A DOHC and Flat-Plane Crank Explainer
Editor’s Note: The majority of this story originally appeared in October 2019, when the official C8.R reveal, and some boring homologation rules led our author on a speculative journey into the 5.5L V8 with dual overhead cams and a flat-plane crankshaft that he thought would be found behind the cockpit of the C8 Z06. After yesterday’s incredible tease, this information seems more relevant and likely to be correct than ever; enjoy!
The glorious soundtrack accompanying Chevrolet’s first public admission of a Z06 variant of their mid-engine eighth-generation Corvette left a lot of jaws on the floor, but it also sparked a great deal of curiosity. Why does that new version of America’s Sports Car sound like an exotic from Maranello? Well, friends, it all has to do with the all-new layout of the Z06’s V8 that already has constructors and driver’s championships under its belt. The C8.R engine destined for Z06 duty in the coming years will be the second production powerplant in Corvette history to feature dual overhead cams. While General Motors aficionados will remember this alternative to traditional small-block pushrods from the C4 ZR-1’s legendary LT5, Cadillac’s NorthStar program, and, more recently, in the short-lived Blackwing V8. It will also be the first product in GM’s 113-year existence to employ a Ferrari-style flat-plane crank inside its V8.
What are DOHC and FPC, and how do they differ from a traditional Chevrolet V8?
We will start with the slightly more familiar Dual Overhead Camshaft piece of the new engine equation.
Fancy Answer: A traditional Chevrolet V8 uses a camshaft located inside the engine block. This camshaft actuates long rods that go up through the block into the head, where they move (push) the rockers/valves. A DOHC setup, on the other hand, places two camshafts per cylinder bank (four total in a V8) within the cylinder heads, one operates the intake valves, and the other one takes care of the exhaust valves. This allows for less-restricted airflow, purges the inertia caused by the pushrods, and most importantly, allows the engine to rev higher! If you are looking for a surprisingly entertaining, more in-depth article on this, check out Jason Torchinsky’s explanation on Jalopnik. You may be also be asking yourself why GM has stuck with pushrods for so long if DOHC is so great? Road & Track’s Chris Perkins has you covered on that one as well.
Layman’s Version: Dual Overhead Cams let your engine rev higher, which adds excitement to the driving experience!
Next, let’s tackle that exotic Flat Plane Crankshaft
Technical information: The crank is a lobed shaft that connects to (and is rotated by) the pistons. Basically, it is where the engine’s “spinning” power meets up and starts to head out to the rest of the vehicle. In Chevrolet’s usual cross-plane design, the pistons are connected at 90-degree increments (right angles), meaning every time a cylinder fires, the crankshaft rotates 90°. In a flat-plane crank, the crank features 180° piston connecting points. FPCs are naturally more balanced and don’t require the use of counterweights like their CPC equivalents. The result is a more compact size, reduced rotational mass, and a lower point of inertia. Opposed to the 1-8-7-2-6-5-4-3 firing order that creates the signature burble of an American V8, FPCs play their eight-piece ensemble in a 1-5-3-7-4-8-2-6 order, causing a completely different sound to flow out of the exhaust.
FPCs for non-nerds: A flat crankshaft is lighter and better balanced than a regular cross-plane crank. This lets it rev higher and faster. This, combined with their unique firing order, creates a different sound than a typical V8.
If you can stomach a video that prominently features two Mustangs, Jason Cammisa put together a great video explaining some differences between FPCs and CPCs with his drummer friend a few years ago. Just make it through the first four minutes if you can over on MotorTrend.com.
The unit in the C8 Z06 is expected to carry the LT6 designation and mirror the C8.R’s 5.5L (336 cubic inch) displacement. Without Balance of Performance regulations to satisfy, the LT6 will be allowed to make more than the 500 ponies of the racecar. Expect somewhere in the neighborhood of 600-650 horses from the production version, an insane feat for a naturally aspirated V8!
The other side of the power coin, torque has, unfortunately, never been a hallmark of the FPC V8. The sublime Ferrari 458 Italia that GM has been benchmarking only has 70.8% as much torque as it does horsepower, while Ford’s famed foray into FPC territory with the 2015-2020 Shelby GT350 managed to push that figure to 81.56% with an extra .7 liters of displacement.
To longtime devotees of GM’s LS and LT V8’s, even the B- grade in our twist to horsepower assessment of The Blue Oval’s “Voodoo” V8 is laughable when the last Z06 had identical torque and HP figures; aka; a score of 100%. Fortunately, with another .3 liters of displacement over the Voodoo, the C8.R is capable of producing a peak torque number that equals 96% of its peak HP rating while revving around 2,000 RPM higher than the Corvette’s previous redline record-holder, the 2006-13 LS7 (which scored a 93% on our torque/HP test). If the road car can achieve more than 85% (510-552 lb-ft) in this metric, even the most faithful LS disciples will have a hard time bemoaning anything about the new powerplant. Luckily, we are just one season away from finding out all of the juicy details about the next great chapter of Corvette performance.
Bring on the 2023 Corvette Z06 Reveal!
[VIDEO] Chevrolet Confirms the 2023 Corvette Z06 to Be Revealed This Fall
With 2022 Corvette Details Announced, the C8 Corvette Z06 Will Likely Debut as a 2023 Model
[VIDEO] Comparison of the 9000 RPM Ferrari 458 to the Sound Made by the C8 Corvette Z06