Thanks Freud. Some of this stuff I learned in dinosaur times. Maybe someone will tell us about new and better ways to do things. The following must be on a spreadsheet somewhere.
Some figuring is needed before the cutting and grinding commences.
The intake valve closes and a pressure wave reflects back along the inlet tract. The pressure wave exits the velocity stack and another pressure wave is reflected back into the intake tract. The pressure wave, if it arrives at the wrong time, can hinder flow into the cylinder through the open intake valve. The wave, if it arrives at the desired time, can push additional mixture past the intake valve and into the cylinder. The intake tract length can be adjusted so the pressure wave arrives at the intake valve at the correct moment. This is ram tuning.
There are some pretty fancy formulae to figure out intake tract length based on cam timing and other factors. This is a bit complicated for this post. A simple method will be explained. It gets a person close to the optimum solution. This procedure is from a 1970 article "Four Stroke Tuning" by Jerry Branch and Le Roi Smith in Petersen Publishing's "Motorcycle Repair Manual." Jerry Branch was an expert in engine air flow. This is, as Jerry says, "A formula from an English engineer" and it is best suited for racing engines.
First, the speed of sound is needed at the race location. Bonneville is at 4200 feet elevation and it is 70 degrees when we race, on the average. The speed of sound is 1090 feet per second in these conditions. This is "V" in the equation. Next the engine speed is needed during the peak ram effect. This is a matter of personal preference. I use 80% of redline for initial trials. 0.80 x 9,000 = 7,200 rpm for the little Honda. This is "N". "L" is the intake tract length in inches.
The formula is L = (90 X V) / N L = (90 x 1090) / 7,200 = 13 5/8 inches for the little Honda.