This is a question that is asked on every forum. As you have seen in my past articles, I have tried to explain the theory of pressure waves in the exhaust and how timing them correctly can enhance the scavenging of the engine. I have also mentioned that there are pressure waves in the also in the induction system.
The intake valve opens and the air starts to travel down the intake, past the valve and enters the combustion chamber. When the intake valve closes, air traveling down the induction system still has momentum to travel in the direction towards the intake valve. This air slams into the closed intake valve then bounces back up the intake. This wave will eventually travel back and forth, up and down the induction system, getting weaker each time. The trick is to adjust the diameter and length of the induction system so that that wave will arrive at the intake valve when the intake valve opens. This will force more air/fuel charge into the combustion chamber.
Here are two dyno charts from an engine that I built this past winter. The customer brought it to me to dyno tune this week. This is a 2003 Softail Standard, 103 ci, HTCC heads, Woods TW-6H cams, 49mm T/B, SERT, V&H Pro Pipe, SE air cleaner...
The first graph is with the SE air cleaner with the factory cover on the filter.
I removed the air cleaner and had no noticeable change. That graph is not shown since there wasn't much gain.
Notice the A/F ratio at the bottom of the graph. The AFR is a flat 13:1 until around 4300 RPM, then I start to lean until AFR until it reaches 13.5:1 to gain more HP. Compare the AFR of the first graph to the second graph. The AFR goes to 13.75 after 3500 RPM. This shows that engine has increased it's volumetric efficiency. Basically it is now sucking more air through the engine and it needs to be richer to get the correct AFR.
The second graph: I remover the air cleaner and attached a velocity stack that I had taken off of an automobile and cut it to fit the HD air filter. This changed the length and diameter of the induction system which in turn changed the timing of the waves inside the induction system.
By just adding the velocity stack, it increased the TQ and HP in the lower RPM range (right where you need it).
I said all of this to show that even an air cleaner can make a difference in power. Not by just freeing up air flow but also by changing the induction length and diameter. This will account for why some air cleaners may work well on some engines and not on others. Most people don't realize that just adding an air filter such as a "Force Winder or Heavy Breather" will increase the induction system's length and diameter which will change the pressure wave timing. This change may help or it may hinder depending timing of the wave.
What is the best air cleaner? 
Linear Mode
