1.) David Vizards Formula The general rule is that you should begin with a runner length of 17.8 cm for a 10,000 rpm peak torque location, from the intake opening to the plenum chamber. You add 4.3 cm to the runner length for every 1000 rpm that you want the peak torque to occur before the 10,000 rpm.
For 6000 RPM, Length = 35cm
2.) Another Formula to Calculate Runner Length for a Specific Peak Torque RPM: from Steve Magnante at Hot Rod magazine N x L = 84,000 where N represents the desired engine rpm for peak torque and L is the length in inches from the opening of the runner tube to the valve head.
For 6000 RPM, L = 14.0" or 35.56 centimeters to the valve head
I also did a lot of reading about plenum volume and plenum to throttle body ram pipe length and diameter from the book "How to Tune and Modify Engine Management Systems" by Jeff Hartman. Look it up on Google Books, pages 119 to 121 are all you need. Anyways, I ended up with a plenum roughtly 60% of the displacement of the engine, and a 12" throttle body ram pipe. Interestingly, the stock GM 96-7 throttle body arm diameter was dead on at 2.75" ID, which I repeated when I made mine from polished aluminum pipe.
Anyways, A picture right after I rolled the car out of the bay (still needs polishing/painting, and final routing of hoses and wiring to make it pretty):
[Edit]: Plenum is 60%, not 40%
