Engine Tuning and Diagnostic Formulas
References:

"Two Stroke Tuners Handbook" 
by Gordon Jennings

"Design and Simulation of Two-Stroke Engines" 
by Dr. Gordon P. Blair


Horsepower  (added by request)

     1 HP = 152 lbs.  moved 220 ft. in 1 min.  (James Watt)
 
        HP = Torque X RPM  / 5252 (in ft.-lbs.)
        HP = Torque X RPM / 63024 (in in.-lbs.)
        HP = Kilowatts X 1.34  

Torque

     Torque = Distance X Weight

Specific Power of any Engine

        SP (in hp/liter) = Horsepower / Displacement (in liters)


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

Brake Mean Effective Pressure (BMEP)

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

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

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

Piston Acceleration
          
   Gmax = ((N2 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[cos-1 (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(c2 - (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

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

Exhaust Systems Tuned Length

   Lt = (Eo x Vs) /  N

   Lt is the tuned length, in inches
   Eo is the exhaust-open 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( D12 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

Rotary Valve Timing

   The Rotor Valve timing sets the low end power.  Insure a snug fit
   and flat surfaces between the rotor and backplate.
   The point of opening for a rotary valve is best established at the
   point of transfer port closing if you want a very broad range of 
   power.  Maximum power is obtained by opening the intake port
   somewhat earlier: from 130 to 145 degrees before top dead center (BTDC).

Exhaust Port Timing

   Sets the RPM range of the engine.  Lower the Exhaust timing for
   low end power and raise the timing for high end power.
   The higher the Exhaust timing the higher the RPM band. 


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