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Ford Cobra Motor Modifications - Kenne Bell Blower - Substantial Boost Solution-Tech
This 4.6 Dohc Gets A Serious Bump In Power With A Kenne Bell Blower
While we have yet to exhaust the available normally aspirated modifications for our early 4.6L Four-Valve motor, we have decided it is high time for boost. Previously, our high-mileage '96 Cobra mill was subjected to IMRC eliminator plates, a modified (shortened) stock intake, and a larger throttle body. While we had it on the dyno, we also engaged a Zex wet-nitrous system to the tune of 100 hp. After those mods, we installed a set of Comp XE262AH cams with excellent results.
In addition to properly degreeing the cams, we also demonstrated just how far off they can be when installed using the factory keyways. With 3 degrees of slop in each cam/sprocket interface, you're looking at a wide variance using the factory-keyed sprockets (to say nothing of using the early non-keyed sprockets). Though not specifically featured in the "Mods For Early Four-Valve Mods" series, we also subjected the motor to further testing with the Comp cams by retarding the intake cam from 110 degrees to 116 degrees. The results were interesting in that the power was up with the stock manifold, but adjusting the cams actually reduced the power with the VRI intake designed by the author. Equipped with the VRI intake and XE262AH cams (adjusted at 110 degrees), the motor eventually produced 411 hp and 342 lb-ft of torque (35 hp over the stock intake).
For this episode, we removed the VRI intake and replaced it with the stock manifold and throttle body. Previous testing has shown the stock dual-blade throttle body (in fact, the entire lid and plenum volume) had little effect on power at this level. The motor still featured the Comp XE262AH cams installed with the intakes set at 116 degrees and the exhaust cams at 112 degrees. In this slightly retarded position, the motor produced its highest peak power with the stock intake.
The normally aspirated motor was set up with the FAST management system, 36-pound injectors, and Hooker long-tube headers. Meziere sent us a new electric water pump that featured an idler pulley, which allowed us to run the electric pump with the Kenne Bell blower. Run at an air/fuel ratio of 13.0:1 and 29 degrees of total timing, the normally aspirated 4.6 produced peak numbers of 374 hp at 6,500 rpm and 342 lb-ft of torque at 5,200 rpm. Torque production exceeded 300 lb-ft, from 3,600 rpm to 6,500 rpm. While 300 lb-ft of torque at 3,600 rpm is fairly impressive from just 281 ci, that number was about to change by more than 100 lb-ft with the installation of the Kenne Bell blower.
While the '03-'04 Cobras grab a lot of attention when talk turns to supercharging, the '96-'98 Cobras are the cars that actually got things started. True, there is not much comparison between the power of an early normally aspirated Cobra motor and the later supercharged version, but like its big brother, there is plenty of power in the N/A version that's just waiting to be unleashed. All it takes is a little forced induction.
With that in mind, we decided to step up the power production of the early motor with the installation of a Kenne Bell supercharger. Unlike the later-production Cobras, which rely on an Eaton Roots-style blower, the Kenne Bell utilized a twin-screw design. Compared to a Roots-style, the twin-screw offers much more power potential (per liter of displacement), reduced inlet charge temp, and it requires less power to drive. The combination of these attributes means the Kenne Bell twin-screw blower will provide more power per pound of boost than a Roots-style. That it is a positive displacement design means efficient boost is provided the minute you stomp on the throttle.
One of the reasons we decided to include a separate "Mods for Early Four-Valve Mods" was because the early motors differed from their late-model counterparts. Though they share similar short-blocks, which displace 4.6 liters, the heads and intake manifolds are substantially different. This difference was one of the reasons why the early Four-Valve motors required a different kit from Kenne Bell. Where the late-model Cobras can enjoy an intercooled kit similar to the one used on the '03-'04 Cobras (but with the more efficient twin-screw blower), the kit for the early Cobras comes sans air-to-water intercooler. What this means is that an early Cobra will not likely produce as much power as a late-model one equipped with an inter-cooled kit, but an early Cobra owner should be able to equal or surpass the power offered by a stock '03-'04.
After all, the difference in the rated power output between the early Cobra and the '03-'04 was only 90 hp. Despite the lack of the inter-cooler, we had high hopes for the blower kit on the '96 Cobra motor. With the exception of the nitrous, none of the normally aspirated mods made much of a power difference, at least not compared to a blower.
The Kenne Bell twin-screw blower kit for the '96-'98 Cobras featured a 1.7L super-charger. While Kenne Bell also offers larger blowers for more serious power gains, we have exceeded 600 rwhp with the 1.7L unit, so we have no fear about the power potential. Since the supercharger was driven off the six-rib serpentine belt shared by the rest of the engine accessories, we had to rig up a drive system using a borrowed A/C compressor, the Meziere idler water pump assembly, and the factory belt tensioner.
After measuring the proper belt length, it took only four trips to the local auto parts store to finally arrive at a suitable belt length. Having the belt extra tight provided us the room to drop down in pulley size for even more boost pressure. Driving the blower was the factory crank/damper, but Kenne Bell also offers larger crank pulleys as well. The larger crank pulley will allow you to increase the blower pulley size (while retaining the same drive ratio), which helps eliminate belt slippage. Increasing the crank pulley also increases the speed of the engine accessories, which will reduce power slightly (though not nearly as much as the extra power provided by the increase in boost). For our runs, we equipped the blower with a 3-inch blower pulley, which produced just over 9 psi of boost.
To ensure adequate fuel delivery, the 36-pound injectors run on the normally aspirated motor were replaced with a set that flowed 65 lb/hr. A Kenne Bell Boost-a-Pump and Boost-a-Spark were employed on the engine dyno to ensure adequate voltage supply to the fuel pump and ignition system. Denso IQ16 Irridium spark plugs were replaced by a set of colder IQ24s. Colder plugs are necessary when running boost, especially on a blower kit not equipped with an intercooler.
After some tuning on the FAST manage-ment system (by Westech's Tom Habryzk), the supercharged combination produced 486 hp and 433 lb-ft of torque. As we have come to expect of a positive displacement supercharger, the Kenne Bell kit offered immediate boost response and a ton of torque right along with it. Down at 3,500 rpm, the blower improved the torque production by over 120 lb-ft. While the extra 110 hp is what everyone talks about, it is this extra torque that is most often enjoyed from behind the wheel.
You know how we roll at MM&FF, so you don't think we could just stick the Four-Valve motor on the dyno and run it at just one boost level, right? Pulley swaps on the Kenne Bell are so easy, we decided to replace the 3-inch pulley with a smaller 2.75-inch version. Of course, the extra boost (a tad over 12 psi total) required some minor tuning (thanks for staying late, Tom). We were rewarded with peak power numbers of 506 hp and 463 lb-ft of torque. It should be noted that we hedged our bets with the stock cast pistons by topping off the fuel tank with 100-octane race fuel.
The last thing we wanted was to damage the test motor so graciously loaned to us by Accufab's John Mihovetz. With the pulley change came another significant jump in torque production. The 4.6 motor now exceeded 450 lb-ft of torque from 3,600 rpm to 5,500 rpm. It is this impressive average power gain that really helps accelerate the vehicle. Sure, we all like to talk about peak numbers (with over 500 hp this motor is no slouch), but the reality is, that big smile on your face came from the immediate (and substantial) torque production offered by the twin-screw blower.
Now that we've tried the twin-screw blower, we suppose we have to give equal airtime to the centrifugal and then maybe the turbo.