Why Chevy Switched to a Flat Crank in the Corvette – Jalopnik

The Chevrolet Corvette was already a star and went supernova when the mid-engine version debuted in 2020, but that was about six years ago, and the location of the car’s engine isn’t as important as it used to be. These days, there’s more fuss about the type of engine it uses: a 5.5-liter V8 with a flat-plane crankshaft is standard in the Corvette Z06, ZR1 and hybrid ZR1X, with the latter capable of covering the quarter mile in under 9 seconds as the fastest, most powerful production Corvette ever. With that twin-turbo V8 and hybrid assist combining for 1,250 horsepower, the ZR1X can also outpace many aftermarket builds. It’s the kind of extreme performance that requires a cutting-edge engine, and that’s where the flat-plane crank industry comes into play.

The engine’s distinctive design provides a level of excitement for supercars, partly because the engine can rev so high. In the naturally aspirated Z06, which can deliver 670 horsepower, the redline is well above at 8,600 rpm; the ZR1 reaches its peak four-figure power at 7,000 rpm. Flat plane engines are also generally lighter and more compact than typical cross-plane V8 engines and better at removing exhaust gases. And the flat-plane V8s sound so distinctive that it’s hard to mistake them for anything else.

Of course, that’s all just the beginning. Let’s take a look at the nuts and bolts of flat-plane crankshaft engines to see how they do what they do.

When a reciprocating engine turns, those pistons move up and down in their cylinders. The crankshaft, in turn, is the first step in transforming that vertical motion into the rotary motion needed to get your wheels moving. Without going too deep into the details, the bottoms of the pistons are connected to the crankshaft, which resembles a specially designed metal rod that runs in the same direction as the engine itself. But it’s not just a purely straight length like a broomstick. Some straight pieces have been replaced by crank pins to which the pistons attach.

Imagine a very simple two-piston arrangement: when one piston is at the lowest point of the cylinder, the crankpin bump points straight down, so that the connection point is below the crankshaft centerline, and when the other is at the highest point, the crankpin bump points up. As the pistons move up and down, they push and pull the crankpins to rotate the shaft.

Expand that to a four-cylinder engine and we see the difference between cross-plane and flat-plane crankshafts. The first has all four crankpins offset 90 degrees from the next. So when they look straight at the end of a crankshaft, they form a cross pattern. In a flat plane motor, the crank pins are at a 180 degree angle, creating a flat plane.

Let’s put it all together by putting a few I4 engines together to make a V8. In a cross-plane V8, the crankshaft requires counterweights to help compensate for the vibrations caused by the movement of the pistons. That means the crank itself is heavier, so more engine power is wasted turning that axle instead of the wheels. The light weight of a flat engine allows it to spin faster to deliver more power. A heavier, cross-plane crank, which also includes a heavier crankcase, is one of the reasons why your daily driver can’t tour like a supercar.

Flat aircraft engines have second-order vibrations, which is a major reason why they are not as common on the street. But when racing, a little extra shaking is worth it for the extra power you get – and the same goes for street-legal Corvettes. Additionally, Chevy has made some concessions to reducing second-order vibration, such as shortening the piston stroke (but at the same time increasing the bore to preserve engine displacement).

Another factor is the firing order of the pistons. In a cross-plane V8, the firing intervals within each cylinder bank are unequal, while in a flat-plane arrangement the firing order alternates from bank to bank. This improves exhaust gas evacuation, using the exhaust gas flow to get more exhaust gases out of the cylinder while increasing the amount of fuel/fresh air entering. With all that, it’s no wonder so many people consider the C8 the best ‘Vette yet.