H-Pipe, X-Pipe, 1, 2, 3...Written by on March 1, 2002 An Exhaust System Based On A Universal X-Pipe Crossover And A Pair Of Stainless Steel Street Series Mufflers
Do you really need an exhaust system on your car? Thats a good question. If you have neighbors, drive it on public streets anywhere there are cops, or race it on virtually any NHRA track in the country, the answer is probably yes. Will it cost you power compared to open headers? Although that answer is also frequently yes, it can be no with the right system.
Weve been running open headers on our 86 Mustang project car since day one because it was the easiest thing to do and it seemed cool. It was also brutally loud, but it did pass muster at our local track because Los Angeles County Raceway doesnt enforce a 95-decibel rule or require mufflers like many street-legal drag programs do. However, we always wondered if the open headers were costing us a little low-end torque due to a lack of backpressure. To find the answer, we took the car to Magnaflow Performance Exhausts research and development center. After discussing our combination and the way the car is used (100 percent at the track), Magnaflow built a system based on its universal X-pipe crossover and a pair of its stainless steel street series mufflers.
For any performance exhaust system, some type of crossover connecting the two sides of a dual exhaust system is important because it acts to balance the two banks of the engine. The common H-style crossover is good at balancing sound pulses between the two halves, but does little to promote scavenging because the exhaust gases tend to follow the path of least resistance, which is straight through each pipe rather than taking the 90-degree turn through the H-pipe into the other half of the system. In an X-pipe system, however, where the two sides of the system intersect, the gasses have no choice but to intermingle as they pass through the junction. This promotes improved scavenging effects by smoothing out uneven exhaust pulses from the engines firing order. It also helps quiet down the exhaust, resulting in a mellower, less raspy tone. According to Magnaflow, the faster acceleration of the gasses through an X-pipe causes them to flow in a linear fashion parallel to the walls of the tubing rather than tumbling. This laminar flowing gas is much quieter than tumbling gas, resulting in an exhaust tone up to 8 decibels quieter than a traditional H-pipe.
We left the car at Magnaflow for about a week, and when we came back we found that after some preliminary testing, the guys had built a really trick system consisting of 2-½-inch tubing from the header collectors into the X-pipe, 3-inch out of the X flaring into 3½-inch tubing running for about 26 inches before necking back down into a pair of 3-inch mufflers with turn-downs. The theory behind this design is that it will keep the velocity of the exhaust gases moving quickly through the headers into the X-pipe to maximize the scavenging effect, while the larger-diameter tubing ahead of the mufflers prevents the gasses from stacking up as they pass through the mufflers to avoid excessive backpressure in the system.
Sounds good, but would it work? To find out, we tested three exhaust-system configurations on Magnaflows in-house Dynojet. For a baseline, we ran the car with open headers and saw 333 hp at 6,300 rpm and 304 lb-ft of torque at 5,200 at the rear wheels. Next, we ran a 2-½-inch bolt-together system consisting of a BBK short off-road H-pipe designed to fit the companys full-length headers connected to a set of race-type 2-½-inch welded mufflers with turndowns. Our Mustangs carbureted 302 didnt like this combo, as power fell to 323 hp at 6,300 rpm and 295 lb-ft of torque at 5,200. The increased backpressure also richened up the air/fuel mixture by about two carburetor jet sizes.
Finally, we swapped on the stepped X-pipe setup and were impressed to see power levels equal to the open headers: 335 hp at 6,300 hp and 302 lb-ft at 5,200. Thats a gain of 12 rear-wheel horsepower, although it turns out we werent really giving anything up or gaining anything extra from the open headers. So the cars happy, and our ears are even happier!
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Can an exhaust system make as much horsepower as open headers? To find out, we brought our ’86 Mustang project car to Magnaflow Performance Exhaust to have a trick X-style crossover system tested against a traditional H-pipe and wide-open headers.The carbureted, hydraulic-roller-cammed 302 made 406 hp naturally aspirated at the flywheel when we dyno-tested it and has run a best of 12.41 at 112 mph. How much power would we see at the rear wheels?Think of the port shapes of a cylinder head when you look at the internal surfaces of Magnaflow’s stamped X-pipe. The smooth radiuses and transitions contribute to airflow just like a well-designed short-side radius or a good valve job (in fact, its designer says the shape was inspired by the ports in a Dart Big Chief Pro Stock head!). Magnaflow’s universal X-pipe (PN 10789) is designed for 21/2-inch tubing in and out. For our system, they split the exit side open to accommodate the larger 3-inch pipe.To standardize airflow on its SuperFlow bench, Magnaflow flows mufflers through a 1.875-inch orifice— the maximum size that will allow the bench’s electric motor to produce 28 inches of water depression, which is the same standard used to flow cylinder heads. This is smaller than most people would use on a performance exhaust system, but it standardizes the playing field. Magnaflow’s 3-inch center inlet/offset outlet PN 14229 muffler as used on our system flows about 250 cfm at the standard depression on this bench. For comparison, the generic 2-1/2-inch mufflers we used with our smaller system flowed a mere 215 cfm, hence the cork effect.Every muffler manufacturer offers various combinations of inlet/outlet configurations to fit various requirements. What we didn’t know is that these different combinations affect both sound levels and airflow. These Magnaflow mufflers are arranged from left to right in order of highest flow and loudest sound level: center/center, parallel offset, center/offset, staggered/offset. The variation in flow from best to worst is only about 10 cfm, so this isn’t a big issue.Prior to having a permanent exhaust system built, we finally got around to swapping the correct headers into our Mustang.When we originally slapped the car together, the 1-3/4-inch primary long-tube BBK headers we wanted to use were out of stock, so we borrowed a set of BBK’s 351 Windsor swap headers from a friend. They use the same primary tube diameter but hang about an inch lower to accommodate the 351’s wider block. They didn’t really fit on our 302, so we lived with a nasty exhaust leak until we finally got our hands on the right pair. Fitment is excellent, and the extra-thick flanges contribute to good sealing. They’re available in chrome or with ceramic coating for longevity.The intermediate system we tested consisted of a BBK 21/2-inch short off-road X-pipe and a set of used unknown brand race-type mufflers with turndowns. This system was a big improvement soundwise over open headers, but we immediately noticed that the engine ran considerably richer, suggesting that the exhaust was acting like a big cork. A test with an 02 sensor exhaust probe showed the engine to be running about two jets sizes fat.