Since its introduction in 1953, the Corvette has been the quintessential definition of the American sports car. It truly represents the American take on the sports car genre, with its stylish design, ample V8 power and outstanding performance for the money. Much like the US auto industry, Corvette has had its ups and downs throughout its history. The mid-1970s to the early 1980s was not a great period for performance, and Corvette also suffered during these years. By 1982 the Corvette was highly out of date and hardly near the forefront of sports cars. In 1983, Dave McLellan’s all new advanced fourth generation Corvette was released. The C4 finally allowed the Corvette to be competitive with contemporary sports cars from Europe and Japan.
By the mid-1980s performance was back and Corvette was in search of ways to keep up with the competition. Despite the all-new advanced design, the 1984 Corvette soldiered on with the 205-hp 350cid V8 with the antiquated crossfire throttle body fuel injection system. Things improved in 1985 when the venerable small-block Chevrolet 350 got a bump to 230 hp with the fitment of the more advanced multi-point Tuned Port Fuel-injection system. This L98 350 would later see further power boosts to 240 hp, 245 hp and finally to 250 hp with further refinement. Engineers could have easily made more power from the engine if there were no regulations. However, to meet contemporary emissions and fuel economy standards, Chevrolet engineers believed they had reached the limit for power for the three decade old small-block Chevrolet.
Nevertheless, Lloyd Reuss kept pressure on the Corvette engine team to come up with more power. A turbo charged V6 engine was initially explored as a solution. The smaller displacement would help meet fuel economy standards, while the turbocharger would allow the V6 to make more power the V8. However, after experimentation it was found that the NVH levels of the turbo six were unacceptable. The turbo V6 was abandoned, but it led to experimenting with twin turbos on the small-block 350. This proved to be a highly successful experiment, boosting power of the small-block Chevy to well beyond the capabilities of the remainder of the drivetrain. The experimental turbo Vettes could reach 180 mph.
Lloyd Reuss had other ideas though. He felt that the Corvette needed to be high-tech, and that turbo charging the ancient pushrod small-block Chevrolet V8 would hardly be perceived by the public as high tech. Reuss wanted a modern alternative – specifically a 4-valve, dual overhead cam V8. So, the turbo charging was abandoned, however, it was not lost. Callaway took over the program. The $26,995 B2K Turbocharged option package was offered through Chevrolet dealers but installed by Callaway from 1987-1991.
At this time, GM owned a stake in Lotus. Lotus engineer Tony Rudd and his team were designing a DOHC 4-valve 4.0L V8 engine for the stillborn Etna super car. Lotus had lots of experience designing high performance and race engines. So Reuss decided to meet with Tony Rudd of Lotus to use his expertise. Initial discussion was to design a DOHC 4-valve cylinder head that could be installed on the existing small-block Chevrolet V8. However, Tony Rudd believed this would limit the power potential of the engine to about 350 hp. He argued that a clean slate design could be engineered with better piston, cylinder, and cylinder head cooling, improving the engines resistance to detonation to make more power. He believed an all-new engine design could attain 400 hp.
So an all-new engine designed by the Lotus team was given the green light, meaning the quintessential American sports car was to receive a British heart. There were some stipulations from GM. Roy Midgley from GM insisted that the new engine retain the 4.4” bore spacing of the small-block Chevrolet. This would allow for the cylinder heads to be potentially retrofitted to the old engine in the future. The new engine was to also use the same bore and stroke as the existing small-block 350.
The 4.0L V8 Lotus had already developed seemed like a logical starting point for the new Corvette V8, but it was too large to fit into the Corvette. The challenge for the team was to design an engine with nearly 50 percent more displacement, but in a significantly smaller physical size.
Soon the new engine, later to be designated the LT5, reach the prototype stage. The engine block was a deep skirt design and was made of cast aluminum alloy. It used a nitrated forged steel crankshaft with 2.76” main journals. Connecting rods were forged steel. The cylinder heads were also cast aluminum and used a pent roof combustion chamber with a centrally located sparkplug. The valve sizes were 1.54” for intake and 1.35” exhaust, and the compression ratio was 11.0:1.
Tony Rudd was concerned about thermal management of the new Corvette engine, so he decided to use forged aluminum wet cylinder liners. The liners were freestanding and supported in place by the cylinder head and head gasket once they were bolted in place. They liners were coated with Nikasil, a nickel silicon coating, to minimize engine wear. The wet cylinder liner design required more space between the cylinders compared to a conventional one piece cast block. As a result the 4” bore was too large at the 4.4” bore spacing. Accordingly, the bore was reduced to 3.90” but that raised concerns that the 400 hp goal would difficult to attain. To maintain the 5.7L displacement, the stroke was increased to 3.66.”
The cylinder head design required high velocity coolant flow to help remove the boiling bubble vapours, in particular around the exhaust valves. However, the engine block did not need high flow coolant and the radiator could not take the high flow required in the cylinder heads. So the engineers developed a bi-flow cooling system which recirculated most of the coolant in the cylinder heads, and a smaller amount through the engine block and radiator.
Since the Corvette’s engine bay had limited space, Tony Rudd’s team used timing chains rather than the timing belt of the Lotus 4.0L V8. A chain allowed for more compacted camshaft sprocket diameters and would make the engine more compact longitudinally. The Corvette’s space constraints caused other design compromises. The exhaust valves were limited to an 11 degree angle and the camshaft bearings were made to integral in the valve covers. These changes allowed the cylinder heads to clear the frame rails. Nevertheless, even with these changes, a special procedure was required on the production line to install the LT5 into the Corvette.
In the days before variable camshaft timing, Lotus needed to devise a way to help meet emissions, and fuel economy standards. Their solution was two camshaft profiles, one for each intake valve. A mild set of camshaft lobes designed for low RPM use would operate one set of 8 intake valves below 3000 RPM, producing good low and mid-range torque. At 3000 RPM, the second set of 8 camshaft lobes would start to kick in for the second set of intake valves. This second camshaft profile was designed to be a more aggressive high RPM design. To make this work, the intake manifold had 16 runners, three throttle bodies and 16 fuel injectors, one for each intake valve. Below 3000 RPM, the intake valves fed by the aggressive camshaft profile were closed off by an upstream by a butterfly valve in the ports. Those corresponding fuel injectors are also disabled making that set of 8 intake valves ineffective. After 3000 RPM, the butterfly valves for the secondary set of intake valves are opened and the 8 injectors activate allowing the high RPM camshaft lobes to become effective.
This complex system allowed for the engine to have a broad RPM range and meet the emission, fuel economy and power level goals. It made for an extremely docile engine at low RPM, but it would pull hard to the redline. It also allowed for a “valet mode” which locked out the high RPM intake valves, significantly limiting the power and performance. However this system also required the largest and most complex computer Delco Electronics had made to that date.
The LT5 first ran on a dynamometer in May of 1987. The engine ran to 7000 RPM, but oiling system problems caused the bottom end of the engine to starve for oil. In focusing on making the engine compact, the cylinder heads were unable to properly drain back to the sump at high RPM. This required a major redesign to the oiling system and the addition of an oil windage tray. The redesign cost six months of time; however, the defective engines were still able to be used for emission certification as they operated fine when limited to 5000 RPM.
Once the oiling system problems were resolved, the LT5 produced 375 hp @ 6000 RPM and 370 lb⋅ft at 4800 RPM, just shy of the 400 hp goal. With the design finalized, the engine performed durability testing. GM had previously used a 200 hour full throttle test only for its small displacement engines, due the fact that those engines spend more time at full power than larger V8s. However, the LT5 was the first V8 subjected to the 200 hour test. Initially, it only lasted 50 hours, with cracks forming in the crankcase bulkheads. Cast iron main-bearing cap reinforcements were added and the engine was able to successful pass the 200 hour test.
This video shows the LT5 being produced by Mercury Marine. It also shows the Wet Vette, a LT5 powered boat.
Since production of the LT5 engine was expected to be very limited and it shared nothing with other GM production engines, production could not occur on a regular production line. The diesel engine divisions were considered to handle production, but ultimately it was decided to go outside GM. Mercury Marine was chosen due to their experience with manufacturing aluminum marine engines and their ability to meet the tolerances of the new engine. They also had experience with GM automotive engines, which were used by its Mercruiser division.
The LT5 engine was used exclusively in Chevrolet’s new “King of the Hill” Corvette, called the ZR-1, introduced at the Geneva Auto Show in 1989. Along with the new high-powered LT5 engine, the ZR-1 had special rear body work to accommodate larger 315/40ZR-17 tires, a 450-ft-lbs rated ZF 6-speed transmission, zero scrub front suspension with larger 11.5” heavy duty brakes, improved rear suspension geometry, adjustable Bilstein shocks, more robust wheel bearings and lower spring rates to give a lower frequency of ride. The ZR-1 had supercar like performance and while cheaper than many Euro exotics, it did come at with a hefty $27,016 premium over a standard Corvette. Road and Track obtained a 0-60 time of 4.9 seconds and a quarter mile went past in 13.4 seconds, certainly on par with the 1960’s Big Block Corvettes, and better than many of the contemporary European exotics.
In the spring of 1990, GM used the LT5 to attempt to beat some FIA (Federation International de l’Automobile) speed and endurance records. The LT5 powered Corvette set an impressive seven FIA records, for the Group II, Class 11 category. They were as follows:
- 100 miles at an average speed of 175.600 mph
- 500 miles at an average speed of 175.503 mph
- 1,000 miles at an average speed of 174.428 mph
- 5,000 km at an average speed of 175.710 mph (World Record)
- 5,000 miles at an average speed of 173.791 mph (World Record)
- 12 Hours Endurance at 175.523 mph
- 24 Hours Endurance at 175.885 mph for 4,221.256 miles (World Record)
For the 1993 model year the Lotus made modification to the LT5’s cylinder heads to improve power. The power rating increased to 405 hp at 5,800 RPM and 385 lb⋅ft of torque at 4,800 RPM, meeting the original 400 hp goal. The main bearing caps were also strengthened by switching to 4-bolt caps and the EGR system was revised to improve emissions.
Production of the LT5 engine ended in 1993, but Mercury Marine produced enough engines to supply Chevrolet Corvette ZR-1 production until it ended in 1995. ZR-1 production peaked in 1990 with 3,049 produced, but then steady dropped off from until its demise. A total of 6,939 ZR-1s were produced. While it offered supercar performance, the cost of admission was significant. Furthermore, the 300hp LT1 Gen II small-block introduced in 1992, closed the performance gap considerably between a base Corvette and the ZR-1. Despite its good performance, like other C4 Corvettes, ZR-1s also suffered from quality control and ergonomic issues. An updated LT5 was initially explored, capable of more than 450 hp, but the idea was abandoned. Instead GM focused its efforts on an all-new engine designed in-house to replace the aging small-block Chevrolet. Engineers used the experiences learned from the LT5 to help develop the highly successful Gen III small-block Chevrolet, the basic design of which still remains in production today. The so-called “LS” series small-blocks have since well surpassed the LT5 engines in power and performance, but for a long while, the King of the Hill Corvette was the one with the British heart.
5,000 miles at an average speed of 791 mph (World Record) no kidding?
or is it 5,000 miles at an average speed of 173.791 MPH mph (World Record)?
Used to live by the Mercruiser plant in Stillwater, Oklahoma that built these engines. A very interesting, if logical, outsourcing. They still had a ZR-1 displayed in the lobby of the plant last time I saw it about 15 years ago, before Mercury closed it down.
Thanks Vince. I always had a crush on the 1990 ZR1 and it’s LT5 engine
I remembered the ZR-1 being on the cover of every car magazine for months and months, it seemed. But I had never understood the effort that went into this engine, so thanks for this dive.
I got to check out that very boat in person.
I am not a boat person, but that is a cool boat. Impressively fast too. I like how they used a ZR-1 to tow it in the video.
For those that haven’t seen the video above on the manufacturing of the ZR-1 engine, it’s a worthwhile watch.
And now Mercury Marine is going full into 4 stroke engines. They are designing, engineering and building their own high tech V8 outboard engines. I also found a video too on how they are offering crate engines for racing cars.
GM does a SHO.
It was a bit disappointing to me, as this really isn’t an SBC anymore, and unlike the past there was no interchange. But it was the ’90s, and it did give the Corvette a lot of PR for a while, even if the actual numbers were modest.
400 horsepower and sub five second to zero to sixty times in the early 90s are modest?
For 1990 it was definitely impressive, The Ferrari Testarossa had 380 back then, but we have definitely been spoiled by horsepower in recent times, and in the case of the ZR1 while it was technically impressive, the C5 with its simple LS engine was also achieving sub-5 second 0-60 times, low 13 second 1/4 mile passes with 345 horsepower, and later would surpass that with 385/405 via the z06 5-6 years after the ZR-1 ended production.
Modest isn’t really a knock, for any engine to make that much power in the early 90s and meet emissions targets with fixed cams is a highly impressive feat. I don’t know of the ZR-1’s aftermarket tuning background but I imagine there is a huge amount of untapped potential with that engine
Sales numbers were modest. Despite all the PR.
Yes, expensive supercars do tend to have modest sales numbers.
You missed my point, again. I meant that overall Corvette sales didn’t seem to benefit from all of the PR around the ZR-1. In fact, total Corvette sales dropped in 1990 considerably, and continued to drop for several years.
Do you now get what I was originally getting at? I certainly hope so.
It’s interesting that the engine used in the latest Corvette is still a pushrod activated OHV design. The engine can be lighter, much more compact, and therefore be “packaged” within the car more easily. It gets the job done without an exotic specification. DOHC engines are very bulky on top and they were difficult to fit within an existing body shell. As a motorcycle enthusiast I’ve always had a keen interest in engines.
The Cadillac NorthStar engine ticked off all the enthusiast boxes for me when I bought mine. All aluminum construction, 4.6 liter, fuel injection, dohc, four valves per cylinder, distributor less ignition. It produced 295 hp. with 300 ft/lbs. of torque. Now that power output is slightly exceeded by the new Mustang’s base turbocharged, 2.2 four valve dohc four!
My old ’97 Jaguar has the marque’s best straight six cylinder motor, only produced for three years. 4.0, fuel injection, all aluminum, dohc, four valves, coil on plug ignition produces 245 hp. Of course it’s output was exceeded by the supercharged XJR model and later V8s and now turbo charged models.
Even my humble ’96 Mustang GT can boast of a sohc aluminum head 4.6 V8 engine with fuel injection, distributor less ignition and the basic design was continually improved over it’s lifetime. This robust design has been one of Ford’s best motors.
If I pop the hood on my ’51 Jaguar Mark VII I’ll find the fourth year of production, aluminum head, DOHC, twin carb version of that famous motor. That motor was the sensation of the immediate post war period. I hopefully should be able to get it running soon. For now, I just enjoy looking at that beautiful mill.
Finally my garage tour comes to a end with a motor that is very interesting, but seems totally extravagant. The 5.3 liter, all alloy, sohc, fuel injected, V12 in my XJS. This is an engine that was a bit before it’s time. The presence of a big alarm clock of a distributor in the middle of the motor was always a problem. Thankfully it has electronic ignition. But it forced a convoluted intake system that was terrible when it was equipped with four carbs in the E Type series three. Fuel injection was a major improvement, but eliminating that distributor would have allowed the intake to nestle in the V like any reasonable design. Still it was viable enough to sell over 120,000 units over it’s long production life. The XJS won the Cannonball run one year, being capable of cruising at speeds of 130 mph. for long periods of time.
Happy Thanksgiving to all. Stay safe. Keep the faith.
I will never understand why, given the ZR-1 got a new and unique rear treatment, they didn’t integrate the CHMSL into the rear valance like it was on the convertible. It couldn’t have been a regulatory issue as the ZR-1-like later C4 convex rears had the light mounted in the same location.
From a functional perspective the roof location is probably the safest place for it to be, C4 Corvettes are very low, even among lower sedan traffic, the CHMSL is probably about the same height as it was in the lower back window in many sedans of the era.
As stated in the article, the ZR-1 required wider bodywork at the rear to accommodate the larger rear tires and wheels. The body on a ZR-1 flares considerably wider in the rear. This meant that the ZR-1 could not use the existing rear end of the base Corvette. Since at that time it was already known that the 1991 Corvette would get a restyle which included the updated rear end, they decided to use the new rear end design on the ZR-1 one year early. However, the CHMSL was on the roof for all other 1986-90 Corvettes coupes, and so that’s why the 1990 ZR-1 had it in that position.
For 1991, when the Corvette was restyled, for whatever reason they moved the CHMSL to the rear fascia for all other Corvettes. My guess is this was to save money, since the coupe and convertibles could use the same rear end. However, they weren’t going to design another rear end for the ZR-1 just to move the CHMSL to match the other Vettes. For the 1991 and newer C4 Vettes, having the roof mounted CHMSL made for a very easy way to spot a ZR-1 in traffic when they roamed the street.
Great article, Vince schooled me on the ZR1 engine some time back where I alluded to it being derived off the SBC, I certainly now see the error of my ways! It’s interesting that it was Ford that really ran with the DOHC V8 concept afterwards, the original InTech 4.6 was conceptually quite similar down to the butterfly activated 16 runner intake system(minus the extra injectors and valet mode) Supposedly the cam lobe profiles are even different per valve too, but I never verified. Besides the Northstar GM pretty much abandoned it for the LS engine, which essentially phased out the Northstar in Cadillacs as well, it was Ford with the Coyote that really realized the fruits of the technology, with 100 more horsepower with .7 less liters with variable cam timing.
These were interesting cars growing up as a kid, Corvettes were kind of passé to us in the 90s and by the time my friends and myself had mutual interests in cars the C4 was in its waning years with the transition to the C5. But the ZR1 in effect was almost like a myth to us, and despite the widebody, which I still have a hard time noticing on the 91 up models (the 90 is easy with the bubble butt), that was THE corvette. Funny, 375 horsepower seems so modest now but that legit top tier Exotic territory back then, and under that stunning C4 clamshell hood that engine was almost as impressive as looking at a mid engined Ferrari mill under the louvered lexan.
Thanks Matt, I appreciate the feedback. I didn’t know that the InTech used the similar design to the LT5. While people may criticize the LS engines for the old pushrod technology, they are excellent performers and I think it was the better route GM to move away from complex designs like the LT5. And as mentioned by someone else, the packaging of the LS is very compact. While the Coyote makes great power for its small displacement and is an excellent engine, the LS motors are considerably smaller and simpler.
Like others, I also had a crush on the ZR-1 when it came out. It certainly was a great halo car that breathed some life back into the Vette. I remember going to the Chevrolet Dealer with my Dad when we got our first one locally – a 1990 Black ZR-1 with a red interior. There was a big ad in the paper encouraging people to come and see it in the showroom. They were very cool cars at the time.
Yes I agree, with the later coming of the LS series engine within the very same decade the LT5 proved to be kind of high tech for the sake of being high tech, it’s an amazing piece of machinery for us gearheads, and it’s far more visually impressive to look at than the LS with its plastic covers over utilitarian ignition coils, but it certainly was a mission creep from the smallblock V8s inherent compact nature. 4.6 and Coyote swaps rarely happen outside of retrofits in other Ford’s as a consequence, a novelty swap really, and it frankly took Ford too long to truly compete with the LS engines via the OHC technology (not helped by the kneecapped displacement)
“GM owned a stake in Lotus”, that’s why they didn’t go back to Cosworth (for instance) to do the engine. Cosworth was vastly more experienced at V8 DOHC 4V engine and cylinder head design than Lotus. Toyota even had more time in the V8 DOHC 4V game, at that time, but that wouldn’t have gone over too well with anyone unless they really hid the details. Still they got a good engine and I love this generation of Corvette, as long as it’s a ZR1. It’s a shame the best GM engineering is not really GM engineering.
The ’96 LT4 car matched the LT5 car’s performance but only made 330hp. The. LT4 was underrated and the LT5 overrated. Had GM put their “Hot cam” in the LT4 it would’ve been quicker than the upcoming c5. Couldn’t have that lol.
The Corvette’s space constraints caused other design compromises.
Actually, it has to do with the body and chassis marriage process during the assembly. The powertrain, drivetrain, and suspension systems are assembled as a single unit (lower chassis). The steel frame and glass fibre body are built in two separate processes before being combined as one unit (upper chassis). The lower chassis is lifted up to the underneath of upper chassis. During the marrying process, the engine must be able to clear the narrow steel frame in the front. Thus, the constraints and limits placed upon the engine design.
See the time-stamped YouTube videos:
During the marrying process, the engine must be able to clear the narrow steel frame in the front.
Which are space constraints I was discussing in the article. I am aware of the installation process, I just didn’t get into the details in the article so it was simplified to one sentence. Thanks for sharing the video.