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.
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.
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.
To illustrate the differences between the carbureted and TPI systems, we took what we thought was an otherwise-stock L98 Corvette motor and installed it on the engine dyno. The engine was configured with an electric water pump and a set of Flow Tech long-tube headers with open (read: un-muffled) collector extensions. We also employed the FAST XFI engine-management system, which made it unnecessary to run a mass airflow sensor or intake tract with the TPI. The FAST system was used to control the factory fuel injectors and provide an air/fuel ratio of 13.0:1 (slightly richer below the torque peak). The motor ran best with 34 degrees of total timing, though we reduced the timing at the lower engine speeds. All testing was performed on 91-octane pump gas.
After some minor tuning, the TPI small-block surprised us by producing 332 hp at 4,700 rpm and 394 lb-ft at 4,000 rpm. Given that the L98 was originally rated at 245-250 hp and 345 lb-ft (1991 figures), we suspect that this particular motor was equipped with a mild aftermarket cam. Compared with L98s we'd run in the past, torque was actually down a bit, while peak horsepower was up. Despite the questionable cam scenario, the TPI motor produced impressive torque up to 4,500 rpm, offering over 350 lb-ft at just 2,500 rpm. As expected, the horsepower curve fell off rapidly after peaking at 4,700 rpm.
After running the TPI system we switched over to the Barry Grant 650 Speed Demon carburetor and Edelbrock Performer RPM intake. The swap went very smoothly, thanks to the engine dyno, and in no time we were up and running. Using an MSD billet distributor, we dialed the same advance curve into the carbureted L98 and spent some time jetting the carburetor to replicate the 13.0:1 air/fuel ratio provided by the TPI system. The results were impressive, with the carbureted L98 producing 363 hp at 5,900 rpm and 387 lb-ft at 4,100 rpm.
As expected, the carbureted intake produced much more peak power, offering a gain of over 50 hp out near 5,500 rpm. But down low, the TPI reigned supreme, generating better numbers from 2,500 rpm to 4,300 rpm. It all boils down to where you want your power-down low or up top-and how much of an emphasis you place on the TPI's superior fuel economy and visual appeal.
Carburetor vs. TPI
It's not terribly surprising that the stock L98 TPI system produced more low-speed power than the conventional carb and dual-plane intake. What was interesting was the difference in power out past 4,500 rpm. Whereas the TPI signed off with 332 hp at 4,700 rpm, the carbureted setup continued pulling strong, reaching a peak of 363 hp at 5,900 rpm. Though the peak torque production differed by just 7 lb-ft, the TPI combination made a solid 20-25 lb-ft more from 2,500 rpm to 4,300 rpm. For a drag-race application, there's no question the carbureted setup would be the way to go. For throttle response, fuel mileage, and overall driveability, the story would be quite different.