Forced Induction And Compression Test - The Multiplier Effect

By no design of our own, the tests run on this 5.4L 4V motor precisely mirrored those run on our 4.6L 2Vs. I guess great minds think alike, though it can be added that fools seldom differ. The 5.4L motors were assembled with drag racing in mind and featured all the right hardware, including forged-steel cranks, forged rods (actually aluminum on the low-compression version), and forged pistons. The change in compression ratio came from a change in piston design, as both motors relied on the same heads, cams (including cam timing), and induction system. The swap components also included the fuel rail and injectors, headers and exhaust, and MSD ignition system (all run by a FAST management system).

The 5.4L 4V motor was built with high horsepower in mind. The 4V heads came from a Navigator, but don't let the humble soccer-mom beginnings fool you. The Navigator heads featured large intake ports, which were further enhanced with extensive porting to unleash an additional 50 cfm per runner. Naturally, the ported heads required something other than a long-runner (Navigator) intake manifold. Knowing the motor would require substantial intake flow as well as the proper tuning to determine the effective rpm range, the 5.4L was topped off with a Sullivan intake casting. Designed to accept a conventional carburetor, the short-runner aluminum intake also featured provisions for fuel injectors. A 90-degree inlet elbow was used to combine the 90mm Accufab throttle body with the intake flange. The 90mm throttle body ensured adequate airflow to the high-rpm 5.4L motor, while 150-pound-per-hour injectors and an Aeromotive A1000 fuel pump (with Kenne Bell Boost-A-Pump) guaranteed sufficient fuel delivery. The heads received a quartet of Sean Hyland 4V cams. The supercharged motor was equipped with Stage 2 intake cams (0.452-inch lift and 225 degrees of duration) and Stage 3 exhaust cams (0.474-inch lift and 235 degrees of duration). The cams were installed at 107 degrees.

The 5.4L 4V motor was also set up with a set of custom 1 3/4-inch to 2-inch step headers, an MSD coil pack, and a FAST engine-management system. Once the testing was performed on the low-compression motor, the components were swapped onto the high-compression short-block. Obviously, a number of different timing and fuel curves were tried to optimize each combination, and in the end the low compression motor produced 478 hp at 7,300 rpm and 395 lb-ft at 5,500 rpm. Running the very same components on the high-compression short-block upped the power output to 543 hp and the torque to 439 lb-ft. The additional 3.5 points of compression upped the power output by 65 hp. According to the old rule of thumb that every point in compression is worth roughly 4 percent in power, we can calculate that increasing the compression by 3.5 points should yield a gain of roughly 14 percent. If we multiply 1.14 times 478 hp, we get 545 hp or close to our actual peak power of 543 hp. The compression formula is fairly accurate, although it should be mentioned that the gains in power are actually greater in the 8.0:1 to 11.0:1 range and tend to diminish slightly thereafter. This means that upping the compression ratio from 8.0:1 to 9.0:1 will likely yield greater gains than going from 12.0:1 to 13.0:1.

Since we added 65 hp to our normally aspirated motor, what would happen once we added the ATI ProCharger F2M blower to the mix? Would the high-compression motor still make only 65 more horsepower than the low-compression version, or would the boost pressure alter the power gains? If you read the results of the "Mods for 2V Mods 4" in MM&FF, you already know the answer. Adding boost pressure supplied by the ATI blower (and air-to-water intercooler) dramatically increased the difference in power between the high and low-compression motors. Equipped with a 73-tooth crank pulley and a 46-tooth blower pulley, the ATI supercharger provided a peak of 25 psi to the 5.4L motor. In low-compression form, this meant upping the power output from 478 hp at 7,300 rpm to an impressive 1,135 hp at 7,200 rpm. The peak torque jumped from 395 lb-ft at 5,500 rpm to 882 lb-ft at 6,400 rpm. According to the power/boost formula, 25 psi should have increased the power output to 1,291 hp, but once again the formula does not take into account the parasitic losses associated with driving the blower. The power/boost formula is usually only accurate on turbo motors that do not suffer the losses associated with driving the supercharger. Supercharged combinations get much closer to the ideal boost/formula calculation at lower pressures and impeller speeds.