Exploring the Engineering of the Corvette ZR-1 Engine

Starting to think that all the drivetrain engineering attention is going toward EVs? Consider the Corvette ZR-1. This 5.5-liter V8 is rated at an astounding 1064 horsepower and 828 lb-ft of torque. How is this engine, known as the LT7, so powerful? Chevrolet of Spring Valley offers some interesting highlights of this incredible V8.

Before the LT7 was the LT6, the naturally aspirated V8, which powers much of the rest of the Corvette model line. The LT7 is much more than a turbocharged LT6, but there are some base similarities. They have the same block casting, which means the 104.25-millimeter bore and 80-millimeter stroke. The dual-overhead-camshaft valvetrain is largely the same, with 45mm intake valves and 33mm exhaust valves. Both have the fuel sprayed into the cylinders from the exhaust side of the head to increase turbulence for an improved mix. The dry-sump oil system carries the same eight quarts, although there is an additional seventh scavenge stage in the ZR1 to keep the turbos lubricated. While this engine is a far cry from the classic Chevy small block, it has the same 4.4-inch bore spacing as the original Chevy small-block V-8 that first appeared in 1955.

Straight From the Engineer’s Mouth

Interestingly, the engine wasn’t originally planned to have over 1,000 horsepower. The initial goal was 850 hp. Here is the story according to Jordan Lee, Chevrolet’s Chief Engineer of Small-Block Engines:

“The C7 ZR1 was 755 horsepower and we figured 850 was a pretty good target. We did our analysis, we did our benchmarking, we did our computer simulations, and it looked like, yeah, 850 was going to work. Then we built our first engines and we put the first engine on the dyno. I remember it very distinctly. I was in the dynamometer running the engine, and the engine looked like it was loafing along. I asked, ‘How much power is it putting out?’ ‘Well, it’s about 830 horsepower.’ The wastegates were wide open, so we weren’t even building much boost. It was then we had an inkling this thing was going to make a ton of power.

The danger of overachieving on the power is that you overtax the vehicle. Is the cooling system designed to handle that much power? We want to make sure we don’t break the transmission or break the half shafts, so we have to run more analysis and more validation tests to make sure it’s up for the challenge,” Lee continued. “Nine hundred horsepower became 950, and when we were in the mid-900s, we all figured we had to go for four digits. We knew that the Dodge Demon 170 on E85 gets 1,025 horsepower. We wanted 1,026 on regular pump gas. The calibration engineers were in the dyno, we started making some runs, and then the dyno wasn’t happy. This engine was so powerful, we had no dynos early in the program that could handle the power. We had to buy two new dynos that were rated over 1,000 horsepower.”

Why Switch to Turbocharging?

Both the C6 and C7 ZR-1 models were supercharged, not because Chevrolet preferred supercharging but because the turbocharging system would not fit under the low hood of the front-engine Corvettes, even with a raised hood bump. The wedged-shaped C8 body created more room for the turbochargers in the mid-engine configuration, allowing the needed boost without the pull of engine power incurred by a supercharger.

To shorten the path of the boosted air, each turbocharger feeds its own cylinder bank and cools the air down with its own intercooler. Without any real intake of exhaust exchange between the two banks, each essentially acts as its own 4-cylinder engine until you reach the common flat-plane crankshaft. That crankshaft is similar to the one on the LT6, except with additional machining on the counterweights to make space for other beefed-up internals.

A Lot of Hot Air

The turbos contain 76-millimeter compressor wheels within ball bearings to produce up to 20 psi in normal operating conditions and as much as 24 psi if heat starts to impact power output. The engine monitors the compressors’ rotational speeds to enable them to spin closer to their mechanical limits, whereupon the blade tips move as fast as 1.7 times the speed of sound and deal with exhaust gasses as hot as 1900 degrees.

For some interesting perspective on the air running in and out of the engine, at full throttle, the two turbocharges could fill an Olympic-sized swimming pool in four minutes, while the exhaust emerging from the four rear tailpipes provides 37 pounds of thrust. To avoid predestination from the air pressure coming from the turbos, the tops of the pistons invert from convex to concave, which drops the compression ratio from 12.5 to 9.8:1. Camshafts and head castings are also bespoke for the LT7.

While the turbos supply the air, the fuel is supplied by a combination of both port and direct injection. Port injection is what you hear at idle. A mix of the two systems is used across the engine’s operating range, with all 16 ports open at full throttle.

A Legend?

Today, plenty of attention is paid to the high power of EVs, even if not especially for track-based vehicles. And those of us at Chevrolet of Spring Valley can’t help but think that the LT7 will become legendary as the pinnacle of V8 Internal combustion performance for production cars. But who knows? A decade or two ago, few would have predicted the LT7.