Intake Manifold: Description and Operation
Intake System
Each cylinder has two intakes of different lengths and diameter. One short one (1) and one which is about twice as long (2) and has a slightly smaller diameter than the short one.
Both intakes start from a plenum chamber (3). The shorter intakes have a damper (4) upstream of the cylinder head.
These dampers are connected and controlled by a vacuum actuator (5) via a lever. The vacuum in the regulator is controlled by a solenoid valve which is controlled by the EZ 129K distributor Ignition System (D1 system) Control Module (ICM).
The steel dampers have a rubber seal. When open, the dampers fit into a recess in the intake so as not to interfere with the inlet air flow.
The dampers can only be either fully open or fully closed. This is controlled by RPM and throttle opening signals sent to the ICM.
The intake manifold is mounted directly on the cylinder head. The manifold has a 2.5 liter plenum chamber and is balanced for each cylinder. This ensures uniform gas flow and charging of the combustion chambers.
Injectors are located on the intake manifold close to the inlet valves to ensure optimum fuel/air mixing.
The Throttle Body (TB) located on the intake manifold and is 65 mm in diameter.
The TB for the 2.5 litre engine has a plastic profile on the throttle disc. This reduces airflow when the throttle opening is small ensuring the exact correspondence of throttle opening and accelerator pedal movement. This means that air volume, and therefore engine output, is more closely related to Accelerator Pedal (AP) movement.
Intake Manifold, Pulses/RPM
The intake manifold is designed to exploit pulses in the intakes at different speeds - to better charge the cylinders, allowing the engine to deliver an even, high torque over a wide speed range.
A: At Rpm below 1,800 rpm; both intakes are open regardless of throttle opening. This allows a rapid increase in airflow when the throttle is opened, which gives a rapid response at low Rpm - when starting and accelerating at low speed.
B: At Rpm between 1,800 and 4,200 rpm; and if the throttle is more than 80° open, the dampers on the shorter intakes are closed, and only the longer intakes are used. These are tuned for this speed range so that the air in the plenum chambers and intakes is made to resonate with the movement of the pistons and valves. This means that air in the intakes pulses in time with the opening of the valves and is pumped into the cylinder when the intake valve opens. This charges the cylinders better than a conventionally aspirated unit.
A new parameter has been introduced for model year 1994 and later which takes "load" into account. This means that the dampers on the shorter intakes can be closed before 80°.
C: At Rpm above 4,200 rpm; both intakes are open regardless of throttle position. Air can flow through both intakes to the cylinders doubling airflow. At high Rpm the combination of the long and short, larger diameter intakes provides the right resonance for maximum torque. The B5254 delivers 215 Nm (168.3 ft lb) at 5,000 rpm.
The long intakes are designed for high volumetric efficiency at Rpm between 1,800-4,200. Using the long intakes alone at high Rpm would restrict air supply because the air pulses would no longer correspond to the opening of the valves.
The damper spindles (a long spindle for three cylinders and a short spindle for two cylinders) are supported in bearings in the intakes. The linkage mechanism is designed so that dampers seal completely as leakage between damper and seat will impair engine torque at those speeds where the dampers are closed.
NOTE: In the event of vacuum system faults, the dampers remain open, resulting in reduced torque at intermediate speeds.