Muscle Mustangs & Fast FordsProject Vehicles
347 Short Block Fast Ford Formula Part 2
Makin' Power With Bolt-Ons, Boost, Or Both.
Note that the rising boost curve offered by the centrifugal design increased the speed where the motor made peak torque. Where the normally aspirated combination produced peak power at 4,700 rpm and peak torque at 3,300 rpm, the supercharger upped these values to 5,500 rpm and 4,200 rpm. It seems obvious that an efficient blower like the Novi 1220 (or Vortech S-trim) is an ideal way to improve the power output of stock 5.0L, even one equipped with a Demon stroker short-block.
Once we were satisfied with the results of adding the Paxton/Vortech supercharger to the stock configuration, it was time to add boost to the bolt-ons. Off came the stock heads, cam and GT-40 intake and on went the RHS aluminum heads, Comp XFI hydraulic roller cam and Edelbrock RPM II EFI intake. Ever present was the Accufab throttle body and matching EGR spacer.
To keep things consistent, no changes were made to the drive pulleys (either crank or blower). This would help illustrate any changes to the boost curve brought about by the bolt-ons. The FAST XFI management system made quick work of tuning the 36-pound injectors and we were eventually rewarded with peak power numbers of 665 hp at 6,300 rpm and 559 lb-ft of torque at 5,500 rpm. In hindsight, we should have run something larger than the 36-lb/hr injectors, as even with 65 psi of static fuel pressure (but no boost reference), we were well and truly out of fuel. The power curve was still climbing rapidly, but we shut off the fun at 6,300 rpm due to the resulting lean mixture.
With sufficient fuel and engine speed, power would easily have eclipsed 700 hp, but for our needs, the peak power was actually irrelevant, as we were looking for differences in power and boost. A comparison of the power output between the stock components and bolt-ons reveal that the bolt-ons were worth 141 hp in normally aspirated trim, but once we added boost to the equation, the gains increased to 244 hp, though additional fuel and engine speed would surely have increased this figure.
Just for fun, let's apply our Power/Boost Formula to this test. As a reminder, the Power/Boost Formula states mathematically that any changes in power in normally aspirated trim are actually multiplied by the boost present. By this we mean than if you take a 300hp motor and add 1 bar (14.7 psi), you should double the power output to 600 hp. If you then increase the power output of that 300hp motor to 400hp (with bolt-ons) and then add the same 14.7 psi of boost, the power output will double to 800 hp. Increasing the power output of the theoretical base engine by 100 hp resulted in an increase of 200 hp once we added the boost.
If we apply this formula to our 307hp motor, we see that adding 8 psi should increase the output by 0.5442 percent (8 psi/14.7[1 bar] = 0.5442) for a total of 474 hp. We obviously did not reach that power level when we applied 8 psi to our 307hp normally aspirated stroker, but there are at least two very important reasons why it failed to do so.
The first is that the formula is not absolute and assumes a 100 percent increase in power based on doubling the atmospheric pressure. Though we sometimes reach or even exceed the suggested power levels with turbos, some of the power from the formula is required to drive the supercharger itself. The parasitic losses associated with driving the supercharger must be subtracted from the total. The second reason is that the combination itself was somewhat self-limiting. Combining the stock heads, cam and (to a lesser extent) GT-40 intake dramatically reduced the ability of the motor to process air. It is true that the blower improved airflow into the motor, but that air must also come out and the mild cam timing and restrictive exhaust ports on the E7TE heads made life difficult to process the additional airflow. So restrictive were the stock components that the bolt-ons increased the power output of the stock configuration more than 8 psi of boost from the supercharger.
Things changed once we improved the power output of the motor with the performance components. Replacing the stock heads, cam and intake with the RHS heads, XFI cam and RPM II intake allowed the supercharger to shine as the modified motor produced 448 hp. According to our formula, adding 8 psi of boost should increase the power output by the same 0.5442 percent, for a total of 692 hp. Our motor produced 665 hp, but this peak number was more a function of our lack of fuel. We suspect that we would be much closer to the number suggested by the formula with our modified motor than with the stock configuration.
What all this boils down to that the best supercharged motors start out as powerful normally aspirated combinations. It sounds simple, but the more powerful the normally aspirated motor, the more powerful your supercharger combination. Not only that, but the gains applied to the normally aspirated combination area actually multiplied by the boost. Thus it is possible to add 100 hp to your normally aspirated motor and have that equate to 200 hp on a turbo or supercharged combination (at 14.7 psi). If you have to choose between boost and bolt-ons, the best choice is always both!