In 1990, for the first time, a Corvette was sold to the public with more than one camshaft. With engineering assistance from Lotus, which GM owned in the late 1980s through the early 1990s, the Chevrolet Corvette team stuffed a 5.7-liter all-aluminum dual overhead cam V8, called the LT5 small-block, under the hood of a special wide-body Corvette that we all know as the C4 ZR-1 Corvette.

The C4 ZR-1 was full of firsts for Chevrolet and American auto manufacturers, which we’ll touch on more of later, but one of the bigger of those was the fact that the special LT5 small-block was not developed entirely under GM’s roof or by its usual team of small-block engineers. It was also expensive, and only a few thousand were ever made in just a few short years. So was the LT5 small-block a triumph of performance engineering? A doomed and costly failure? Or, maybe, both?

Compared to the L98 and LT1 small-blocks, the LT5 filled the entire engine bay.

A Brief C4 ZR-1 History Lesson

By the mid-1980s, car manufacturers were finally starting to shake off the cobwebs from the Malaise era and look towards higher-performance vehicles again. With Japanese sports cars like the turbo Toyota Supra, turbo Mazda RX7, the upcoming Acura NSX, and eventually the Dodge Viper upstaging the new C4 Corvette on the track and at the stoplights, Chevrolet needed something more than the limp-wristed L98 small-block that barely made 240 horsepower.

Chevrolet played around with a few different ideas to get the performance of the C4 Corvette up enough to surpass the competition. From turbo V6s from the Buick Grand National to turbocharged versions of the current small-block, the Corvette team knew that naturally aspirated simplicity was the way to go, and eight cylinders is what the Corvette deserved.

They turned to Lotus, the British sports car manufacturer that GM bought majority ownership of in 1986. Lotus was working on its own 4.0-liter DOHC all-aluminum V8 (that never made it to production), so the Corvette team went around their own internal small-block V8 engineering department and teamed up with Lotus to make Chevrolet’s own DOHC all-aluminum V8 exclusive to the Corvette we’ve all come to know as the Gen II LT5 small-block.

While GM designated the LT5 as a Gen II small-block, it shares almost nothing with Chevrolet’s traditional 2-valve pushrod V8. It has four times the camshafts, and they’re now mounted in the heads instead of in the block. The LT5 small-block also has double the fuel injectors and intake runners compared to the base model L98 small-block. That monstrous 16-runner intake on top of the beefy DOHC cylinder heads gives the LT5 a commanding presence under the hood.

When the LT5 small-block was introduced exclusively with the ZR-1 package, it took the world by storm and shocked diehard Corvette fans with its blistering performance and price. The LT5 produced 375 horsepower at 6,000 rpm and 370 lb-ft at 4,800 rpm, 135 horsepower more than the base model L98 in 1990 out of the same 5.7-liter displacement. In 1993, revisions to the exhaust, tuning, and other minor changes upped the power to 405 horsepower at 5,800 rpm and 385 lb-ft at 4,800 rpm.

To call the C4 ZR-1 a supercar might sound strange to people today, as it’s just a 375-horsepower C4 to most, but the C4 ZR-1 set several performance records and outperformed almost everything out of Europe with a 0-60 time in the mid-four-second range and a 180-plus MPH top speed. It also broke records for Detroit, being the single most expensive option on any production car to be produced by an American manufacturer. On top of the base Corvette’s $31,979 MSRP, the ZR-1 option cost a staggering $27,016 premium. This brought the price to $58,995, and while that was a bargain compared to Ferraris, Lamborghinis, and the Porsche 911 Turbo, it was still a stunning asking price for a Corvette.

This is a 1993-1995 LT5 block. The iron main caps were cast with the aluminum deep-skirt saddle.

The Aluminum Block

Other than the 1969 ZL1-equipped Corvettes, of which only two were ever sold to the public, this was also the first time the Corvette received a mass-produced aluminum block V8. The aluminum block is a wet-sleeve, open deck, deep-skirt design incorporating a cast-aluminum lower girdle with integral steel captured main caps. Individual forged aluminum cylinder liners from MAHLE are plated with a special Nikasil coating instead of the more common iron sleeve design. While the LT5 small-block shares the same bore spacing as other small blocks at 4.40 inches, the same bellhousing bolt pattern, and a deck height of 9.03 inches, which is very close to the pushrod small-block’s 9.025-inch deck height, that’s where the similarities come to a screeching halt.

Instead of cast-in-place iron sleeves, the LT5 had forged aluminum cylinders that were Nikasil-coated.

The cast-aluminum crankshaft saddle incorporates integral cast-iron bearing caps, with two center bolts from 1990 to 1992, and four center bolts from 1993 to 1995. It, in turn, bolts to the lower block, making it a 4-bolt or 6-bolt main bearing cap design, depending on the year. The saddle extends the lower crankcase structure below the crank centerline to complete the deep-skit design and offers maximum strength. The front of the block houses the camshaft chain-drive system and the crank-driven oil pump behind an aluminum timing cover, while the rear maintains the original Chevy bolt pattern (engine swap, anyone?). The starter mounts in the valley between the cylinder heads.

An integrally cast oil passage runs through the saddle from the oil pump at the front to the centrally located pickup tube. The crank-driven oil pump was a huge departure from the usual rear-mounted small-block oil pump that’s driven off the distributor and camshaft. It is a georotor design, similar to what we see today on the Gen III and IV LS family, and drinks roughly 9 quarts of synthetic Mobil 1 from a cast-aluminum oil pan.

The Rotating Assembly

The rotating assembly for the LT5 small-block consists of a forged steel nitrided crankshaft, forged steel I-beam connecting rods, and cast aluminum pistons. With a 99mm bore and 93 millimeter stroke (3.90 inches and 3.66 inches respectively), the displacement comes to 349 cubic inches. The traditional Gen I small-block has a 4.00-inch bore and 3.48-inch stroke that come out to a near identical 350 cubic inches.

The crankshaft utilizes the same 2.100-inch rod journal like other Chevrolet small-blocks, but massive 2.760-inch main journals, which are a hair bigger than production big-blocks. The thrust bearing was moved from the traditional small-block’s rear main journal to the center journal, where the integral crankshaft reluctor is located for the crankshaft position sensor.

The forged steel crankshaft is very strong. Note the integral reluctor on the center main journal for the crankshaft position sensor.

At first glance, the connecting rods look like any other production I-beam rod. But placing it side-by-side with a Gen I 350 rod will start to showcase its differences. The design itself is meatier, with more material at the pin bore, beam, and big end bore. The pin bore had dual oil feed holes for forced pin oiling for the floating wrist pins. They measure slightly longer than a Gen I 350 rod’s 5.700-inch center-to-center length at 5.740 inches.

The pistons in the LT5 small-block, made by MAHLE, are cast aluminum with a skirtless barrel design, unlike other small-block piston designs. While cast aluminum isn’t as strong as forged aluminum, cast has superior expansion rates compared to forged, and the skirtless barrel design is plenty strong for the LT5’s power output. The top of the piston has a spherical bowl design, and when combined with the shallow 40cc combustion chambers, the compression comes in at 11.0:1, which happily runs on pump gas.

Usually, dished pistons in a small-block is a dead giveaway that it was a low compression, low-performance V8. But when combined with the 40cc combustion chamber, the LT5 had an 11.0:1 compression ratio.

Flow, Flow, And More Flow

The LT5 small-block was not GM’s first DOHC production engine, but it was their first DOHC V8, and those cast aluminum 4-valve cylinder heads and 16-runner induction system play a huge role in this 5.7-liter V8 outperforming its 2-valve 5.7-liter V8 baby brother. At 0.500 inches of valve lift on the intake, these heads flow 263 cfm, which is about 70 cfm more than the L98 heads or 50 cfm more than the Gen II LT1 heads.

The combustion chamber is a shallow, pent roof design that resembles a four-leaf clover and measures out at 40cc. Each chamber is filled with two intake valves that measure 1.54 inches in diameter and two exhaust valves that measure 1.39 inches in diameter. The valve stem is 8mm in diameter, unlike the common Gen I small-block’s 5/16-inch diameter.

Each cylinder had two intake ports to go with each intake valve. A primary port that was slightly smaller, and a larger, secondary port that only opened up for max performance when your foot mashed the pedal past half throttle or the engine went past 3,500 rpm.

Each intake valve has its own port and intake runner, for a total of 16 on this 8-cylinder engine. While both intake valves are the same size, one of them for each cylinder has a smaller diameter port, making it the primary port. The larger, secondary port has a butterfly valve in the cylinder head that stays closed until about 3,500 rpm, or your foot goes past half throttle. It then opens up, along with the injector for that runner, for full flow and performance. Each ZR-1 came with a secondary key that the driver had to turn on the dash for this “full engine power” mode; otherwise, the secondary ports and injectors would stay dormant, turning your high-dollar Corvette into a 210-horsepower ZR-1.

The LT5 has shallow, pent-roof combustion chambers that resemble a four-leaf clover.

The standard intake and exhaust lobes measured at 252 degrees of duration and 0.390 inches of lift, but the lobe on the camshaft for the secondary intake valve was larger than the rest as well, acting like a dual-cam-profile system. A small cam for smooth drivability and low-end torque, and the larger intake lobe to match the larger runner for high rpm horsepower.

This is an original cutaway display engine GM made when the LT5 was released. Here you can see that the valve covers pulled double duty as covers and camshaft caps.

The throttle body was also designed for low RPM drivability and high RPM airflow. It features three butterfly valves, two secondary valves measuring 58mm in diameter, and a smaller single primary valve measuring only 22mm. At low RPM and low throttle operation, the LT5 small-block breathed through just the primary, until you stood on the gas and the two secondary valves opened up for full airflow.

The throttle mounting pad on the upper intake plenum shows the three throttle blade openings. The smaller hole in the top center was for the idle-air control valve, the two big openings are for the secondary 58mm throttle blades, and the medium-sized hole in the bottom center is for the 22mm primary butterfly.

Electronic Control For The LT5 Small-Block

The 16-injection EFI system was unique, but it was for more than just high-performance. In the early days of electronic fuel injection, it was hard to regulate a small duty cycle at idle on a fuel injector that was large enough for 400 horsepower. By having a smaller primary injector and port, you get a world-beating supercar that can drive like a pedestrian economy car. The LT5 used four twin-tower ignition coils mounted under the intake manifold. This is a waste-spark design that can be found on other GM four- and six-cylinder engines of the time and provides higher-powered spark along with cleaner and more complete combustion. Because of the unique EFI system and ignition, the C4 ZR-1’s emissions and fuel economy were exceptional, allowing it to dodge the government’s gas-guzzler tax.

Can You Modify The LT5?

Most of today’s C4 ZR-1 owners keep their cars tucked away in the garage to keep the miles low, as often happens with most collector cars. But some have modified theirs for more performance, and companies that supply some performance parts. Like all other Corvettes, you can swap out the stock exhaust system for free-flowing headers and exhaust, as well as higher-flowing air filter and intake systems, combined with computer tuning.

But a few daring owners have taken it a step further and rebuilt their LT5 small-block into larger-displacement strokers with ported heads and custom camshafts. Building a 427 cubic-inch LT5 is never going to be a beer-budget endeavor, but a big-inch, 500-plus horsepower C4 ZR-1 is probably the most unique late model Corvette at the local cars-and-coffee meet.

This 1991 ZR-1 was sold at a Mecum auction in Dallas, Texas with a 675-horsepower 427 cubic inch LT5 under the hood.

All Good Things Must Come To An End

From 1990 to 1995, Chevrolet sold 6,939 ZR-1 Corvettes, with a majority of those sold in the first two years. When Chevrolet introduced the 300-horsepower LT1 for base model Corvettes in 1992, it closed the performance gap between the base model and ZR-1s, so most buyers weren’t willing to fork over the extra $27k for the ZR-1 option.

GM cites a few reasons why 1995 was the final year for the LT5 small block. First, to update the LT5 for the OBD-II system, which was going to be mandatory beginning in 1996, would’ve cost GM over one million dollars. Second, the LS1 was on the way for the C5 generation. The LS1 would produce 345 horsepower, cost several thousand dollars less to manufacture for each engine, and weigh a couple of hundred pounds less than the LT5. Third, the C4 ZR-1’s sales figures were dwindling. During its final four years, there were fewer than 500 ZR-1s sold each year, so GM wasn’t interested in investing more into such a low-volume seller.

This is all that’s left of the pre-production test mule for the next-gen LT5 that never was. Note the 8-runner intake manifold and cylinder heads that eliminated the 16-injector setup.

Lotus and the GM engineers were working on a next-gen LT5 that would produce up to 475 horsepower, but it never made it past the development phase. The push was to install it in the C5 generation for a C5 ZR-1 (that we never got), but the hood would need to be redesigned to be about 20mm taller, and the cost was not worth the trouble.

In the end, the LT5 small-block represented a bold leap forward in American performance engineering. An all-aluminum, 32-valve, DOHC powerhouse that pushed boundaries far beyond the traditional pushrod small-block V8 formula. While its production run was short and costly, the LT5 showcased what GM and Lotus could accomplish when innovation took priority over big profit. Today, it remains a technological marvel from a pivotal era. An exotic American V8 that bridged the gap between old-school muscle and modern precision, and a lasting reminder that sometimes, the best ideas are the ones that dared to break tradition.