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 Players
Tunnel-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 Test
One 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.
After a thorough engine cooldown, we got to work swapping intakes. Having grown accustomed to the look of the tunnel-ram, a single four-barrel just seemed modest by comparison. Once bolted down, the Holley 950 was mounted, the linkage was connected and adjusted, and the beast was fired. We adjusted the idle and made our first partial pull to determine how far off the fuel mixture was. It looked good enough to pull all the way to 7,000 rpm. The air/fuel ratios were right on the money. No tweaking, no tuning, no jetting, no nothing. Out of the box, the Holley proved lethal. We cooled the Barracuda down and pulled it to 7,300 rpm. We were rewarded with just under 513 hp at the rear wheels. This Indy/Holley combination was proving very effective. We performed a backup pull to make sure the 513 horses weren't a fluke and were handed another 513 hp ticket. No controversy here.
What We Learned
Well-designed tunnel-rams work. They make big torque and horsepower everywhere and carry the torque a great distance. You will notice by the dyno charts that the ram produced prodigious amounts of torque and horsepower well past 7,000 rpm, while the single-plane dropped off rather rapidly past its peak of 7,000 rpm. The ram usefully extends the powerband of the engine, enabling you to run a numerically higher gear than you would be able to with a single four-barrel induction. But the tunnel-ram comes with a price. The initial outlay will be significantly greater as the intakes typically cost more, two carburetors must be used, and the necessary linkage must be obtained. Then there's the tuning effort needed to make the system work properly. Do not expect to bolt on a tunnel-ram, set the idle, and go. Considerable toil went into making all of the essential adjustments to the carburetion for the correct fuel metering particularly at high rpm where tuning is even more critical. Then there's the hood clearance issue. The tunnel-ram will more than likely not fit under any stock hood.
As far as the Indy manifold is concerned, we were surprised to find a four-barrel intake that came as close to the output of the tunnel-ram as this one did. Indy certainly did its homework when it produced this clean-slate design. For sheer user-friendliness, look no further than the Indy manifold. We bolted this induction on and laid down well over 500 hp at the wheels. It doesn't get any simpler. Hood clearance with anything more than a 1-inch filter element may be a problem, but the packaging is much more user-friendly than it was with the ram. Indy Cylinder Heads has produced a winner. Not only did it keep the tunnel-ram in sight, it proved much less complex to set up. With the Indy intake, you don't need an expert tuner to get you dialed in. Just bolt it on, make big power and go. For many, that's good enough.
Double the trouble, or double the fun? Our tunnel-ram-equipped, W-2-headed 340 cranked out 532 horses and 430 lb-ft of torque at the rear wheels. We know some big-blocks that can't come within 100 hp of this little small-block. The Mopar Performance tunnel-ram was topped with a pair of hybrid Holley/Proform carbs rated at roughly 750 cfm each for a whopping 1,500 cfm total. Too much for 346 inches? Hardly. |
After a quick engine cool-down period, we pulled the ram off, cleaned all surfaces, and dropped the Indy single-plane onto the little Mopar. |
After the tunnel-ram, the Indy unit looked tame. |
Indy Cylinder Heads has developed a line of high-rise small-block intake manifolds that feature super-tall plenums and beautifully formed runners. |
We topped off our state-of-the-art Indy intake with another trick piece--a Holley 950 HP. |
This trick adjustable throttle-cable bracket from AR Engineering was just what the doctor ordered. We were able to use the same bracket with both intakes. It made the swap much easier. |
Did the Indy/Holley intake disappoint? Not a chance. It exceeded our expectations with an outstanding 513 hp at the wheels. If you want the ultimate in single-plane induction, look no further than Indy. |
These photos demonstrate why the tunnel-ram design is so effective. This Pro Stock example clearly shows a direct shot from the carburetor to the intake valve. |
The common plenum certainly doesn't hurt. |