Intake Valve Size

As a good estimate, use the formula below. To find out more details, read below the picture.

intake valve size = (bore)(0.6)


Intake valve size is the diameter at the bottom of the valve and is measured in inches.


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 an intake valve size that gives you the power you want at the rpm you want.


Factors to consider:

1. Intake valve size is the diameter of the intake valve measured in inches. Valve size is limited to 0 - 4 inches.

2. The larger the intake valve size, 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.

3. The smaller the intake valve size, the more torque your engine will have. Small intake valve sizes cause the air to reach high velocities at lower rpms. This translates into more air in the cylinders at lower rpms and more power at lower rpms.

4. If the intake valve size is too small, it will choke air flow, which means less power. Air velocity is limited to the speed of sound, which is 767.58 mph (miles per hour) or 67547.4 fpm (feet per minute). An intake valve size that causes the air to reach the speed of sound too early could choke air flow and limit maximum power.

5. Intake valve size + exhaust valve size must be less than the size of the bore. If the exhaust valve diameter + intake valve diameter is bigger than the bore, one of those valves is hitting the deck of the block and not opening at all.

6. Exhaust valve lift does not have air separation problems, so the lift can be higher, allowing smaller exhaust valve size.


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 diameter, intake valve duration and intake valve lift factor in together to determine air velocity in the intake system. Large valve diameter, long duration 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. Small valve diameter, short duration 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