Richard Holdener
August 13, 2010

Choosing the right cam for a supercharged 4.6L Three-Valve motor is no different than choosing one for any other motor-or is it? Right off the bat, the Three-Valve motor has that odd cylinder-head configuration, with a pair of intake valves and a single exhaust valve. The ratio of intake-to-exhaust flow obviously has an effect on the ideal cam profile, which means that picking a cam for a Three-Valve is not the same as picking one for other head configurations.

Naturally, the intake flow will be considerably higher with a pair of intake valves compared to just a single valve. Add the presence of boost pressure and things really get crazy. Does positive pressure alter the cam requirements on a Three-Valve, or will just any old cam profile work? Obviously the former is more accurate than the latter, but just what constitutes a good Three-Valve blower cam?

To answer that question, we put a pair of profiles from Comp Cams to the test on a supercharged Three-Valve. Not just your run-of-the-mill blower motor, mind you, but a serious stroker effort making manly boost levels.

The test motor, or more accurately, the test Mustang, belongs to ST Motorsports. The Kenne Bell-supercharged '06 GT itself is worthy of a story, having already ripped off a 9.79 at 140 mph, with an automatic no less. Putting things into perspective, this run was made with the motor producing roughly 750 hp at the wheels-some 170 less than its current output (after our cam test), but more on that figure in a moment.

Even without the Kenne Bell, the GT was far from stock. Thanks to a bore and stroke combination of 3.72x3.80 inches, the stroker displaced 331 ci and was topped with Stage 3 ported heads from Livernois Motorsports. The short-block consisted of a forged steel crank, Manley forged connecting rods, and Diamond forged pistons. The Diamond pistons featured a dish to drop static compression to 9.2:1. Also present was a set of 13/4-inch long-tube headers from American Racing Headers, a Milodon oil pan, and a Precision Industries torque converter with a triple-plate lockup clutch that was hooked to an ST-built 4R70W.

The 331 was built with boost in mind-in this case, from a Kenne Bell 2.8L twin-screw. Capable of supporting 1,000 hp, the H-series blower featured a huge 4.5-inch intake tube and MAF assembly, combined with a Mammoth intake manifold and dual 75mm throttle body. Inlet restrictions reduce both the power and boost produced by the supercharger, so every effort must be made to maximize airflow to the supercharger.

Additional features included an oversized, trunk-mounted intercooler reservoir. Filled with ice water, this reservoir maximizes efficiency of the air-to-water intercooler. The fuel system on the GT was upgraded with a GT500 dual-pump fuel system along with 80-lb/hr injectors and a dual 20V Boost-a-Pump from Kenne Bell. Note that this power level was achieved with a stock fuel rail!

The Three-Valve was first tested with the Stage 1 NSR (no spring upgrade required) cams (PN 127050) from Comp. The Stage 1 NSR cams offered 0.450 lift, a 214/227 duration split and a seemingly blower-friendly 14-degree lobe separation angle. Testing has shown these cams offer 20-25 hp over the production cams, and work well on both normally aspirated and supercharged street combinations. All testing was run with the Kenne Bell blower, sporting a 3.0-inch blower pulley combined with an 8-inch crank pulley from Innovators West. The timing and air/fuel mixture were locked at 23 degrees and 11.8:1, respectively, while the coolant and intercooler water were kept constant through data logging. In short, every effort was made to keep any variables consistent for all testing.

Running the Stage 1 NSR cams, the supercharged Three-Valve produced 751 hp and 685 lb-ft of torque on the DynoJet. These are stout numbers, especially for one running through an automatic.

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After backup runs demonstrated that these numbers were perfectly repeatable, the Mustang was pulled from the rollers and out came the Stage 1 cams. Having run the majority of my testing on the engine dyno, I don't envy having to perform cam swaps in the car, but the performance enthusiasts at ST Motorsports made short work of the cams, and we were up and running in no time. The Stage 1 cams were replaced with XE280PH-13 cams. The Stage 3 blower grinds (PN 127650) offer more lift and duration, not to mention a slight change in the lobe separation angle.

The Stage 3 cams feature a 0.559/0.560 lift split, a 245/256-degree duration split (measured at 0.050), and a 113-degree lobe separation angle. Unlike the NSR Stage 1 cams, these ballistic bumpsticks required increased valvespring pressure and cam phaser limiters. Lucky for us, the Livernois Stage III heads already feature sufficient spring pressure (and coil bind clearance) to work with both cams, while Comp Cams supplied the necessary Phaser limiters. The limiters are used to reduce the amount of cam phasing available, from near 50 degrees down to just 20 degrees, to maintain proper piston-to-valve clearance. Since major adjustments are made only at part throttle, little or no power is lost with the installation of the limiters at wide-open throttle.

Once the Stage 3 blower cams were installed, the GT was positioned back on the DynoJet. It was evident that these cams are more aggressive than the Stage 1 grinds. Adjustments were made to tame the idle quality, but it was obvious this car was running something other than stock sticks.

Keeping the air/fuel mixture at 11.8:1 and the total timing locked at 23 degrees, output jumped to an amazing 836 hp and 713 lb-ft of torque. The Stage 3 blower cams were worth 85 hp over the already impressive Stage 1 cams. Since this is an automatic, the start rpm was roughly 3,500 rpm, but no power loss was experienced with the cam swap in the tested rpm range.

Where the Stage 1 cams failed to reach 700 lb-ft of torque, the Stage 3 cams bettered the 700-lb-ft mark from 4,600 rpm to 6,100 rpm. Instead of falling off after 6,500 rpm, the motor pulled hard all the way to 7,000 rpm. Remember, this was a car that already ran 9.79 at 140 mph when the motor was making just 750 rwhp. The extra 85 hp should easily knock 0.30-0.40 off the e.t.'s, maybe more.

Impressed as we were with the gains offered by the Comp Cams Stage 3 blower cams, the boys at ST Motorsports weren't done yet. Installation of the Stage 3 cams actually dropped the boost pressure by as much as 2 psi, which is something we've come to expect from improving the efficiency or displacement of the motor.

Knowing they had smaller blower pulleys in hand for the 2.8L Kenne Bell, they installed a 2.5-inch blower pulley and let 'er rip. Keeping the air/fuel and timing the same as the previous runs, the motor just touched 900 rwhp and over 880 lb-ft of torque. Torque production exceeded 800 lb-ft from 3,200 rpm to 6,200 rpm, making for one serious torque curve.

From the looks of the curve, there seems to be an airflow restriction somewhere in the system. The guys at ST Motorpsorts are looking to install an even larger throttle body and follow that up with an exhaust upgrade, and then the larger 3.6L supercharger in their quest for even more Three-Valve power. For now, we are plenty impressed with any supercharged Three-Valve pumping out 900 rwhp through an automatic.

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