Intake Runner Length (L)
Knowing that the pressure waves (positive or negative) must travel 4 times back and forth from the time that the intake valves closes to the time when it opens and the speed of the pressure waves, we can now figure out the optimum intake runner length for a given rpm and tube diameter. We must take into account the intake duration, but you want the pressure waves to arrive before the valve closes and after it opens (air wont pass though a closed valve). To do this you must subtract some duration, typically you take off 20-30° from the advertised duration. 30° works well for most higher rpm solid cammed drag motors. So the Formula to figure effective cam duration (ECD) will be:
ECD = 720 - (Adv. duration - 30)
For a race cam with 305° of intake duration it will look like this:
ECD = 720 - (305 - 30)
The ECD of that cam would be 445
The formula for optimum intake runner length (L) is:
L = ((ECD × 0.25 × V × 2) ÷ (rpm × RV)) - ½D
Where:
ECD = Effective Cam Duration
RV = Reflective Value
D = Runner Diameter
If our engine with the 305 race cam needed to be tuned to 7000 rpm using the second set of pressure waves (RV = 2) and had a 1.5" diameter intake runner the optimum runner length formula would look like this:
L = ((445 × 0.25 × 1300 × 2)÷ (7000 × 2)) - 0.75
So 19.91 inches would be the optimum runner length.
ECD = 720 - (Adv. duration - 30)
ECD = 720 - (288 - 30)
ECD = 720 - 258
ECD = 462
The ECD of that cam would be 462
__________
The formula for optimum intake runner length (L) is:
L = ((ECD × 0.25 × V × 2) ÷ (rpm × RV)) - ½D
V = ???
ECD = Effective Cam Duration
RV = Reflective Value (why does it = 2?)
D = Runner Diameter (Inner diameter)
If our engine with the 288 race cam needed to be tuned to 7500 rpm using the second set of pressure waves (RV = 2) and had a 1.50" diameter intake runner the optimum runner length formula would look like this:
L = ((462 × 0.25 × 1300 × 2)÷ (7500 × 2)) - 0.75
L = ((300,000)÷ (7500 × 2)) - 0.75
L = (300,300 ÷ (15000)) - 0.75
L = (300,300 ÷ 15000) - 0.75
L = (20.02) - 0.75
L = 20.02 - 0.75
L = 19.27
__________
Well out comes the firewall.
I want to know what the v stands for... if the value is wrong so is my calculations.
the formula I got is from browsing the net.
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