More With Less: Power vs. Boost

Words And Photos: Richard Holdener

Is it possible to make More power with Less boost? Everyone knows it’s possible to make more power with more boost, so how can the opposite possibly be true? The answer requires understanding that there is more to power production than boost alone, and that boost alone should never be considered a true indication of power. Though we often label it positive pressure, boost is more accurately an indication of back pressure in the intake track. Yes folks, that wonderful boost gauge is actually reading how much restriction your engine represents to the airflow provided by the supercharger.

This test revolved around a very simple principle–get rid of those restrictions then watch the power climb and the boost drop. Expressed as a simple equation to determine horsepower, we combine boost plus the power output of the engine itself. To increase power you can alter either of the two components in the equation. Increased power can be accomplished by increasing boost pressure from the supercharger OR increasing the power output of the normally aspirated motor you are applying the boost to. You can of course opt to do both, but we kept things simple by altering only the engine side of the equation for this test.

Both the 4.8L and 6.0L were tuned using the FAST XFI/XIM management system.

Both the 4.8L and 6.0L were tuned using the FAST XFI/XIM management system.

The boost provided by a supercharger is a function of the impeller speed relative to the engine speed. Coupled to this equation is the power output of the normally aspirated motor. To illustrate what happens to power and boost when we alter the engine side of the equation, we ran a Vortech TI-Trim supercharger on two different test motors. The Vortech was run at the same impeller speed (using the same blower and crank pulleys) with the same intercooler and at the same air/fuel and timing values. The only variable in the equation was the displacement and power output of the two test motors. Test motor number 1 was a mild 4.8L equipped with JE forged pistons, a set of TFS Gen X 205 heads and mild BTR Stage 1 truck cam. Test motor number two was a larger 6.0L equipped with CP forged pistons (and Total Seal rings), TEA Stage 2 243 heads and a healthy COMP cam. Both motors were run with the same early truck intake, 83-pound Holley injectors and FAST XFI/XIM management system.

Before adding boost, we ran each motor in normally aspirated trim to help illustrate both the gains offered by the impressive Vortech supercharger and to see if the difference in power normally aspirated remained constant once we added boost. First up on the dyno was the mild 4.8L. Sporting a mild cam and TFS heads, the 4.8L produced 398 hp and 353 lb-ft of torque. Adding the Vortech supercharger to the mix with the 3.8/7.5 pulley combo and air-to-water intercooler resulted in peak numbers of 746 hp and 592 lb-ft of torque. It should be noted that we ran the 4.8L slightly higher than the 6.0L (6,700 rpm), so we will make our comparison at the peak engine speed of 6,500 rpm. Boost supplied by the Vortech started out at 3.1 psi at 3,100 rpm then rose to a peak of 14.6 psi at 6,500 rpm (15.2 psi at 6,700 rpm). Using the Vortech supercharger, we nearly doubled the power output of the little 4.8L, but now it was time for the larger 6.0L.

Off came the 4.8L and on went the more powerful 6.0L. Thanks to increased displacement, compression and cam timing, the 6.0L offered considerably more power than the smaller 4.8L in normally aspirated trim. Run on the dyno with the same FAST XFI/XIM management system, the 6.0L produced 549 hp and 481 lb-ft of torque. After adding the Vortech system to the mix, the peak numbers skyrocketed to 896 hp and 724 lb-ft of torque. The power certainly jumped but what happened to the boost?

The boost curve started out at 2.9 psi at 3,100 rpm and rose to a peak of 12.8 psi at 6,500 rpm. Run on the larger 6.0L sporting an extra 150 hp (over the NA 4.8L), the peak boost dropped by 1.8 psi. The change in boost between the two motors increased with engine speed, so the difference would be even greater at 7,000 rpm. The benefit of adding more motor instead of adding more boost is that a reduction in boost will decrease the charge temperature and chance of detonation, while an increase in boost will do the opposite. Of course, you can always do both and crank up the boost on the bigger motor for maximum effect, just know that it is possible to get More with Less.

Modified 4.8L vs Modified 6.0L-NA

Before illustrating the effect of increased normally aspirated power on boost, we had to test each of the two combinations in normally aspirated trim. We ran both the mild 4.8L and slightly wilder 6.0L on the engine dyno. Run in normally aspirated trim with TFS heads and mild BTR cam, the 4.8L produced 398 hp at 6,300 rpm and 353 lb-ft of torque at 5,600 rpm. By contrast, the larger and more powerful 6.0L produced 549 hp at 6,900 rpm and 481 lb-ft at 5,000 rpm. Even before adding boost, the 6.0L offered an additional 150 hp over the smaller 4.8L. The question now was what would happen when we added the Vortech supercharger to the mix?

Modified 4.8L vs Modified 6.0L-Vortech Supercharged (HP)

Both the 4.8L and 6.0L were equipped with the same Vortech supercharger kit, right down to the pulley combination and air-to-water intercooler. After running on the smaller 4.8L, every component of the Vortech supercharger kit was pulled and installed onto the awaiting 6.0L, including the 3.80-inch blower pulley and 7.5-inch ATI damper/crank pulley. The test was designed to illustrate the change in power and boost pressure between the two motors using the same impeller speed. Run on the 4.8L, the Vortech produced peak numbers of 746 hp at 6,700 rpm and 592 lb-ft of torque at 6,300 rpm. Running the same combination on the larger 6.0L resulted in 896 hp at 6,500 rpm and 724 lb-ft of torque at 6,400 rpm. Measurements taken at the same engine speed indicate the same 150 hp difference between the two combinations.

Modified 4.8L vs Modified 6.0L-Vortech Supercharged (Boost)

The net result of increasing the displacement and power output of the test motor was a decrease in boost pressure. Once again measured at the same engine speed of 6,500 rpm, the increased power output of the larger 6.0L dropped boost pressure by 1.8 psi. Note that the change in boost increased with engine speed, starting at just .2 psi difference at 3,100 rpm then increasing to 1.8 psi at 6,500 rpm. This increasing trend would continue had we elected to increase the engine speed during testing.

Sources

ATI
atiracing.com

Brian Tooley Racing
briantooleyracing.com

COMP Cams
compcams.com

CXRacing
cxracing.com

DNA Motoring
dnamotoring.com

FAST
fuelairspark.com

Holley/Hooker
holley.com

JE Pistons
jepistons.com

Lucas Oil
lucasoil.com

Total Engine Airflow
totalengineairflow.com

Total Seal Rings
totalseal.com

Trick Flow Specialties
trickflow.com

Vortech Superchargers
vortechsuperchargers.com

 

 

About the author

PPN Editor

Power & Performance News is the source for news, tech and products that help you get more performance from your vehicle. If powertrain performance projects and hardcore technical content are your interest, Power & Performance News is the publication designed for you. Our acclaimed editorial staff covers all aspects of engine and driveline upgrades with a mission of presenting information that is both interesting and achievable for the “average car guy”.
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