[ QUOTE ]
What is the advantage, if any, this set up? Looking at pictures in my GT40 books it appears that it is not done to equalize the tube lengths. I have heard that the exhaust note will be different.
[/ QUOTE ]
I have heard numbers in the range of a 20 to 40 hp improvement with the crossover design.
It is not done to get more equal lengths to the runners. In fact the crossover design exacerbates the unequal length situation.
You start with an equation based on the engine volume, rpm range, exhaust valve duration and a few more items. This gives you a starting header length and the user is expected to make multiple dyno runs to derive the optimal length. On my engine this length came out to be close to 6 feet in length. This is a bit too long for the GT40 if you also want mufflers.
With the 180 degree crossover you get an effective header length of the first runner AND the runner to the cylinder that's 180 degrees out. So with two 36 inch runners you (nearly) get the proper reverse pulse from a single 72 inch runner.
The crossover design conserves accoustic energy. The effect of this is a quieter engine so you can use less muffler and still keep the dB levels low.
You can hear my car run in this parody I made:
Porsche guys hate this movie /ubbthreads/images/graemlins/mad.gif
It's 14Meg so it takes a while on a dial up but should give you a good idea of the sound of a crossover system. My mufflers are straight through with ceramic fiber packing.
Hope this helps.
Mark