Richard Holdener
December 3, 2014

In a world where even stock Shelbys exceed 660 hp, what does it take to impress the mod motor world? Since 700 hp is just a pulley swap, tune, or air intake away, the big number must be 800, 900, or even 1,000 hp. Surely 1,000 hp is impressive, especially when it comes from a motor sporting just 5.8 liters of displacement, right? While 1,000 hp should be more than enough for most enthusiasts, the supercharged bar has been officially raised by the folks at Accufab and Kenne Bell.

Fresh off their last effort that smashed through the 1,500hp barrier with a peak reading of just over 1,600 hp, this latest project incorporated a number of upgrades, including a supersized new supercharger, Bigun intake, and a matching intercooler upgrade. Combine these new offerings from Kenne Bell with the mod motor wizardry from Accufab, and it made for some Rare Air indeed. How rare? How does 1,739 hp sound?

If you are looking to make some serious power with your rod motor, Accufab's John Mihovetz is the guy you want working on it.

The Mod Squad: Accufab 5.4L Mod Motor

If there is someone with more mod motor racing experience than Accufab’s John Mihovetz, we haven’t met him. All that experience and knowhow was force-fed into this 5.4L Shelby motor that starts with the aluminum Ford GT block. After being deburred, the block was machined and prepped to receive a complete forged rotating assembly. Not your average street assembly, the combo included a Bryant forged steel crank, Manley rods built specifically to Accufab specs, all swinging a set of custom JE forged pistons. Not your average shelf pieces, these JE slugs were built with serious boost (and 12.0:1 compression) in mind. Topping the short-block was a set of extensively ported Ford GT heads. In addition to the porting, the Four-Valve heads also received larger (38mm/32mm) valves and custom valve seats designed to withstand the harsh, boosted environment. For maximum sealing, the GT heads were secured to the block using lock-wired, copper head gaskets and ARP 2000 head studs. The finishing touch included a fully ported GT500 intake and 225-pound injectors.

Bigger & Better: Kenne Bell 4.7L Blower

Far and away the most radical change to the supercharged mod motor was the blower itself. Having eclipsed 1,600 hp previously with the 3.6L and 4.2L blowers, Kenne Bell decided it was time to step things up. The simple concept behind a positive displacement supercharger is that each revolution delivers a given amount of air. The faster you spin it, the more air it provides—to a point. Naturally, when you run out of available revolutions, you have to increase the air supplied by each one. Increasing the displacement of the supercharger does just that. Though bigger is definitely better, Kenne Bell did more than just increase displacement from 4.2L to 4.7L. Along with adding capacity, KB also improved the 4x6 rotor profile and added case injection, all while maintaining the patented Liquid Cooling and Seal Pressure Equalization. Need more? Kenne Bell then stuffed all this supercharged goodness inside the same package that housed the smaller 4.2L.

While it may seem like magic that allowed the 4.7L to fit in the external package of the smaller 4.2L, it was actually science. The only way to add displacement without an external change was to gain the extra 0.5 liter internally. This was accomplished by boring out the case of the 4.2L 0.125 inch to accept the reprofiled 4x6 rotor pack. Extensive testing on its own supercharger dyno, chassis dyno, and Westech’s engine dyno verified the efficiency and capacity of the revised rotor pack. Run on the supercharger dyno at the same blower speed and boost (data backed up by the engine and chassis dyno), the new 4.7L offered an extra 130 cfm, dropped the discharge temps 32 degrees, and required 22 less horsepower to spin the blower! Tested at higher boost and rotor speeds, the difference exceeded 75 hp. Did we mention the 4x6 rotor pack can also be spun faster than the 3x5 combo? More airflow, a lower charge temp, and less power to drive equate to more power out of the motor. The new 4.7L is more than just bigger; it’s bigger and better.

The Big Guns: Bigun Inlet & Throttle Body

Given the flow, boost, and power capability of the new 4.7L blower, it was only natural that Kenne Bell would design an intake system to feed the beast. Despite the fact that the Mammoth inlet already surpassed the flow of the competitors, Kenne Bell designed the Bigun series inlet manifold to take supercharged motors to the next level.

Even the biggest, baddest supercharger can’t overcome restrictive inlet. Each inch of vacuum drop between the throttle body and supercharger is worth roughly 2 psi of boost. Less air going in means less power coming out. The massive Bigun inlet offered a 106-x237mm opening flowed 2,417 cfm. This can be combined with a new Bigun dual 106mm throttle body system that flowed 2560 cfm. One of the tricks incorporated into the new Bigun throttle body was to move the throttle bores closer together. This centralized the airflow into the Bigun intake manifold and increased the flow rate of the combination. This Bigun inlet system also positioned the throttle opening in the high-pressure cowl area to maximize flow into the system. KB also offered the single oval (168x106mm) throttle body. While this matched the flow rate of the dual 106s when tested alone, the combined flow was down compared to the Bigun system. This type of testing is a clear indication of Kenne Bell’s dedication to performance and the premium the company places on inlet restrictions.

Chill Out: Intercooler

When Ford stepped up the power of the GT500, it also increased the capacity of the intercooler. More power and boost requires more cooling. Kenne Bell recognized the fact that a supercharger capable of nearly 2,000 hp required more cooling. The limited space constraints offered in the GT500 packaging required considerable R&D to come up with an intercooler that offered both more flow and increased cooling, all while fitting in the limited available space.

When it comes to intercoolers, there must be a balancing act between flow and cooling. Maximum flow means minimizing flow restrictions, but this is at odds with maximum cooling, which requires maximum surface area. Kenne Bell designed the Bigun intercooler upgrade to increase both airflow and enhance cooling (no easy task). By increasing both the number of cooling rows and the fins per row, Kenne Bell was able to increase the cooling capacity. The company was also able to increase the size of the intercooler core from 45.6 square inches to 56.25. This increased the flow rate (as measured on Kenne Bell’s flow bench) from 1109 cfm to 1232 (the earlier ’07-’12 core flowed just 995 cfm).

The flow bench is but one test of the potential of the intercooler, so naturally Kenne Bell performed plenty of engine and chassis dyno testing as well. This testing included not only power numbers but enough data logging to choke a Cray supercomputer. By measuring boost pressure and temperature before and after the stock and Bigun intercoolers, Kenne Bell was able to determine pressure and temperature drops to correlate with the numbers generated on the flow bench. Only with accurate data can you determine what is really happening as boosted air is forced through the intercooler core. Testing on the Accufab 5.4L revealed the new intercooler dropped the charge temps by an amazing 332 degrees using ambient dyno water (ice water dropped this another 40 degrees). The pressure drop registered at 1,250 hp was just 0.5 psi. This compares to over 2 psi on the stock intercooler. From a flow and heat rejection standpoint, the new intercooler was obviously doing its job. Using ambient dyno water through the core, the Bigun intercooler upgrade was worth 22 hp over the ’13-’14 core, so the power gains were right in line with the improvements in temperature and flow. Employed by Accufab on its highly modified 5.4L Shelby motor, the new Kenne Bell blower, intercooler, and Bigun inlet pushed the peak power output to 1,739 hp at 31 psi of boost. Best of all, more power was left in the blower and engine combination.

01. Kenne Bell was able to increase the displacement of the 4.2L, while maintaining the external dimensions, by increasing the internal bore size and installing larger, revised rotor profiles.

02. The new 4.7L offered not only increased displacement and power potential but also provisions for direct water/methanol case injection. The position of the case injection eliminated the possibility of bearing damage caused by injection into the throttle opening.

03. Before releasing it to the public (or engine dyno), Kenne Bell extensively tested the new 4.7L on a blower dyno against not only the company’s own superchargers but also many competitors. The revised 4x6 rotor profile offered more flow, lower charge temps, and a significant reduction in the parasitic losses associated with spinning the blower.

04. The new 4.7L also incorporated patented features called Liquid Cooling and Sealed Pressure Equalization. Not to be confused with intercooling or case injection, the Liquid Cooling helped minimize the temperature differential between the cool (inlet) and heated (discharge) sides of the blower casing.

05. For most supercharged Shelby applications, the Mammoth intake would more than suffice, but Kenne Bell designed the Bigun intake for the flow needs of the massive 4.7L.

06. The design of the Bigun inlet, throttle body, and intercooler required extensive flow-bench testing. The massive flow rate of the intake and throttle body required dedicated test fixtures.

07. Looking to keep pace with the new 4.7L blower and Bigun intake, Kenne Bell stepped up to an even larger throttle body. To maximize flow when combined with the Bigun intake, the dual 106mm bores were moved closer together to centralize the flow away from the tapered walls on the intake. This design change improved the flow rate by nearly 150 cfm.

08. More flow and boost required more cooling. Space limitations required extensive testing to arrive at the optimum combination of cooling and pressure drop. The new Kenne Bell intercooler upgrade dropped temps by 332 degrees and was worth 22 hp over the factory 5.8L core.

09. Though the new core is a drop-in replacement, owners can machine the stock lower intake to maximize the flow potential of the new core.

10. Kenne Bell is big on data logging. Check out all the lines and hoses used on the chassis dyno to monitor boost and temperature before and after the intercooler, not to mention air/fuel, timing, engine speed, wheel speed, engine and ambient air temp, and so on.

11. When combined with the Accufab mod-motor wizardry, the new 4.7L and Bigun intercooler upgrade allowed the methanol-injected 5.4L to produce a maximum of 1,739 hp and 1,252 lb-ft of torque at 31 psi.

12. The hero run netted some huge power numbers. Equipped with the new 4.7L twin-screw blower from Kenne Bell, the Accufab race motor produced 1,739 hp and 1,252 lb-ft of torque.