What Is a Tuned Port Injection?by Lee Sallings
Chevrolet's version of multi-port fuel injection called Tuned Port Injection was introduced to the public in 1985 on the Corvette and Camaro. Featuring tuned length intake runners, one individual injector per cylinder ( multi-port), and mass airflow sensor fuel management. The system was initially designed to help the smallish 305 cid engine with a boost in low and mid range torque, and quickly adapted to the 350 cid engine found in the Corvette.
The venerable TPI system proved to be versatile and tunable, and is now a mainstay for hod rods and street rods alike.
The intake manifold
The center piece of the TPI system is it's unique intake manifold. A centrally located plenum, and two sets of runners on each side, give it the distinctive TPI look. Attached to the front of the plenum is the dual inlet throttle body and MAF sensor, connected to the front mounted air cleaner housing. The long, narrow tuned length runners are largely responsible for the increase in torque and horsepower output over the previously used carburetor or throttle body injection. These runners allow the engineers to determine and control at what engine speed peak torque will be seen and then engineer the rest of the system around the target engine speed.
Fuel delivery is handled by a high pressure fuel pump located in the fuel tank. This pump supplies 35 psi regulated fuel pressure to the Bosch style fuel injectors, and enough volume to handle all the requirements of the engine while under load. Unlike batch fire injectors, these injectors are not opened several at one time, but one at a time to allow more precise control of air fuel ratio. Fuel is added by increasing the on time of the injector, and at wide open throttle they see as high as 80 percent duty cycle.
Engine Management System
In the TPI system, a more versatile mas air flow sensor replaces the familiar manifold absolute pressure sensor as the main engine load sensor. Because the MAF sensor measures air mass entering the engine, and not engine manifold vacuum it is unaffected by changes to camshafts and ignition timing. The computer is able to compensate for those changes through the use of the MAF sensor because it does not rely on manifold vacuum readings that change drastically due to camshaft design. The familiar feedback loop of the oxygen sensor tells the computer when the desired air fuel ratio is reached, and idle speed continues to be controlled by the idle air control valve.
Air delivery is handled by the twin opening throttle body feeding metered air into the engine from the MAF sensor and the rest of the intake tract. The familiar IAC valve and TPS are mounted externally, and help monitor engine load and control idle speed.
The Cheverolet TPI system has proved to be tunable, and forgiving. The fact that it has aggressive looks, ample support from the aftermarket, and produces gobs of low and mid range torque assure it will be with us for a long time to come. Even though it is no longer produced by the company that introduced it in 1985.
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