Take one supercharged four-valve...
Take one supercharged four-valve motor, add performance parts, and stir as necessary.
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 in "Mods for 4V Mods, Part...
As in "Mods for 4V Mods, Part 1," we relied on the tuning abilities of the F.A.S.T. engine management system (and Westech's Tom Habryzk) to optimize every combination.
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.
Power Curves--NA vs. Supercharged...
Power Curves--NA vs. Supercharged Cobra 4.6
The first modification performed to the Cobra motor supplied by Ford Racing actually reduced the power output. In an effort to see how much power the Eaton M112 supercharger was responsible for, we decided to run the supercharged Cobra motor in normally aspirated form. To do so, we replaced the Eaton supercharger and intake assembly with an intake manifold from a normally aspirated '01 4V Cobra. The intake bolted right in place without any problems. The stock '01 intake was run with the stock dual-blade throttle body, as well. Equipped with the '01 intake, the low-compression '03 Cobra produced 370 hp and 377 lb-ft of torque. This compares to a right-at-500-hp run in the same configuration (headers, electric water pump and optimized fuel and timing curves) with the Eaton M112 supercharger at nearly 9 psi of boost.
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.
 Power Curves--Eaton Roots...  Power Curves--Eaton Roots vs. Kenne Bell Twin-Screw (Same Pulley Ratio)
Having already run the Eaton supercharger up in boost with the Dial-Ur-Boost pulley system from South Florida Pulley Headquarters in "Mods for 4V Mods, Part 1," we decided to take up where we left off and look for more power. The motor was first run with a 7.5-inch DUB crank pulley and a 3.2-inch DUB blower pulley to verify the baseline power. So equipped, it produced 537 hp at 6,500 rpm and 530 lb-ft at 3,500 rpm at a maximum boost pressure reading of 12.1 psi. As expected, the Eaton supercharger produced peak boost pressure in the mid range (4,200 to 4,800 rpm), but fell off to just 10.9 psi at 6,500 rpm. After a repeat run, we removed the Eaton supercharger and installed the Kenne Bell 422 twin-screw supercharger. We expected and were rewarded with big power gains from the more efficient twin-screw design. Equipped with near-identical pulley ratios (the Kenne Bell was actually spun slightly slower with a 3.25 blower pulley), the Kenne Bell supercharger pumped up the power to the tune of 682 hp. The timing and fuel were kept constant at 23 degrees and 11.8:1 using the F.A.S.T. system. The peak torque was up as well, from 530 to 595 lb-ft. |  Boost Curve--Eaton Roots vs....  Boost Curve--Eaton Roots vs. Kenne Bell Twin-Screw (Same Pulley Ratio)
Check out the difference in the boost curves supplied by the two blowers. Even though the two blowers were equipped with the same pulley ratio (the twin-screw was actually spun slightly slower), the twin-screw blower was up by 2.5 psi at 3,100 rpm and carried that advantage for several hundred rpm. As the Eaton M112 started to loose boost pressure, the twin-screw just continued to climb, maxing out at 16.5 psi at 6,500 rpm compared to just 10.9 psi for the Eaton. The Eaton M112 works very well on the Cobra motor and allowed the guys at Ford to achieve a specific power goal at the lowest desired cost. The design and displacement of the Eaton M112 is limited for maximum power usage and huge power gains are available by swapping out the blower for a more efficient design. |  16.5 vs. 19.5 psi--Kenne Bell...  16.5 vs. 19.5 psi--Kenne Bell 422 Blower (7.5 vs. 8.5 crank pulley)
After witnessing the power gains available from the twin-screw supercharger, we decided to crank up the boost on the Kenne Bell blower. To up the boost pressure supplied by the blower, we replaced the 7.75-inch DUB crank pulley with a larger 8.5-inch version. The DUB pulley system made the pulley change a snap. The pulley change necessitated a belt swap as well (from a 760 to a 780). Swapping out the crank pulley increased the peak boost pressure from 16.5 psi at 6,500 rpm to 19.5 psi at the same engine speed. The increase in boost pressure upped the power output by as much as 40 hp and 47 lb-ft of torque. The peak power increased to over 700 hp, despite experiencing some belt slippage at the top of the rev range. |
 Exhaust Test: Stock cat-back...  Exhaust Test: Stock cat-back vs. Bassani vs. Headers and 3-inch
The three curves depicted here illustrate the power gains offered by changes in the exhaust system. The '03 Cobra motor was run first with the stock exhaust manifolds feeding a Bassani X-pipe (no cats) and a stock Cobra cat-back exhaust. So equipped, the Kenne Bell supercharged motor produced 666 hp and 578 lb-ft of torque at 17.9 psi. Replacing the stock cat-back exhaust with a Bassani cat-back resulted in a jump in peak power to 677 hp and 587 lb-ft of torque. Installation of the Bassani cat-back exhaust upped the power by as much as 12-13 hp while decreasing the peak boost pressure to 17.2 psi (always a good sign). The final test involved replacing the stock exhaust manifolds with a set of Flowtech (Hooker) 1 5/8-inch headers and the Bassani cat-back with a simple 3-inch exhaust consisting of two 18-inch sections of 3-inch pipe (basically collector extensions). The headers and open exhaust improved the peak power output to 682 hp with torque now up to 595 lb-ft. Improving the exhaust flow once again decreased the boost pressure to 16.5 psi. Remember, flow improvements made after the blower (heads, cam or exhaust) will usually decrease boost pressure, while improvements made before (air inlet, MAF and throttle body) will increase boost pressure. Both can result in power gains. |  The first order of business...  The first order of business was to run the Cobra engine in near stock trim to compare the boosted motor's power output of 501 hp to the normally aspirated combination. |  After running the Cobra motor...  After running the Cobra motor with the Eaton, we removed the blower and installed an intake from an '01 (normally aspirated) 4V Cobra. The low-compression crate motor produced 370 hp and 377 lb-ft of torque in normally aspirated form. |