Back in our "Mods for Four-Valve Mods" series, we experimented with a Kenne Bell supercharger to replace the factory Eaton M112 blower. In Part 2 of the "Mods" series, we managed to exceed 700 hp with the Kenne Bell supercharger on the otherwise stock '03 Cobra crate engine (supplied by Ford Racing).
In our subsequent "Boost Bash" series, we once again exceeded 700 hp with the '03 Cobra 4.6, but at a reduced boost level thanks to the introduction of a set of Comp XE262AH cam profiles.
If you have ever read any of our testing on forced induction, the power output of any supercharged motor is a function of the power output of the normally aspirated engine multiplied by the boost pressure. Thus, improving the power output of the normally aspirated combination can, and often does, yield larger power dividends once you add boost to the equation. Of course, it is imperative that the normally aspirated combination be able to withstand the extra power offered by the supercharger.
After reviewing the power gains offered by the Kenne Bell twin-screw supercharger, we began to wonder just how much power was available from a more serious supercharged Four-Valve combination. Having run the 4.6 up to 20 psi with a pair of turbos from HP Performance (to the tune of 990 hp), we decided it was high time to see just how much power was available from the Kenne Bell supercharger at a similar boost level. Having already exceeded 700 flywheel horsepower during the testing for "Mods for Four-Valve Mods," we knew we would easily surpass that power number. We have personally witnessed a modified '03 Cobra exceed 700 wheel horsepower on the Dynojet, so we know that translated to more than 700 flywheel horsepower. The question now was, how much more?
As luck would have it, the very same powerplant that exceeded 700 wheel horsepower (at 28 psi of boost) was used for this high-horsepower Four-Valve buildup. Of course, the engine was removed from the Cobra, and freshened up (and modified) in order to increase the power potential, but it was nice to know that we were working with a familiar face.
The proper route to more power is a bit more complicated than simply cranking up the boost. When this 4.6 exceeded 700 wheel horsepower, it was equipped with a 9.5-inch crank pulley and a 3.39-inch blower pulley, giving us a drive ratio of 2.80:1. Running 6,500 rpm, this pushed the blower speed over 18,000 rpm. According to Kenne Bell, there was a little more blower speed available, but both the blower speed and boost levels (28 psi) were nearing the danger zone. The ideal situation would be to increase the power output while simultaneously reducing the boost pressure.
Wanting not only more power but more reliable power, the '03 4.6 was taken to Accufab's John Mihovetz. No stranger to huge modular power, he took on the buildup and allowed us to follow along during the assembly and subsequent dyno thrash. Naturally, the dyno testing involved more than just the set-it-and-forget-it tuning session, as Mihovetz tried a number of different trick components (many custom machined) in his quest for serious Kenne Bell power. Before he could get to the induction, intercooling and exhaust tricks, he had to build a suitable long-block. He took the stock steel Cobra crank and added a set of forged connecting rods and forged pistons, and stuffed them inside a race-prepped aluminum Cobra block. The forged pistons deserve special mention as they not only featured crown and skirt coatings, but were flat-top slugs (with valve reliefs), which upped the static compression to 10.0:1. The pistons were equipped with stainless steel top rings to withstand the extreme conditions generated by the blower motor.
Any time John Mihovetz gets...
Any time John Mihovetz gets involved in a modular project, you can expect big power numbers.
The buildup started out with...
The buildup started out with a fresh set of forged pistons. The flat-top pistons (coated on both the skirt and crown) upped the static compression ratio from 8.5:1 to an even 10.0:1. These are the same pistons used by Mihovetz on his 1,500-plus-horsepower, twin-turbo mod motor.
The factory H-beam rods were...
The factory H-beam rods were ditched in favor of a set of stronger I-beam rods. Naturally, the factory never intended the 4.6 to produce anywhere near 900 hp.
The new forged rods and pistons...
The new forged rods and pistons required the factory steel crank to be rebalanced (note the weld marks on the counterweights).
The forged reciprocating assembly...
The forged reciprocating assembly was stuffed into a fully prepped aluminum Cobra block. The block was also treated to a main stud girdle to further improve bottom end strength. Mihovetz even went to the considerable trouble to source custom ARP cross bolts for the buildup.