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The “Original” LT5 & What Could Have Been

HOT ROD logo HOT ROD 2018-06-19 Barry Kluczyk
LT5-engine-Lingenfelter-gen-iii-001© Hot Rod Network Staff LT5-engine-Lingenfelter-gen-iii-001

The automotive world is filled with near misses, vehicles and technologies that made it this close to production, only to get the hook just before rolling into the center ring.

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We know of dozens of next-generation vehicles and even variations on existing models that never made it to the production line. We'll never know whether the four-door Pontiac Fiero would have made a splash had it gone into production, but the stillborn next-gen HUMMER H3 we saw about a decade ago looked pretty cool.

Curiously, the C4 ZR-1 belongs on the list of near misses. Engineers were warming up their torque wrenches for the third-generation DOHC LT5 engine when Chevrolet pulled the plug.

The original LT5, of course, was a technological marvel of its day. Famously developed jointly by Chevrolet and Lotus, and assembled by marine engine manufacturer MerCruiser, the 375-horsepower DOHC engine was universally praised for its high specific output and exceptional refinement. Unfortunately, that wasn't quite enough to sustain it.

It's a well-known narrative: Despite the initial stampede for the "King of the Hill" Corvette, sales quickly waned, thanks largely to its hefty window sticker and loss of visual distinction by 1991. It was a lamentable end to a project with stellar ambitions, but not everyone in GM's Powertrain division was shedding tears.

LT5-engine-Lingenfelter-gen-iii-001

There had always been some bruised pride that the engine wasn't entirely developed in-house or built by the company. That lingered through the end of the car's production run and by then; work was well underway on the revolutionary LS1 engine, so there weren't many champions at GM for the expensive LT5 that was built by someone else.

In fact, Chevrolet reportedly notified Lotus as early as 1991 of their intention discontinue the car, announcing they'd build it through 1995 and call it good after that. They'd made their statement and it was time to move on.

The last LT5 production engine was built in November 1993. They were the "Gen II" versions, which delivered 405 horsepower through head and cam changes. They also had strengthened blocks, with four-bolt main caps.

Graham Behan was a Lotus engineer who oversaw development of the Gen II LT5. He was also asked to helm a Gen III version of the engine that would address forthcoming emissions regulations while also pushing output significantly higher. Work on it had progressed beyond the drawing board to the tooling of prototype parts. The team was just about to bolt it all together when Chevy "eighty-sixed" the project.

"It was disappointing when the end came, because there was so much potential in the basic design," says Behan. "We had a team of young, eager and dedicated individuals who gave the maximum effort to bring the LT5 to life; and we felt there was more to do with it."

Behan eventually left Lotus, settling in the United States after a stint at GM's proving ground near Detroit during the LT5 days. He then went to work for Lingenfelter Performance Engineering, in Indiana, and in 1998 he received a package from a former colleague containing a variety of prototype parts from the Gen III project, including cylinder heads, pistons, an intake manifold and more. According to Behan, there were still machining chips in the cylinder head when he opened the crate, as the parts were unceremoniously boxed up and stashed. They were headed for the dumpster when that colleague rescued them.

Behan carefully stashed the one-off parts and cultivated a plan to one day build that next-gen LT5. The opportunity came nearly 20 years later, when Behan's boss Ken Lingenfelter greenlighted the project on company time. And more than simply building a dyno queen, Lingenfelter even purchased a 1993 40th Anniversary ZR-1 (1 of 245 built) to use as driveable showcase of the Gen III's capability.

"The parts had been created, but we never got to build the first prototype Gen III engine," says Behan. "As an engineer and I wanted to see whether the engine would achieve the target power originally conceived for it."

High power and low emissions

Despite the plethora of parts in the crate, there were still some elements missing that would prevent the prototype engine from being built absolutely as intended. For one thing the block was to change to eliminate hydrocarbon-producing fire rings in the bore liners, but there wasn't a block in the crate, so a Gen II version was substituted.

The Gen III cylinder head design was the most significant change, incorporating an all-new, emissions-driven combustion chamber design and redesigned ports. The chamber was larger and had a more conventional pent-roof shape than the high-swirl "cloverleaf" design of the Gen I/Gen II. Again, the change was designed to reduce emissions, but overall airflow through the new head design increased about 10 percent over the Gen I/Gen II, according to Behan. The heads also use only eight fuel injectors rather than the Gen I/Gen II's 16-injector design.

"We were working toward OBD II standards of emissions and engine control," he says. "The 'squish' pattern of the original cloverleaf combustion chamber design just wouldn't cut it for the upcoming emissions standard, but the change nonetheless proved effective at building high-rpm horsepower."

When it came to the camshafts, Behan had to compromise somewhat, because the box of parts contained a number of high- and low-lift intake and exhaust cams, which were originally intended to support an early variable valve lift system. Unfortunately, a pair of high-lift intake cams wasn't in the mix, so he made do with a quartet of high-lift exhaust camshafts, when nevertheless offered 0.445-inch lift and 239 degrees duration at 0.50-inch lift.

"The intake cams would have delivered a little more lift and little more duration," says Behan. "Overall, that probably cost us about 30 horsepower in the end, but we worked with what we had, using all the prototype parts at our disposal."

As for that variable valve lift system, the idea was to incorporate a finger-follower design with low- and high-rpm lobes, with the high-lift lobes supporting high-rpm performance. Lotus patented the technology and sold it a few later, where Porsche and Honda eventually developed variable cam systems based on it.

Spot-On

Despite not having the optimal camshafts or variable valve lift, the Gen III LT5 nonetheless wound out on Lingenfelter's dyno to a spine-tingling 7,200 rpm and produced 528 horsepower and 431 lb-ft. of torque. That was without any accessories dragging on the engine, a test procedure that duplicated the original SAE 24 tests about 25 years ago. According to Behan, the Gen II engine produced 445 horses without accessories during testing, which is about 10 percent more than the 405hp rating in the car. Likewise, the Gen III's output is pretty much spot-on for the 475hp target when the accessories are added to the engine, proving the engineers a quarter-century ago had their slide rules accurately calibrated.

"One of our guys worked out the numbers himself on a piece of paper, using a calculator," Behan says. "He projected the peak horsepower would come at 7,200 rpm and that was exactly where we saw it with this engine, along with horsepower that was also on the mark. That was very satisfying to verify after all these years."

Behan is philosophical about the engine and his involvement in the project, suggesting GM did what it needed to do, even if the potential for the Gen III engine wasn't realized.

"The original LT5 was a statement Chevrolet and GM had to make at the time and they succeeded," he says. "I saw mockups of the LS1 while we were working on the LT5, so I knew what the future held. A lot of good people put their best effort into the LT5 and the fact the engine was short-lived doesn't diminish the overall accomplishment. It was a hell of an engine with lasting influence."

As near misses go, this one was a heartbreaker. Vette

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