When talk turns to tunnel ram intake manifolds, often times it seems they are placed in the same magical category as superchargers and individual-runner injection systems. These trick induction systems are bitchin' to look at and can be found on all manner of race hardware, which obviously means they have no place on a street motor, right?
While the streetability of a trick tunnel ram remains to be seen, we followed along on a comparison test that illustrated that at the very least a tunnel ram is much more than a simple high-rpm race manifold. Often placed in a different category than the more popular single and dual-plane intakes, the tunnel ram actually combines several beneficial design features of the two common intakes to produce what can be (on the right engine combination) the best induction system of the bunch. When you throw in the stunning visual effect of having a polished, dual-carb tunnel ram sticking out in the wind for all to see, the tunnel ram has a great deal to offer any performance big-block.
Intake manifolds for a typical V-8 like our 476 big-block Chevy test motor can be broken down into two basic categories, single plane and dual plane. A complete technical rundown would require more pages than we have available, but know that a dual-plane intake typically features long runners designed to promote low and mid-range torque production while keeping the power peak below 6000 rpm(in most cases).
Dual-plane intakes also effectively divide the V-8 engine into a pair of four cylinders by isolating the fuel and air supplied by the carburetor to each half of the engine. Isolating the two sides improves the signal to the carburetor. Combining the improved signal with the longer runners in the dual-plane, makes for an impressive street system. Dual-plane intakes are also available with the divider directly under the carburetor (used to split the motor) reduced or machined. This obviously does not transform the dual-plane intake into a single plane, but it does effectively shift the power curve (much like the installation of an open carb spacer).
Not surprisingly, single-plane intake manifolds differ from their dual-plane counterparts by way of a common plenum under the carburetor. Single-plane intakes typically offer shorter runners than the dual plane in inmost cases the design necessitates four shorter (inner) runners combined with four longer (outer) runners.
The use of a common plenum and shorter runners enhances high-rpm power. Unfortunately, the extra power production that occurs at the top of the rev range comes at the price of a reduction in power down low. Much like a wilder cam profile, the single plane manifold effectively shifts the torque production higher in the rev range (compared to a dual plane). Producing the same torque value at a higher engine speed will result in an increase in horsepower.
What this all means is that the choice between a single-plane and a dual-plane intake really comes down to the intended application. In most applications, the dual-plane design offers more low-speed power while the single-plane intake maximizes peak power production. For maximum street/strip acceleration (or maximum ultimate speed), the top-end power produced by the single-plane intake is usually the best choice. The dual-plane will provide the best overall torque curve, throttle response and fuel mileage.
Technically speaking, the tunnel ram manifold falls into the single-plane design category, as the tunnel ram shares the common plenum under the carburetor. The difference between the tunnel ram we used from Dart and the single-plane (Edlebrock 454R) was basically the runner length, plenum volume and use of an additional carburetor. The elevated position of the carburetor pad(s) on the tunnel ram allowed for longer and straighter runners, to say nothing of the fact that all eight of the runners were pretty much the same length (a trait not share by the single-plane intake).
The extended runner length helps to promote power production over a given rpm range. Basically speaking, the runner length (combined with cross section and taper) determines where the motor makes power. Naturally the intake runner length (and overall design) should be combined with the proper cam timing and cylinder head flow (and to a smaller extent header design) to optimize power production in a given range. Your choice of intake would certainly be different for a low-rpm towing engine for your dualie than for your 10-second Chevelle. The engine components (including the intake) should all be chosen to help reach a desired power curve to best suit the intended application.
Speaking of applications, to properly demonstrate the performance merits of the tunnel ram, we needed a cool big-block Chevy test motor. Rather than order a GM crate mill or screw together a rebuilt 454, we took Westech's Steve Brule up on an offer to use the race motor from his record-setting jet boat. The 476 was a serious piece, featuring 13.6:1 compression, CNC-ported heads from AFR and a serious sold roller camshaft. The 476 was basically a .100-over 454 that featured a Scat 4340 forged crank and matching rods combined with a set of JE pistons.
Breathing came from a set of 335cc (intake port volume) AFR aluminum heads. According to the flow bench, the heads flowed 410 cfm at .800 lift (not an unrealistic lift value given the wild cam timing). Comp supplied the hot roller cam (and the remainder of the valvetrain) that featured a .780/.744 lift split and a healthy 282/288 duration split (measured at .050). Additional components used in the build up included a Moroso oiling system (including marine pan and vacuum pump), a complete MSD ignition system (including crank trigger) and a set of 2.25-inch (primary size) Hooker headers.
As mentioned previously, an intake manifold should be chosen for a particular engine combination as well as the intended application. On this particular engine, the effective operating range was rather small, given the fact that the jet boat work much like a high stall converter. Once you hit the throttle to start the quarter-mile pass, the engines speed instantly climbs (up to around 6800 rpm) and the boat accelerates as the flow through the jet pump catches up with the engine speed. This means that the change in engine speed during the run is very minimal (500-800 rpm), and that every attempt should be made to maximize the power production in that rev range.
Having run the motor successfully with a combination of an Edelbrock Super Victor intake (with Dominator car flange) and a Barry Grant 1195 King Demon RS carb, Brule was looking to further improve the power output of his race motor. Given that it made peak power near 7400 rpm and the relatively narrow operating range, Steve decided the engine might be a good candidate for a tunnel ram. As he found out, not only was the tunnel ram impressive at elevated engine speeds, but it shined down low as well.
The first order of business was to establish a baseline with the Super Victor intake and 1195 Barry Grant King Demon carb. Equipped with the single four-barrel, the 476 produced 870 hp and 644 lbs-ft of torque. The power output was impressive considering the relatively small displacement. After successful back up runs produced the same power numbers, we replaced the Super Victor with the Dart tunnel ram and dual 1095 King Demon carbs. The longer runners in the Dart tunnel ram combined with the increased plenum volume and additional breathing offered by the pair of BG carbs to produce some impressive power gains.
Equipped with the Dart tunnel ram and dual King Demons, the 476 pumped out an even 900 horsepower and 670 lbs-ft of torque. Not only did the tunnel ram improve the peak power numbers, but the impressive induction system elevated to power and torque curves from 6000 rpm to 7400 rpm. Basically, the tunnel ram offered more power throughout the tested rev range. At these elevated specific output levels, it is difficult enough to even improve the power output, let alone show consistent gains throughout the rev range. The Dart tunnel ram was definitely the hot set up on this marine motor.
While this particular test was run on a dedicated race motor, we also ran another tunnel ram test on a much milder combination. The test mule was a low-compression 496-inch (.060-over and 4.25 stroker crank) big-block equipped with AFR 315 heads, a mild hydraulic roller cam (255/262 duration) and a set of Hooker Chevelle street headers. It was tested with a single-plane Holley 300-5 intake and a Weiand Hi-Ram (street/strip tunnel ram). The Holley intake was run with a 950 HP carb, while the Hi-Ram was run with a pair of tunnel-ram specific 750 cfm carbs.
Equipped with the single-plane Holley intake, the 496 produced 652 hp at 6300 rpm and 578 lbs-ft at 4800 rpm. After swapping on the Weiand Hi-Ram, the peak power numbers jumped to 687 hp and 618 lbs. ft. of torque. As with the 476 race motor, the tunnel ram improved the power output of the 496 stroker throughout the rev range (from 3000 rpm to 6500 rpm), further illustrating the impressive low-speed and mid-range power offered by the long-runner design.
Of course all the extra power comes with the cost of cutting a hole in your hood, but hood scoops are cool too, especially when they cover a trick (and effective) dual-carb tunnel ram.