Here is a sizing chart I have found to be quite useful when running a full length muffled exhaust system......
As you can see, a dual 2-1/2" system starts becoming a flow restriction at around 400 HP and a dual 3" system starts becoming a flow restriction around 500 HP. Size accordingly and be sure to run a full sized crossover (balance) pipe connecting the left and right side exhaust wherever you can fit it behind collectors, but before muffler inlets. And don't sweat the hype about X-type crossovers. With a full length muffled system, an H-type crossover will work just as well. Even Dr. Gas (inventors of the X-type) on their web site mention that each engine/chassis/exhaust system is different and for many, an H-type works as well or better than the X-type.
If your engine will make over 400 HP, but you cannot fit 3" dual tail pipes...... Look at whether or not you could run twin pipes on each side (4 pipes in all). 2 pipes 2-1/2" in diameter will flow nearly as much as a single 3" pipe. If this will work for you, look at mufflers configured like a Suburban muffler (single 3" inlet with dual 2-1/2" outlets) and run one of these on each side.
Definitely go with the DynoMax super turbo mufflers. They flow better than Flowmasters and are quieter as well. Use the longest ones you can find in the largest oval body dimension that will fit under your car and make sure you use plenty of muffler hangers and pipe hangers to support the system.
FWIW, David Vizard did quite a bit of research some years back regarding building a zero sum loss full length muffled exhaust system for race cars. His findings involved 2 elements......
* A chamber (pressure wave terminator box) equal in volume to twice the cubic inches discharged into it placed between the point at which a tuned length header collector would terminate on an open header system at the track and the muffler inlet with the inlet and outlet of the terminator box(es) configured in a specific way.
* Vizard determined that mufflers flowing a minimum of 2.2 cfm per HP generated would not restrict flow. Dynaflow used to list below each of its mufflers the cfm it would flow and the HP that would support. I believe they only list cfm now. So take your engine's HP at peak X 2.2 and then divide that by 1/2 and that will give you the minimum cfm of each muffler needed for your application if running duals. You can also apply this formula to a single exhaust system.
Note...... Most street cars will not have sufficient space to install Vizard's pressure wave terminator boxes beneath them (the exception would be running large dia. pipe(s) and locating the muffler(s) as far to the rear as possible and in so doing, having sufficient volume to do the same thing as Vizard did). But if you cannot do this on your car, just size the system correctly and make sure you have mufflers that won't restrict flow and you will be good to go.
Hope this gives you some ideas,
Harry
