Curbside Tech: V8 Engine Crankshafts and Firing Orders – Good Vibrations

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– Posted on April 13, 2020

Chevrolet small-block cutaway

The V8 is ingrained in American culture.  It was the engine that dominated the North American automotive landscape from the 1950s to the end of the 1970s.  The V8 has long been a symbol of power, performance and smoothness, but this wasn’t always the case.  While V8 engines have been around since the beginning of the 20th century, the earliest iterations used a flat plane crankshaft, making them fundamentally different and rougher running than most of today’s V8s.  In the early 1920s, Cadillac and Peerless pioneered the smoother crossplane crankshaft, a design that revolutionized the American V8.

flat plane crankshaft geometry

The earliest V8 engines used flat plane crankshafts.  This looks like two four cylinder engines that have been joined together at the crankshaft.  Like a 4-cylinder engine, there are four crankshaft journals set 180 degrees apart.  The front and rear journals are completely opposite in position to the two center journals, forming a flat plane.  Also like a 4-cylinder engine, the flat plane V8s are prone to vibration.

crossplane crankshaft geometry

In search of a smoother V8, Cadillac and Peerless developed the crossplane crankshaft, first introduced by Cadillac in 1923 and Peerless in 1924.  A crossplane crankshaft has the front and rear crankshaft journals oriented in opposite positions, 180 degrees apart.  The two inside crankshaft journals are also 180 degrees apart. The front and rear journals are set at 90 degrees to the two center journals.  So the crankshaft has journals every 90 degrees, forming a cross shape from the front.

Ferrari Flat Plane Crankshaft

A modern Ferrari Flat Plane V8 Crankshaft


The crossplane V8 is significantly smoother than a flat plane V8 engine and it gave the V8 its reputation for smoothness.   The crankshaft geometry eliminated vibrations of the second order.  This crankshaft design also gave the crossplane V8 its distinctive exhaust note as it required a unique firing order compared to a flat plane V8.  The design was adopted by other manufacturers and it was used by the majority of V8 engines.  Some manufacturers, in particular European high performance makes like Ferrari, continued to use flat plane crankshafts, for they offer some advantages in high RPM engines.

Flat Plane V8 Piston Forces

On a flat plane V8 engine, the inertia forces of each piston are cancelled out. In this diagram the two outside pistons are moving up to and reached top dead center and the two inside pistons are moving down and reached bottom dead center.


Vibrations of the first order are caused by the inertia force produced by the piston mass as it moves up and down in the cylinder.  The maximum force occurs when the piston is at top dead center or bottom dead center.   A flat plane crankshaft doesn’t have vibrations of the first order, as there is always a counter force from another piston which results in a net force of zero.  As the one piston reaches top dead center, this force is countered by piston immediately adjacent reaching bottom dead center. Each of the eight pistons are paired up with an opposite, resulting in no vibration of the first order.

Crossplane V8 Forces

This diagram shows that the first two pistons are moving down after passing top dead center, while the second two are moving upwards after passing bottom dead center. These forces create a moment around the center axis of the crankshaft.  When the crankshaft rotates 180 degrees, this diagram reverses. These forces create vibrations of the first order that cause the crankshaft to vibrate in a seesaw action as it rotates.


The journals at each end of the crossplane crankshaft do not move together, which causes vibrations in the first order.  As the first crank journal travels down from the top of the cylinder, so does the second crank journal, but the third journal is travelling up from the bottom along with the fourth journal. So, one end of the crankshaft has net force upwards, and the other a net force downwards. This creates a force that attempts to rotate one end of the crankshaft around the center of the engine, much like sitting on one end of an unoccupied seesaw. Of course, when the crankshaft rotated 180 degrees these forces reverse. So as the crankshaft rotates it generates seesaw effect, vibrating each end of the crankshaft up and down. These forces can easily be countered.  By using heavy counterweights on the crankshaft that oppose the pistons inertia force as they move up and down, the net force will be zero.  This eliminates the vibrations of the first order.  The downside to the heavier counterweights is that it causes more rotation inertia compared to a flat plane crankshaft, making a flat plane more advantageous in high RPM engines.

The large counterweights used on a crossplane V8 engine counteract the forces shown above.  Using counterweights eliminates the vibrations of the first order.


A flat plane V8, however, has vibrations of the second order.  The piston rod geometry dictates that a piston will travel at a higher velocity when on the top half of its travel compared to the lower half. When a crankshaft journal is at 90 degrees or 270 degrees from top-dead-center, the piston is actually below the halfway point in the cylinder. This means that the piston travels at a higher velocity on the top half of its travel than it does on the lower half as it covers more distance over the same time. As the piston accelerates to the higher velocity, this creates more force (remember F= ma).  Due to the piston locations on a flat plane V8 crankshaft, the net velocity of the pistons is not zero.  Just like a 4-cylinder, this difference in piston velocity causes vibrations of the second order, non-sinusoidal vibrations, as it spins.

A crossplane crankshaft does not have any vibrations of the second order, as the net velocity of all the pistons is always zero.  For each piston movement, there is always a corresponding piston movement that is in the opposite direction at the same velocity, counteracting the forces each creates. With 90 degrees between each crankshaft journal on the crossplane V8 and it being a 4-cycle engine, each cylinder should fire once over 720 degrees of crankshaft rotation.  If we divide 720 degrees by 8 cylinders, it means every 90 degrees a cylinder should fire.  However, the crossplane crankshaft layout limits the order in which the cylinders can be fired.

Cylinder numbering is not consistent between manufacturers. Ford is on the left, Chevrolet on the right.


Before firing orders are discussed, there is one caveat; not all manufacturers use the same cylinder numbering convention.  Ford, in particular uses, its own convention.  Most V8s number the left front cylinder as number 1 cylinder with the left bank the odd numbers, 1-3-5-7 and the right bank the even numbers 2-4-6-8.   Ford V8 engines have the right bank slightly forward of the left bank, so the front right cylinder is labeled as number 1.  However, unlike the other manufacturers, Ford labels the right bank 1-2-3-4 and left bank 5-6-7-8.  Some other manufacturers also label the right front as cylinder number one, but unlike Ford, most often the right bank is numbered as 1-3-5-7 and the left bank as 2-4-6-8.

Nevertheless, to compare firing orders, the cylinder numbering must be standardized.  For simplicity sake I will use the conventional cylinder numbering, with left bank 1-3-5-7 and right bank 2-4-6-8.  In the chart below, I also listed the equivalent firing orders using other numbering conventions.   As seen below, there are only eight possible firing orders.  Of these, only the first three are commonly used.

Chart to compare firing orders for American V8 Engines

Note that all references in the main body to 1st, 2nd, 3rd and so on firing orders are those listed in this chart. Click to see a larger version of the chart.


Due to the layout of the crossplane crankshaft there will always be an instance when at least two cylinders on each cylinder bank are fired sequentially over 720 degrees of crankshaft rotation.  This is what causes that distinctive V8 sound we all know.  Two high pressure exhaust pulses are being forced into the exhaust manifold in succession causing a change in tone.  In comparison, a flat plane crank will fire evenly between each cylinder bank, left-right-left-right-left -right-left-right, which creates its own distinctive sound and better exhaust scavenging.

 

Listen to the difference in sound for a GM LS V8 engine with a flat plane vs crossplane crankshaft.

The bottom four firing orders are unique, as they are bank to bank firing orders.  This is where all cylinders in one bank fire, followed by all cylinders in the next bank.  These firing orders are not used, although some racers have experimented with them and found no advantages. They result in more vibration and a different sound from the others.

The first firing order is by far the most common on traditional American V8s.  Most GM, Mopar, Ford and AMC V8s used this firing order.  The second firing order was commonly used on many of the early V8 engines, including the Ford Y-block, Olds V8, and Buick Nailhead.   In the late 1960s Ford adopted the third firing order on the 351W and the 335 series engines.  This firing order wasn’t something new though, as was used by Cadillac on its 429 and its second generation V8 engines.  GM also used the third firing order for the LS series engines.

Cadillac used a number of different cylinder numbering conventions for its V8s. Although it looks like it used four different firing orders, there are actually only two firing orders here once the cylinder numbering is standardized.


The top four firing orders listed each have one cylinder bank where two adjacent cylinders fire in sequence. The other cylinder bank has two cylinders fire in sequence, but they are not adjacent to one another.  Changing between these four firing orders, changes which of the two adjacent cylinders on each bank are fired sequentially.  So for the first firing order, which is the most common for V8 engines, the number 5 and 7 cylinders (or the 7 and 8 cylinders on a Ford) fire sequentially.  These two cylinders are at the back of the engine block.  Both Ford and GM later switched to the 3rd firing order, where the number 3 and 1 cylinder (6 and 5 on a Ford) are fired sequentially which is at the front left corner of the engine.

Here are some other early V8s that weren’t included in the diagrams above.


By moving the location of the two sequentially fired adjacent cylinders, this can have effects on the cooling of the engine, the induction, and the crankshaft harmonics.  Firing two cylinders side by side causes extra stress on the crankshaft journals and main bearings, extra heat, and both cylinders may fight for fuel and air.  GM switched to the third firing order for the LS engines as it provided better cooling by having the sequentially fired cylinders at the front near the water pump where the engine is generally cooler.  It also resulted in better crankshaft harmonics and it improved main bearing durability/wear. Induction issues can result from two cylinders firing side by side, as both cylinders are fighting for the same air/fuel mixture in that same area of the manifold. However, these are can be resolved by intake manifold design.

These two different camshafts show that how the locations of the lobes change for different firing orders.  The larger camshaft is from a GM LS V8 and the other from a Chevrolet small block.


Engine builders have also experimented with these firing orders on existing engines, in particular on the Chevrolet small-block.  A camshaft swap is required, as it controls the cylinder firing sequence.  Obviously this requires a custom ground camshaft, although some are available off the shelf for popular engines like the Chevrolet small-block.  These swaps are called the 4/7 swap for the second firing order or the 4/7 2/3 swap for the third firing order.  The 4/7 2/3 swap can result in slightly more horsepower due to the better crankshaft harmonics, but only in extreme high horsepower/high RPM builds, so it’s not really advantageous unless you need every last horsepower in a racing type venue.

This is a set of 180 headers. These headers are routed so that each collector sees no sequential firing. This is for a Ford 1-5-4-2-6-3-7-8 firing order (1-8-4-3-6-5-7-2) and if you follow the paths of each cylinder you will see that the exhaust will fire between the left and right collectors evenly. This creates an exhaust sounds similar to a flat plane V8.


After reading this article, I hope you understand that the crossplane and flat plane crankshafts have a significant effect on how smooth an engine is, how it sounds and its ability to rev.  We also learned that the crossplane crankshaft is limited to eight firing orders, of which only three are commonly used.  These firing orders always have two cylinders on each cylinder bank that fire sequentially for every 720 degrees of crankshaft rotation.  By changing firing orders, one can move which two cylinders fire sequentially, which may have some advantages for a given engine design.  And it’s that sequentially firing of two cylinders on the same bank that creates the distinct V8 sound.