What Is A Flat-Plane Crank And Why Is It So Loud? An Explainer

One of the things you may have noticed among all those big, exciting numbers and words we used to talk about the 2015 Shelby GT350 Mustang were these three words: flat-plane crank. Many of you may be wondering just what the hell that means, and why you should give a rat's rectum. Here's what and why.

Most V8 engines we know and love today — the ones with that distinctive V8 burble-sound and those 'bundle of snakes'-types of exhaust manifolds — are not flat-plane crank designs. They're something known as cross-plane crank engines, and they're the norm enough now that this flat-plane business is actually news.


The fundamental difference in cross-plane and flat-plane crankshaft designs can be pretty well inferred from their names. Flat-plane cranks are, well, flat. If it helps, you can use this handy equation:


That means that the crankshaft is really a sort of 2D-shape: just the square-wave up-and-down style crankshaft you probably picture in your head when you think of a basic crankshaft. Inline (and flat) four-cylinder engines generally use this sort of crankshaft.

Cross-plane cranks are called that because when you look at them head-on, they seem to form a + or cross shape, where a flat plane will form a simple | shape. This is because cross-plane crankshafts have 90° angles between the crank throws, where the pistons connect. Here, this picture explains it all:

Illustration for article titled What Is A Flat-Plane Crank And Why Is It So Loud? An Explainer

Easy, right? Okay, it's easy to see how they differ, but the question is why.

Early V8 engines were all flat-plane cranks, because that's a simpler, more straightforward design. A flat-plane crank V8 is really like a pair of mated I-4s, and there's some issues with them, the biggest issue being balance and vibration issues, specifically secondary balance and vibration issues.


For the purposes of this quick explainer (there's much more detailed information available, of course) we won't go too deep into the details, but suffice it to say that a flat-plane V8 can generate a significant amount of vibration, especially on large displacement engines.

To correct this, the cross-plane crank was developed (way back in the 1920s) primarily to make everything nice and smooth. To achieve this, a cross-plane crank sets things up so that there's unevenly-spaced firings in each cylinder bank, but even cylinder firing over both banks, which keeps things balanced and gives cross-plane V8s that burble sound we all love.


The price of this balance and smoothness is weight and size. A cross-plane crank is inherently larger, since it's got crankpins on two axes, and it's much more heavily counterweighted and balanced, all of which require a larger, heavier crankcase to hold it all, which makes a less favorable overall weight and center of gravity.

Illustration for article titled What Is A Flat-Plane Crank And Why Is It So Loud? An Explainer

Also, the heavier, counterweighted crankshaft simply can't be spun/revv'd as quickly as a lighter one. These disadvantages all bring us to the reasons why a flat-plane crankshaft may be desired at all: it's lighter, it's smaller, and it can rev much, much higher.

How much higher? Some say up to 8000 RPM. Which, on a V8, I think we can all agree is bonkers.


Of course, this all comes at a cost of much more engine vibration, but if you look at all the advantages, they point to one very specific application: go-fastification, sometimes called "racing."


A racing engine that's light, small, and revs to ungodly numbers is far more important than one that has a bit of the shakes. Ask Ferrari — they use them on all their V8s. And, with modern, lightweight materials and interesting new designs, a surprising amount of that vibration can be mitigated, as Ford has most likely done. They already proved they could make a surprisingly well-balanced 3-cylinder engine — it's very reasonable to assume many of those lessons would be applied to much larger powerplants.


A flat-plane engine can also scavenge exhaust from the cylinders in a more efficient way, which offers performance and efficiency boosts as well, with less complex exhaust manifolds. Interestingly, people have been thinking about all the advantages of the flat-plane crank in relation to Mustangs for quite a while, and some folks have predicted this at least as far back as last year. By our own Oppo folks, too!


If you'd like a more watchable explainer, this guy does a fine job:

As far as why the engine is so loud, I think it's really a combination of several factors: the higher revs is absolutely a factor, since it's just cramming more and more noisy power strokes into a smaller and smaller period of time. Also, the use of lighter weight materials throughout to counteract the vibrational forces is likely making things a bit louder as well, since thinner, lighter parts are probably less effective at deadening sound than a heavier, denser material would be.


And I'm sure there's some shrieking of the driver as he or she realizes this big V8 is revving up like a sportbike engine or something. That can get loud, too.

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Here's an idea for an article Jason:

Why do different kinds of engines sound the way they do? We all know that a V8 has that burble, but why exactly does it do that? Why does a Subaru flat-4 sound like it does? Why does a V12 have that gloriously smooth 'hum'?

I'm not asking in terms of exhaust tuning, but what about the number of cylinders, engine construction, etc leads to such variety of engine sounds.