Ford428CJ
FE Freak!
I do plan on using a Flowmaster Outlaw Series race muffler on my 4" pipe. A buddy of mine tried a straight 4" pipe and one with a race muffler. He said that it gained 4 to 5 mph going up a good size hill. Here is the one that I'm going to try:
http://www.flowmastermufflers.com/race.html
http://www.flowmastermufflers.com/products/index.php?main_page=product_info&products_id=3205
http://www.flowmastermufflers.com/race.html
http://www.flowmastermufflers.com/products/index.php?main_page=product_info&products_id=3205