Richard Holdener
June 1, 2003

When it comes to maximum effort Mustangs, right near the top of the list has to be the new '03 SVT Cobra. Jim, Evan and the boys from New Jersey have already gone mid-11s with a basically stock '03 SVT, the major mods being a blower pulley and Mickey Thompson ET Streets. Think for a second about all those poor schmoes dropping serious coin on fancy Z06 Corvettes. While the SVT will never offer the handling of the Vette in factory trim, the gold chainers better think twice about running a modified Cobra at a stoplight.

Perhaps the best thing about purchasing the SVT is the way the Cobra takes to performance mods. Like the portly, but powerful, 5.4L Lightning, building a killer Cobra is a matter of a few simple modifications. It is possible with relatively minor mods to up the power output of the 4.6 SVT motor dangerously close to 500-rear-wheel horsepower. To put that kind of power into perspective, it is possible to put a 3,800-pound Cobra solidly into the 10s. How many honest-to-goodness, 10-second, fully loaded, daily-driven street machines do you see running around? Such is the beauty of a supercharged SVT Cobra.

As good as the '03 car is in stock trim, there is always room for improvement. Even with a motor as powerful as the supercharged four-cammer, you soon tire of stock performance levels. Like any good drug, the boost offered by the supercharger is very addictive. That addiction only grows, requiring the boost more and more often. Soon enough, even large doses of the stock boost level are just not enough. After a while, it becomes necessary to increase your dosage.

Luckily, a number of performance tuners currently manufacture blower pulleys to increase the power output of your SVT Cobra motor. Changing blower pulleys works by increasing the blower speed relative to engine rpm. On a positive displacement supercharger such as the Eaton M112 used on the SVT Cobra, the amount of air supplied by the blower is dependant on two variables. The first variable is the displacement of the supercharger. The M112 designation refers to the displacement of air supplied by one revolution, or 112 ci (slightly less than 2 liters). The second variable is the speed of the blower relative to the engine speed.

If the blower supplies slightly less than 2L of air-per-revolution, how fast must it spin to supply the power needs of a 4.6? Actually, math can be a big help here, as the formula for calculating boost pressure for a given engine displacement is as follows:

Math being math and reality being reality, running the M112 supercharger with a drive ratio of 2.14 actually produces a peak boost reading of 8.3 psi according to our gauges. Things such as inlet restrictions can cause a reduction in the observed boost versus the calculated boost, and the stock air intake system on the SVT was responsible for up to 2 psi of boost at elevated boost levels. Maybe math is a little closer to reality than we thought?

The heart of the force-fed Ford is the Eaton M112 supercharger. Pullied to produce roughly 8 psi in stock trim, the M112 is capable of even more power when provoked.

Enough calculations, let's get on with making some horsepower. The first test run on the '03 Cobra was to establish a baseline. The Snake used for testing had been treated to a few minor modifications, the first of which was a cat-back exhaust system. The Ford Racing cat-back exhaust was trick in that the system incorporated an X-pipe. While X-pipes have become commonplace in the aftermarket, this system was unique in that the X-pipe was positioned behind the mufflers rather than in front. A great deal of research and development went into the design, including noise calibrations. Though we did not get a chance to perform a back-to-back test, we hope to very soon. The exhaust sounded great, without being obnoxiously loud. Some of the aftermarket systems we've heard installed on supercharged Cobras border on criminal insanity. The Ford Racing piece was authoritative, but would not likely scream to the local constabulary every time you went hunting Vettes.

When testing at Kenne Bell, be prepared to gather data, as the data logging equipment is extensive and absolutely necessary when you go looking for answers.

Modification number two came in the form of a K&N replacement filter. We all know supercharged SVTs respond well to intake and exhaust modifications, so it was only natural the owner opted for these two initial mods. Run with the stock blower pulley, the mildly modified Mustang produced 417 hp at the wheels with a like amount of torque.

As is always the case when running tests at Kenne Bell, we came away not just with power numbers, but with a veritable stack of data thanks to the data logging employed during the test. The data logging indicated the M112 supercharger pumped out a peak boost pressure of 8.6 psi. Oddly enough, the boost pressure was not constant through the entire rpm range. The boost pressure started at 6 psi as low as 2,000 rpm, made its way to 8.6 psi at 4,000 rpm, maintained that level until 4,500 rpm and then proceeded to fall off to 7.8 psi at 6,000 rpm. The drop in boost is an indication either the Eaton M112 blower did not offer sufficient flow to keep up with the airflow demands of the motor or an inlet restriction existed elsewhere (maybe a bit of both).