Time To TPI
The Who, What, Where, And Why Of TPI
During the mid-'80s, automobile manufacturers made the decision to jump into electronic fuel injection (EFI). Computer-controlled engines benefit from improved fuel mileage and crisp throttle response derived from accurate control of the fuel and spark curves.
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In 1985, GM introduced its Tuned Port Injection (TPI) manifold on the Corvette and the Camaro line. The TPI system includes a manifold base, two sets of runners, a plenum, and a throttle-body.
The TPI manifold was originally designed for 305ci engines with 19 lb/hr fuel injectors operating at 36 psi, and employs a long and narrow runner to enhance bottom and midrange torque. The 350ci TPI-equipped engines use the same intake as the 305ci with larger 22 lb/hr injectors at 43.5 psi. The additional cubic inches broadened the power curve, but the long-runner intake limited rpm. The LT1 engine replaced the TPI in the Corvette in 1992, and in the Camaro in 1993.
Now that TPI intakes are both plentiful and inexpensive, it's tempting to snatch one up for your hot rod. Before hunting the wrecking yards, we dove into several books for some technical advice to make sure we bought the right stuff. TPI-equipped Corvettes and F-body cars rolled off the assembly line between 1985 and 1992, but mixing and matching parts from different engines can cause problems. The main rule to follow when TPI hunting is to obtain as many parts as possible from the same donor engine. The TPI design can be identified by the long arching runners that extend from the base manifold to the plenum. However, the basic TPI has experienced several revisions over the years.
In 1985, the Corvettes and Camaros used an iron small-block cylinder head. The following year, Corvettes were offered with two cylinder-head options. Corvettes with aluminum heads used an >> intake manifold that placed the ERG flange near the distributor, while the '86 iron-head Corvettes used the '85 design that placed the EGR valve in the middle of the manifold. Then, in 1987, all Corvettes came with aluminum heads using the '86 aluminum-head base-manifold design. Another major change to the TPI system came on the '87-'92 Camaro cylinder heads. These cast-iron heads feature revised center bolt-hole angles. These altered bolt-hole angles required a third base with matching center bosses.
GM revised the TPI plenums as well. All plenums feature an EGR passage, but the '85-'88 plenums are cast with an additional cold-start passage controlling a ninth fuel injector. The '85-'88 TPI plenums also feature a triangular idle-air control (IAC) port located between the two throttle-bore openings. This IAC port requires a compatible throttle-body, while '89-'92 plenums without an IAC port can use either throttle-body design.
Once you have all the right manifold parts, it's time to hunt down the proper electrical components. Obtaining the correct sensors, electronic control module (ECM), and wiring harness is crucial to completing the TPI swap (see "EFI Basics, Part I," Jan. '02). Using the factory wiring harness can be cumbersome due to many nonessential connectors. Several aftermarket companies including Painless Wiring and Howell Engine Developments offer easy-to-use TPI wiring harnesses that will make this swap much easier.
The key to going fast for less is doing the research and understanding what it takes to get your TPI combination right. With the proper aftermarket changes, your TPI-powered Chevy will be frying the tires and smoking the competition.
