A Look At The C4 ZR-1’s Jewel-Like, All-Aluminum DOHC LT5 Engine

In 2017, it seemed that the new ZR1 would never arrive. Of course, back in the olden days, car magazines published their “New Cars” editions in August to stoke enthusiasts for the arrival of the new cars in September. It was an annual tradition that Corvette inventor, Harley Earl created. Fast-forward to the digital age and there was a non-stop flow of leaks, spy photos, and teasers. There was just no rest for weary Corvette fans!

Finally in November 2017, Chevrolet unleashed the hottest street Corvette in one of the world’s hottest places, Dubai, in the United Arab Emirates. Just like the Z06 Corvette, the ZR1 is a complete package and is just a few ticks away from being a street version of the C7.R, but with all the creature comforts imaginable, plus being available in coupe or convertible configuration, and with an 8-speed paddle shift transmission. Unlike the Grand Sport that can be easily mistaken for a Z06 (nothing wrong with that!), the new ZR1 has a dedicated front end, hood, side vents, and a rear wing. Under the hood, the new LT5 engine delivers 755-horsepower and 715 lb/ft of torque. At just 2,200-rpm, the LT5 has 600 lb/ft of torque. Yes, the new LT5 is a torque monster!

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The 1994 Corvette sales brochure has this dreamy image of the ZR-1. Image Credit: GM Archives

And in the spirit of Zora Arkus-Duntov’s “racer kit” options of the past, (1957 RPO 684, 1963 RPO Z07, 1967-1969 RPO L88, 1970-1972 RPO ZR1, and 1971 RPO ZR2) for customers that want to turn their new ZR1 into a serious track machine, there the ZTK Track Performance Package that includes: an adjustable carbon-fiber high rear wing, carbon-fiber end caps on the front splitter, Michelin Pilot Sport Cup summer-only tires, stiffer front and rear springs, and optimized Specific Magnetic Selective Ride Control. Yes, this is all very heady stuff that would never have happened were it not for the world-class C5-R, C6.R, and C7.R racing Corvettes. When it comes to building world-class street sports cars, a factory racing effort is essential.

But, to see how far we’ve come, let’s take a look at the first street ZR-1 that was made possible by Lotus Engineering, a company steeped in the fruits of racing. After the C4 Corvette was introduced and started kicking some serious butt in the SCCA Showroom Stock Series, Corvette Chief Engineer Dave McLellan set out to build a Super Vette. Since GM had recently purchased Lotus Engineering, why not use their racing engine expertise to make something special for the Corvette? Chevrolet Engineering oversaw the project and Mercury Marine, in Stillwater, Oklahoma was contracted to build the new LT5 engines because of their expertise in building high-performance, all-aluminum marine engines.

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Assembly of the racer-like engine was subcontracted to Mercury Marine of Stillwater, Oklahoma because of their extensive experience building aluminum performance boat engines. The Mercury Marine 21,000 square foot facility used state-of-the-art machine tools with extraordinary tolerance capabilities. After the block and heads were machined, they were examined with a Cordax measuring machine that checked every three-dimensional measurement in only 45 minutes. Next was a four-step assembly process.

First, the heavily-ribbed short-block was assembled with aluminum cylinder sleeves coated with Nikasil, a nickel-silicon alloy. The crankshaft and connecting rods were forged steel, and the pistons were aluminum. The five main bearings were held in place with an aluminum cradle that attached with 28 bolts. Next, the double-overhead-cam aluminum heads were assembled. Each bank had two camshafts, one for the intake valves, and the other for the exhaust valves. The valve stems were actuated directly off the cam lobes.

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During the third step, the assembled heads, valve train, and cam covers were installed. The fourth and final step saw the installation of the ignition hardware, alternator, complete induction system, and accessories. The electrical system was then computer checked, followed by a dyno test. The completed engine had a 14-minute dyno run for initial startup; break in schedules, and a full-throttle run to establish power rating.

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The new engine measured 350 cubic-inches, and from 1990 to 1992 was rated at 375-horsepower. For 1993 to 1995 ZR-1s, engineers hot-rodded the LT5, upping the power to 405-horsepower. Measured the old way; the “gross” horsepower was close to 500! Not only was it a nearly completely hand-made engine, the LT5 was the only official Corvette engine that was not built by Chevrolet.

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Chevrolet used the Morrison Speed Record ZR1 in their print ads. Image Credit: GM Archives

How was the LT5-powered ZR-1’s performance? Outstanding! Abner Jenkins’ “Mormon Meteor III” had previously held speed records for over 50 years! The Duesenberg-built, 21-foot-long streamliner was powered by a 1,570-cubic-inch Allison V-1710 (aka, Curtiss Conqueror) V-12, 750-horsepower aircraft engine! The production ZR-1’s engine had 350-cubic-inches and 375-horsepower! 

In March of 1990, with a race-prepared, but nearly stock ZR-1, Tommy Morrison’s team blasted the following records: 5,000 kilometers at 175.710-mph, 24-Hours at 175.885-mph, 5,000 miles at 173.791-mph, plus, four FIA International Category Class A-G2-C10 records.

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Even the Smithsonian loves racing Corvettes. Image Credit: Smithsonian Institute

Speed record racing wasn’t the only arena C4 ZR-1 Corvettes competed. Morrison Motorsports, Doug Rippie Motorsports, Kim Baker and a few others raced ZR-1s in the early 1990s with modest success. Production-based sports car racing has always been very difficult. In the classes with factory-supported European cars, it is difficult-to-nearly impossible to win a championship.

While the early C4 Corvette romped in SCCA Showroom Stock racing, the IMSA GTO and GT-1 classes were a whole other world. While none of the C4 ZR-1 Corvettes won any major races or championships, they were all privateer efforts, slugging it out with fully-supported factory cars. It was unfortunate that there wasn’t a factory “C4-R Racing” program.

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Here’s the Rippie Motorsports ZR1 at LeMans. Image Credit: www.DougRippie.com

From 1990 to 1995 only 6,922 ZR-1 Corvettes were sold. So why wasn’t Chevrolet able to sell more ZR-1s and why were they so expensive? “Price” was the major issue. In 1990, the Corvette cost $31,979 and the ZR-1 option was $27,016, making the 1990 ZR-1 start at $58,995. By 1995, the Corvette cost $36,785 and the ZR-1 option had ballooned up to $31,258, making a 1995 ZR-1 start at $68,043.

1990 was the ZR-1’s best year with 3,032 cars sold, 1991 saw sales drop by one-third to 2,044, 1992 saw a 75-percent drop to only 502 units, and 1993, 1994, and 1995 only sold 448 units each year.

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The Pirate Racing ZR-1 racer resides at the National Corvette Museum. Image Credit: www.ZR1.net

The nosebleed cost of the ZR-1 wasn’t just because of the LT5 engine. The ZR-1’s body widened from the doors back in order to cover what was then considered ultra-wide rear tires. Wheels, tires, and suspension were all upgraded to handle the 125-horsepower difference between the base 250-horsepower L98 and the LT5’s 375-horsepower. Corvette sales were almost half what they’d been in the mid-80s. Had the economy been better, and if Chevrolet would have been able to order more engines from Mercury Marine, the price would have come down. 

LT5 Engine Specs:

* Displacement: 350 CID
* Horsepower: 1990-1992-375 @ 5800 RPM, 1993-1995 – 405 @ 5800 RPM
* Torque: 1990-1992-460 LB-FT @ 4000 RPM, 1993-1995 – 385 LB/FT
* Bore & Stroke: 3.90” x 3.66”
* Firing Order: 1 – 8 – 4 – 3 – 6 – 5 – 7 – 2
* Compression Ratio: 11.0:1
* Cylinder Block: Cast aluminum,
1990-1992 – open deck 2-bolt per bearing
1993-1995 – open deck 4-bolt per bearing
* Cylinder Liner Material: Forged aluminum extrusion
* Cylinder Head Material: Cast aluminum, 2.44-cid pent roof combustion chamber
* Intake Manifold: Cast aluminum
* Intake System: AC Rochester, Extruded aluminum multech injectors
* Valve sizes: Intake – 1.54“, Exhaust – 1.39”
* Camshaft: Alloy cast iron, intake & exhaust-lift 0.39
* Camshaft Drive: Primary – silent chain, Secondary – duplex roller chain
* Valve lifters: Alloy cast iron, hydraulic
* Main Bearing: Cast leaded bronze steel-backed
* Crankshaft: Nitrided forged steel, 2.76 dia. main journal
* Connecting Rods: Forged steel
* Piston Material: Cast aluminum w/expansion control-spherical bowl
* Fuel Type: Premium
* Exhaust Manifold: Stainless steel
* Engine Weight: 596
* Engine Redline: 7200 rpm
* Manufacturer: Mercury Marine

Also, the LT5 was only used in the ZR-1, so it could not be amortized down by being available in other GM cars. It’s really very simple; if GM had used the LT5 or a variant of the LT5 engine in more cars, the cost would have been much lower. 

All the parts were literally thrown away. Yes, really! – Geoff Jeal, Lotus Development Team

Often times, within a large company, development programs run quietly in the background. Sometimes they come to fruition, most of the time they fizzle out. We saw recently that before Ford went ahead with the new Ford GT racer, a Mustang-based Le Mans racing program called, “Project Silver” was in development. The project was canceled after Ford determined that the racer would cost $250,000 each and had a serious aerodynamic disadvantage.

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Morrison Motorsports ZR-1 were fierce competitors in IMSA racing in the early-to-mid 1990s. Image Credit: www.CarGuyChronicles.com

For a brief time, there was to be a next generation LT5 to be used in the 1996 ZR-1. The website ZR1NetRegistry.com reported that, according to Geoff Jeal, a former member of the Lotus development team, they were working on a 450-hp/450/lb-ft version of the engine with full OBD2 compliance!

The engine would have new heads with porting, modified valve angles, and an improved combustion chamber. A new intake manifold was designed with heater pads to improve cold starting and vaporization, and all-new headers were made with heated catalytic converters. The new plenum intake replaced the existing speed/air density system with new twin mass air-flow sensors for improved fuel circulation.

The new parts were engineered with enough parts to hand-build 20 engines, when the plug was pulled. What happened to the prototype parts? According to Geoff Jeal, “All the parts were literally thrown away. Yes, really!” Sister publication Engine Labs, did a story of the buildup of one of these engines using some of the parts that didn’t make it to the shredder. 

Another interesting piece of LT5 trivia comes from Jerry Watts and the ZR-1 Registry, concerning the strength of the LT5 bottom end. An IndyCar twin-turbo version was built on the basic LT5 structure and produced nearly 1,000-horsepower!

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Haibeck Automotive Technology specializes in LT5 maintenance, repair, and refinishing.

LT5 engines are available on eBay for those looking for a replacement or something exotic for a project car. One seller is asking $4,899.99, plus a $3,500 core charge. LS1, LS2, and LS3 engines can be purchased for less, but they don’t look nearly as exotic. “Exotic looks” is going to cost ya!

If you are okay with the basic C4 platform, ZR-1 Corvettes are currently the performance Corvette bargain of the decade, with many selling for between $15,000 to $25,000, depending on mileage and condition. Looking back, the ZR-1 stands as the most advanced, most powerful of the C4 Corvettes. If you’re longing for a classic, exotic Vette, a C4 ZR-1 might be for you! 

About the author

Scott Teeters

Scott Teeters has been a contributing artist and writer for Vette Magazine since 1976, and in 1997 launched his Vette Magazine monthly column, The Illustrated Corvette Series. His stories focus on the historical, technical, and people aspect of the world of Corvettes, off and on the racetrack. He contributes to Power Automedia as a freelance writer.
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