V8 Crankshaft

There are two common crankshaft design for V8 engines: flatplane and crossplane. Most road car V8's, including the M5, use a crossplane crankshaft design.

In the crossplane design, opposing cylinders in the V8 attach at all four points of the compass. In the flatplane design, they attach to two-points of the compass, each repeated once.

The pistons attach as shown below for the 90 degree V8 configuration.

The important thing in crankshaft design is to make sure there is no imbalance that leads to a shaking car. The piston heads, pins, and connecting rods are all heavy, and they move very quickly. In a single-piston engine, this would cause vibrations.

As the crank goes around its circle, the piston moves straight up and down, causing an up and down shaking. As well, the crank pin and a component of the connecting rod are thrown side-to-side, causing a side-to-side vibration. All crankshafts have some counterweights to counteract this side-to-side action of their own rotating parts (in other words, the crank pin and the big end of the connecting rod need to be balanced by a weight on the other side). But the rotating counterweight can never counter the motion of a piston, pin, and small end of the connecting rod traveling up and down.

Thus the point of having multiple cylinders and clever crankshaft designs is to make all of these shakes cancel out, leaving a silky smooth engine with no vibrations at all.

In the flatplane design, for every piston moving down there is an opposing piston moving up to cancel the vibrations. As well, this is symmetrical about the middle of the crankshaft, so neither is there any net torque on the crank.

However, this is not true of the crossplane V8. While the ups and down all cancel each other out, there is a net back and forth rotation about the middle of the crankshaft.

Fortunately, these can be cancelled out by the strategic placement of counter-weights on the crankshaft. While all crankshafts have counterweights to balance their own internal rotating weight, putting in place a large enough weight to also balance the pistons and rods is unique to the V8.

Is is actually a fortunate accident of a 90 degrees V8 engine that these weights do the job. Normally, one could not expect a rotating weight to balance an up and down motion. However, in a 90 degree V8 where two pistons are connected to one crank pin, the weight does double-duty for both pistons at once, and can cancel things out properly.

Once these weights are in place, the crossplane V8 also has first order balance, as does the flatplane, but at the expense of the added rotating weight which means the car cannot rev as high or as fast as the flatplane.

So if they are both the same, but the crossplane is heavier, why ever use the crossplane? The answer is that while neither arrangement has any first-order imbalance, the flatplane has a second order imbalance, whereas the crossplane does not. I'll now explain where this second-order imbalance comes from.

How the up-and-down location of the piston head (at P) relates to the crank angle A from TDC (Top Dead Centre) is actually quite complex, and is given by the formula above, solving for quantity x (l and r are constants of the engine).

One implication of the formula is that the way the piston moves with the crank angle is slightly different at the top of the motion than at the bottom.

In the graph above, looking at the blue plot, I am plotting the piston position in inches around the midpoint of its motion. Notice the shape of the top hump is considerably broader than the shape of the bottom one. To make this clearer, the red plot is the blue one 180 degrees back and flipped over so you can directly compare peak to trough. The green line plots the difference between the red and blue plots. If they overlaid perfectly, it would be flat. It is clearly not flat, and this is where second-order imbalance comes from.

Another way of looking at the red and blue plots above is that I am plotting the position of the two pistons whose motion is opposing one another. This is made clearer by flipping over one of the lines.

Imagine the blue is the motion of the cylinder on one end of the crankshaft, while the red is the opposing motion of a cylinder on the other end of the crankshaft. They almost cancel each other out (no first-order imbalance so no extreme shaking), but not quite as shown by the wavy green line which is the engine shaking up and down as a result of the second-order imbalance. It is called second-order because the frequency of the vibration is twice the crankshaft rotational frequency (note how the small wavy line waves twice as fast as the larger piston ones).

The flatplane V8 has no first-order imbalance, but it does have a second order imbalance that cannot be practically corrected. This is why flat-plane designs can only be used with small light V8 engines found in some sportscars with smaller displacements, shorter strokes, and lighter piston heads and rods (such as F1 racing cars, Ferrari V8s, and the Lotus Esprit V8).

Lotus Esprit flatplane V8

Even given the lightweight engine construction, they will tend to shake you up a bit, but folks say they enjoy it!

With the crossplane design, the second order imbalances actually cancel each other out completely, so it runs a lot smoother as a result, even in larger and heavier engines (such as the M5's relative to those other sportscars). This property of the crossplane crankshaft was only discovered in the 1920's (by Cadillac and Peerless at the same time - they share the patent), and before then all V8s were flatplane, and shook!

Other than the extra weights, there is another problem with the crossplane relative to the flatplane, and that is the piston firing order.

The ideal firing order for any "V" engine is to alternate cylinders firing on first one bank and then the other. This would spread out the heat and smooth out the exhaust pulses which leads to better exhaust flow. The flatplane firing order is like this. Very predictable right bank, left bank, right bank, and so on.

However the crossplane V8 does not have the ideal firing order. There must always be a cylinder fired on the same side at least twice, once per bank, and at least once in the firing order adjacent cylinders will fire in sequence, which leads to excess heat issues on the wall between those adjacent cylinder that must be designed around with extra cooling. The numbering and firing order (in brackets) for the cylinders in the M5 is as follows.

(3)4--------8(4)
(6)3--------7(7)
(8)2--------6(5)
(1)1--------5(2)
      FRONT

There are therefore heat issues in the wall between cylinders 1 and 2, and both 1,2 and 8,6 are on the same side leading to uneven exhaust pulses. With the crossplane design this is completely unavoidable.

This firing order also gives a V8 its distinctive "burbling" sound owing to the uneven exhaust pulses coming from the right and left cylinder banks each of which is typically fed through to its own tail pipe: ti-ta-tii-ta-ti-taa. Other than the sound, this leads to a slowdown in the exhaust which backs things up and robs power and fuel efficiency.

The S63 engine, however, uses a cross-bank exhaust manifold that take exhaust pulses from both cylinders banks and feeds them to the turbos, and then out the exhaust for a very smooth exhaust flow.

While the crossing over of cylinder banks to exhausts is not unheard of, it requires a complex exhaust manifold, as the exhaust valves are typically on the outside of the V, and crossing them over early is a messy business. Some of the problem can be alleviated with an H or X exhaust that crosses the two exhaust banks over half-way to the tailpipes. This alleviates some of the problem but is still not ideal.

In the S63 engine, the exhaust valves are flipped to the inside of the V, and all exhaust directed to the turbos. The exhaust pulses are crossed over right inside the V before they hit the turbos, all in the space of a foot or so, and the smoothed exhaust outputs of the turbos can each go to its own tail pipe.

So in the M5 you do not hear the V8 rumble nearly as much as a result of this clever design, and it solves all the efficiency problems associated with the crossplane V8 exhaust.