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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.
Now that we had the N/A power figures, we could compare them to the supercharged output at a given boost level. Using the baseline runs in "Mod for 4V Mods, Part 1," we know that the Cobra motor produced 501 hp with 23 degrees of timing, an optimized 11.8:1 air/fuel ratio and long-tube headers at a peak boost pressure of 8.8 psi. Here is where things get tricky with the calculations. The Eaton supercharger produced a peak boost reading of 8.8 psi at 4,700 rpm, but only 8.0 psi at the peak of 6,500 rpm. Using the maximum boost reading of 8.8 psi, we see that the gain of 131 hp offered by the supercharger equates to roughly 35 percent (of the original 370 hp).
By comparison, 8.8 psi equates to almost 60 percent of 14.7 psi, thus we increased the boost pressure by 60 percent and only improved the peak power by 35 percent. Even using the lower boost pressure reading of 8.0 psi, we still see that the pressure increased by 54 percent, yet the power was up only 35 percent. This is an indication that the Eaton (Roots) supercharger is not terribly effective at improving the power output compared to other forms of forced induction. We will take a much more in-depth look at all the forms next month as we subject the '03 Cobra mule to no less than four different types of forced induction in a no-holds barred shootout.
With our normally aspirated comparison completed, we turned our attention to finding more power. The Eaton supercharger was reinstalled and equipped with a 3.2-inch blower pulley. We installed a 7.75-inch crank pulley and tuned the air/fuel and timing curves. Running 23 degrees of timing and a steady air/fuel ratio of 11.8:1, the supercharged motor produced a peak boost pressure of 12.1 psi. Despite a dropping boost curve (to 10.9 psi) the 4.6 managed to produce 537 hp at 6,500 rpm and 530 lb-ft at 3,500 rpm. A back-up run showed the power curve to be repeatable, as did a comparison with the previous testing from Part 1, where we ran an identical combination.
After we were satisfied with the power output of the Eaton, we swapped on the twin-screw Autorotor supercharger upgrade from Kenne Bell. Wanting a direct comparison, we decided to install the same 3.20-inch blower pulley on the Kenne Bell supercharger. Unfortunately, we only had a slightly larger 3.25-inch blower pulley that would slightly reduce the blower speed (and boost) supplied by the twin-screw Autorotor. Undaunted, we swapped over the factory Cobra intercooler and installed the 3.25-inch blower pulley. Like the Eaton supercharger, the Kenne Bell was equipped with an Accufab throttle body and run with dyno water running through the air-to-water intercooler.
To say that we were impressed by the results of the blower upgrade would be an understatement. Installing the Kenne Bell supercharger (with the larger blower pulley no less) increased the power output to an amazing 682 hp. Think about it: the Kenne Bell added more power (145 hp) to the Eaton supercharged Cobra motor than the Eaton supercharger did to the normally aspirated motor (just 130 hp). Where the power curve leveled off with the Eaton, the horsepower kept climbing with the twin screw.
The Kenne Bell blower upped the torque peak as well, from 530 to 595 lb-ft. As further testament to the efficiency of the twin-screw, the blower not only pumped out more pressure at 3,000 rpm (14.2 vs. 11.5 psi), but the boost curve kept climbing with the twin-screw to a maximum reading of 16.5 psi (vs. 10.9 psi for the Eaton) at 6,500 rpm. The Eaton is simply not large or efficient enough to compete with the twin-screw supercharger at the elevated power levels.
The same can be said (to an even greater extent) when comparing the Eaton to a centrifugal supercharger, the only difference being that the centrifugal supercharger will lose out dramatically at lower engine speeds. Not so for the twin-screw supercharger, as the positive displacement Autorotor makes more power everywhere than the roots blower. Too bad SVT didn't equip the Cobra with a twin-screw in the first place.
After running the Kenne Bell blower, we began to think like any good Cobra owner. Hey, let's crank up the boost! While the peak boost pressure was already over 16 psi, we knew the Autorotor had a bunch more to offer. Knowing this, we replaced the DUB 7.75-inch crank pulley with a larger 8.5-inch version. The DUB system made pulley swaps something to look forward to rather than dread. The pulley change took all of two minutes, including changing the drivebelt to suit the larger crank pulley. Remember that our Cobra motor was previously upgraded with a Metco idler system to help minimize belt slippage.
It should be mentioned that previous testing with a nitrous system (for another magazine) necessitated the installation of a Kenne Bell Boost-a-Spark. The ignition upgrade cured a high-speed misfire and allowed us to successfully combined nitrous and boost (from the Eaton) without misfiring. We are confident that the Boost-a-Spark-augmented ignition is what allowed us to run such elevated power levels with our coil-pack ignition (from a 2V 4.6L). The larger DUB crank pulley increased the peak boost pressure from 16.5 to 19.5 psi, upping the power output everywhere. The power gains were as high as 40 hp and 47 lb-ft of torque. The pulley swap allowed us to exceed 700 hp for the first time with our Ford Racing crate motor.
To this point, all of our testing had involved induction improvements. Now it was time to focus on the exhaust side of the equation. Though run in reverse order on the dyno, it is probably easier to understand if we describe the test procedure in ascending order. Originally we wanted to install a complete factory Cobra exhaust system to compare against a complete Bassani system (including headers). Unfortunately, the factory cat pipe supplied to us (on loan) was clogged. We discovered this only after hooking up the entire exhaust and attempting to run the motor. With the boost gauge showing in excess of 25 psi and the dyno indicating an output of less than 200 hp, we knew a problem existed. In the end, we replaced the clogged cat pipe with the supplied X-pipe from Bassani, giving us the stock exhaust manifolds feeding the Bassani X-pipe and the stock Cobra cat-back exhaust. The ordeal took several hours to straighten out, but we finally had a compete system ready to run. Credit the excellent Westech dyno facility for the ability to test complete exhaust systems on the engine dyno.
The exhaust test was run with the Kenne Bell supercharger equipped with the 3.25-inch blower pulley and the smaller 7.75-inch crank pulley. Equipped with the stock exhaust manifolds, Bassani X-pipe and factory cat-back, the supercharged Cobra motor produced 666 hp and 578 lb-ft of torque at 17.9 psi. The boost reading is important as improvements to the exhaust system actually lowered the boost pressure. The first test involved replacing the factory Cobra cat-back exhaust with the Bassani 2.5-inch stainless-steel cat-back system. I would be remiss if I failed to point out the fact that the Bassani X-pipe sounded great. I've always liked the effect X-pipes have on the exhaust note and the Bassani system was no exception.
Installation of the Bassani cat-back upped the peak numbers to 677 hp and 587 lb-ft, while lowering the peak boost pressure to 17.2 psi. The Bassani cat-back exhaust improved the power output by as much as 12-13 hp elsewhere in the rev range. To illustrate what is possible with headers and open exhaust (for a drag race vehicle), we installed a set of 1 5/8-inch Flowtech (Hooker) headers, each running through an 18-inch section of 3-inch exhaust (no mufflers). The free-flowing exhaust system upped the peak power to 682 hp, while peak torque checked in at 595 lb-ft. Like the Bassani exhaust, the gains offered by the headers and open exhaust were more pronounced elsewhere in the curve.
This exhaust test completed our mods list in Part 2, look for a ported Eaton supercharger, CNC-ported heads and a whole gaggle of cams in a future episode of "Mods for 4V Mods."