L98 Engine Induction - Stealth Mode TPI

Building a 458-hp L98 street sleeper

Richard Holdener Mar 10, 2007 0 Comment(s)
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While it has been a decade since its demise, there is plenty to like about the original GM Tuned Port Injection system.

Small-block Chevys have come a long way since their introduction back in the mid '50s. They have grown in displacement, technology, reliability, and (most importantly) power production. The modern LS2 is a worthy successor to the original Mouse motor. Despite the impressive efficiency and all-aluminum construction offered by the modern Mouse (to say nothing of the 505-hp offered by the factory LS7), we here at Super Chevy have not abandoned our 23-degree roots.

Sure, the modern Mouse is lightweight, powerful, and efficient, but let's not forget where we came from, or the fact that the original small-block can be powerful. With millions of small-blocks still in existence, it will be some time before the LS-series motors replace the original as the powerplant of choice for Chevy enthusiasts. Given the dedicated following, it is not likely that we will ever see the demise of the small-block, at least not until current reserves of fossil fuels dry up.

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Looking like an arachnid-based alien warship, the long runners in the stock TPI system were designed to promote impressive mid-range torque production.

Before Chevy introduced the current LS1 architecture, it offered an intermediate update to the original Mouse motor in the form of the LT1 (and more powerful LT4). Prior to that, the final configuration was code named L98. Fans of Tuned Port Injection will remember that the final small-block came equipped with a modern direct-port fuel injection. While the earlier Cross-Fire (and TBI) injection introduced fans to the merits of electronic fuel injection, it was TPI that launched the modern EFI performance era back in '85. Unlike previous carburetors (including the computer-controlled varieties), the TPI system offered precise metering of the fuel under all operating conditions. This was especially important to help meet the ever-tightening emissions laws. Fuel efficiency and emissions were optimized by balancing fuel delivery to each individual cylinder. Unlike carbureted, TBI, and even the Cross-Fire EFI systems, TPI provided fuel injectors for each port, thereby balancing power production (and fuel usage) in each cylinder. Even fuel delivery is difficult (if not impossible) in a typical carbureted (or TBI) application, so the air/fuel is tuned to the leanest cylinder. Unfortunately, this also means other cylinders run rich. This cylinder imbalance decreases power while increasing fuel consumption and exhaust emissions.

In addition to the introduction of individual port injection, the TPI systems also provided a unique induction system. Designed to flow air (the injectors are positioned at the base of the intake to flow fuel only into the head port), the TPI systems incorporated tuned runner lengths to optimize power production at the lower rev ranges. With long, small-diameter intake ports, the TPI system enhanced low- and mid-range torque production.

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The TPI induction system featured a dual-blade throttle body. Though adequate for our mildly modified L98, it proved inadequate to feed the needs of our Stealth Mode stroker.

To illustrate this point, torque production from a typical L98 Vette motor exceeded horsepower production by roughly 100 lbs-ft. Rated at 250 hp, the TPI system helped the 350 pump out an amazing 350 lbs-ft of torque. Naturally this over abundance of mid-range torque came with a penalty. The same runners in the TPI system that were designed to enhance power production below 5000 rpm, lost efficiency rapidly thereafter. TPI motors were all about instant gratification. There was never any waiting to come on the cam, just plenty of torque to get things going in a hurry. Unfortunately, the long runner lengths quickly put an end to the party.

A stock Tuned Port motor offers impressive looks and torque production. But who wouldn't like some more power from their L98? More power is what it's all about. While it is possible to turbo or supercharge an L98 (anyone remember the Callaway Corvettes?), we decided to travel the normally aspirated route for this exercise. For this exercise, not only will the L98 retain its TPI setup, we even hoped to conceal most of our mods, thus officially taking our modified TPI into Stealth Mode.

Given the constraints of the build up, we decided that the power gains would take a three-fold approach. The three methods of improving the power output of the 350 cubic-inch L98 would include displacement, compression, and breathing. Before tackling any of the three, we needed to run an L98 test engine to illustrate what to expect from a healthy TPI small-block. What we thought was a stock L98 pulled from an unsuspecting Corvette turned out to have a mild Comp cam. Sometime in its life the owner replaced the factory hydraulic roller cam with a 270HR Magnum cam from Comp Cams. The single-pattern Magnum profile offered .500 lift, 215 degrees of duration, and a 110-degree lobe separation angle. Given our accelerated dyno schedule, we didn't have time to replace the cam with a stock version. Thus our L98 was run with the cam, a set of Hooker long-tube (1 5/8-inch) headers, and the FAST XFI engine management system.