Single or Dual-Plane?

Words And Photos: Richard Holdener

One of the most difficult questions we have to answer when building a carbureted street/strip motor is which intake to use, single plane or dual plane? The question is realistic, as, despite similar peak power numbers, the two intakes offer decidedly different power curves. For the uninitiated, the single-plane, dual-plane intake debate actually comes down to a simple matter of desired engine speed. The dual-plane manifold, like the Performer RPM tested on our 6.0L, was designed to maximize low and mid-range power production.

This makes the dual-plane ideal of most street applications, but it does give up power at higher engine speeds compared to the more race-oriented single-plane design. At one end of the performance spectrum, we have the stock or mild (daily driven) motors that should only be run with a dual-plane manifold. Occupying the opposite end of the spectrum are the dedicated race motors that can take full advantage of the high-rpm, single-plane design. Intake selection for those two extremes is quite simple, but picking a manifold for the vast armada of motors positioned between the poles poses quite the conundrum.

To illustrate the difficulty associated with proper intake selection, we chose to run the test on a carbureted LS (actually LQ). Truth be told, similar result can be had on almost any test motor, but the LS engine family is the hottest thing going and we loved how easy it was to make power with something that started out life as a simple truck motor. Since the induction choices for both stock and dedicated race motors were straightforward, our test motor was purposely positioned squarely between the two extremes.

Starting with a 6.0L truck motor, the iron block was machined to receive the stock crank swinging Carrillo rods and CP, flat-top pistons along with a Total-Seal ring package. The forged short block also received a COMP LSr cathedral-port cam (pt# 54-459-11) that offered a .617/.624 lift split, a 231/239 duration split and 113-degree lsa. The cam was tamed with a set of 850-16 hydraulic roller lifters and 7.35-inch pushrods. Topping this modified sort block was a set of Stage 2, CNC-ported 243 (LS2) heads from Total Engine Airflow. Fuel was supplied by a Holley 950 Ultra HP carburetor while spark was supplied by an MSD ignition controller designed specifically to convert the fuel-injected LS to carburetion.

To evenly test the merits of the single and dual-plane intakes, we were hoping this combination would top the 500-hp mark, and it did just that. In fact, we overshot the target by over 50 hp in the case of the Edelbrock Victor Jr. intake used to represent the single-plane contingent. To illustrate the benefits offered by each intake design, we ran them both on the engine dyno under the same conditions, meaning same air/fuel ratio, timing and temperatures. The test motor also featured components from Meziere, Lucas Oil and a set of long-tube headers feeding a pair of dyno mufflers.

First up was the dual-plane Performer RPM from Edelbrock. The quintessential dual-plane, the Performer intake offered both plenty of power and an exceptional torque curve. The peak numbers checked in at 545 hp at 6,600 rpm and 482 lb-ft at 5,200 rpm, but torque production exceeded 450 lb-ft from 3,500 rpm to 6,250 rpm, making the available torque curve both useful and broad. Truth be told the peak power was likely another 100 rpm (or so) higher in the rev range, but we only ran the Performer to 6,600 rpm. What this test showed us was that the dual-plane offered not only good low and mid-range torque, but plenty of rpm capability as well.

Next up was the single-plane, Victor Jr. intake. Designed with a common plenum feeding all eight cylinders, we expected the single-plane manifold to excel at the top of the rev range and that is exactly what happened. The Victor Jr. not only bested the Performer in peak power (554 hp to 545 hp), but also bettered the absolute torque production of the dual plane (488 lb-ft to 482 lb-ft). Judged solely by the peak numbers, the single plane was the clear winner but the power curves tell another story.

Though the single-plane intake offered more peak horsepower and torque, the dual-plane Performer out-powered the Victor Jr. from 3,000 rpm (and below) to 4,800 rpm. Down at 3,500 rpm, the Performer offered an additional 28 lb-ft of torque. The question on the table now becomes does the extra 28 lb-ft of torque below 4,800 rpm offset the loss of 7-8 hp above that point. Being somewhat application dependent, that debate will continue to rage but know that in addition to the torque loss, the single plane also reduced idle vacuum and low-speed drivability. Check back with us as we change the engine combo and add boost to the equation in Part 2. A carb conundrum indeed! Read Part 2 HERE!

Tested on the modified 6.0L, the intake swap showed the quintessential single/dual plane conundrum. Looking specifically at the peak numbers, the single-plane Victor Jr. seemed to easily best the dual plane by offering 554 hp and 488 lb-ft to just 545 hp and 482 lb-ft for the dual plane. Peak numbers, however, do not tell the whole story, as the dual-plane offered as much as 28 additional lb-ft of torque down low while losing out just 7-8 hp at the top of the rev range. The question now is will you miss the extra grunt offered below 4,700 rpm more than the missing 7-8 hp above that point? The choice obviously comes down to the intended application, but both manifolds offered plenty of power on this modified 6.0L.

Sources

Aeromotive
aeromotiveinc.com

COMP Cams
compcams.com

CP Pistons/Carrillo Rods
cp-carillo.com

Edelbrock
edelbrock.com

Holley/Hooker
holley.com

Lucas Oil
lucasoil.com

MSD
msdignition.com

Total Engine Airflow
totalengineairflow.com