We answer the gnawing question of the difference between tunnel-ram and single-quad.
What's the ultimate naturally aspirated intake-manifold design? Is it a modern single-plane or a tunnel-ram? That's a question we intend to answer with this dyno-test.
The PlayersTunnel-ram intake manifolds made their first known dragstrip appearance on the legendary Ram Chargers "High and Mighty" '49 Plymouth in 1959. A group of hobbyists consisting largely of Chrysler engineers, the Ram Chargers created a new style of manifold by mounting a pair of Carter AFB carburetors over a common plenum and runners made from industrial-grade rubber hose. Thus the first tunnel-ram was born, attached to a 354-inch Hemi. Popular in the '60s and '70s with car crafters, the tunnel-ram enjoyed quite a following until the advent of modern, technologically advanced single four-barrel intakes that offered easier packaging and the simplicity of tuning one carburetor.
In racing classes with no limitations on carburetion, tunnel-rams live on in high-tech sheetmetal and carbon-fiber form. In the ultimate expression of normally aspirated performance, NHRA Pro Stockers are exclusively equipped with tunnel-ram-style induction. But, like most performance components, tunnel-rams are combination specific; we don't recommend slapping one on a 300-inch 8:1 compression engine.
According to Air Flow Research's Tony Mamo, a tunnel-ram-equipped engine should flow more air than one with a conventional single four-barrel intake manifold because it fools the motor into thinking it has better heads-even if the heads are very good. If, on a flowbench, a single four-barrel intake manifold is substituted for a radius plate (normally being attached to the port being tested), it is not uncommon to see significant decreases in flow because of the turn in the runner inherent to a single-plane design.
Our test vehicle was equipped with a Mopar Performance tunnel-ram for W-2 cylinder heads. Unlike its predecessors from the '70s, this modern design is a race unit with relatively short runners. The carburetion perched atop the intake consists of a pair of Holley 650 mechanical secondary double-pumpers equipped with Proform carb bodies. The Proform bodies feature no choke horns, nicely sculpted air entries, down-leg boosters, and screw-in air bleeds. It appeared somewhat cobbled together, but the dyno indicated that the carbs were working pretty well, especially after we got the air/fuel ratio dialed in with the high-speed air bleeds.
To pit the venerable tunnel-ram against a contemporary single-plane design, we had to locate an intake with a large plenum volume. Representing modern single-plane technology is a relative newcomer in the realm of small-block Mopar single-plane intakes. Designed and manufactured by Indy Cylinder Heads, the Indy 360-3 was designed for use with the company's own aluminum small-block cylinder heads, which are similar to the venerable W-2. The W-2, born out of the Pro Stock wars in 1976, is an iron, open-chamber design with oval intake ports featuring relocated pushrod holes that necessitate offset rocker arms. Though designed over two decades ago, they still prove formidable.
The manifold is a direct bolt-on for standard W-2 heads, featuring beautifully shaped runners with consistent port volume and a very large plenum. The high-rise Indy intake promised to give the mighty ram a run for its money, topped with an equally serious Holley 950 HP carburetor. Also a mechanical-secondary double-pumper design, the 950 HP comes with all the features a custom carb shop would equip a race carb with, including screw-in air bleeds, a milled-off choke tower, double-step down-leg boosters, and improved metering circuits. Both intakes represent state-of-the-art technology, and if any single-plane can be a contender in this arena, this is the one.
The TestOne week prior to our showdown, we spent an entire day working the bugs out of our tunnel-ram carburetor combination on Westech's SuperFlow chassis dyno. What we ended up with was a seriously stout small-block posting an output of 532 hp at the rear wheels.