Fuel flow to the cylinders...
Fuel flow to the cylinders on the TPI motor was supplied by individual injectors. Using one injector per cylinder ensured a consistent cylinder-to-cylinder air/fuel ratio.
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.
By contrast, the carburetor...
By contrast, the carburetor does not deliver a consistent air/fuel mixture to each cylinder. Nevertheless, there is no denying that a great many formidable small-blocks have been powered by traditional carbureted combinations. Shown here is the Barry Grant 650 Speed Demon used for our testing.
Airflow to the common plenum...
Airflow to the common plenum on the TPI intake was supplied by an OEM 48mm dual-blade throttle body. It was clear the factory throttle body represented a restriction, as there was more than twice as much vacuum (an indication of a flow restriction) in the TPI manifold as there was with the carbureted combination.
Compared with the stock throttle...
Compared with the stock throttle body, the 650 Speed Demon carburetor (right) featured twice as many throttle blades. The increased airflow offered by the higher-flowing throttle openings reduced the vacuum in the intake from 1.4 inches (with the TPI) to just 0.6 inch.
The TPI fuel injection relies...
The TPI fuel injection relies on a computer (we ran ours with a FAST XFI management system) to precisely dial in the air/fuel ratio and timing curves for all driving conditions. Though not quite in the same league in terms of flexibility and precision, carb jetting and distributor advance weights can make for a pretty effective combination.
On the engine dyno, the TPI...
On the engine dyno, the TPI L98 produced 332 hp at 4,700 rpm and 394 lb-ft of torque at 4,000 rpm. In looking at the power curve, we suspect that this engine was sporting something other than the factory cam. (We did not know the history of the motor before installing it on the dyno.)
Replacing the TPI and EFI...
Replacing the TPI and EFI systems with the Edelbrock manifold and Barry Grant carburetor resulted in a sizable jump in peak power. Equipped with the carb and dual-plane intake, the L98 produced 363 hp at 5,900 rpm and 387 lb-ft of torque at 4,100 rpm.
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.