We've been using Pipemax , it is an awesome program to calculate pipe sizes and crossover pipe positioning .
I'm not going to say I understand it all exactly but we have made some real nice torque + HP gains on a friends 393
It came about because we dynoed the car with an old pressbent 2.5" system , mufflers cheap redbacks .
Exhaust was changed to all 3" Mandrel bent , X pipe and nice Ultraflow mufflers . Sounds reasonable for a 393 you say
Well we lost power and has taken a while to get it back with some changes in H pipe positioning .
My friend has posted the charts of our results from memory on Facebook in one of the Cleveland pages .
If I had some specs Sean I can run yours through the program . It's not the be all and end all but does give some nice data if you can understand it all
I should add , the changes we did . Originally the X pipe was at 26" from the collector . By pipemax that was the worst spot it could of been. With dyno testing we had to trial where the exact point was needed for the cross pipe . Pipemax told us that 17" was the magic point . We started longer and kept shortening it back . This was done on a chassis dyno with open headers and extensions , reducing the pipe length by 1 inch each run. As we approached the ideal length , believe it or not the AFR started to show lean and we had to alter carb settings . We got rid of the X pipe and went to a simple H pipe . Turned out 17" was the ideal spot for the H .
The exhaust was altered and the car has shown strong increase in midrange power and toque and has picked up MPH at the track
All may seem a bit strange and if I hadn't of seen the tests myself I'd find it a bit hard to believe .