2003 Ford Mustang Cobra Increases Power - Mods For Mods Part 1

The first order of business was to establish a baseline. As indicated, we installed the factory timing curve using the F.A.S.T. system and to our surprise, the supercharged crate motor thumped out some pretty impressive numbers. How does 483 hp and 427 lb-ft of torque sound? We all know that the Cobra motor is rated at 390 hp and 390 lb-ft of torque. Where did the extra 90 hp come from you ask? First off, we suspect the supercharged Cobra motors are underrated by Ford. Every one we have tested on the chassis dyno has produced much more than the 390hp flywheel rating would suggest. It is also important to understand that the rating from Ford is an SAE number derived with a full exhaust including cats, full inlet system (silencer, mass air and all inlet tubing) and full accessories.

Regardless of the rating, our test motor now produced 483 hp and 427 lb-ft of torque at a peak boost reading of 8.8 psi. As is the case with these supercharged Cobra motors, the boost curve provided by the Eaton supercharger dropped off with engine speed. This drop off in boost pressure would become much more severe as we increased the drive ratio on the blower.

After establishing a baseline, we decided that the first modification would be to tune the existing combination. Using the F.A.S.T. engine management system, Tom Habryzk from Westech dialed in a new timing curve that supplied 23 degrees of total timing everywhere (from 3,000 to 6,500 rpm). In stock trim, the WOT timing was as low as 13-14 degrees according to the gang at Kenne Bell. Increasing the ignition timing had a dramatic effect on the power curve. Since the supercharged engine was already equipped with 23 degrees of ignition timing from 6,000-6,500 rpm from the factory, there was very little difference in peak power but boy howdy did the power improve down low. The extra timing improved the torque production by as much as 40 lb-ft. No changes were made to the air/fuel mixture, which was kept constant at 11.8:1. We know that the factory air/fuel mixture can be considerably richer than this, having witnessed many Cobras running in the 10s on the chassis dyno. The difference would likely have been even greater had we elected to run the factory air/fuel curves as well on our baseline. No wonder the new Cobras respond so well to custom chips.

The next performance modification on our "Mods for 4V Mods" list was a set of 1 3/4-inch (primary) Flow Tech (Hooker) headers. Keeping the ignition timing at a steady 23 degrees and the air/fuel ratio at a constant 11.8:1, the long-tube headers and 18-inch collector extensions allowed our supercharged test mule to exceed 500 hp for the first time. The headers upped the peak power to 501 hp, while the torque peak was up slightly to 461 lb-ft. Interestingly enough, the boost pressure actually dropped with the installation of the headers. The reason for this is that the exhaust scavenging helped improve the efficiency of the engine. Improving the (pumping) efficiency of the motor will result in a decrease in boost (or back) pressure. Installation of (the right) cams, ported cylinder heads or increasing the displacement of the motor will all result in an increase in power combined with a drop in boost pressure. Please don't be mislead into thinking that we should now increase the boost pressure to compensate for the drop. The real test of the effectiveness of the headers is to install them with no other change.

After improving the flow rate of the exhaust system, we turned our attention to the inlet system. Since the stand-alone F.A.S.T. system allowed us to eliminate the air silencer, mass air and air cleaner inlet assembly, we were left with the stock throttle body and inlet tube connecting it to the supercharger. In retrospect, we should have measured the inlet vacuum present between the throttle stock body and the supercharger, but we were more interested in monitoring the boost pressure. In place of the stock dual-blade throttle body and cast aluminum inlet, we installed a billet aluminum, single-blade throttle body and free-flowing inlet system from Accufab. Like most pre-blower airflow improvements, installation of the Accufab throttle body increased the boost pressure slightly (.2 psi) and upped the peak power numbers from 501 to 508 hp, while the peak torque was up 2 lb-ft to 463. As expected, the power gains improved with engine speed. A later comparison between the stock and Accufab inlet systems (at a higher boost and power level) revealed gains of 10-12 hp.

With improved intake and exhaust systems, it was time to crank up the boost pressure. The boost pressure on the factory Cobra is supplied by a Roots-style Eaton (positive displacement) supercharger. The blower is able to supply the motor with more air than it could ingest of its own accord in normally aspirated form. The extra airflow is seen as boost. The boost pressure is controlled by the speed of the supercharger relative to the engine. This speed is determined by the drive ratio or the size of the crank and blower pulleys. The stock pulleys measured 3.6-inches (supercharger) and 7.5 inches (crank). This pulley ratio produced a blower speed 2.08 times the engine speed, meaning that our Eaton supercharger was spinning over 13,500 rpm at our self-imposed engine redline of 6,500 rpm. (6,500 rpm x 2.08 = 13,541 rpm).

Given that additional power is just a pulley change away, it is not surprising that blower (and crank) pulleys are two of the lost popular upgrades among Cobra owners. Gains of 50 hp and 50 lb-ft (or more) are not uncommon with pulley changes, making them one of the best bang for the performance buck available.