INDUCTION SYSTEM TYPES

Posted: July 30, 2011 in Induction Systems

Dual Plane

     This type of manifold has a divided plenum (or two smaller plenums). It is a good choice for low rpm power and gives better throttle response than most other manifolds. The small plenum area gives good carburetor signal strength and low-end drivability.

Single Plane

     Also known as 360° manifolds. All intake runners come form a common plenum. The open plenum smooths out the induction pulses better than a dual plane manifold and can give better top-end power, at a cost of low rpm power. If you have a high revving engine, a single plan would probably be the best choice.

Tunnel Ram

     Really this is just a more exotic version of a single plane. All the intake runners are straight and meet at a common plenum (the tunnel). This type of manifold gives excellent fuel distribution and flow for top-end power. The large plenum area reduces signal strength and throttle response, so it takes some good tuning to make these responsive for street driving. When tuning in one of these, you’ll need a quick accelerator pump and more ignition timing down low. In most cases, you can lock your distributor to total advance. You might need a retard box to retard the timing while you start it, but for the most part tunnels rams run best with a lot of advance at an idle. If you want an advance curve on a street tunnel ram set up, use a vacuum advance and hook it directly to manifold vacuum. The poor mixture at low rpm, requires a lot of timing at idle and cruise conditions.

Individual Runner (IR)

     This type manifold has one throttle bore per cylinder. It enhances low and mid-range power by increasing peak velocity through the venturi. There is no plenum to dampen the induction pulses, so it is difficult to get them to work at high rpm (It is common for fuel to splash out of the throttle bore at high rpm). The carburetor is also very critical, an IR set up will need each throttle bore to flow enough for peak airflow. This means a 350 cubic inch engine can have almost 3000 CFM and not be over carbureted. If this setup is used with dual 4 barrels (Holley dominators are common), you’ll need to make the linkage a 1:1 ratio so the secondary’s open at the same rate as the primaries.

Cross Ram

     Mostly used on bigger cars to help low to mid rage torque. The long runners can help low-end power. Hood clearance can be a problem with long runners, so by crossing the runners to a carburetor located on the other side of the engine they can be longer but not higher. This was common with Mopars and worked very well for it’s time. Fuel drop out was a problem, so these set ups ran very rich at low rpm and sucked up gas. Long runners with a wet flow system give the fuel more time to form into large droplets at low rpm thus decreasing atomization.

Tuned Port

     Tuned Port manifolds can come in various shapes and forms. They are usually associated with fuel injection, but the Tuned Ports idea is not related to EFI at all. Tuned port simply means that the intake runners are tuned to a specific rpm range. Most factory tuned port set-ups are sized to help mid range torque. The Chevy TPI’s work very well to give a little power boost in the 3000-3500 rpm range. The problem with them is they run out of air by 4500 rpm.
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