Intake Valve Duration

Valves_123rf                    Cylinders_valves_cam_cutaway_123rf

Pictured above are some valves on the left and the valves in their place in the engine on the right. The valves factor in to the amount of air that gets into the cylinders.

What you do:

Choose a duration that gives you the power you want at the rpm you want.


Factors to consider:

1. The longer the duration, the more the engine will breath. This is good if you want more power at higher rpms. This also means your engine takes longer to develop power.

2. If the duration is too short, it cuts off air flow too early. Cutting off air flow early means less power.

3. Duration is limited to 0 - 330 degrees.


How it all works:
Cam_istock

This is a picture of a cam. The lobes on the cam are the egg shaped bumps. These lobes have lift and duration specifications that you dictate. The lift is how high the lobe on the cam will lift the valve. The duration is how long, in crank degrees, the valve will be open.

Intake valve duration is determined by the intake lobe on the cam. Intake valve duration is measured in crank degrees. When we set the duration in crank degrees, we are determining the point on the cam at which the lobe begins pushing the lifter up and where it lets the lifter back down. Since the cam rotates 1 time for every 2 times the crank rotates, an intake valve duration of 300 degrees means the intake valve is open for 150 degrees of a single revolution of the engine. Intake valve duration, intake valve diameter and intake valve lift factor in together to determine air velocity in the intake system. Long duration, large diameter and high lift all cause the engine to breath better at high rpms, but also cause the air to move slower at lower rpms, which reduces power at lower rpms. Short duration, small diameter and low lift all cause the air in the intake system to go faster, which increases power at lower rpms, but chokes air flow when the air velocity reaches the speed of sound. The speed of sound is 767.58 mph (miles per hour) or 67547.4 fpm (feet per minute).

Since the cam rotates 1 time for every 2 times the crank rotates and cam duration is measured in crank degrees, the HiPerMath equation for intake cam degrees is:

intake cam degrees = intake valve duration
                                                 2

Next, we calculate the angle from the x-axis to the point at which lift begins. We call this angle "theta".

theta = 90 - intake cam degrees
                                 2

Next, we calculate the x and y values of the point at which lift begins, using the angle we just calculated:

x = 0.5cos(3.14/(180 / theta))

y = 0.5sin(3.14/(180 / theta))

Next, we calculate the b-value for the equation y = ax2 + b, which is the equation for a parabola you have probably seen in your high school math classes. We use a parabola because it closely represents the shape of a cam lobe:

b = intake valve lift + 0.5

The 0.5 in the equation above is the radius of what is called the cam base circle. The cam base circle is the minimum radius the lifter will rest on.

Now that we have values for x, y and b, we can calculate the a-value for the equation y = ax2 + b :

a = -(y - b) / x2

Now that we have the a and b values for the equation, we plug in -0.4, -0.3, -0.2, -0.1, 0, 0.1, 0.2, 0.3, 0.4 for the x values (since the x values for our parabola will range from -0.5 to 0.5, to get values for y:

yn4 = -a(-0.42) + b
yn3 = -a(-0.32) + b
yn2 = -a(-0.42) + b
yn1 = -a(-0.12) + b
y0 = -a(0.12) + b
y1 = -a(0.12) + b
y2 = -a(0.22) + b
y3 = -a(0.32) + b
y4 = -a(0.42) + b

We calculated values for y above because we are going to use them in the distance equations below where we calculate the distance from (0,0) to (x,y) and subtract the radius of the cam base circle:

d1 = &radic (-0.42 + yn42) - 0.5
d2 = &radic (-0.32 + yn32) - 0.5
d3 = &radic (-0.22 + yn22) - 0.5
d4 = &radic (-0.12 + yn12) - 0.5
d5 = &radic (02 + y02) - 0.5
d6 = &radic (0.12 + y12) - 0.5
d7 = &radic (0.22 + y22) - 0.5
d8 = &radic (0.32 + y32) - 0.5
d9 = &radic (0.42 + y42) - 0.5

Now we calculate the average lift:

average intake lift = d1 + d2 + d3 + d4 + d5 + d6 + d7 + d8 + d9
                                                                    9

Next we calculate average open intake valve area:

average open intake valve area = average intake lift x intake valve size x 3.14

Now we can calculate the air velocity, in fpm (feet per minute) through the intake valve, during the time it is open, at any rpm we want:

air velocity through intake valve in fpm = 0.327 x number of cylinders x 2 x stroke x rpm x 3.14 x (bore / 2)2
                                                                                               average open intake valve area x 12

Finally, we can put 67547.4, which is the speed of sound in fpm, in place of "air velocity through intake valve in fpm", solve the equation for rpm and find out what the maximum rpm is from that result! The HiPerMath equation is below:

maximum rpm =                    67547.4 x average open intake valve area x 12                   
                                0.327 x number of cylinders x 2 x stroke x 3.14 x (bore / 2)2

We just found out how much this cam allows the engine to breath!


If you know the cubic inches of your engine, a simpler equation would be:

maximum rpm =                    67547.4 x average open intake valve area x 12                   
                                                              0.327 x cubic inches x 2


If you know the liters of your engine, a simpler equation would be:

maximum rpm =                    67547.4 x average open intake valve area x 12                   
                                                              0.327 x liters x 61.02 x 2


Definitions:

fpm = feet per minute
rpm = revolutions per minute