lbft) / 5252

The Weight of Air
 14.7 lbs. per sq. inch at sea level.

Air Density Calculation
 StdAirDensity = 1.22556 and is defined at
59.0F degrees, 0.0% humidity, and 29.92 inches
on the barometer.

 Temp_c = (Temp32.0) * 5.0 / 9.0;
 Temp_k = Temp_c + 273.0;
 Baro_mb = Barometer / (29.92 / 1013.0);
 Baro_pa = Baro_mb * 100.0;
 SaturationPressure_mb = 6.11 *
pow(10,(7.5*Temp_c)/(237.7+Temp_c));
 VaporPressure_mb = Humidity *
SaturationPressure_mb / 100.0;
 TempVirtual_k = Temp_k / (1.0 
(VaporPressure_mb/Baro_mb)*(1.0.622));
 // D = P/(T*R)
 AirDensity = Baro_pa /
(TempVirtual_k*GasConstant);
 % Std Density =
AirDensity/StdAirDensity*100

Additional Conversion Factors

 1 Centimeter  0.0328084 foot; 0.393701
inch

 1 Circular Mil  7.853982 x 10 to the
negative seventh square inches; 5.067075 x 10 to
the negative sixth square centimeters

 1 Cubic Centimeter  0.061024 cubic inch;
0.270512 dram (U.S. fluid); 16.230664 minims
(U.S.); 0.999972 milliliter

 1 Cubic Foot  0.803564 bushel (U.S.);
7.480520 gallons (U.S. liquid); 0.028317 cubic
meter; 28.31605 liters

 1 Cubic Inch  16.387064 cubic
centimeters

 1 Cubic Meter  35.314667 cubic feet;
264.17205 gallons (U.S. liquid)

 1 Foot  0.3048 meter

 1 Gallon (U.S. liquid)  0.1336816 cubic
foot; 0.832675 gallon (British); 231 cubic
inches; 0.0037854 cubic meter; 3.785306
liters

 1 Grain  0.06479891 gram

 1 Gram  0.00220462 pound (avoirdupois);
0.035274 ounce (avoirdupois); 15.432358
grains

 1 Hectare 2.471054 acres; 1.07639 x 10 to
the fifth square feet

 1 Inch  2.54 centimeters

 1 Kilogram  2.204623 pounds
(avoirdupois)

 1 Kilometer  0.621371 mile (statute)

 1 Liter  0.264179 gallon (U.S.
liquid);0.0353157 cubic foot; 1.056718 quarts
(U.S. liquid)


 1 Meter  1.093613 yards; 3.280840 feet;
39.37008 inches

 1 Mile (statute)  1.609344 kilometers

 1 Ounce (U.S. fluid)  1.804688 cubic
inches; 29.573730 cubic centimeters

 1 Ounce (avoirdupois)  28.349523 grams

 1 Ounce (apothecary or troy)  31.103486
grams

 1 Pint (U.S. liquid)  0.473163 liter;
473.17647 cubic centimeters

 1 Pound (avoirdupois)  0.453592 kilogram;
453.59237 grams

 1 Pound (apothecary or troy)  0.3732417
kilogram, 373.24172 grams

 1 Quart (U.S. dry)  1.10119 liters

 1 Quart (liquid)  0.946326 liter

 1 Radian  57.295779 degrees

 1 Rod  5.0292 meters

 1 Square Centimeter  0.155000 square
inch

 1 Square Foot  0.09290304 square meter

 1 Square Inch  645.16 square
millimeters

 1 Square Meter  10.763910 square feet

 1 Square Yard  0.836127 square meter

 1 Ton (short)  907.18474 kilograms

 1 Yard  0.9144 meter

POWER  AC CIRCUITS
 Efficiency = 746 x Output HP / Input
Watts
 3ø KW = Volts x Amps x PF x 1.732 /
1000
 3ø Amps = 746 x HP / 1.732 x Eff. x
PF
 3ø Eff. = 746 x HP / 1.732 x Volts x
Amps x PF
 3ø PF = Input Watts / Volts x Amps x
1.732
 1ø KW = Volts x Amps x PF / 1000
 1ø Amps = 746 x HP / Volts x Eff. x
PF
 1ø Eff. = 746 x HP / Volts x Amps x
PF
 1ø PF = Input Watts / Volts x
Amps
 HP (3ø) = Volts x Amps x 1.732 x Eff.
x PF / 746
 HP (1ø) = Volts x Amps x Eff. x PF /
746
 1 KW = 1000 Watts

 Eff. = Efficiency, PF = Power Factor, KW =
Kilowatts, HP = Horsepower

 POWER  DC
CIRCUITS

 Torque = HP x 5280 /
RPM
 HP = Torque X RPM /
5280
 1 HP = 36 lb.in. @ 1750
RPM
 1 HP = 3 lb. ft. @ 1750
RPM

Eff. = Efficiency, HP = Horsepower
OHMS LAW

 Volts (E) = Amps (I) x Ohms
(R)
 Amps (I) = Volts (E) / Ohms
(R)
 Ohms (R) = Volts (E) / Amps
(I)

R=Ohms, E=Volts, I=Amperes

 To figure miles per hour, multiply the
engine RPM by the Wheel Diameter in inches and
divide this by the Gear Ratio times 336
 or
 MPH = RPM * wheel diameter (in inches) /
gear ratio * 336

To figure engine speed (RPM), multiply by the
Speed in MPH, by the rear axle gear ratio times
336. Divide this by the tire diameter in
inches.
or
RPM = MPH * gear ratio * 336 / tire diameter
Minimum engine horsepower required to reach a 1/4 mile speed:
HPq = (0.00426 x MPH) x (0.00426 x MPH) x (0.00426 x MPH) x WEIGHT

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