Back in February when we reported future Corvette learnings from a GM insider we dubbed “Deep Burble,” our technical staff pushed back hard on the notion of a new 5.5-liter flat-plane-crankshaft V-8 engine. Flat-plane-crankshaft V-8s vibrate like two four-cylinder engines strapped together. That’s why the performance-engine world largely held the line on displacement of such V-8s to around 4.5 liters, until Ford pushed the envelope with the Ford Mustang Shelby GT350’s 5.2-liter Voodoo engine. Ford’s bold move came at the expense of some durability issues, however, so when we were invited to ride along with development engineers in the 2023 Chevrolet Corvette Z06, our very first questions were all about how the sports car’s 5.5-liter flatty solves the vibration question—especially with its 8,600-rpm redline.
Flat-Plane vs. Cross-Plane Cranks
We have a great explainer article outlining the differences and benefits of each crankshaft design here, but in a nutshell, a flat-plane V-8 crankshaft looks just like an inline-four crank with its throws 180 degrees apart and with room for two rods on each throw, whereas the cross-plane crank divides up the throws at 90 degree angles. The earliest V-8s all used flat-plane cranks, until in 1923 Cadillac introduced a cross-plane crankshaft with counterweights that quelled the nasty secondary vibrations (those that happen twice per revolution) inherent in a flat-plane crank for a smoother, more luxurious experience. Sports cars care less about comfort and more about squeezing the most power out of each drop of fuel, and flat-plane cranks help do that. But the bigger the pistons and the faster they move, the greater the secondary vibration, which can ultimately prove damaging to the engine and things that bolt to it. Hence why a V-8 as large as the Z06’s 5.5-liter is so surprising a choice for a modern engineering squad.
All-New Oversquare Design
Chevy manages the 5.5-liter LT6 V-8’s shake primarily by minimizing piston speeds, which the team accomplished by specifying a big 104.3-mm bore and a short 80.0-mm stroke. That results in a 103-mph peak piston speed at redline. That’s lower than the redline peak speeds of the Porsche 918 Spyder’s 4.6-liter (104 mph), the Ferrari 458 Italia’s 4.5-liter (109 mph), and way lower than that Shelby Mustang’s 5.2-liter (114 mph). Ford’s flat-plane crank was adapted to fit an existing engine architecture, which is probably why it ended up barely oversquare, with a bore and stroke of 94.0 x 93.0 mm.
Chevy was not constrained by any legacy architecture. The Z06’s LT6 engine shares nothing with the production small-block V-8 (save for its 4.4-inch bore-centers measurement). It does share a whole lot with the Corvette C8.R racing engine, thanks to what Chevy claims is the closest collaboration between any street and racing program yet. And the deviations mostly allow the street engine to outperform the racing version, given how it isn’t constrained by homologation rules and restrictor plates.
Clean-sheet benefits aside, another key countermeasure to the flat-plane-crankshaft shaking conundrum is use of forged titanium connecting rods from Austria-based Pankl Racing Systems pushing forged aluminum short-skirt pistons. An aluminum harmonic balancer removes a bit more rotational inertia and further assists the LT6’s ability to blip the revs to 8,600 rpm and back to idle in a heartbeat or two, sport-bike style.
Good, Good, Good, Good Vibrations
The LT6 engine still ends up vibrating more than a V-8 with a cross-plane crank does, but among GM engineers there was no appetite for adding the power-robbing weight and inertia of counter-rotating balance shafts. Similarly, there was considerable reluctance to fit soft engine mounts that might isolate the vibration at a cost to the sense of immediacy that is to be this frenetic engine’s principle attraction.
Instead, the Z06 development team decided to embrace the vibes as a part of this car’s very different, very hardcore track mission. The team did expend considerable effort engineering the many bits and pieces that mount to or immediately around the engine to prevent them from rattling or humming. Heat shields and other thin metal components were the trickiest to calm, but even the transaxle bellhousing gets additional ribbing for strength and fastens to the engine with longer bolts. Trust us, the sound and vibration from this engine—not to mention the resulting 670 hp at 8,400 rpm and 460 lb-ft of torque at 6,300 rpm—is well worth any and all such countermeasures.
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