In terms of the induction system, rising complexity and cost impact the subject of tunnel ram intakes. Essentially, two issues have been obstacles to common usage. First is cost and second is installed height.
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If your installation can accommodate the height, figure that a tunnel ram setup costs at least 80 percent more than a single-carb and intake manifold. In practice, though, this can escalate to as much as 300 percent more, especially if you elect to have a custom-made sheet-metal manifold. But before writing off a tunnel ram induction system on the basis of cost, let’s thoroughly investigate its performance potential.
The power advantages of a tunnel ram intake are fourfold. First, the acquisition of adequate carburetion capacity becomes easy. Second, carburetor placement allows the carburetor throttle bores to be positioned almost directly over the port runner entrances. Third, the runners can have an almost straight shot to the head ports for an essentially unimpeded airflow to the intake valve and into the combustion chamber. And finally, with the right plenum volume and the nearly straight runners, pressure wave tuning becomes more effective.
All of these attributes add up to a measurable power advantage but there is more, especially if the engine is equipped with a big cam. Eight idle and possibly eight transition circuits are available depending on how they are staged. The fuel-metering circuits can be progressive or simultaneous, so that there is enough airflow for low-speed driving. And when the system is optimally tuned, it can be more “traffic friendly” than a single 4-barrel carb setup. This aspect prompts the question of a tunnel ram’s streetability. With eight-carb barrels, the induction system has eight idle circuits. Also, if simultaneous opening of the butterflies is used, the system accesses eight progression circuits; if progressive butterfly opening is used, the system accesses only four circuits. Either way, more airflow is available on the low-speed circuitry for street driving, thus delaying the need to use the main jet circuits. Correctly calibrated you can achieve more traffic-friendly performance than delivered by a single carb on a single-plane intake.
This tuning practice contradicts the commonly held belief that a tunnel ram is good for race applications only, and thus it implies that it is less than capable of good low-speed manners. If anything, the reverse is actually true. My experience with tunnel ram setups on engines with big street and/or short race cams has shown a 650 idle rpm is achievable. But the “race only” reputation is not without some validity.
The idle jets for a tunnel ram setup need to be much smaller than for a single-carb setup and the idle air bleeds typically need to be at least a couple of sizes bigger. Also, don’t go for the instantaneous opening of all eight barrels unless the carbs are relatively small, such as 600 or so cfm.
If you are looking for good top end numbers, carbs of 750 cfm or more can deliver, but they need a progressive throttle linkage so they don’t present too much venturi area too soon.
Vacuum secondaries can also work well on the street, especially if the cam is short enough to produce at least 9 inches of idle vacuum. As for main jets and air bleeds, these are usually somewhat similar to what you might expect for a single carb on a single-plane intake.
As for the carbs themselves, don’t go overboard on CFM just because you can. For almost any street performance application, a pair of performance 650s is good for engines capable of up to 800 to 850 hp, and 750s are suitable for 850 to 950 hp. Moreover, it seems that an annular-discharge booster is often best for low- to mid-range drivability. On a 500-ci or larger engine, annular-discharge boosters can bump the torque at 3,000 or so rpm by as much as 40 ft-lbs. About half the time, the top-end output is marginally improved, and when that is not the case, any output reduction is minimal.
Having looked at the theoretical reasons that a tunnel ram should work, let’s take a look at a real-world dyno test. (See Figure 7.8.) The test engine was a high-effort 525-inch unit so the builder has properly prepared all the components: chamfered bores, ported intake manifold, well-matched ports, and modified pistons. It used a Mk IV block, which was bored 0.060 over, a Scat crank and rods, and some custom skinny-ring Wiseco pistons. The block deck was relieved and the heads were a set of Dart 320-cc port items that were ported to good effect. With the heads milled to 110 cc, the compression with the particular piston crown was 11:1.
The single-carb intake was a Sniper Jr. equipped with an AED 1150, and the tunnel ram intake was from Weiand and carried two 750 carbs reworked in the butterfly and booster (annular-discharge type) areas to flow 850 cfm each. The all-important cam and valve-train was the result of a COS-Cam spec computed at TWPE. Although totally streetable, this combination produced a torque curve better than most street 572s make.
If the race rules permit, your only carbureted option for maximum output is a tunnel ram. This conjures up a picture of an expensive sheet-metal intake, but that need not be the case. Some very functional, and more to the point, affordable intakes are available from Pro-Filer and Edelbrock.
If using a tunnel ram intake now interests you, be advised: Get your carbs as a purpose-built pair from a reputable custom carb shop such as AED. At the end of the day it will save you both money and frustration.
Written by David Vizard and Posted with Permission of CarTechBooks