A Note About Flat-Plane Crankshafts

With the release of the new Ford Mustang GT350, we now have a very special type of motor within our reach. Once reserved for exotics and race cars, flat-plane cranks offer distinct advantages in track and race applications, with some drawbacks for street and daily use. To understand these differences, let's first take a look at how these cranks differ from a standard 90-degree cross-plane crankshaft.

Cross-plane (90 Degree) Crankshaft

1,5

3,7                        2,6

4,8

Flat-plane (180 Degree) Crankshaft

1,4,5,8

2,3,6,7

Above, we have a visual depiction of how both types of crankshafts if viewed from the front. You need to visualize these as rotating assemblies, viewed head on, with rods and pistons attached. Each number corresponds to the cylinder in a V8. You notice that the top crankshaft, the traditional cross-plane, evenly distributes the rotating assembly by dividing the pistons across four 90 degree angles. Flat-plane cranks, on the other hand, stack four cylinders and associated rods/pistons in one of two opposed 180 degree angles. This means two things, one a benefit, and the other an issue, as far as road cars are concerned. The direct opposition of these rotating masses on the flat-plane versus the cross-plane means no counterweights are required, leading to much faster revving. If you've ever heard a Ferrari wail to its redline and back with the tap of the throttle, you can start to understand how this is an advantage. What this causes, however, is a noticeable vibration to the motor, as the directly opposed sets of pistons alternate in position, creating a back-to-forth shift in overall weight noticeable by the driver.

This unique arrangement of pistons also requires a unique firing order. Because there are only two "points" to the crankshaft, the firing order of a flat-plane motor always alternates cylinder banks. Whereas a cross-plane car distributes the firing order across a constant rotation of cylinders.

To illustrate:

Traditional Cross-Plane

BANG

BANG                                                                                                            

BANG
BANG

BANG                                                                                                            

BANG

BANG                                                                                                            

BANG                                                                                                            

Modern Flat-Plane

BANG

BANG                                                                                                            

BANG

BANG                                                                                                            

BANG

BANG                                                                                                            

BANG

BANG                                                                                                            

In this flat-plane car this effect not only scavenges exhaust gasses faster and more thoroughly, leading to increased power potential, but delivers an unmistakable sound. As opposed to the rolling, burbling note of a cross-plane V8, the flat-plane develops more of a shriek. That trademark Ferrari wail mentioned before is one such example of that sound. Traditionally associated with exotics, this change in sound will be a unique element of this new Mustang not previously found in any American production sports car.

Another example of race technology trickling down to the standard driver, the development of a flat-plane standard production car excites me. Offering fast revving, soaring redlines, a trademark scream and a visceral rumble through the steering wheel, a flat-plane V8 is a tremendous experience one once had to be able to afford a supercar to experience. Now, when these cars come up for discussion, or appear at your local meet, you'll have a bit more knowledge regarding what's under the hood.

See you on the road,

Alexander