Donald Farr
Former Editor, Mustang Monthly
August 19, 2007

At last year's Mustang Club of America 30th Anniversary Celebration in Birmingham, Alabama, Paxton's Gil Cormaci was impressed with Steven McCarley's '65 Mustang hardtop ("Homebuilt Hot-Rod," Mar. '07, p. 36). The only problem, Gil said, was that it wasn't equipped with a Paxton supercharger. We figured we could remedy that situation. Since Steven and his Mustang are located near Atlanta, we called our old friend Chuck Gutke at Cobra Restorers for installation assistance. With more than 25 years of experience with Cobras, Shelbys, and high-performance Fords, Cobra Restorers is well versed in adding superchargers to carbureted Ford engines. Tech Patrick Kelley handled the installation, with frequent comments and an occasional helping hand from Jimmy Grindle.

After some initial fuel delivery challenges which turned out to be little more than a new-but-defective fuel filter, Scott Milner at Coupe Performance made minor adjustments to the QuickFuel carburetor to take Steven's 289 from 240 naturally aspirated horsepower to 355 hp with the Paxton supercharger. That's an impressive 115hp gain for a small-block Ford.

Steven put his own spin on how it feels to drive a 355hp supercharged Mustang: "First of all, it's amazing the respect a blower gets on the street. Nobody expects a '65 or '66 Mustang to have a supercharger underhood. It just blows-excuse the pun-people away. Of course, the car is a handful. If you just dump it, First, Second, and Third gears are basically useless, sending the BFGs up in smoke. With some pedaling, you can keep the tail out all the way up an interstate entrance ramp-ask me how I know."

Steven is ready for more. His high-mileage 289 will be replaced by a roller-cammed 306 small-block with a steel crank and prepped rods. The goal is 450 rwhp.

On the Dyno
Bolting on the supercharger was a simple task at Cobra Restorers; Patrick had the engine running and the supercharger whining within a day and a half. Immediately afterward, Patrick and Jimmy trailered Steven's Mustang to Jeff Harris and his Dynojet at ProSpeed Performance, where it became apparent from the first aborted pulls that the engine was leaning out at higher rpms. Instead of risking damage, the dyno testing was postponed until the problem could be investigated. After haggling with carburetor tuning and adding larger fuel lines, the culprit turned out to be a defective fuel filter with an internal restriction, purchased new from a local auto parts store. Sometimes you need to look for the small things.

Back on the chassis dyno, this time at Coupe Performance and with additional tuning by Scott Milner, the 289 responded with 355 rwhp at 6,000 rpms, a 115hp increase over the 240 hp without the supercharger.

Tuning for Top Performance
Boost crams more air into the engine, usually meaning more fuel is needed than a factory-style fuel system can handle. Because tuning for boost is different than tuning a naturally aspirated engine, we turned to Paxton's Bob Endress for advice.

"You need a fuel line that's large enough to provide an additional 150 to 200 hp worth of fuel," Bob says. "In some cases, people begin chasing around the carburetor looking for a problem that doesn't really exist. They get futher away from their base tune until they discover the fuel pump isn't getting enough fuel in the first place. It has to be there."

Bob notes the fuel pump needs to add a pound of fuel pressure for every added pound of boost. "If you have a static fuel pressure reading of 6 or 7 pounds with a total of 5 pounds of boost, you're going to need 12 or 13 pounds of fuel pressure at wide-open throttle. That's critical because you must have more fuel pressure coming into the float bowls than the pressure in the enclosure. Otherwise, fuel will stop flowing into the carburetor."