Watts Shop Performance
 Calculations and Caculator
How to figure MPH at any given RPM, tire diameter, rearend gear ratio and transmission gear or
vise versa.
RPM       x        Tire Diameter
-------------------------------------------------------------------        =  MPH
Rearend gears  x  Trans Gear Ratio  x  336
How to figure Carburetor CFM.
Engine CI                           Maxium RPM
----------------------        X      --------------------------           = CARB. CFM
2                                         1728
For easer use.
Engine CID   x    Maxium RPM
------------------------------------------------------------    = Carb. CFM
3456
How to figure valve-open duration from cam card.
BTDC   +   ABDC  +  180 = Number of Degrees (Duration)
180 is the degrees of a full intake stroke
Formula can figure Exhaust also   ATDC  +  BBDC  +  180 = Duration
How to figure pressure ratio for boost.
14.7  +  Boost
PR = ------------------------------
14.7
Example for 5 psi of boost
14.7  +  5 (psi)
PR = ---------------------------  =  1.34
14.7
1.34 is 34% more air going into the engine
Example #2 for 12 psi of boost
14.7  +  12
PR = ---------------------------  = 1.82
14.7
That 82% more air going into the engine
How to figure Air Flow Rate.
CID  x  RPM  x  0.5  x  EV
Air Flow Rate  =  ----------------------------------------------
1728
Example of a 302 Mustang
302 x 5500 x 0.5 x 0.85
Air Flow Rate = ------------------------------------  = 408 CFM
1728
How to figure Air Filter Flow Lost
Standard Barometric Pressure
Air Filter Flow Loss = -----------------------------------------------------------------------  -1 = ?
Standard Barometric Pressure - Loss through filter
30
Air Filter Flow Loss = ------------------------------------  -1 = 11% Loss
30 - 3
How to convert cc/min to lb/hr.
Cubic Centimeters                     Pounds
---------------------------      =     ----------------  X  10.5
Minute                                     Hour
How to calculate injector size for a given application.
Expected BHP  X  0.55
Pounds of Fuel Per Hour Per Injector  =  ------------------------------------------
Number of Injectors
How to figure Duty Cycle.
Stock Pulse Duration
Duty Cycle  =  --------------------------------
Time of one Revolution
Example :
5.0 msec
Duty Cycle  =  ------------------  =  0.67  = 67%
7.5 msec
How to figure Performance Gain.
Desired Horsepower
Performance Gain = ----------------------------------  -1 = ?
Original Horsepower

Example :
275 + 100
-----------------------  -1 = 0.36  =36%
275
How figure Horsepower.
Horsepower = Torque  x  RPM
-------------------------
5252