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Forced Induction And Compression Test - The Multiplier Effect
Effect of compression ratio-supercharged 5.4L 4V.
The "Mods for 2V Mods" series in our sister publication, Muscle Mustangs & Fast Fords, took a hard look at the effect of changes in compression ratio on a two-valve 4.6L GT motor. The idea behind the test was to illustrate the change in both power and boost curves (on supercharged applications) offered by a drop in compression ratio. Obviously, this was an important (if time consuming) series of tests, as in almost all instances, a drop in static compression is recommended when adding forced induction to your motor. This is especially the case when we're talking about street motors, as the limiting factor in terms of power is almost always the available octane rating of the pump gas being used. Sure, you can toss in a tank of 100-octane race fuel and go a bit wilder with boost, compression, or total timing, but running 91 octane definitely limits power production given the increased risk of detonation.
It's interesting to note that when we ran the back-to-back tests on high-compression and low-compression test motors equipped with superchargers (both Kenne Bell and Vortech), the boost pressure was lower on the low-compression versions. This drop in boost was despite the fact that we ran the motors (and blower) with identical drive ratios. The uninitiated may be tempted to attribute the drop in power on the supercharged applications to the drop in boost pressure, but the reality is that the drop in power was present on the normally aspirated versions, and the additional boost pressure only compounded the losses.
To better understand this relationship, we can first look back at the results of the drop in compression on the 4.6L 2V motors. The 10.1:1-compression 4.6L produced 401 hp and 389 lb-ft of torque, where the 8.1:1-compression 4.6L produced just 365 hp and 368 lb-ft of torque. A good rule of thumb is, every point of compression is worth roughly 4 percent in power. Using this rather general rule, we can multiply 401 hp (the output of the high-compression motor) by 0.92 percent (equal to 2 times 4 percent for each point of compression) to achieve 369 hp. The difference between 365 hp and 369 hp is well within tolerances for the 4 percent rule to be considered valid. Adding forced induction to the equation tends to increase the complexity, but the power loss actually still seems to follow a pretty straightforward and familiar equation.
Longtime readers of MM&FF should recognize the Holdener boost/power formula, which states that the gain in power offered by forced induction on a normally aspirated motor is a function of the original power multiplied by the pressure ratio (boost pressure/14.7 plus 1 times NA power). The numbers from our high-compression motor can serve as an example, as the normally aspirated motor produced 401 hp while the Kenne Bell supercharger upped the power output to 600 hp at 9.3 psi. Using our formula, we see that the 9.3 psi is 0.63 percent of 14.7. According to the boost/power formula, the supercharger should have produced 1.63 times 401 hp equals 653 hp. We can attribute the difference to the drive losses associated with spinning the supercharger, as 50 hp is not an unrealistic amount of power to spin the twin-screw supercharger at this speed and power level.
Plugging the numbers from the low-compression version equipped with the Kenne Bell, we see that the 365hp motor run at a 8.7 psi should have produced 581 hp (1.59 times 365 hp), yet it produced only 533 hp. The difference between the formula number and the actual number was again pretty close to 50 hp. We can now apply the power/boost formula to the power difference between the high and low compression (supercharged) motors. The boosted 10.1:1 motor produced 600 hp, while the 8.1:1 produced 533 hp for a difference of 67 hp. Remember, the power difference between the NA version of the motors was 36 hp (401 hp minus 365 hp). If we multiply the original power difference between the two NA motors (36 hp) by the pressure ratio (1.63), we get a calculated difference of 59 hp. OK, so 59 hp is a tad off the 67 hp actually achieved, but remember that the drop in compression also resulted in a slight drop in boost pressure, which should be factored in as well. Regardless of the slight difference in power between the actual and calculated, the power/boost formula did get us close and clearly showed that the power differences were in fact a multiple of the boost pressure times the difference achieved normally aspirated.
The reason for backtracking to the 4.6L GT test is that the power/boost formula can now be applied to the tests run on these wild, supercharged 5.4L 4V race motors-a viable option for those of you who are hard-core Lightning racers. As luck would have it, mod-motor guru John Mihovitz was running just such a test and we were allowed to tag along for photos and results as he put a high-compression and low-compression short block to the test using the very same external components.
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.
The final test was to install the ATI blower and intercooler on the high-compression motor. Running the same pulley setup (46-tooth and 73-tooth), the peak power jumped from 543 hp at 7,100 rpm to an amazing 1,350 hp at 7,400 rpm. The torque was equally impressive, as the elevated compression made itself known by pushing the torque curve past 1,000 lb-ft (peaking at 1,020 lb-ft at 6,700 rpm). Once again, the formula suggested a peak number of 1,466 hp (2.70 times 543), but the drive losses cost more than 100 hp at this rpm and boost level. Remember, the difference between the two normally aspirated motors was 65 hp. Adding the supercharger to the mix upped the power difference to a whopping 215 hp. Like the normally aspirated versions, the added compression upped the power output-not just at the power peaks, but also throughout the tested rev range. When you combine the results of this test with those generated by the 2V 4.6L GT motor, it's pretty safe to say the changes in the power output of a normally aspirated motor are actually multiplied when you add boost to the equation. These tests illustrate why we are so adamant about the fact that the best route to a solid forced induction motor is to start with a stout normally aspirated version. As added incentive, any gains you achieve on your normally aspirated motor (by adding ported heads, aftermarket cams, or a free-flowing intake) will actually be multiplied after you install the supercharger.
Effect of Compression (8.3:1 vs. 11.8:1) Normally Aspirated 5.4L 4V.
As expected, increasing the static compression ratio by 3.5 points had a noticeable effect on the power curve. Given that additional compression is always present, power gains were recognized throughout the rev range. The low-compression 5.4L produced 478 hp and 395 lb-ft of torque, while the 11.8:1 version upped the peak numbers to 543 hp and 439 lb-ft of torque. Note that the relative shapes of the power curves remained the same and the additional compression ratio simply elevated the entire torque curve.
Effect of Compression (8.0:1 vs. 12.0:1) Supercharged 5.4L 4V.
Run on the low-compression motor, the ProCharger F2M supercharged upped the peak power to 1,135 hp and 882 lb-ft of torque. This supercharged 5.4L 4V race motor was certainly impressive, but not nearly as impressive as the high-compression version. The ATI blower jumped the power peak of the high-compression motor to a whopping 1,350 hp, while the peak torque now stood at 1,020 lb-ft. If there is any question about whether compression ratio affects normally aspirated and supercharged motors, these graphs should quickly put them to rest. The gains offered in supercharged form were much greater than those achieved normally aspirated. Boost pressure is a multiplier effect, essentially increasing the gains achieved normally aspirated. Remember, this 5.4L 4V was destined for a drag-race vehicle running a steady diet of C16 (high-octane) race fuel. Attempting to combine a high static compression ratio and elevated boost levels is a dangerous affair on pump gas. When fuel octane isn't limited, however, the power gains can be fairly impressive.