What Is the Difference Between a Flat Plane and Crossplane Crankshaft?
If you are wondering the difference of a flat plane crankshaft vs. a crossplane crankshaft, this article reviews what they are, how they work, their differences and more.
What Is a Flat Plane and Crossplane Crankshaft?
V8 engines are great. They're big, they're powerful, and best of all, they're loud. Now if you have a really good ear, you may have noticed that typical V8s like the ones in Corvettes or Mustangs tend to sound different than the ones in super high-end sports cars like the Ferrari 458. That sound difference is a side effect of a difference in the configuration of the crankshaft.
There are two types of crankshafts that can be used in V8s. They are called flat plane or crossplane crankshafts. Crossplane cranks are used in most road cars today. Flat plane cranks are used in some racing engines and in all Ferrari V8s. You may have recently heard that 2016 Ford Mustang GT350 is going to use a flat plane crankshaft (where previous V8 Mustangs have used a crossplane crank). You may be wondering why that's such a big deal. As it turns out, which kind of crank an engine has affects engine balance, weight, size, rev limit, exhaust scavenging, and, yes, sound. We'll walk through the differences and explain how the whole thing works along the way.
How a Flat Plane Crankshaft Works
The flat plane crank actually came first and was used in the earliest V8 engines. In a flat plane crank, all the crank pins are separated by 180 degrees. So, when the first crank pin is at its highest position, the fourth crank pin is also at the top. The second and third crank pins are at the bottom. If you looked at the crankshaft head on, you would see a straight line up and down, hence the "flat plane" name. This is the same setup as the crank in a typical inline four cylinder engine, except, in this case, each crank pin connects to two pistons, rather than one.
How a Crossplane Crankshaft Works
In 1915 Cadillac and the Peerless Motor Company started work on the crossplane crankshaft design. Cadillac released a car with a crossplane V8 in 1923 and Peerless followed in 1924. When the first crankpin of a crossplane crank is at the top, the second pin is 90 degrees away, pointing to the right. The third one is 270 degrees away, pointing to the left, and the fourth is 180 degrees away, pointing down. If you faced the crankshaft dead on, you would see a cross shape. As a side note, Yamaha has produced a crossplane inline four cylinder motorcycle engine, but that design is exceedingly rare.
So, why did manufacturers switch to crossplane V8s?
Primary and Secondary Force: How Engine Balance Affects the Crankshaft
The crossplane V8 has better balance, particularly secondary balance, than the flat plane V8. We're going to have to get pretty technical to dive into the details here, so bear with us as we explain a bit about engine balance.
The pistons moving back and forth create reciprocating forces on the crankshaft. If these forces aren't balanced out, they can create vibration, which over time would wear on the engine.
There are two main types of forces that engine designers are concerned with, called, unsurprisingly, primary and secondary forces. Primary forces occur once per revolution. Secondary forces occur twice per revolution.
Flat Plane Crankshaft and Primary and Secondary Forces
The primary forces come from the motion of the pistons up and down. As we mentioned before, in the flat plane V8 or in a standard inline four, when pins one and four are at the top, pins two and three are at the bottom. That means, as the crank rotates, pins one and four are headed down and pins two and three are headed up. The upward and downward forces balance out. So, a flat plane V8 has primary balance.The trouble for flat plane cranks comes from the secondary forces. There is an imbalance of forces when the crankshaft is at 90 and 270 degrees. Now, if the piston was able to move straight up and down, then at 90 degrees, the piston would be halfway down its travel. The connecting rod goes around a circle, though. So, if you drew an imaginary line from the center of the crankshaft to the piston (piston height) and one from the center of the crankshaft to the crankpin (the crank radius), then your connecting rod piston height, and crank radius would form a right triangle.
Remember when you asked your high school trigonometry teacher "when are we ever going to use this?" Well, this is a good example. If you use your Pythagorean theorem, you'll find that the piston height is the square root of the difference between the square of the connecting rod length and the square of the crank radius. Keep in mind that the piston height has to be the long arm of your right triangle. If the piston height were smaller than the crank radius, then the piston would smash into the crankshaft at 180 degrees, which would be bad. Then you have to compare that piston height to the height at the top and bottom of its travel.
We'll spare you the gory mathematical details, but trust us when we say that the piston is actually past half of its downward travel when the crankpin reaches the halfway point of its downward travel. Similarly, when the crankpin is at 270 degrees, halfway through its upward travel, the piston will be below halfway through its upward travel. So, between 90 degrees and 270 degrees the piston travels more slowly than it does from 270 degrees, back to 90 degrees.
When crankpin one and crankpin four are turning from the top to 90 degrees, they are going down fast. At the same time pins two and three and turning from 180 degrees to 270 degrees, and they are coming up slow.
Perhaps you remember your high school physics better than your trigonometry. Force is mass multiplied by acceleration. Higher velocity means higher acceleration which means higher force. The higher force is pushing down and the lower force is pushing up, so there's an overall downward force.
As crankpins one and four travel from 90 to 180 degrees, they are going slowly downward. At the same time, pins two and three are going quickly up from 270 degrees to the top. The overall force is upward now. This will reverse again in the next quarter turn and one more time. So the vibration goes up-down twice per revolution. This is the undesirable vibration that Cadillac engineers wanted to eliminate with the crossplane crank.
Crossplane Crankshafts and Primary and Secondary Force
In a crossplane V8, when crankpin one is turning from the top to 90 degrees, pin two is turning from 90 degrees to 180 degrees, pin three is travelling from 270 degrees to the top, and pin four is turning from 180 degrees to 270 degrees. Pin one is going down fast, pin two is going down slow, pin three is going up fast, and pin four is going up slow. All the forces balance out. So the crossplane V8 has secondary balance.
What about its primary balance? When pin one is headed down, so is pin two. Pins three and four are headed up at the same time. That's two pins going up and two going down. The primary forces are balanced. There's one catch, though.
In the flat plane engine, the two outside pins move together and the two inside pins move together. When there's upward force in the middle, there's downward force on the outside, and vice versa.
In the crossplane engine, when the first pin is headed down, so is the second. The third and fourth pins are headed up. That means that the two front pins are going down and the two rear pins are headed up. Halfway through the revolution, this will be reversed. That creates a see-saw rocking force across the length of the engine.
Pros and Cons of Crossplane and Flat Plane Crankshafts
To avoid having the engine rock back to front, counterweights have to be added to the crankshaft. That adds weight. That means that the engine can't rev as high or as quickly. The crossplane crankshaft itself, because of the way it sticks out in all directions, also takes up more space inside the crank case. That necessitates a bigger engine overall.
Flat plane V8s tend to be smaller, higher-revving engines. The new engine for the Mustang GT350 will reportedly rev to 8,000 rpm. It's that high revving quality that performance and racing car makers are seeking, even at the expense of the vibration caused by the secondary imbalance. They figure their cars won't necessarily see everyday use or high mileage, so the vibration won't have time to take its toll. They can also use shorter piston strokes and special lightweight materials to reduce the force of the secondary imbalance.
Differences in Firing Order, Exhaust, and Burbling
On two V8s with different crankshaft designs, different pistons will be at the top and bottom of their stroke at the same point in a revolution. That means they'll have to have different firing orders. The simpler flat plane design allows for firing orders where the cylinders fire in cross-bank pairs. The front cylinder on the driver side is followed by the front cylinder on the passenger side, for example. All the cylinders fire in pairs like this.
Crossplane V8s can fire in a number of different orders depending on the manufacturer. Something all these orders have in common, though, is that they will always result in two cylinders in the same bank firing one after the other at some point in the firing order. When that happens, you get higher than normal gas pressure in the exhaust. The changes up and down in exhaust pressure over time create the familiar rumbling or burbling noise associated with big American V8s. Flat plane V8s have a steady exhaust flow that creates their characteristic wail.
Performance car designers prefer the more even exhaust flow, because it keeps backpressure from reducing the efficiency of combustion. Ford's LeMans winning GT40 used a specially designed exhaust manifold to get better exhaust scavenging from a crossplane V8. Some critics said it looked like "a bundle of snakes."
Although racing teams may prefer the smooth exhaust of a flat plane V8, many drivers have a soft spot for that classic crossplane burble. If you happen to be one of those drivers, then, hopefully you know now where it comes from.
