How a V8 engine works

Only some of a V8's eight cylinders push at any instant. So why does it feel so smooth, always ready to pull? Two banks of four cylinders in a ninety-degree V, with the crankshaft running along the bottom between them. Follow one gulp of air, intake to tailpipe, as the crankshaft turns twice.

Here is the whole itinerary. Air enters through the filter, into the intake manifold, past the intake valve, into the cylinder — then out the exhaust valve, through the exhaust manifold and down the tailpipe. Pistons, rods, crankshaft, camshaft and spark plug are the machinery that drives it. Three waypoints lie ahead: two banks of four cylinders set at ninety degrees on one crankshaft; the four strokes, intake, compression, power, exhaust; and a firing every seven hundred and twenty degrees — one power stroke every ninety.

The crankshaft is the roundabout we keep circling. Start with the shape. A V8 has eight cylinders, arranged in two banks of four, set at ninety degrees to each other, forming a V, and all eight of them turn one crankshaft below. When you see a five litre V8, that number is just the total swept volume: five thousand cubic centimetres shared by eight cylinders gives six hundred and twenty-five cc each.

Now follow one cylinder through its four strokes. Intake: the intake valve opens, the piston travels down, and falling pressure fills the cylinder with air and fuel. Compression: both valves shut tight and the piston rises, squeezing that charge down to roughly a tenth of its volume, about ten to one. Power: the spark plug fires, the burning gas expands hard, and the piston is driven down.

This is the only stroke that makes power. Exhaust: the exhaust valve opens, the piston rises again, and pushes the burnt gas out. Four strokes, two turns of the crankshaft, seven hundred and twenty degrees. So each cylinder fires once per seven hundred and twenty degrees, and its power stroke lasts only one hundred and eighty of those degrees.

Half a turn. Eight cylinders share that same seven hundred and twenty. Divide it by eight and you get ninety: one firing every ninety degrees of crank rotation. The crank throws are set ninety degrees apart, and the firing order, something like one, eight, four, three, six, five, seven, two, staggers the two banks, instead of emptying one bank and then the other.

And because each push lasts one hundred and eighty degrees while the next arrives after only ninety, the power strokes overlap. On average, two of the eight cylinders are on their power stroke at any instant. The camshaft keeps all of this on schedule. It opens and closes the valves, and it turns at half crankshaft speed, one cam turn for every seven hundred and twenty degrees of crank, driven by the timing chain.

Eight cylinders across seven hundred and twenty degrees — that's exactly ninety degrees apart. Each power stroke lasts a hundred and eighty, so the next push begins before the last one ends. The crankshaft is never between pushes. Take a V8 turning at three thousand rpm.

Divide by sixty: the crankshaft turns fifty times a second. One full cycle takes two crank revolutions, so that is twenty-five cycles a second. Each cycle fires all eight cylinders, so twenty-five times eight is two hundred power strokes a second — one firing every five milliseconds. And it scales with speed: a thousand rpm gives about sixty-seven firings a second, three thousand gives two hundred, six thousand gives four hundred.

Compare it with the four-cylinder. Both charts count how many cylinders are pushing the crank, across one full turn. In the four-cylinder, each power stroke lasts a hundred and eighty degrees, and the next starts exactly as it ends — one push at a time, never two. The V8 fires every ninety degrees, so its hundred-and-eighty-degree pushes overlap by ninety: two are always driving.

That's why eight cylinders feel smooth and pull hard, even though only about two are firing at any moment. One more time, the whole loop. Air in through the intake valve, four strokes per cylinder, out through the exhaust valve. That's two crankshaft revolutions — seven hundred twenty degrees — a firing every ninety, and one hundred eighty degrees per power stroke.

So roughly two of the eight cylinders push at once, and the camshaft turns at half crank speed. Eight overlapping pushes, one every ninety degrees, are why a V8 pulls smoothly instead of lurching.