Michael Galimi
December 15, 2009

Once the exhaust valve closes, the gases in the header tube lose velocity due to the loss of that force from the piston and from the fact that the pressure equalizes. However, the slowing gases in that tube are helped by the scavenging effect of the header collector (and X- or H- crossovers). The other cylinders that are pulsing exhaust through the appropriate tubes merge at the collector and actually help draw the slowing gases from the tube that has the closed exhaust valve.

The tubes of our new headers feature two different sizes, a primary size and a secondary size. The step increase is a trick that is more popular in racing applications but is becoming more common on the street. "When the tubing diameter is increased in size the gasses decrease the tract pressure and increase positive exhaust flow. That creates a raised scavenging effect and allows more air and fuel to be utilized. That is the more air in, more air out effect," said George Kook Jr. of Kooks Custom Headers. "Also, a stepped header helps on collector scavenging as well because it cancels the reflective frequency back to the cylinder head. An increase of horsepower and torque is obtained because the efficiency is elevated because the clean cylinder charge is not re-burning the unused fuel."

Dez Racing's Brian Machie handled the installation and it was easiest for him to drop the K-member and A-arms in order to get the headers in. We encountered one problem-in our excitement, we goofed the order. The stepped headers utilize a race-style collector that measure 3.5 inches. The X-crossover is designed for use with a Kooks 3-inch collector due to the ball/socket technique for bolting it together. The two pipes wouldn't mate together. Luckily, Dez Racing has stockpiled old exhaust pipes-small and large. Machie fired up the welder and built a conversion pipe to go in between the two. The Kooks 1 3/4-inch long-tubes (PN 6000S) are designed to work with the 3-inch X-crossover, not the stepped headers with the 3.5-inch collectors (P/N 6002S).

Once back on the dyno with the new Kooks stepped headers and X-crossover, we noticed the air/fuel ratio changed down low and the engine pulled to a higher rpm. "It leaned out down low as the header helped the engine. Up top we were fine because I have it tuned a little rich for the supercharger. Torque increased as much as 10 lb-ft in some areas and it always made more torque up to the redline over the shorty headers," said Dezotell of the more efficient exhaust. Peak torque went from 465 rwhp up to 476 rwtq. We also saw a healthy increase of rwhp, to 579 rwhp with the long-tubes and larger X-crossover versus 549 rwhp with the short-tubes and 2.5-inch X-crossover.

"I pulled the car to 7,100 rpm with the long-tubes and it rolled over at 6,600 rpm. But you can see the power is flat from 6,600 to 7,100 on the graph. It didn't fall off and carried it through. You can definitely rev the motor higher at the track. But now, unfortunately, the exhaust is no longer the restriction. It is somewhere else and we think it is the intake manifold," proclaimed Dez.

Right now, the engine wears an unported P-51 intake that is typically used in applications up to 6,600 rpm. Dez also said he didn't change the timing and the more efficient exhaust would allow him to do so safely. The exhaust gases move out of the chamber faster-keeping it cooler and allowing more ignition timing before pre-ignition occurs. But for this test, the timing was kept consistent for a true A-B comparison.

Our search for more power continues-next month we will be getting our boost and timing back, then we will add ported heads and make a change in the intake department. How far can we go? Stay tuned as we add one last dose of horsepower.

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