CALCULATING THE CAMSHAFT REQUIREMENT

I have read all the “How to Select the Right Cam” articles published in the automotive magazines over the past few decades, and most of them posted on the internet.  None have ever explained how to select the right cam, or any cam.  Many of the articles give incorrect information, and the best of the articles only tell you how having the right cam parameters enhance your engine performance.  But none of these articles give any instructions, formulas or equations, so the reader can determine the right camshaft parameters, or even one of the cam parameters.  

The worst articles admit how the camshaft requirement is related to, and can be affected by many variables, such as valve diameter, stroke, port flow CFM, port area and others, but cannot show or explain their relationships, or how they affect the cam requirements.  Then they proceed to inform you there isn’t any simple formula or equation for choosing the right camshaft.  These MoTards have been telling the same story for many years, and will be for many more. 

I want to assure you that there is a simple equation that accurately calculates the cam requirements for a given engine combination, and has been in use for decades.  I am going to explain to you, which engine parameters determine each camshaft parameter, and explain their mathematical relationships.  It will be short and to the point.

MATHEMATICALLY RELATED PARAMETERS

If you know the piston diameter, you can calculate the piston disc area.  So, piston diameter and piston disc area are related.  Increasing the piston diameter by 10% will cause a 21% increase in the piston disc area.  So, they have a square / square root function relationship.  Pretty simple stuff.

Every parameter in the engine has a specific relationship with at least one other parameter.  Only related parameters can be used in calculating another parameters value.


The Valve Area Required is determined by the cylinder displacement and the max HP RPM.  They both have a 1:1 relationship with the Valve Area Required.

Theoretical Flow is the CFM that an unrestricted orifice area should produce.  No confines.  To gauge the efficiency of our ports, we calculate the unrestricted theoretical flow from the given valve area, then divide it into our actual port flow values measured on a flow bench.  This gives us a Coefficient of Discharge %.

The Effective Valve Area is related to and determined by the intake port flow CFM at the convergence valve lift, and the flow bench test pressure in in/h2o.  The Effective Valve Area is the theoretical unrestricted orifice area required to produce the measured port flow CFM at the given in/h2o.  The theoretical flow of a one square inch orifice @ 28” h2o would be 145.98 CFM.  A one square inch orifice test plate will only flow about 60-61% of the theoretical value of 145.98.  That is 89 cfm @ 28” h2o.

To calculate the valve open time, or cam seat duration, we need to understand what parameters affect the seat duration requirement.  RPM obviously affects the seat duration and is one of it’s related parameters, but it is not directly related.  The mean piston velocity and mean port velocity, both in FPM, are the only dictating factors in calculating the cam seat duration, and are directly related to seat duration by a square/square root function relationship.

The Intake Net Valve Lift is determined by the Minimum Required Intake Valve Area and the Effective Valve Curtain Area.  And our profiles rate of lift curve is determined by the rod to stroke ratio, or piston motion.  



HERE ARE THE EQUATIONS FOR SEAT DURATION AND NET VALVE LIFT


VE = Volumetric Efficiency %
PI = 3.141592654
B = Bore in inches
PDA = Piston Disc Area = (B^2 * PI / 4)
S = Stroke in inches
CD = CylDisp = PDA * S 
R = Rod Center distance in inches
N = Number of Cylinders
VAR = Valve Area Required = CD * HPRPM / 108,000
HPRPM = RPM at Peak HP
HPFPM = Mean Piston Velocity at Peak HP RPM
IVD = Intake Valve Diameter
IVSTD = Intake Valve Stem Diameter
IVA = Intake Valve Area = (IVD^2*(PI/4)) -  (IVSTD^2 * (PI/4))
ICFM = Intake Port Flow CFM @ convergence valve lift
In/H2o = Bench Test Pressure
FPS = Mean Port Velocity

EVA = Effective Valve Area = ((ICFM / SQRT(In/H2o)) / 27.58)
EVD = Effective Valve Diameter = SQRT(EVA * 4 / PI)
EVC = Effective Valve Circumference = EVD * PI

ReqILift = (VAR * (300 / FPS)) / EVC

ISeat = Intake Seat Duration = SQRT((HPFPM / (FPS * 60)) * PI) * 360

You can’t compare these seat duration values to a conventionally designed profile’s seat duration.  Our inverse radius, modified sin wave designed profiles will have 8 to 14 more degrees at seat, with the same duration @ .050”.  Select your cams with comparable durations at .050” and .100”.


CAM PROFILE GENERATION – ONE SIDE FROM TOP OF LOBE DOWN

R2S = Rod to Stroke = R / S
theta = 0  
deg = 0
idura = iseatdura + 4 ----  add 4 so we can cut off 2 from each side to mate in the clearance ramp
deg = 0
theta = PI ----TOP OF IS BOTTOM OF STROKE 
Do Until deg >= idura / 2
     vel2 = vel1
     cam1 = (((r2s + 0.5 * 1 - ((1 / 2) * Cos(theta) + r2s) * Sqr(1 - ((1 / (2 * r2s)) * Sin(theta)) ^ 2))

     / 1) * (reqilift / irock)) + icamlash
     rad = Sqr((r2s) ^ 2 - 0.25 * (Sin(theta)) ^ 2)
     vel1 = Abs(0.5 * Sin(theta) * (1 + 0.5 * Cos(theta) / rad) * (2) * (PI / 180) * (360 / (idura)) * (reqilift / irock))
     theta = theta + (((PI / 180) * (360 / idura)) / 4)
     deg = deg + 0.25
Loop

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