Gearhead Quandary: Be Flat or Be Crossed?

Most, if not all, of us know that deep rumbling sound of a V8-powered muscle car or even your garden variety V8-powered daily driver. There is almost something primal about the deep throaty sound of a V8. In modern times V8 engines are becoming more and more of a rare breed, with turbo-charged four-cylinder engines making more power than the V8s of yore; But this blog is not about the pros and cons of the modern small, high-power engines. This is about V8 engines.

In general, there are two types of V8 engines, and the difference lies in the crankshaft design and the dynamic changes that makes to the engine. The two basic designs are the Cross-Plane crankshaft and the Flat-Plane crankshaft.

In a Cross-Plane crank engine if you look down the end of the crankshaft the image of the crankshaft makes a cross.  There are piston throws every 90° of the crankshaft, so there are two pistons at 0°, two at 90°, two at 180°, and two at 270° (for a total of eight pistons). The “standard” American V8 engine utilized a Cross-Plane crankshaft and this type of engine accounts for most American cars and trucks with V8 engines.

In a Flat-Plane crankshaft engine, if you look down the end of the crankshaft the image of the crankshaft is flat. There are piston throws every 180° of the crankshaft, so there are four pistons at 0°, and four pistons at 180° (for a total of eight pistons). This type of V8 engine is typically associated with exotic/supercars. Flat-Plane crank engines are used in such cars at McLaren, Ferrari, Porsche 918 Spyder, V8 powered BMW M3, the Ford Mustang Shelby GT350 and the 2023 Chevrolet Z06 Corvette (all other Mustang and Corvette variants have Cross-Plane crank engines it’s only the GT350 and the most current Z06 that have Flat-Plan crank engines).

The two types of engines sound and drive very differently. Cross-Plane crankshaft engines have a lot of low-end torque, but the engine speed is limited, while a Flat-Plane engine has less torque, but relatively high RPM redlines, the sound of the engine is very different too, a Cross-Plane crankshaft engine rumbles while a Flat-Plane crankshaft engine screams.

Most people don’t have an opportunity to drive Cross-Plane and Flat-Plane crank engines for comparison, but I happen to be fortunate enough to have access to both. For a comparison, when driving my V8 powered pickup down the highway, the engine lobs along at about 1,500 revolutions per minute (rpm), however, while driving my more exotic daily driver down the highway, the engine speed is typically around 4,000 rpm (with a maximum allowable rpm north of 8,000 rpm).

The dynamics of the two engine design requires a little bit of engineering nerd-fu. There are two types of main reciprocating forces that cause vibration in a piston engine, these are referred to as the Primary Forces and the Secondary Forces. The Primary Forces occur once per revolution of the crankshaft and are caused by the inertia of the reciprocating mass (which is primarily the pistons moving up and down). The Secondary Forces occur twice per revolution of the crankshaft and are caused by the geometry and relative motion of the crankshaft and connecting rod and the inconsistent speed it imparts on the piston. Because of the geometry, a piston travels faster just before it reaches top‑dead‑center than it does just before it reaches bottom‑dead‑center. Because the speed of a piston near top‑dead‑center and a piston near bottom‑dead‑center is different, the forces that the pistons cause are not equal and therefore do not cancel each other out. The amplitude of the Secondary Forces is typically 25% of the Primary Forces. The reasons for the different personalities of the engines lie in how the two engine designs deal with these forces (vibration):

• In a Cross-Plane crank engine, large counterweights are added to the crankshaft to counteract the Primary Forces; however, the secondary forces cancel each other out. The combination of the counterweights' balanced primary forces and the naturally balanced secondary forces make for a very smooth engine; however, the downside is that the added weight of the counterweights limits the speed (rpm) of the engine and impacts the responsiveness of the engine because of the added inertia.  Additionally, because of the piston firing order, sometimes two pistons are firing in succession of the same side of the engine (Left Bank, Right-Bank, Right-Bank, Left-Bank…), this gives the Cross-Plane crank engine it’s uneven rumble sound and makes engine exhaust scavenging less effective.
• In a Flat-Plane crank engine the primary forces cancel each other out and therefore there is no need for the addition of large counterweights; however, the secondary forces do not cancel each other out. Because of the lack of large counterweights, the crankshaft is lighter, and the engine can safely reach much higher RPM’s; however, because the secondary forces do not cancel each other out the engine isn’t as smooth as the Cross-Plane crank engine. Additionally, because of the piston firing order (Left Bank, Right-Bank, Left-Bank, Right-Bank …), engine exhaust scavenging is more effective on Flat-Plane crank engine and the even exhaust flow give the engine it’s “screaming” sound track.

How the engines make “power” is a little different too, the basic equation to calculate engine power is:

Where:
Power is in Horsepower
Torque is in Pounds - Feet
Rpm - revolutions per minute
5,252 is a constant to make the units all work

Because horsepower is essentially torque x angular velocity (or revolutions per minute - rpm), there are two ways to make more power from any given engine:  1) increase the torque or 2) increase the engine speed (rpm’s).  And there lies the difference between the two engine types. Cross-Plane crank engines typically have higher torque output at lower rpms but operate at lower maximum speeds (rpms). Flat-Plane crank engines eagerly rev but typically have less available torque. There are several reasons for the different torque characteristics for the two engines. Flat-Plane crank engines typically have a shorter stroke than a Cross-Plane crank engine which can lead to lower torque output (but makes the engine rev easier), there is also the issue that designing an air inlet and valves to handle the high volume of air for the higher rpm’s can also lead to lower torque output. As in most engineering problems, there are design trade-offs.

There are pros and cons for each engine type.  The Cross-Plane crank engine is smooth running, has a lot of low-end torque and is very drivable.  The Flat-Plane crank engine is more responsive, smaller and lighter and has more power for the same displacement, it isn’t as smooth as the Cross-Plane crank engine and can be less “drivable,” especially in stop and go traffic.  You will typically find Flat-Plane crank engines in racing and exotic cars where the added performance gains outweigh the additional vibration and drivability issues inherent in a Flat-Plane crank engine.  Additionally, because of the higher operating speed (rpm’s), Flat-Plane crank engines can have a shorter life expectancy.

About the Author

Timothy B. Hatch, B.S.M.E., P.E. is a consulting engineer with our Houston Office. Mr. Hatch is a consulting engineer specializing in the evaluation of industrial accidents, the analysis of losses involving machinery and equipment, the assessment of damage, and the estimation of repair/replacement cost. You may contact Tim for your forensic engineering needs at thatch@edtengineers.com or (281) 463-4548.

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