The stock Ford parts (manifolds, muffler, etc.) are very restrictive. The factory cared mostly about noise reduction, not backpressure. Running a stock single muffler exhaust is said to take from 10 to 20 HP from a hopped up engine. This is like giving back that fancy intake or heads you just installed. The secret is adding smooth flowing exhaust headers, dual exhaust pipes, and dual low restriction mufflers. Back in the 1950s, this meant a steel pack muffler. This is most similar to a modern glasspack in construction, except that it used steel mesh (like steel wool) instead of fiberglass. The inner core of the muffler is a straight through pipe with holes, slots, or louvers, resulting in little flow restriction while the packing helps cut the high frequency part of the exhaust note. High frequencies are more annoying in the exhaust sound, which is why most of us can put up with the loud but deep tone of the glass- or steel-packed muffler. Many people donít like to know exactly where your car is by the sound, however. Modern technology has made efficient AND quiet mufflers possible. I must admit, even a stock flathead sounds great through duals and Ďpacks. (Check out the ad for mufflers in the old ads section.)
The flathead does have one big problem with the exhaust - the center two cylinders share an exhaust port. Thatís why most flathead headers only have three tubes, instead of the four tubes per side you would expect from a V-8. Many fixes have been tried for this problem. Many sources recommend installing a divider in this shared port. I have seen conflicting plans for dividers, and some reports insist that they are not needed. Some engine builders have even experimented with creating new ports exiting out of the top of the block! You can imagine that this involves considerable machining and careful planning. Iím not sure Iíd suggest this step for a street engine. Again, talk with an experienced engine builder.
You may want to add cutouts to your pipes before the mufflers. This lets you open a cap or a plate and allow the exhaust to exit before the mufflers. This was more important in the days before good mufflers, and still applies to race engines that need to be driven to the track. On the street, it is a quick way to annoy the neighbors and get ticketed. (Sounds good in the driveway, though.)
David Fort (firstname.lastname@example.org) says:
"The Fentons are the cast iron headers of the 50's which are now being repoped. They do make for a quieter engine compartment and they look neat. As for the splitting of the center exhaust port, I've heard both sides of that subject. The cast iron divider that goes into the block is held in place by the head bolt that passes through the exhaust port. Though the divider is suppose to prevent the turbulent cross mixing of exhaust gases between the two center cylinders, it also takes up a major portion of the exhaust port space. Some old time rodder contacts told me not to go that route as the exhaust flow is restricted more than enhanced."
RMeier@micro.honeywell.com (Meier, Roger) adds:
"Yes exhaust crossovers affect both the sound and the fury. Back in the flathead days, all self respecting hot rodders would add dual exhausts to their wheels. The flathead engineshave an exhaust tube under the carb that ran from the #2 and 3 cylinder exhaust to the #6 and 7 cylinder exhaust. Although only .6 inch diameter this was a crossover of sorts. Those who really wanted an awesome exhaust note would lift up the intake manifold and block this tube, a penny was commonly used but they would only last a few weeks then they would burn out and need to be replaced. It would decrease the torque, but what a sound . It would also cause some cold weather running problems. "