Tuned Port Injection may seem primitive by contemporary standards, but in its day, "TPI" represented an important step forward in the evolution of Corvette-engine technology. Compared with its carbureted predecessor, the new-in-'85 L98 TPI motor not only offered superior peak power, but it also greatly reduced emissions, improved driveability, and-thanks to its long, small-diameter intake runners-yielded a tremendous increase in low-speed torque.

Unfortunately, the same intake runners that provided such impressive bottom-end grunt also limited the engine's high-rpm output. And while replacing the TPI system with a simple carb and dual-plane intake would yield impressive peak power gains, it would dramatically reduce the huge low-speed torque numbers that made the L98 famous.

The question confronting power-starved L98 Vette owners, then, is clear. To wit: Is it possible to improve the output of a TPI motor without incurring the penalties associated with an induction-system swap? One need only look back at the '87-'91 Callaway B2K C4s to find the answer. Whereas the '87-spec L98 produced 240 hp at 4,000 rpm and 340 lb-ft of torque at 3,200 rpm, the twin-turbocharged B2K engine-which used OEM cylinder heads, a factory TPI intake, and virtually stock cam timing-produced 345 hp at 4,000 rpm and 465 lb-ft of torque at only 2,800 rpm. (Those output numbers grew to 382/562 the following year.) What Callaway's turbocharged L98 offered was big-block torque in a small-block package.

Seeing the potential of the L98 as a forced-induction engine, New Mexico-based HP Performance recently introduced a bolt-on turbo system for '85-'91 C4s. Designed specifically for the needs of TPI engines, the new kit includes a single 60mm turbo, a front-mounted air-to-air intercooler, a compressor-bypass (or "blow-off") valve, and a TiAL 44mm wastegate that regulates boost to 7-8 psi.

According to HP, a stock L98 will withstand 8 psi, provided the air/fuel mixture and timing curves are spot-on. In fact, whereas the Callaway turbo motors featured low-compression pistons, the higher compression provided by the standard L98 slugs actually enhances throttle response, fuel economy, and outright power production. The HP kit provides the necessary extra fuel and timing via a set of 42-pound injectors and a revised PROM.

The 60mm turbo used in the HP kit provides near-immediate boost response and tire-annihilating low-end torque. Anyone who has ever driven a modified 427 or 454 big-block Vette will immediately recognize the huge shove in the backside available from the HP-blown L98.

Subjective impressions are one thing, but we were really anxious to see what an L98 equipped with the HP Performance kit could do on an engine dyno. Our test motor was an aluminum-headed L98 that appeared to be completely stock. There was no porting evident on the heads, the TPI system was unmodified, and the block still had the factory markings. Our only question concerned the cam profile, as the motor came from the old Chevy Race shop, and we have no idea what sort of testing it underwent before it ended up in our facility.

Before installing the turbo kit, we ran the L98 in normally aspirated trim to establish a baseline. The engine was configured with an electric water pump, a set of long-tube headers, and the factory injectors. Run with the FAST engine-management system, it produced 332 hp at 4,800 rpm and 394 lb-ft at 4,000 rpm. Given the relatively high engine speeds at which the peak horsepower and torque values occurred, we suspect the cam was a tad hotter than the factory L98 stick. Still, the TPI motor offered impressive low-speed torque, with the curve exceeding 350 lb-ft from 2,500 rpm to 4,900 rpm and topping 375 lb-ft from 3,200 rpm to 4,700 rpm. Whatever its cam specs, the 350 seemed to be in excellent running condition and ready for some boost.

Installing the turbo system was easy on the engine dyno and did not appear as if it would be terribly difficult in the car. In fact, anyone who has ever installed headers should have no trouble with the job.

As installed in the vehicle, the turbo is positioned in the space normally occupied by the battery. The exhaust from the turbo runs through a 3-inch single tube that splits to feed the factory exhaust system (it's compatible with aftermarket exhausts, too). Lacking a C4 exhaust system, we simply let the turbo exhale through a 3-inch section of tubing with no muffler. The long-tube headers used for baseline testing were replaced with the turbo manifolds and a crossover tube that runs under the oil pan. The manifolds resembled "shorty"-style headers and came Jet-Hot coated to maximize the heat energy to the turbo.

We also drilled and welded the necessary drain fitting in the stock oil pan. We relied on the oil-pressure source to feed the turbo using the supplied length of braided line and a 90-degree fitting for the top of the turbo. The only deviation from the kit as run in the car was the deletion of the mass airflow sensor.

Adding boost to the normally aspirated L98 required careful tuning. Whereas the normally aspirated combination may run best with 32 degrees of ignition timing, the boosted motor may only tolerate 18 to 20 degrees.

After tuning to provide 20 degrees of total timing and a safe air/fuel mixture of 11.75:1 (with the supplied injectors), we were rewarded with an even 500 hp at 4,800 rpm and an incredible 611 lb-ft of torque at 3,800 rpm. As expected, the addition of 8.3 psi of boost pressure simply amplified the naturally aspirated power curve. The horsepower still peaked at 4,800 rpm, and the engine speed for the torque peak was actually reduced from 4,000 rpm to 3,800 rpm.

Even more impressive is that the HP Performance system has room to grow should you decide to build a dedicated TPI turbo motor. Seven hundred pound-feet at the tires, anyone?

Hp Performance
301 E 4th St.
NM  88202
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