Forced Induction And Compression Test - The Multiplier Effect

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.