Posted: August 2, 2011 in Induction Systems

Induction Waves

     Let’s first look at what happens in the manifold to better understand how to use it to our advantage. When an engine is running, there are high and low pressure waves moving in the manifold caused by the inertia of the air and the opening and closing of the valves. The idea of port tuning is to have a high pressure wave approach the intake valve before it closes and/or just as it opens, forcing in a little more intake charge.

Pressure Wave Causes

     The most commonly known cause of a pressure wave is the piston as it moves down the bore. On the intake stroke, the piston makes a negative pressure wave that travels form the piston to the intake tract. Once that negative pressure wave reaches the plenum area, it is reflected as a positive pressure wave. That positive pressure wave travels back toward the cylinder. If it reaches the intake valve just before it closes, it will force a little more air in the cylinder. The second, less realized, cause of pressure wave is the exhaust. If you have a good exhaust system that scavenges well, during the overlap period there will be a negative pressure wave as the exhaust is scavenging and pulling in fresh intake charge. The same thing happens, it travels up the intake and is reflected at the plenum area as a positive pressure wave. If the length is correct for the rpm range, the positive pressure will be at the valve just prior to its closing and help better fill the cylinder. This will also help by reducing reversion with long duration cams. To get the benefits form this you need a well tuned exhaust system (another tech article). The third and most complex cause of pressure waves is when the intake valve closes; any velocity left in the intake port column of air will make high pressure at the back of the valve. This high pressure wave travels toward the open end of the intake tract and is reflected and inverted as a low pressure wave. When this low pressure wave reaches the intake valve, it is closed and the negative wave is reflected (it is not inverted due to the valve being closed), once again it reaches the open end of the intake tract and is inverted and reflected back toward the intake valve. This time the valve should just be opening (if the port is tuned to the rpm range) and the high pressure wave can help.

Pressure Wave Speed (V)

     The pressure waves travel at the speed of sound. In hot intake air it will be about 1250 – 1300 ft. per second. Engine rpm does not affect the speed of the pressure waves and this is why induction wave tuning only works in a narrow rpm range.

Combined Effects

     On a well tuned intake set up there will be a high pressure wave at the intake valve as its opening, at the same time the engine is in its overlap period (both valves open). If the exhaust is tuned to the same rpm range as the intake, there will be low pressure in the exhaust (due to scavenging) at the same time. Since the intake port near the valve is higher than atmospheric pressure and the cylinder is a great deal lower, the air will start to fill the cylinder quickly. The higher pressure area will quickly drop in pressure as the piston travels down the bore; this creates the low pressure wave that travels away from the cylinder. Just as this starts to happen, the piston starts moving down the bore creating another negative pressure wave, so there are actually two negative pressure waves, one right after another. In a well tuned intake system there can be as high as 10 psi of air pressure at the intake valve due to these pressure waves. So you can see that it can have a very large influence on the volumetric efficiency of the engine.

Reflective Value (RV)

     Getting an optimum runner length may be hard to do due to engine compartment space and/or the engine configuration. A small cammed engine operating at lower rpm will need a long runner length, so instead of trying to fit such long runners under the hood, you can just tune the system to make used of the second or third set of pressure waves and make the system much shorter.

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 won’t 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 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

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.

Intake Port Area

     Unlike intake runner length which effects power over a narrow rpm range, the size (area) of the runner will affect power over the entire rpm range. If the port is too small it will restrict top-end flow and flow and if it’s too large velocity will be reduced and it will hurt low-end power. The larger the port is, the less strength the pressure waves will have. Since the intake valve is the most restrictive part of the intake system, the intake runners should be sized according to how well air can flow through the valve area. Most decent heads will have an equivalent flow through the valve area as an unrestricted port of about 80% of the valve area; this is if the camshaft it matched to the heads. In other words a 2.02″ valve, which has a 3.2 square inch valve area, in a decent flowing head will flow the same air as an open port with about 2.56 square inches of area (80% of 3.2). So the port area should be about 2.56 square inches just prior to the valve (this is in the head port). Some well ported race heads may have an actual flow of an area up to 85%, but for the most part it is around 78-80%.

Intake Port Taper

      To further help fill the cylinder, it helps to have a high velocity at the back of the valve. To do this the intake port can be tapered. To be effective, there should be between 1.7 and 2.5% increase in intake runner area per inch of runner, which represents a 1-1.5 degree taper. For an example, let’s say you’re looking for a 2% increase per inch taper on the 2.02″ valve we discussed earlier. We already came up with a port area of 2.56 square inches at just before the valve. Now let’s say the total runner is 10 inches from the valve to the plenum and we’re looking for a 2% per inch taper. This turns out to be a total of 3.12 square inches where the port meets the plenum. As you get near the 2.5% per inch taper point, you are pretty much at the limit of helping air flow.  A larger taper will only hurt signal strength at the carburetor.

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