Inside the Shelby GT350R – What Is a Flat-Plane-Crank V-8? [Inside Automotive]

The 2016 Shelby GT350R made its debut Monday at the 2015 Detroit Auto Show.
The 2016 Shelby GT350R employs a high-revving, flat-plane-crank V-8 engine architecture.

In your opinion, which would sound better – the low-pitched, earth-shaking rumble and howl of a traditional V-8, or the high-pitched, barking shriek of an exotic V-8?  Why have Ferrari’s eight-pots almost always sounded like the latter, and why have American trucks and muscle cars always sounded like the former?

The answer (mostly) resides within the design of a V-8 engine’s crankshaft, or “crank.”  The crank’s job is to perform a conversion between reciprocating motion and rotational motion – essentially, it turns the reciprocating motion of the pistons into rotational motion to put power to the ground.

In the earliest days of the automobile, most V-8 engines employed a “flat-plane-crank” design, as it was much simpler in concept and execution.  In a flat-plane-crank engine, the crankshaft’s pins are placed opposite of each other in a vertical, 180-degree pattern.  For example, one side of the crank will fire cylinders 1, 4, 5, and 8, while the other side fires cylinders 2, 6, 3, and 7.

The flat-plane-crank design fires cylinders two at a time.  Image courtesy of Wikipedia.
Flat-plane-crank design. Image courtesy of Wikipedia.

The inherent issue with a flat-plane-crank design is the lack of balance.  Essentially, with only two weighted positions on the crankshaft causing two cylinders in each bank to fire at the same time,  it shakes and vibrates like crazy.  Thusly, automakers felt the solution was to develop a new “cross-plane-crank” design.  Instead of the flat-plane’s vertical, 180-degree orientation, the cross-plane placed its pins in a plus-shaped, 90-degree alternating pattern to fire a bank’s cylinders one at a time.

Problem is, the cross-plane design was inherently weaker, especially at certain points of its phase, and required heavy counterweights on the crankshaft to counter vibration caused by its own natural plane imbalances.  When balanced by counterweights, however, the cross-plane’s advantage outweighed the problem, with its ability to be scaled up to large displacements without causing destructive vibration.  However, due to its counter-weighted mass, it became a slow-revving design that could not speed up or slow down very quickly.

In most manifold configurations, the cross-plane design resulted in uneven firing between the left and right cylinder banks, which produced the traditional “burble” which we associate with the all-american V-8 engines found in muscle cars and pickup trucks.

The cross-plane-crank design fires cylinders independently.  Image courtesy of Wikipedia.
The cross-plane-crank design fires cylinders independently. Image courtesy of Wikipedia.

Granted, manufacturers could have used a pair of balance shafts (versus counterweights) to further drown out the cross-plane V-8’s harshness.  They deemed it easier and more cost effective, however, to simply use the cross-plane design with counterweights in a vast majority of their modern V-8 engines.

The benefits of a flat-plane-crank are less rotating mass (two “arms” versus four), and lighter weight (no counterweights), which result in a quick-revving nature and added strength during high-RPM operation.  In the Shelby GT350R, this allows the 5.2-L V-8 to wail freely up to its 8,200-RPM redline.  The downside is the increased complexity and cost in adding the balance shafts to the crank’s design.

Still, there’s a reason Ferrari has almost exclusively used flat-plane-crank V-8s in its eight-cylinder supercars.  While to many nothing can ever replace an old-fashioned, all-american V-8 rumble and howl, to others, nothing can ever replace the high-pitched, high-revving, barking shriek of a flat-plane-crank V-8.

What do you think?  Given the choice, would you be willing to pay a bit more and give up some refinement for a flat-plane-crank V-8’s inherent benefits?  Or would you be best served by a cross-plane-crank V-8’s high-displacement, low-RPM robustness?

Watch the full reveal of the new Shelby GT350R below, with a glimpse its flat-plane-crank V-8’s wail starting at 3:16: