L98 Induction System - Inductive Reasoning

Carbed or computerized: Which one's better for your L98?

Richard Holdener Jul 1, 2006 0 Comment(s)
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Should you consider ditching the factory TPI system in favor of a simple carb and intake?

Ask any Vette owner to choose between two particular performance components, and chances are he'll opt for the one that makes more power. While this logic may appear sound prima facie, the reality is that there are many variables to consider beyond peak power alone. For example, what if you were given a choice between an L98-style Tuned Port Injection (TPI) system and an Edelbrock Performer RPM Air Gap intake topped with a Barry Grant 650 Speed Demon carburetor? Which one would you choose? Is your answer still the one that makes the most power?

In order to make an informed decision, you'll need to understand how each of these induction systems functions. While both the carbureted and fuel-injected setups provide metered air and fuel, they go about it in decidedly different ways. In terms of fuel metering, the carburetor relies on airflow across the venturis to draw fuel from the main jets. The fuel supplied to the air stream is dependent upon the air speed and the size of jetting (or fuel-flow) orifice. Tuning the carburetor to alter the air/fuel ratio is accomplished by changing jet sizes. The four-barrel design of the carburetor allows you to change the four available main jets independently, though this is rarely done on a dual-plane intake such as the Performer RPM. The main jetting is augmented by idle-mixture screws and, in some cases (though not ours), adjustable high-speed air bleeds. This combination provides a range of tuning sufficient to create an acceptable air/fuel curve for most driving conditions.

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The L98 small-blocks came equipped with a unique intake manifold consisting of long runners fed by a common plenum. These long runners promoted impressive torque production.

The electronic fuel-injection system employed on the TPI provides even more-precise control of the fuel metering, thanks to the single injector situated at each intake port. These high-pressure injectors, combined with computer control, allow the EFI system to provide unequaled fuel metering for every conceivable load, throttle angle, and driving condition.

Where EFI really shines is in delivering improved fuel economy and reduced emissions under part-throttle cruise conditions. Part of the improvement comes from the use of elevated fuel pressure and a very small flow orifice in the tip of the fuel injector. The high pressure and small flow orifice combine to shear the fuel into ultra-small droplets. The greater surface area provided by the reduced droplet size (compared with a similar amount of fuel in one large droplet) improves the burn and energy-conversion rates of the fuel.

Certainly, enhanced economy and emissions compliance are two of the more desirable characteristics of EFI, but how do these systems fare at wide-open throttle? In fact, the superior atomization and more-precise mixture control provided by a properly calibrated EFI give it peak-power potential beyond that of a comparable carbureted setup. That said, the real key to the power curve in both carbureted and fuel-injected configurations can be found in the design of the intake manifold.

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For our carbureted setup, we chose an Edelbrock Performer RPM Air Gap manifold. The dual-plane intake is the manifold of choice for nearly all street applications, combining impressive torque with plenty of top-end power.

All of which leads us, in a roundabout sort of way, to our test. For our carbureted intake, we selected the Edelbrock Performer RPM Air Gap, arguably one of the best intakes on the market for a street-driven application. We chose the dual-plane design for its desirable combination of a broad powerband and improved driveability as compared with the racier single-plane type. While a single-plane intake might produce more peak power at the top of the rev range, a dual-plane design will provide much more average power across the useable rev range. Up to 6,000 rpm-as high as we were willing to push our well-used L98-the dual-plane would be the hands-down choice. The divided plenum not only aids torque production, but also improves the signal to the carb to improve throttle response and fuel delivery.

While impressive compared with a single-plane intake, even the dual-plane pales next to the torque-producing capability of the L98 TPI. TPI might just as well stand for "torque-producing injection," as the extra-long runners greatly enhance the low-speed and midrange torque production of the L98 small-block. In simplified terms, long runners-particularly small-diameter ones-maximize cylinder filling at lower engine speeds. TPI uses long, small-diameter runners to produce huge torque numbers in the low- and mid-rpm ranges. Stomp on the gas of an L98 motor, and you're immediately rewarded with an exhilarating shove in the backside. No waiting to come "on cam"; just instant gratification. The downside to the significant improvement in low-speed power is that the long, narrow runners limit power production at the top of the rev range. Tellingly, the L98 makes peak power at just 4,600 rpm.

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