Engines Exposed: The Flat-Plane V8


We explain the magic behind Ford's GT350.

Since Ford was the first automaker to mass-produce a V8, itwould make sense for them to adopt technology to improve its V8s. Insome cases, that means enhanced efficiency. The roar of an American V8 is a verydistinct sound. The loud purr of power and imbalance is intoxicating and irresistible. What makes it sound so great is a little sadthough. It’s directly related to the exhaust manifold and the crankshaft,and depends on the cylinder layout. While it might be like lions roaring frominside an engine block it’s really one of the most inefficient ways to design aV8 imaginable.

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As an example, we can look at the Mustang’s V8 from 2015. We know it as a 5.0-liter unit, producing around 430 horsepower and can do the quarter mile time (with a manual transmission) in about 12.8 seconds. Beneath the top end of the engine, lurks what’s called a cross-plane crankshaft. Ford V8 owners that use distributors will know the firing order is nothing short of ludicrous. Looking at the crankshaft and how it’s designed shines a lot of light on that inconvenient truth. It’s worked for a long time in American V8s. However, if the Mustang wants to compete with Ferraris, Ford needs to step up its engine game, which brings us to the new Mustang GT350. It uses a 5.2-liter V8 producing around 525 horsepower.

That makes it the most powerful naturally aspirated V8 Ford has ever made. There are a lot of things that make this engine cool, but possibly one of the best changes Ford has decided to make resides with its crankshaft.

We mentioned before how a cross-plane crank is one of the most inefficient designs for a V8. Because it’s inherently imbalanced, this requires counter-weights to be added making for a heavy block. It is also physically bigger, so it revs slower and is less responsive. There is a plus side to using a cross plane engine however, and that is its smooth operation. At no point in time is a piston is not firing. One of the firing orders for a cross-plane crankshaft is 1-8-4-3-6-5-7-2. That makes the pistons move R-L-L-R-L-R-R-L. With a cross-plane crankshaft, that movement changes with each different firing order. A flat-plane crankshaft is best described as two inline-four engines joined by the crankshaft.

The reason is because the flat-plane crankshaft allows the pistons to keep their movement the same on each bank, as they would be if they were in two separate inline-four engines. There are a few different firing orders for flat-plane crankshaft engines, but they all share the same piston movement, R-L-R-L-R-L-R-L. This design makes for a more balanced engine. Racing engines use this design because it’s lighter across the board, and creates more even exhaust pressure. No counter-weights are needed. Now we come back to the new Mustang GT350. It uses a flat-plane crankshaft, which means its better for racing. Does that mean it’s a born racecar? Possibly. Besides the crankshaft it also has a higher compression ratio (12:1).

It also revs higher than any other Mustang engine, at 8200 RPM. Now if only Ford would pony up for their GT350 and add some direct injection, it would be ready for some boost.

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