After all our preaching, you no doubt realize the benefits of a good exhaust system—or more importantly, the drawbacks of a poor one. Hot Rod has done several stories in the last year about exhaust systems, how they work and how to build the correct one for your car. This month’s exhaust story will show how Borla Performance Industries gets maximum power with minimum noise. As we’ve said in previous issues, there are three ways to muffle the sound coming from the combustion chamber: restriction, reflection and absorption. Restriction means just that: The flow of exhaust gas is restricted. This method definitely makes for a quieter exhaust note, but it also chokes off any decent power the engine is trying to make.
Reflective technology involves splitting the flow path of the exhaust gases within the muffler and using wave-cancellation techniques to create a pressure drop within the muffler. That reduces sound pressure and, therefore, the noise level at the muffler’s exit. If you look at a cutaway of a reflective-type muffler, you’ll find a series of baffles and reflectors. That’s the maze the exhaust winds around to set up the wave reflection.
Absorption techniques have traditionally been typified by the “glasspack” straight-through mufflers. That type of muffler is basically a straight tube inside another tube. The inner tube is perforated with louvers, and between the tubes is a packing material, usually fiberglass. As the exhaust goes through the muffler, the packing material absorbs some of the sound. Unfortunately, the louvers present a flow restriction, and that costs power. Adding to the flow restriction is the fact that while it may have a 2-½-inch inlet and outlet, the combination of the smaller inner tube and the protruding louvers can reduce pipe diameter to as small as 1-3/8 inches!
Borla uses the absorption principle for its mufflers but in a much more efficient design that doesn’t interrupt exhaust gas flow. Instead of louvers, Borla uses a tube perforated with small holes, which is surrounded with a stainless-steel wool packing material. Because nothing is hanging down into the flow path, the engine doesn’t even notice the perforations, but these holes allow the packing material to absorb sound without interrupting the exhaust flow.
All of Borla’s mufflers are straight-through designs using these perforated tubes. The case, end caps and entry/exit tubes are also made from stainless steel, which allows Borla to offer a million-mile warranty on its exhaust products. While the standard Borla muffler is a great piece, of more interest to hard-core Hot Rod readers is the XR-1 line of race mufflers.
Whereas the standard street muffler has a single tube and flow path for the exhaust gas, the XR-1 line has multiple tubes, anywhere from three to seven. According to Borla, the multiple tubes actually increase velocity through the muffler, allowing these mufflers to outflow even a straight piece of pipe in most applications. In a muffler test conducted by Hot Rod in the January ’92 issue (“Merrie Melodies”), the Borla was one of several mufflers that did just that; it was up 7 lbs-ft of torque (and the same amount of horsepower) over an equivalent-size straight pipe, and it was the overall horsepower champ over 11 other mufflers. The reason for using more inner tubes is solely for sound canceling. The more tubes, the greater the surface area that the exhaust gas sees and the more sound deadening without the expense of restricted flow. The XR-1 race mufflers are physically larger than the street mufflers due to the multiple inner tubes. And because they’re designed for racing, the noise level is on the loud side for street use. You’d have to be pretty hardcore and on friendly terms with the law to use these on a daily driver.
The current trend among the heavy hitters running giant motors and 8-second e.t.’s is to use mammoth mufflers with 5-inch in-and-out pipes. These huge mufflers increase horsepower potential on these cars, but it is Borla’s contention that the large size is only necessary because of the reflective technology used in this type of muffler. Because the exhaust gas must negotiate its way around the baffles in these mufflers, a bigger pipe is needed for more airflow. Borla claims that the straight-through design of the XR-1 negates the need for a monster pipe, and that’s why the largest XR-1 has a 3-½-inch inlet and outlet, and more commonly 2-½ to 3 inches.
Of the race mufflers, the trickest are the new collector mufflers. These mufflers, known as Vacuum Wave Collectors, actually replace the header collector and slip over the header primary tubes. Inside the case, the lengthened primaries are perforated and surrounded with stainless packing material to absorb sound, then they converge into a specially shaped conical collector, which is also perforated. The engine doesn’t see the perforations, so they don’t present a flow restriction, but they still absorb sound waves.
The collector has a seven-degree taper from the primaries to the outlet, and inside the collector is a “spike” that is also tapered at seven degrees, keeping it parallel to the walls of the collector. Borla says that the relationship between the collector and the spike creates a negative pressure differential in the collector, which further increases velocity at the muffler outlet. That means better exhaust gas flow and scavenging, which translates into increased horsepower. Can you use them on the street? Not really. As race-car mufflers, they bring the sound down to the legal level of most tracks, but for the street, they’re really too loud. Also, because the outside case diameter is 6 inches, it would be tough to make them fit under a street car.
The performance realized by Borla mufflers does come at a price. Because they are constructed of stainless steel, the Borlas are more expensive than traditional mufflers—the 3-inch oval XR-1 muffler retails at $312. But that stainless construction also means that they’ll probably outlast the car, and it allows the aforementioned million-mile warranty.
