By John Willhoit
The twin pipe "Sport" muffler that some of the vendors sell is actually a muffler that was made by Dansk for the early T1 cars that has no S pipes going through the bumper guard. I guess some marketing person at Stoddard had the idea to sell it as a "sport" muffler since they could sell more. That's been going on for 25 years or so. The outlet pipes have restrictors and this muffler has been proven to not make the same power as a stock, later type muffler (see this article). If you have an early car, buy a stock 356A muffler and add extensions to the stock outlets.
The Super Sound muffler from Dansk does in fact have larger pipes inside the muffler, and a larger cross over tube, so it does (should) reduce back pressure and (should) make more power. It is sold with two chrome resonators (very stupid looking BTW), and without these resonators your engine will be really loud and back fire on deceleration. We used one of these mufflers on a car, and the only way to make it run correctly without the resonators was to reduce the outlet size to 1.75" from the supplied outlet size of 2.0"+. I don't recommend this muffler for a 356 unless you plan on changing the outlet size or adding some other type of resonator pipes.
Header exhausts will always make more power at a given RPM range, and this is determined (mostly) by the length and diameter of the primary pipes (the ones before the collector). Headers are good for racing but the problem with a 356 is that you can't make the primaries short enough for high RPM power. To overcome this the pipes can be made larger in diameter, but then you give up low and mid range range power (for racing usually OK). The tuning of the length of a header is the same type of tuning done with the length of the intake, and this can have a dramatic affect on cylinder filling and performance...but only at a specific, and often narrow, rpm range. That's why the varioram intakes make so much more torque on the newer cars...the intake runner length changes as the rpm changes.
A 1.5" diameter Bursch or most other headers will definitely improve mid range torque at the expense of sub 3K torque and (depending on engine size) high RPM torque. I mainly don't recommend headers except for racing because they look stupid.
All stock mufflers were designed to reduce sound, but another purpose of all those tubes and baffles in the stock muffler is to reduce exhaust reversion. Simply put, reversion is a wave that moves back up the primary header pipe (on any muffler) into the cylinder, and on overlap actually through the cylinder and into the intake runner. On big cams with a lot of overlap this wave can actually push the intake charge back up into the carb where it then gets sucked back into the cylinder creating an overly rich mixture that you cannot tune away with the carb jetting. That's why dyno curves with big cams on a 356 almost always have a dip at around 3-3.5K RPM, especially with a non stock muffler. I'll have more on this with testing on my website soon.
We have built sport mufflers that improve performance (and sound) over the stock muffler but not dramatically, and not without a compromise in some lower RPM performance and throttle response. Sport mufflers are mostly for looks and sound. The factory Leistritz or Dansk mufflers do an excellent job of reducing sound and reversion, and (based on our testing and driving) will provide the best power in the 2500 to 5500 driving range. We use these stock mufflers on even our 2002cc engines and they work great. They don't need to flow any more or be any larger to make excellent power, especially on a 1600 or 1720cc engine. The few additional HP that you get from cutting up the oem baffles or adding bigger pipes will be offset by a reduction in average HP.
Those Germans were smart, stick with the stock exhaust.