Goran Malmberg is Swedish Pantera owner who builds guitars as well as exhaust systems. V8 180 and 90 degree headers are apparently more sensitive than bank separated 4-into-1 headers. Rather than trying for equal length, he adjusts the lengths so they play the same note. From his website (http://www.hemipanter.se/#Exhaust
), he explains his method:
"Headers are 31 x 2.11 inches, giving the engine a peak at 5500 rpm. I have my own (guitar tuning) trick of tuning the exhaust primary pipes. By listening to the note of the pipe, it is possible to tune all primary (and secondary pipes respectively) to the same frequency even though this might result, due to bends and such, in some discrepancy in actual physical length. Think of it as an organ pipe. Sing a note in the pipe listening for the resonance to tune in. The frequency is a function of air volume and pipe length. Staggered pipes they may be, but we´ve got the resonance length spot on!. And frequencies is what the engine senses. A fraction of an inch is easily detected in the variance of note. End pipes are 3 inches x 25. To quiet the car I use a large silencer, pictured later in this site.
Another interesting side effect of "guitartuning" the exhaust is that it provides great music! Especially with the very short system of the Pantera which responds to almost any change in pipe design. And as no surprise, good note and performance are more often than not very, closely related."