Header pipe diameter and design
 

Is there a rule of thumb in calculating which header pipe diameter to use with a given engine? Is displacement the only variable or do expected operating RPM, expected torque peak or other variables come in to play? Are different types of merge pipe configurations better for some applications than others?

Here's a reasonable starting point from A. Graham Bell's book "4 Stroke Performance Tuning":

1) Primary Length = ((850 * ED)/RPM)-3
Where RPM is the engine speed the exhaust is being tuned to and ED = 180 degrees plus the number of degrees the exhaust valve opens before TDC. Generally, you want to primarys to be at least 15 inches.

2) Primary Inside Diameter = sqrt(cc/((P+3)x25)) x 2.1
Where cc = cylinder volume in cc and P = primary length in inches.

3) Secondar Inside Diameter = sqrt(ID^2 * 2) x .93
where ID is the inside diameter of the primary pipes.

These are only starting points. According to Bell, a reduction in primary pipe diameter of .125 will move the torque peak down by 500-600 RPM in larger engines, and by 650-800 RPM in engines under 2 liters.

Looking at a 911 2.0 engine, I worked out the following:
Primary Length: 33 inches
Primary ID: 1.28 inches
Secondary ID: 1.69 inches

Actual measurements off of stock early heat exchangers.
Primary Length: ~36 inches
Primary ID: 1.375 inches
Secondary ID: 1.875 inches

Re: Header pipe diameter and design
 

As John outlined, there are rules of thumb for a starting point but IMHO, there is no substitute for good old-fashioned dyno work & development. Needless to say, this is time consuming, labor intensive, and not inexpensive to do,..:)