Blow Through Battle: LS Carb Conundrum Part 2

Words And Photos: Richard Holdener

Back in Part 1 of our Carb Conundrum, we compared a pair of Edelbrock intake manifolds on a carbureted 6.0L LS motor. Not just any intakes, we ran the age-old single-plane, dual-plane comparison by pitting a Victor Jr. against a Performer RPM on the powerful test motor. Since the single plane was designed for top-end and the dual plane for mid-range and bottom end, we decided to test them on neutral ground with a healthy street/strip combination. True to form, the single-plane Victor Jr. made more peak power (and torque), but the dual-plane RPM offered considerably more torque below 4,700 rpm. Hardly definitive, the test left us with more questions than answers, including the two posed for this next test. Question number one was what happens if we alter the displacement (and power) of our test motor? Perhaps even more exciting was question number two. What happens to the equation when we add boost?

Answering these questions required building another test motor. Off came the 6.0L used in Part 1 and on went a smaller 4.8L LS combination. Not wanting to totally skew the results in favor of the dual plane, we decided against using a bone-stock 4.8L. Mods to the mini LS included forged JE pistons with 7cc domes to increase the both intestinal fortitude and static compression. With boost in the future, the wimpy, stock LR4 cam was replaced by a Stage 1 blower grind from Lil John’s Motorsports.

The Stage 1 cam offered a .610/.586 lift split, a 223/238-degree duration split and blower-friendly 120-degree LSA. Actually this cam was designed for a positive displacement blower application and the centrifugal would get a slightly revised profile from Lil John’s but we had the PD blower cam on hand a decided to run with it. Additional mods included TFS Gen X 205 heads, an ATI Super Damper and MSD ignition controller. With the 4.8L ready to go, we decided to first duplicate the normally aspirated test run previously on the 6.0L then add some boost to the equation.

First up was the dual-plane Performer RPM. The smaller 4.8L responded very well to the dual plane design, as the RPM allowed the little 4.8L to produce peak power all the way up at 7,000 rpm. Run with a 750 HP Holley carb, the modified 4.8L produced 440 hp at 7,000 rpm and 352 lb-ft of torque at 5,800 rpm. Those were pretty lofty engine speeds for a dual-plane design. Logic initially told us that the high-rpm nature of the combination was tailor made for the single plane, but boy were we wrong. Run with the Victor Jr., the 4.8L produced just 427 hp at 6,900 rpm and 351 lb-ft of torque at 5,500 rpm. Nowhere was this more evident than below 5,000 rpm, where the dual-plane Performer RPM offered an additional 40-45 lb-ft of torque. That torque deficit was all the more impressive considering the limited torque production offered by the smaller displacement. The displacement and cam timing were simply better suited to the dual plane on this normally aspirated combination, but what about boost?

Given the popularity of a single-plane intake on boosted LS applications, this intake comparison was eye opening. Contrary to popular opinion, the gains offered by a specific intake design (i.e. single plane/dual plane or dramatic changes in runner length on an EFI manifold) do not disappear with the presence of boost. The gains offered normally aspirated are often times multiplied by pressure ratio. Nowhere was this more evident than below 5,000 rpm once we added the Vortech supercharger. Since the blower was run at the same speed on both intakes, and change in the power curve would be unrelated to boost (as can be the case running the same test with a turbo). The big torque gains offered by the dual-plane design on the normally aspirated 4.8L continued under boost. The single-plane did seem to improve slightly relative to the dual plane under boost, but the general trend was still present. This test also showed that it is possible to run a blow-through (carbureted) application on a dual-plane intake. While we didn’t exactly crown a winner in the boosted battle, the results gave us plenty of ammunition for future firefights.

Just as it did on the larger and more powerful 6.0L, the dual-plane intake offered considerably more low and mid-range torque production on the 4.8L. Unlike the 6.0L, the dual-plane Performer continued the trend and made more peak power on the smaller 4.8L. Run with the Performer, the mild 4.8L produced 440 hp at 7,000 rpm and 352 lb-ft of torque at 5,800 rpm. By comparison, the single-plane Victor Jr. produced peak numbers of 427 hp at 6,900 rpm and 351 lb-ft of torque at 5,500 rpm. The RPM was better everywhere on the smaller 4.8L, with the exception of a 600-rpm spread from 5,000-5,600 rpm.

Adding the blower changed things slightly, but the dual-plane RPM still offered considerably more torque (nearly 50 lb-ft) down low (up to 5,000 rpm). The dual-plane out-performed the single plane up to 6,700 rpm, where the Victor Jr. pulley slightly ahead. The curves illustrate that the dual plane is much better suited to the smaller motor even after the introduction of boost. Note that the additional torque below 5,000 rpm offered in normally aspirated trim continued under boost. Boost pressure does not change the tuning effect offered by the intake design.

Sources

Aeromotive
aeromotiveinc.com

ATI
atiracing.com

COMP Cams
compcams.com

CP Pistons/Carillo Rods
cp-carillo.com

Edelbrock
edelbrock.com

Holley/Hooker
holley.com

JE Pistons
jepistons.com

Lucas Oil
lucasoil.com

MSD
msdignition.com

Total Seal Rings
totalseal.com

Trick Flow Specialties
trickflow.com