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Supercharger Shootout - Brothers In Arms
In This Supercharged Slugfest, We Find Out What 20 PSI Of Boost Is Worth On An '03 Cobra, An '05 GT, And A Shelby GT500.
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Much like the Terminator, the limiting factor on the GT500 motor is the supercharger itself. Despite being larger than the blower employed on the 4.6L Cobra, the Eaton supercharger was designed for the GT500 with a specific power and price goal in mind and therefore has certain limitations. It wouldn't make any sense for Ford to install a blower capable of 1,000 hp on a motor that only needs to make 500 hp. Limiting the blower capacity actually works in Ford's favor, as this helps minimize the warranty claims caused by enthusiasts cranking up the boost. Just imagine how many motors would be in for warranty if you could go from 500 hp to 1,000 hp with a simple pulley change? The factory blower was chosen for its combination of power potential and price point, as the Roots-style configuration is considerably less expensive than a comparable twin-screw supercharger. This is why Ford employed the twin-screw design on the more expensive Ford GT. A twin-screw design is more efficient than a Roots-style blower (including the new twisted-lobe TVS blowers), but it is more expensive.
While Ford chose the Roots-style blower for the GT500 motor, the Kenne Bell upgrade featured the more efficient and larger 2.8L H-series blower. What could be better than a bigger, more efficient motor being fed by a bigger, more efficient supercharger? Running the KB blower up to 20 psi on the larger GT500 motor produced exceptional results.
Achieving 20 psi on the 5.4L required a drive ratio of 2.73 (7.5 crank/2.75 blower). Basically it takes more blower speed to fill the larger motor. Run at 20 psi, the GT500 motor produced 756 hp at 6,200 rpm and 684 lb-ft at 4,800 rpm. The combination of increased displacement and blower speed resulted in a serious jump in torque production, too. Compared at 2,300 rpm, the GT500 bettered the smaller 4.6L motors by a solid 100 lb-ft of torque. Can you harness 600 lb-ft at just 2,300 rpm? Probably not, but it's always better to have more and need less than the other way around. Besides, the GT500 needs the extra power to offset the extra curb weight.
With over 700 hp, any of these brothers in arms would make for one serious street machine, but run at the same boost level, it looks like there really is no replacement for displacement.
'03 VS. '05 VS GT500--Same Drive RatioFor our main story, we supplied 20 psi of boost to the three different test Mustangs. Achieving the 20 psi of boost required different drive ratios (pulley combinations) for the three different motors. The reason for this is a combination of displacement and efficiency. The larger 5.4L required more blower speed to produce the desired 20 psi. The same can be said for the low-compression Terminator motor, as the Four-Valve Cobra motor required more blower speed than the Three-Valve combination to produce the same 20 psi.
What if we were to run the 2.8L blower on all three combinations at the same speed (same pulley combination) rather than try to select a particular boost level? For this test, all three combinations were equipped with an identical drive ratio of 2.30. With the blower spinning the same speed, it was the Three-Valve motor that came out on top, bettering even the mighty GT500 motor. Of course, running the blower at the same speed on all three combinations resulted in dramatically different boost levels. The power output and boost level (20 psi) of the Terminator remained the same (it already had a 2.30 drive ratio), while the boost level of the GT500 dropped to 16 psi. The change in drive ratio to the Three-Valve motor increased the boost pressure to 24 psi.