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2003 Ford Mustang Cobra Kenne Bell Twin Screw Supercharger - Mods For Mods Part 2
Mods for Mods Part 2
After running our '03 Cobra engine in "Mods for 4V Mods, Part 1," we were excited about the prospect of returning to the dyno with the supercharged crate motor. Supplied by Ford Racing, the four-valve 4.6 was fresh and new, right down to the Eaton M112 supercharger.
In the first installment, we put the powerplant on the engine dyno and ran it in near stock trim. The motor was equipped with an electric water pump, 2.5-inch open exhaust pipes behind the factory cast-iron exhaust manifolds and no MAF or attending inlet system (just an open throttle body). Equipped with the factory timing curve (though a leaner-than-stock fuel curve), and the factory blower and crank pulleys (thus stock boost pressure), the '03 crate 4.6 pumped out 483 hp and 427 lb-ft of torque.
After the baseline run, we tuned the timing, installed headers and an Accufab throttle body and then proceeded to crank up the boost pressure on the Eaton supercharger. When it was all said and done, the blown Cobra motor produced a best reading of 539 hp and 569 lb-ft of torque using a 7.75-inch crank pulley and 2.93-inch blower pulley.
As SVT Cobra owners have come to expect, the boosted four-valve produced monster torque, but the boost pressure (and attending horsepower) supplied by the Eaton fell off at increased engine speeds.
While Cobra aficionados have been known to run more aggressive pulley ratios than the ones tested previously, the gains experienced are primarily in the lower rev ranges. The extra boost pressure and torque production feels great and can be enjoyed on a regular basis down low on the street, but becomes of limited value on the track at reducing e.t.'s and increasing trap speeds, as the gains are minimal in the rev range used during acceleration.
After finding the horsepower limit of the Eaton supercharger, we were anxious to take four-valve performance to the next level. Before we could go forward, we actually had to take a step back, in this case, back meant without boost. That's right loyal readers, to better illustrate the gains offered by the Eaton supercharger on the Cobra 4.6, we decided to run it normally aspirated. Replacing the supercharger with a non-blower intake from an '01 4V Cobra would allow us to graphically demonstrate the power gains offered by the supercharger. Think about it as installing a supercharger on a low-compression Cobra motor, just in reverse. Thanks goes out to the gang at Kenne Bell for supplying not only the twin-screw supercharger (to be used later), but also the '01 Cobra intake for testing.
Why go to all the trouble of running the supercharged Cobra engine in normally aspirated form you ask? The effectiveness (I hesitate to use the scientific term efficiency here) of a supercharger can be calculated with some simple math. Basically speaking, if the normally aspirated motor has 14.7 psi (atmospheric pressure) helping push the air into the vacuum crate by the downward moving pistons, then any additional pressure supplied to the motor (in the form of boost) can be calculated as a percentage.
For instance, if our normally aspirated motor produced 100 hp at atmospheric pressure and we supplied 1 Bar or an additional 14.7 psi of boost to the motor, theoretically the power output should double. Of course, this doesn't always happen, as things like the parasitic losses associated with driving the blower combined with the efficiency of the blower itself will reduce the eventual percentage gains. The gains offered at lower (or higher) boost levels can be calculated by using percentages of Bar (14.7 psi).
For example, suppose we ran just 10 psi of boost on our 100hp motor. To gauge the effectiveness of the supercharger, we would take 10 (psi) and divide it by 14.7 to get the percentage of Bar or atmospheric pressure (roughly 68 percent). If everything went perfectly, we could expect the power output of our 100hp motor to increase by 68 percent at 10 psi.
Using the simple formula, we can check the effectiveness of the Eaton supercharger on the 4.6. Before we can apply the formula, we need to know how much power the motor will produce in normally aspirated form. Swapping out the Eaton blower for the NA '01 intake was a piece of cake. The intake was run with the same 65-lb-hr injectors and the stock dual-blade throttle body (though no MAF or inlet system). The air/fuel curve was dialed using the F.A.S.T. system to produce 13.0:1, while the total timing was upped to 28 degrees. We tried both 26 degrees and 30 degrees, but the normally aspirated 4.6 ran best at 28 degrees. Equipped with the '01 intake, the low-compression Cobra mill developed 370 hp at 6,000 rpm and 377 lb-ft of torque at 4,900 rpm. It should be noted that the long-runner intake greatly improved the torque production over the short-runner intake used under the blower, but the limited room with the Eaton supercharger dictates the use of a less-effective (in the usable rpm range of a Cobra motor) short-runner intake.