Dale Amy
May 1, 2001

Step By Step

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Paul says the first step in installing the Dr. Gas kit is fitting the passenger-side header pipe in place. Don’t forget to use the supplied gasket, as the modular’s passenger-side header uses a flat, not a ball, flange. Tighten this pipe securely before moving on to the next step.
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Next, fit the X itself, along with the driver-side header pipe. Note that the long leg of the X (that PHP’s Karl Roekle is so kindly pointing to) goes to this forward, driver-side position. Leave the bolts at the ball flange somewhat loose for now.
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Now the pipes behind the X can be slipped in and loosely mated to the after-cat system (note that for street use there’s lots of room to fit catalytic converters behind the X). Check how the after-cat is hanging, adjust as necessary for proper tailpipe orientation, and be sure the X-pipe is tucked up nice and square. If your tranny crossmember has an exhaust hanger, tack the supplied hangar hardware onto the pipes. Then tighten the bolts, tack-weld the junctions at the X, and recheck before removing the X-pipe and welding the full perimeter of the junctions. Bolt the assembled unit back up into place, and get ready for some more power and a unique sound.

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.

Horse Sense:
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.