Tuned Port Injection may seem primitive by contemporary standards, butin its day, "TPI" represented an important step forward in the evolutionof Corvette-engine technology. Compared with its carbureted predecessor,the new-in-'85 L98 TPI motor not only offered superior peak power, butit also greatly reduced emissions, improved driveability, and--thanks toits long, small-diameter intake runners--yielded a tremendous increase inlow-speed torque.
Unfortunately, the same intake runners that provided such impressivebottom-end grunt also limited the engine's high-rpm output. And whilereplacing the TPI system with a simple carb and dual-plane intake wouldyield impressive peak power gains, it would dramatically reduce the hugelow-speed torque numbers that made the L98 famous.
L98 350 TPI: Naturally Aspirated vs. HP Turbo Kit (8.3 psi) - TheseTPI motors have always been known for their massive torque production.The steady diet of boost from the HP turbo kit simply elevated theengine's power curve without changing its basic shape. Imagine stabbingthe throttle and summoning up over 600 lb-ft of torque. That, dearreader, is one serious C4 Corvette.
The question confronting power-starved L98 Vette owners, then, is clear.To wit: Is it possible to improve the output of a TPI motor withoutincurring the penalties associated with an induction-system swap? Oneneed 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 oftorque at 3,200 rpm, the twin-turbocharged B2K engine--which used OEMcylinder heads, a factory TPI intake, and virtually stock camtiming--produced 345 hp at 4,000 rpm and 465 lb-ft of torque at only2,800 rpm. (Those output numbers grew to 382/562 the following year.)What Callaway's turbocharged L98 offered was big-block torque in asmall-block package.
Seeing the potential of the L98 as a forced-induction engine, NewMexico-based HP Performance recently introduced a bolt-on turbo systemfor '85-'91 C4s. Designed specifically for the needs of TPI engines, thenew kit includes a single 60mm turbo, a front-mounted air-to-airintercooler, a compressor-bypass (or "blow-off") valve, and a TiAL 44mmwastegate that regulates boost to 7-8 psi.
Viewed from the outside, it's hard to imagine all this hardware fittinginside the engine compartment of a C4 Corvette. It does.
According to HP, a stock L98 will withstand 8 psi, provided the air/fuelmixture and timing curves are spot-on. In fact, whereas the Callawayturbo motors featured low-compression pistons, the higher compressionprovided by the standard L98 slugs actually enhances throttle response,fuel economy, and outright power production. The HP kit provides thenecessary extra fuel and timing via a set of 42-pound injectors and arevised PROM.
The 60mm turbo used in the HP kit provides near-immediate boost responseand tire-annihilating low-end torque. Anyone who has ever driven amodified 427 or 454 big-block Vette will immediately recognize the hugeshove in the backside available from the HP-blown L98.
The kit includes a 60mm turbo designed to provide immediate boost. Wehave produced over 700 hp using this unit, so there is plenty of room togrow should you decide to build a dedicated turbo motor.
Subjective impressions are one thing, but we were really anxious to seewhat an L98 equipped with the HP Performance kit could do on an enginedyno. Our test motor was an aluminum-headed L98 that appeared to becompletely stock. There was no porting evident on the heads, the TPIsystem was unmodified, and the block still had the factory markings. Ouronly question concerned the cam profile, as the motor came from the oldChevy Race shop, and we have no idea what sort of testing it underwentbefore it ended up in our facility.
Before installing the turbo kit, we ran the L98 in normally aspiratedtrim to establish a baseline. The engine was configured with an electricwater pump, a set of long-tube headers, and the factory injectors. Runwith the FAST engine-management system, it produced 332 hp at 4,800 rpmand 394 lb-ft at 4,000 rpm. Given the relatively high engine speeds atwhich the peak horsepower and torque values occurred, we suspect the camwas a tad hotter than the factory L98 stick. Still, the TPI motoroffered impressive low-speed torque, with the curve exceeding 350 lb-ftfrom 2,500 rpm to 4,900 rpm and topping 375 lb-ft from 3,200 rpm to4,700 rpm. Whatever its cam specs, the 350 seemed to be in excellentrunning condition and ready for some boost.
Feeding the 60mm turbo was a set of tubular exhaust manifolds. Everyexhaust component on the kit was treated to Jet-Hot coating to maximizethe heat energy to the turbo.
Installing the turbo system was easy on the engine dyno and did notappear as if it would be terribly difficult in the car. In fact, anyonewho has ever installed headers should have no trouble with the job.
As installed in the vehicle, the turbo is positioned in the spacenormally occupied by the battery. The exhaust from the turbo runsthrough a 3-inch single tube that splits to feed the factory exhaustsystem (it's compatible with aftermarket exhausts, too). Lacking a C4exhaust system, we simply let the turbo exhale through a 3-inch sectionof tubing with no muffler. The long-tube headers used for baselinetesting were replaced with the turbo manifolds and a crossover tube thatruns under the oil pan. The manifolds resembled "shorty"-style headersand came Jet-Hot coated to maximize the heat energy to the turbo.
This TiAL 44mm wastegate was employed to regulate boost pressure. Oursystem was set to provide roughly 8 psi to the otherwise-stock L98.
We also drilled and welded the necessary drain fitting in the stock oilpan. We relied on the oil-pressure source to feed the turbo using thesupplied length of braided line and a 90-degree fitting for the top ofthe turbo. The only deviation from the kit as run in the car was thedeletion 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 degreesof ignition timing, the boosted motor may only tolerate 18 to 20degrees.
After tuning to provide 20 degrees of total timing and a safe air/fuelmixture of 11.75:1 (with the supplied injectors), we were rewarded withan even 500 hp at 4,800 rpm and an incredible 611 lb-ft of torque at3,800 rpm. As expected, the addition of 8.3 psi of boost pressure simplyamplified the naturally aspirated power curve. The horsepower stillpeaked at 4,800 rpm, and the engine speed for the torque peak wasactually reduced from 4,000 rpm to 3,800 rpm.
Even more impressive is that the HP Performance system has room to growshould you decide to build a dedicated TPI turbo motor. Seven hundredpound-feet at the tires, anyone?
Keeping the charge temperature down and the power up was thisfront-mounted air-to-air intercooler. | 
Larger (42-pound) fuel injectors and a revised PROM were used to tailorthe air/fuel ratio and timing curve to the turbo combination. | 
We employed a FAST XFI engine-management system on the engine dyno tooptimize the tuning of both the naturally aspirated and turbochargedcombinations. |
The TPI induction system was a major factor in determining the shape ofthe torque curve. The long runners promoted low-speed power butrestricted horsepower production past 5,000 rpm. (Crusty plug wires andspare-tie wrap not included.) | 
We first ran the L98 in normally aspirated form. The engine produced 332hp and 394 lb-ft of torque with a set of long-tube headers and anelectric water pump. | 
Equipped with the HP Performance turbo kit, the L98 thumped out an even500 hp and 611 lb-ft of torque at a peak boost pressure of 8.3 psi. |