An EngineLabs Exclusive: The Gen III LT5 That Never Was, Alive Again

301_1301In 1986, General Motors made an acquisition in the form of Group Lotus, the well-known British engineering and manufacturing firm. One of the very first collaborations came in the form of the Corvette’s engineering team seeking to develop the world’s fastest production car, based on the C4 Corvette and what would later become known as the first ZR-1 Corvette.

Group Lotus’ input for that project centered around the development of the LT5 engine, a dual overhead-camshaft, aluminum-block V8 powerplant that had many different features than the traditional small-block Chevrolet engine that powered Corvettes up until its development. The engines were produced and assembled by Mercury Marine.

The intricate design of the underside of the LT5 on full display.

The intricate design of the underside of the LT5 on full display.

One thing that remained constant between the traditional small-block and the LT5 were the block’s bore centers. Cylinder bores were 3.900-inch, and the engine used a 3.660-inch stroke to achieve its 5.7-liter displacement, whereas the traditional 5.7-liter used a 4.000-inch x 3.480-inch arrangement.

The GEN I LT5 as initially released in 1990 produced 375 horsepower, but by 1993 and the GEN II version of the engine, the power numbers jumped to 405 horsepower; the engine platform was retired in 1995 at the end of the C4 ZR-1’s lifespan.

As was the case with many projects taking place at GM in the mid-90s, the LT5 simply disappeared with the debut of the C5 Corvette, with reasons given based around the engine’s cost, weight, and other factors, rumored to be the fact that the engine was not produced in-house.

EngineLabs recently had an opportunity to speak with one of the Lotus engineers, Graham Behan, who worked on the GEN III program that never came to light despite outstanding performance from the prototype engines.

The target for this engine, which began testing in 1993, had 475 horsepower from a naturally-aspirated engine checked off on the top of the engineering sheet. GM cancelled the program, citing OBD-II as an obstacle. Official word from GM has always been that the LT5 would no longer meet emissions standards.

Behan mysteriously got his hands on some of the development parts in 1998  – we’re told they just “showed up”. He tried for several years to get the engine built without success.

Now, as an employee of Lingenfelter, Behan approached Ken Lingenfelter about doing a “what if” build for the 25th anniversary of the ZR-1 in 2015, and Ken gave the green-light for Behan to see the project through to completion. The engine will eventually go in a ZR-1 owned by Ken.

The great part? We’ve got exclusive details on this incredible project, so read on!

The completed engine on the dyno at Lingenfelter Performance. This is a true one-of-one engineering marvel.

The completed engine on the dyno at Lingenfelter Performance. This is a true one-of-one engineering marvel.

The Bottom End

It’s not like building a new engine block for this one-off project was realistic, so Behan is working with a 1993 ZR-1/LT5 engine block and crankshaft that came out of the 1993 ZR-1 owned by Ken Lingenfelter. The engine will be installed back into this vehicle.

As there were no changes to the design, the block and crankshaft were reused from the donor vehickem a 1993 ZR1 owned by Ken Lingenfelter.

At the time of original development, new cylinder liners were designed to make the GEN III meet the upcoming (1996) emissions requirements. The new liners did not have a fire ring on top. Behan says this trapped hydrocarbons and led to the LT5’s tendency to detonate; the GEN III engine could not have this issue for emissions reasons. New flat top liners were designed and used for the LT5, and the fire ring was moved to the headgasket.

“It’s all about having reduced hydrocarbons and crevice volumes,” he explains.

With the advancements in materials and design, Behan made the choice to go with an all-new flat-top piston design from Mahle, with a full floating pin and modern locks. Previous GEN II pistons were dished. The top ring land has been moved up also, and the engine has a 7,500 rpm target redline.

Interestingly, the top rings from Mahle are the same used in a modern LS1. Behan says, sarcastically, “I wonder where they (GM) got that design from.”

(Left) The original LT5 dished piston on the right, the flat-top Mahle piston used in this build on the left. (Middle) On the Mahle piston seen on left, the ring lands have been moved up to provide better combustion. (Right) Note the full-skirt design on the original LT5 piston on the right, compared to the modern piston design seen on the left.

The Heads And Valvetrain

The GEN III LT5 uses eight fuel injectors with a port under the intake runner, versus 16 injectors in the GEN II engine. This required redesigning the heads and intake. This was done to make engine OBD-II compliant; at the time GM told Lotus that the software/computer processing ability would not be available to drive 16 injectors after 1995. The 8-injector design required custom-designed heads.

“You design an engine from the combustion chambers out. We got rid of the traditional LT5 squish-lands in the combustion chambers for the GEN III design,” says Behan.

The target flow for the cylinder heads to achieve the required horsepower figure was 300 cfm on the intake side. These make it all the way to 330 cfm, approximately ten percent better flow than the GEN II LT5 could provide.

(Left) Checking combustion chamber volume. (Right) What good would a set of four-valve overhead-cam cylinder heads be without tuning up the ports?

The combustion chambers are approximately 14cc larger than GEN II LT5, and allow the engine to reach a compression ratio of 11.4:1. In the interest of reducing valvetrain weight, these valves are custom machined with 7mm stems versus the 8mm valves found on the GEN II LT5.

Since these parts aren’t exactly on the shelf at the local auto parts retailer, when the cylinder head needed oil restrictors, Behan used bleed screws from a Holley carburetor as a way to improvise and solve the oiling issues.

Even more importantly, these heads were initially slated to use variable camshaft timing, with a high pressure solenoid system which was patented  by Lotus. The cams were supposed to have two sets of lobes, a low lift for below 4,500 rpm, and a high lift for 4,500-7,500 rpm. With the demise of the LT5 program, the VCT system was licensed to Honda and others – this basic design is what became Honda’s famous VTEC, but could have been in the ZR1 in 1996.

The Low Lift VCT Design

Cam specs are currently .445-inch lift, with 239 degrees of duration at .050-inch lift. There were supposed to be higher lift cams for the intake for the VCT system – .030-inch more lift and 10-20 degrees more duration, but Behan could not get enough of the right cams or VCT parts, as he had left side cams only. Cams that are used in the project as spec’d above are 1993 model exhaust side high lift cams.

Behan speculates there’s at least another 30 hp available with the better cams, and better power under the curve for the ZR1.

Extensive machine work was required to get these heads to fit and work. This included cleaning up the bowls, the valves, and getting the right springs. Of course, once that was all finished, they found oil leaks due to porosity in the castings, so they welded, epoxied, and waved the magic wand in order to get this resolved.

The High Lift VCT Design

The Intake Manifold

Topping off the engine was not an easy proposition, either – the prototype intake that matched the heads was located. There was only one plenum made back then, and this is it. The intake required final machining as it was just the raw casting without bolt holes or any other finishing touches.

There is no throttle body available for this intake. For the dyno test (seen in the video below), they improvised by using a 90mm Mustang throttle body, and are working on having a custom unit built to go in the car.

Another one of one part - the lone surviving intake manifold.

Another one of one part – the lone surviving intake manifold. Note the single injector location at the base of each port.

Megasquirt contributed the control system and helped with tuning. In another parts-bin raid, the ignition system is from a Cadillac Northstar, since there were no OEM parts available.

The engine dyno’d 528 hp and 430 lb.-ft. of torque, with peak horsepower achieved at 7,200 rpm. Behan was concerned after seeing this that the engine would have had a hard time with GM’s durability test. They require going a few hundred RPM past peak; that would have sent the engine to 7,600 rpm, and with higher lift cams and VCT it could have been more like 7,800 or even 8,000 RPM.

The durability test was the reason why the peak power rpm changed for the 1993 ZR1, because of this GM standard.

The Engine on the Lingenfelter Dyno

The power numbers are in dyno cell trim, no induction on the car, no exhaust back pressure to deal with. Graham saysto figure in a loss of three horsepower for every one inch/hg of backpressure.

In this same dyno cell configuration the GEN II engine made 440 hp. Graham is confident that for the final verification, in installed trim, this engine would have easily met or exceeded the 475 hp goal.

Custom connecting rods were required, as the original pieces wouldn't work with the new piston design.

Custom connecting rods were required, as the original pieces wouldn’t work with the new piston design.

After the engine was cancelled for the C4, Lotus mentioned that it would fit the upcoming C5 with just a 20mm hood bulge. GM didn’t bite.

GM told Lotus if they could make a business case to sell 1,500 or more per year to another company, they could keep the tooling and sell the engine. Lotus shopped Mercury Engine, and even Rolls Royce, producing a NA and Twin Turbo version for Rolls, however they didn’t want it.

The project was shuttered and there was no more LT5 – until now.

Thanks to Don Creason and Graham Behan for their invaluable assistance in compiling this article.

About the author

Jason Reiss

Jason draws on over 15 years of experience in the automotive publishing industry, and collaborates with many of the industry's movers and shakers to create compelling technical articles and high-quality race coverage.
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