Accessory exhaust systems are not approved by the VW factory and are not generally sold by dealers. Further, some states with outdated inspection codes disapprove of any exhaust system that is not an original factory equipment type. However, accessory systems do usually give a power boost (up to 10%) and a satisfying sound.
There are two basic types of accessory exhaust systems currently available. The first, the header type, has a separate muffler on each side, one for cylinders 1 and 2, and one for cylinders 3 and 4. This type of system does not allow the use of the heater and will not fit on sedans and buses, unless the rear body panels have been cut away. The header system is rather loud and rough sounding, because it is actually two completely separate two cylinder systems. It is most often seen on dune buggies, where a large ground clearance is necessary.
The second type of accessory exhaust system is known as the 180° tuned, or extractor system. In most such systems, equal length pipes are run from each cylinder, the pipes from each cylinder 180° apart in the firing order are run together, then both pipes are run together into a single outlet. The effect is a small horsepower boost and a smooth exhaust note. These systems are installed in much the same way as the stock muffler, and are available in models which utilize the original heater. Some extractor exhaust kits furnish canvas tubing to connect the heat exchangers to the fan housing tubing. The canvas tubing will rot out quickly, and should be replaced with stanless steel flexible tubing held with screw type hose clamps.
When fitting any low restriction exhaust system, it is advisable to rejet the carburetor to prevent an excessively lean mixture and resultant valve burning. Normally a main jet one or two sizes larger will be sufficient. It may also be necessary to replace the air correction, or air bleed, jet. The spark plugs and the inside of the tail pipe will give evidence of changes in mixture.
The results of rejetting, or of any other engine modifications, must be checked with suitable testing equipment to ensure that exhaust emissions remain within legal limits.