5.0 Mustang & Super Fords
1999 Cobra X-Pipe
This Time We X-Periment On A Four-Valve Modular
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It's generally conceded that an X-pipe's scavenging advantage over an H-pipe increases with rpm. The theory goes that as an engine spins faster, there is insufficient time for the traditional H-pipe's crossover tube to fully equalize the exhaust pressure imbalance that exists between a V-8's right and left cylinder banks. Blame the 90-degree-crank V-8 firing order for this pressure imbalance.
To maintain dynamic balance in such engines, two cylinders in each bank must fire in immediate succession each time the firing order is completed. As this occurs, there is two cylinders' worth of pressure in one bank's exhaust pipe and none in the other's. In the X-pipe design, because the right and left exhaust pipes are siamesed for some distance, exhaust pressures down-stream of the X can be synchronized at all engine speeds. This reduces back-pressure, smoothes the exhaust note, and coincidentally tames the decibel level in the process.
In our last look at X-pipes ("The X-Files," May 2000, p. 39), our test mule was a relatively stock 5.0 Fox that repeatedly produced peak horsepower at around the 5,000-rpm mark. Even at this fairly tame upper limit, however, the five X-pipes tested posted impressive gains over the factory four-cat H-pipe.
This time around we're experimenting on a '99 Cobra, also nearly stock, but whose deep-breathing DOHCammer just naturally winds to heights that would have a stock, pushrod 302 dialing 911. Instead of comparing different X-pipes, this time we decided to install just one set--the original Dr. Gas Synchronizing Crossover Kit--to see how the Four-Valve modular responds.
In discussing the article with Paul Svinicki at Paul's High Performance--who would be doing the installation and dyno testing--he suggested that we also retune to take advantage of the X-pipe's scavenging characteristics. Paul says he can usually coax out a little more power by altering the fuel and even spark curves on cars that are swapping over from an H-pipe, especially if they have upgraded headers, which also alter cylinder scavenging. This is particularly true, he tells us, with the Dr. Gas pipe, which has its X located farther forward than its competitors. "You do have to tune for the air/fuel ratio," Paul says, "and of course it depends on where the X is. The closer the X is to the heat [of the exhaust port], the better it works for the scavenging effect."
Let the X-periment begin. The truth is out there.
Induction and exhaust are always tied together. Improve one and you may well find the limits of the other. Paul Svinicki (pictured) says the cork in the '99 Cobra's inhalation is the intake manifold itself (flowing a maximum of about 390 cfm), not the mass air or throttle body.