It’s a word you’ll hear a lot of if you’re at a vintage hot rod show. Does it have one? Did you swap one? It’s an engine that brought big power to new cars for over 20 years and lived on after that for another 20 in trucks and even longer for enthusiasts. It was named one of the best engines of the 20th century, but it is absolutely antiquated by today’s standards. It’s the Ford Flathead V8, and this is how it works.

I’ll start off by pointing out that Ford didn’t invent the flat cylinder head. It was far from the only company to use the design, and the style was surprisingly common into the 1930s. Cadillac even used it on their V16. But none of those had the staying power of the Ford. And since that’s the best known, that’s what we’re going to focus on.

On a modern, overhead valve engine, the air comes into the cylinder through valves that are located at the end of the tube that makes up the cylinder. There is a small combustion chamber, the valves are in that chamber, and the air comes in. When the air passes through the valves, it has an open path to the top of the piston. The air flows smoothly and everything works great. More or less.

The flathead was different. The air and fuel mix had to pass through a narrow channel from the valves to the cylinder. With two 90-degree turns. And that’s not even the worst part.

It starts with the design of the cylinder block. The cam and crank shafts look just about the same as on a modern engine. They’re in the same place. The cylinders are in the usual place too. But look at the two holes in the block beside the cylinder. No, those aren’t water passages. Or shotgun barrels. Those are the holes where the intake and exhaust valves go.

“But where does the fresh air come into the block and the exhaust leave,” you ask? Look in the vee of the block. See some more big round holes? Those are for the intake air. Now look on the side of the block. Those are the exhaust holes. You might be starting to understand why the flathead has gone away.

The intake air passes directly through the block, right beside the exhaust. Which heats the air even before it enters the cylinder. The exhaust has to work its way around to the side of the block so it can exit. The whole time, it’s heating everything up. It lead to massive overheating problems in a time where water pumps were marginal at best. In a modern head, the air comes in one side of the head and exits through the other. It never touches the insides of the block, except for directly inside the cylinder.

It gets worse.

The flathead is named for the flat look of the cylinder head. Like somebody trimmed a piece of 2×10 to length and just bolted it onto the engine. There aren’t even any moving parts, so in theory you probably could make one from a hunk of lumber.

Look at the thin passage from where the valves are to where the cylinder is. It’s narrow and short. But the intake air and exhaust have to flow through that tiny restriction to get to the cylinder.

It leads to poor airflow, a lousy combustion chamber shape, and it doesn’t exactly encourage high compression ratios. Ford’s Flathead V8s stayed mostly under 7:1, and most of them were well under. Compare that to the 8:1 of a small block Chevy V8 and the near 14:1 of modern engines.

Instead of being above the cylinder, the spark plug (or plugs since some of these have more than one per cylinder) was located between the valves. So the air/fuel ignites, then the flame has to travel all the way to the cylinder, and then it starts to push the piston down. So there’s as much force trying to lift the head in the combustion chamber as there is trying to actually move the car. Maybe more.

The engine was a revelation at the time. Since the alternative was, well, nothing. The Model T also used a side-valve (flat) head. There were many innovations in the design, like how the block was cast, and how the crank was actually steel, which was cheaper.

Despite the difficulty, hot rodders used the engine to make lots more power. Mostly because there weren’t any other widely available and affordable options. It’s where they learned to machine engines and design performance parts. As soon as overhead valve engines started to show up affordably, though, the flathead started to fade. But they are still used today. Because they’re cool. That big and flat cylinder head with about four dozen bolts looks awesome poking out from under the half-hood of your Ford roadster. It’s just that a 302 or an LS would be faster. And cheaper. Sorry, flathead fans.

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