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Big Block Tunnel Ram Intake - Two A "T"

Revisiting That Old Favorite, The Tunnel Ram Intake, On The Dyno On A Pair Of Big-Blocks

By Richard Holdener

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.

By Richard Holdener
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