Michael Galimi
December 15, 2009

The past few issues we have been following a supercharged 2000 Mustang GT as it received a new set of Trick Flow Twisted Wedge Street/Strip 4.6L cylinder heads, the latest Anderson Ford Motorsport (AFM) F82 camshafts, and a larger ProCharger D1SC centrifugal blower. The '00 GT is fairly basic with a built short-block (cast crank, forged rods and pistons) and the aforementioned induction components. The heads were worth 34 rwhp over CNC-ported OEM heads, and we finished at 571 rwhp.

We were expecting 600 rwhp but a slipping six-rib blower belt hampered our lofty-but realistic-goals. The boost gauge touched 17 psi, but the engine fluttered in the upper rpm range. Mike Dezotell of Dez Racing was also disappointed, as the engine should be revved up to 7,500 rpm in order to take advantage of the larger AFM cams. The rwhp level fell off at 6,500 rpm.

Since we couldn't end on a sour note, we are back at it. Our original goal for this issue was to add a ported Fox Lake P-51 intake and also have the company run our TFS heads through its CNC-machine. That would help our engine ingest more air and get to our goal of 7,500 rpm.

Another trick on our list for the 600-plus-rwhp club was to-finally-upgrade to long-tube headers. Kooks whipped up a nasty set of stepped headers (1 3/4 to 1 7/8-inch primary tubes) with 3.5-inch collectors. We also ordered a 3-inch X-crossover, which has provisions to mate to the Magnaflow 2.5-inch after-cat exhaust. The headers and X-crossover are made from stainless steel. Until this point, our test car has employed a set of Ford Racing short-tube headers and 2.5-inch X-crossover.

Deadlines prevented us from adding the ported heads and intake, so we focused solely on upgrading to the Kooks long-tube headers. We made two changes since the car was dyno-tested last issue; the first was the addition of a ProCharger eight-rib blower pulley setup. The unfortunate part was that ProCharger's smallest blower pulley is 3.40 inches, while our six-rib one was 3.20 inches. That meant our baseline would have less boost through the entire curve.

The second difference is that last month Dez knocked off 2 degrees of timing (15 versus 17 degrees), due to the Stang seeing street time. He wanted a safety margin should some low-octane fuel come through the station's pump. In this trim, boost barely touched 17 psi at the top of the pull-thanks to the belt-grip. That was the same boost we peaked at last issue but less timing advance brought output to 549 rwhp and 465 rwtq.

We had high hopes for the long-tube conversion and larger X-crossover due to the supercharged nature of this Mustang."The shorty headers are not worth it," said Dezotell. "They are a major restriction at 500 rwhp and here we were trying to push 600 rwhp as well as rev the engine to 7,500-both goals were not previously achieved and we hoped this exhaust would help us remedy flaws in our system." These headers are of the stepped variety, which means the header tubes start at 1 3/4 inches and then open to 1 7/8 inches-more on that in a bit.

Adding a better exhaust system is a simple concept, what goes in must come out. And this being a supercharged engine means that we have a lot of air getting crammed in to the 281ci powerplant that has to make it way out. But what does that automotive cliché mean? An engine is an air pump and it has to get rid of the spent gases from the combustion process. As the exhaust valve opens, the pressure in the exhaust tube is lower than that in the cylinder, so gases will rush into the cylinder head port and into the header tube(s), to eaqualize the pressure. As this is happening, the piston is moving upward, helping the process by pushing the exhaust past the valve. As the piston is thrust upward, there comes a point (near top dead center) when the intake valve will begin to open. This valve event is called overlap and the rapidly exiting exhaust gases actually help pull-in the fresh air/fuel mixture from the intake valve.

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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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