How do we get more power with less boost? Simple, just add displacement!

Words By Richard Holdener

If you are looking to make your small motor thump out power like a big motor, nothing beats boost. Since adding a blower or turbo to a small block will make it perform like a big block, naturally the ideal combination has to a big block with boost, right? This line of reasoning seems sound, but remember, there are big blocks and then there are BIG blocks!

In this case we don’t just mean bigger displacement, though that was certainly the case for this test. While bigger is certainly better, bigger and badder results in an even more devastating one-two punch. When you add boost to the equation, the additional inches and breathing offered through the various modifications (the badder portion of the equation) pays even bigger dividends. We all know increasing the boost on a blown application will improve power, but what if we told you it was possible to increase the power output with less boost? Interested? Let’s dive right in and examine the relationship between cubic inches and pressurized pounds (psi).

To illustrate the effect of displacement (and mods) on a supercharged application, we had to apply boost to a pair of different big blocks. In the mild corner was a Gen VI 454, while the opponent in this obviously one-sided contest was a 540-inch stroker supplied by Blueprint Engines. As big blocks go, the smaller 454 was pretty tame, sporting a stock short block — high-mileage no less — a mild Comp hydraulic roller cam, and as-cast, aluminum heads from Speedmaster. The oval-port heads actually offered plenty of flow, but the otherwise mild, low-compression 454 simply could not take full advantage of the available airflow. The 454 was finished off with a Speedmaster dual-plane Eliminator intake, Holley 650 XP carb, and MSD distributor (shared with the 540).

By contrast, the 540 Power Adder crate motor from Blueprint Engines was designed from the get-go for boost. It featured a BPE four-bolt block, forged internals, high-flow (rec-port) aluminum heads, and a much wilder roller cam profile. Delivered in long-block form, the BPE 540 was completed with a single-pane, Weiand Team G intake, and larger Holley 950 XP carb.

To start the test on boost, we first had to run the two test motors in normally aspirated trim, and SURPRISE, the big more made more power. Run with long-tube headers and no accessories, the mild 454 produced 469 hp at 5,600 rpm and 508 lb-ft of torque at 3,600 rpm. The BPE 540 upped the power ante by producing 649 hp at 6,100 rpm and 629 lb-ft of torque at 4,500 rpm.

Not surprisingly, the bigger big block was better everywhere, but now it was time for the good stuff, boost! In this case, we decided to add boost to both of the test motors using a roots blower kit supplied by Speedmaster. The kit featured a 6-71 supercharger from the Blower Shop, a dedicated blower manifold, and all the trimmings necessary to apply boost to a big block. Holley came through with a pair of 850 XP blower carbs and dual-quad linkage. The carbs were bolted to the dual-quad adapter supplied with the kit. Speedmaster also supplied a variety of different cog pulleys (and belts), but this test was run with identical pulley ratios (and therefore blower speed) to illustrate the effect the extra displacement and power had on the boost pressure supplied by the supercharger.

Test number one involved installation of the blower kit from Speedmaster on the 454. Off came the dual-plane intake and on went the dedicated blower manifold. The blower was secured using the supplied aluminum blower mounting studs and gasket. The supercharger was equipped with a 50-tooth blower pulley, while we installed a 55-tooth pulley on the crank using the supplied adapter. The Holley 850XP blower carbs were perfect right out of the box, offering a near-perfect 11.6:1 air/fuel ratio under boost.

Running a maximum of 30 degrees of total timing (on race gas), the supercharged 454 produced 653 hp at 5,800 rpm and 625 lb-ft of torque at 5,100 rpm. More importantly, the boost supplied by the 6-71 blower started at 5.6 psi and rose to a peak of 9.1 psi. The blower added 184 hp to the big block, but we were more concerned with what would happen once we installed in on the larger 540.

After removing the 950 carb and Weiand intake from the 540, we installed the blower kit fresh off the 454. Everything remained the same, right down to the carbs and pulley ratios. Naturally, we were interested in the power gains offered by the 6-71 blower on the 540, but we were also curious about the boost curve on the larger motor.

After setting the timing (the carbs were once again spot on), the BPE 540 thumped out 845 hp at 6,400 rpm and 770 lb-ft of torque at 4,700 rpm. The blower added 196 hp to the 540, but what was interesting was the additional power came at a peak boost pressure of just 6.1 psi. That’s right, the blower added more power to the larger 540 at a much lower boost level.

Looking at it another way, the best way to improve power and lower the boost pressure is to make modifications to your existing motor (including a hike in displacement). Measured at the same rpm, the boost on the 540 dropped by almost 3 psi compared to the smaller and milder 454. If you want your blower motor to work well (especially important on a roots blower), make your motor bigger and more powerful before adding boost!

Sources: ATI, atiracing.com; ARP, arp-bolts.com; The Blower Shop, theblowershop.com; BluePrint Engines, blueprintengines.com; COMP Cams, compcams.com; Holley/Hooker/NOS, holley.com; MSD, msdignition.com; Speedmaster, speedmaster79.com