An EngineLabs Exclusive: The Gen III LT5 That Never Was, Alive Again
Enginelabs
May 18, 2015
By Jason Reiss
In 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.
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 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.
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.
More at link
Enginelabs
May 18, 2015
By Jason Reiss
In 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.
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 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.
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.
More at link