I'm building my own! I currently run a 4" side pipe that unbolts (four bolts) at the collector and then I bolt on a little shorty turn out to run open headers. When I want to or can, sometimes you HAVE to run muffled.
So, I'm buying a 36" long 4 inch aluminized "stack" from a truck supply store, I'll cut, weld, modify the basic 36" "tube" as needed. Then I'm buying two 30" Spirals which BOLT from the outside through the "stack" (4" pipe) with button heads screws at each end. The center core of the spiral is an open pipe. Need extra quiet? Plug the outlet of the center pipe, but that WILL cost you horse power. I'm working on a way to temporarily close the small center pipe when I need to, open it up again when I don't. Because they BOLT IN to the stack you can turn them around to face the other way if you want to try that.
Finally, the end piece, the turn out. This will be adjustable, like a jet nozzle for direction. Down or to the side with a ring clamp of some kind. Sound meter reading coming from the left? Turn the turn out away from the meter! Still to loud? Pop in the center pipe plug.
...this is my working theory, it looks great on paper, now I'm hustling the parts to make it reality.
Tom's Motorsports, has shown very little horse power loss with this type of system and has been using it for racing for several years now. Before they went "all spiral on us" they had problems with extreme heat or significant horse power loss. Spiral IS the key!
Talking to the owner of the Spiral patent thing the other day, he made an interesting comment. Fords are harder to get quiet than Chevies, I thought that was interesting. I told him I had an FE, I could hear him cringe on the end of the phone.
