Flat Plane vs Cross-Plane CrankshaftsLast Updated August 4, 2019 | Meghan Drummond
Flat-Plane vs Cross-Plane Crankshaft
When the Voodoo engine was first announced with the GT350, one of its unique features was a flat plane crankshaft. Now the GT500 is coming with a crossplane crankshaft Voodoo variant. Although crankshafts aren’t the most discussed engine component, and certainly won’t be the most significant difference between the GT350 and GT500, they can make a significant change in how an engine performs and sounds.
What’s a Crankshaft?
Crankshafts are typically made from steel and sit underneath an engine’s cylinders. Each cylinder has a piston that corresponds to it, and these pistons have connecting rods extending from them. The connecting rods attach to crank pins, which are turned by the crankshaft. As the crankshaft turns, the pistons move up and down within their cylinders.
This piston movement is responsible for pulling the air and fuel mixture into the cylinder, compressing it, and then pushing out the exhaust gas. Without a crankshaft, internal combustion engines simply wouldn’t work.
As you might expect, there are several types of crankshafts. In the world of V8 engines, the two crankshaft styles that are seen most often are flat plane cranks and cross plane crankshafts.
The primary difference between these two crankshafts is the degree their pistons move in relation to the crankshaft. A flat plane engine’s crankshaft throws its pistons 180 degrees, creating a shape that from an aerial view would look a lot like a straight line. A cross plane engine throws its pistons in 90-degree angles, creating a “cross” design when viewed from above.
There are pros and cons of both crankshaft designs, and even though it’s a small shift, it creates a big difference in everything from performance to sound.
Cross Plane Crankshafts
Most V8 engines are going to have a cross-plane crankshaft despite this crankshaft’s slightly more complicated design. With a cross-plane crankshaft, each crank pin moves at a 90-degree angle. This should lead to an imbalance, but in order to correct that, counterweights are added that allow for smooth operation. Ultimately, this is both the cross-plane design’s greatest strength and weakness. The smooth, balanced feel is desirable for many drivers, but the counterweight’s primary objective is to function as weight. Though this may seem obvious, this weight is a major negative for the crankshaft's style. Not only are engines with cross-plane crankshafts heavier, but also the engine must deal with more rotational weight. Unlike static weight, rotational weight is more variable and puts more of a strain on the engine.
Because the weight is dangling off a part of the car that’s responsible for moving, some of the engine’s power is lost dealing with this additional weight.
Ultimately, it’s due to this rotational weight that cars with cross-plane crankshafts have a lower rev ceiling. Engines with a cross-plane crankshaft do also have a higher torque capacity than a similar engine with a flat plane crankshaft.
Almost all Mustangs that have a V8 have a cross-plane crankshaft, including the upcoming Shelby GT500.
- Lower Rev Ceiling
- Heavier Engine
- Low-end Torque
- Deep, Rumbling Engine Sound
Flat Plane Crankshafts
Flatplane crankshafts predate crossplanes, which makes sense because the design is significantly simpler.
In a flat plane crankshaft, there are 180 degrees between the throws. This means that by their very nature flat planes are better balanced than cross planed crankshafts and subsequently don’t require the use of counterweights to create balance.
Eliminating counterweights from the crankshaft doesn’t just eliminate pounds, it eliminates rotational weight and creates a more responsive engine. The GT350 can rev to 8200 RPM, significantly more than the 7,300 redline that we’re expecting to see on the GT500.
The downsides of the flatplane crankshaft are why you won’t see them in many V8s. The movement is joltier than that of a cross-plane engine, and as such, they tend to be a rougher ride. There’s also less torque, which for many is a dealbreaker.
The GT350 is the only Mustang right now with a flat plane, but they’re common in Ferraris and many other European sports cars like the McLaren.
- Higher Rev Limit
- Simpler Design
- Higher Pitched Engine Sound
- Less Torque
The Coyote engine, thanks to its modular design, can lend itself to many applications, but Ford’s done a great job of making sure that each variant remains unique, which meant that they weren’t simply going to shove a Voodoo into the GT500. Instead, we get a cross-plane Voodoo, a variant of the Coyote found in the “lesser Shelby” GT350.
The Voodoo is basically a Coyote engine, but with a few key differences that we should expect to see in the GT500 as well. The additional displacement (5.2L vs 5.0L) comes from increased bore size. As the purists say, there’s no replacement for displacement, but it’s interesting to see how the same displacement can behave differently based on something as simple as the angle of the piston thrusts from the crankshaft.
The Shelby GT350 has been lauded by many for the intense sound difference of its engine, and it owes that to its crankshaft. The GT500 will almost certainly sound more like a “traditional” Mustang, and though it sacrifices some high-end revving, it gains the low-end torque that many want from their muscle cars.
There are some other key differences between the two engines, most notably that the GT500 will have a supercharger attached to its Voodoo, giving it a competitive edge between the two Shelbys.
Cross planes are the default crankshaft style in American pony and muscle cars. They provide more low-end torque and have a deep rumbly growl that many associate with power and performance.
Flat plane crankshafts aren’t as common, but the GT350 has used a flat plane since its return to the Mustang lineup in 2016. Due to the lack of counterweights, flat plane engines have less rotational mass to account for in their crankshafts and are able to rev significantly higher. The GT350 can reach max revs around 8,200 RPM. Flat planes produce a higher pitched engine note that is rarer.
Sources: Car and Driver | Engine Labs | Jalopnik | Image Credit: AutoEvolution | Jalopnik | On All Cylinders | NASA
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