Why Are 16 Better Than 8, but 20 Wasn’t? More Valves, Explained

Eight valve, 16 valve, even 20 valve. When it comes to engines, for a brief period of time how many valves you had was one of the most important specs on the tin. Why would you want the puny 8v model when you could go even quicker with a 16v? And then, for a brief number of years, you could even get a VW or Audi with a 20v. So why is more v’s better? And what do they do?

When extracting motive power from gasoline inside an engine, getting air and fuel in and out is probably the most important single thing that you need to worry about. Forget spark, pack enough air and fuel in and you don’t need it – like a diesel. But you need to be able to get that air in. When you’re pumping the air, there’s always going to be one big restriction. One bottleneck. The place where there’s enough of an issue that it’s going to be the crucial place you need to work on to make sure there are air and fuel flowing.

In the case of the internal combustion engine, that place is at the bottom of the cylinder head. The valves. You need a strong, durable valve to keep the explosion in the cylinder. You also need it to be smaller than the piston’s surface area. And you need one to let air in and one to let air out again.

For decades, most engines had two valves. One for the intake, one for the exhaust. Simple, easy, effective. More or less. But there were limits. You see, you can only make a valve so big before it hits the piston, the cylinder wall, or the other valve. And even when they get close to that maximum size, air can no longer flow around them without opening the valve extra far. That’s called high lift, named for the camshaft lobe lifting the rocker arm that opens and closes the valve up higher than normal.

So what do you do when you need more air, but can’t make the valve bigger?

You add more.

Because valves are round, which they need to be for flow and cost reasons, they can only get so big. So what do you do if you want to flow more air? There are basically two choices. Add forced induction, like a turbo or a supercharger, or add more valves.

Adding more valves adds complexity. And cost. If it’s a vee-layout engine, you probably need to switch to overhead cams. Even for an overhead cam engine, you have to package more valves, more rocker arms, more springs, and have a camshaft with more lobes. It’s all really complicated. Bigger valves give more open surface area to flow air.

Let’s do some math on why. 40mm is a pretty common size for the intake valves on an 8v VW engine, with 33mm for the exhaust. We’ll explain why the exhaust ones are smaller later.

On a 16v engine, the intakes are 32mm, with 28mm exhaust. But there are two of each per cylinder instead of one of each.

Each 40mm valve has 1,257 mm2 of surface area. Each 32mm valve has just 804. Area of a circle being based on the square of the radius and all that. So the 16v head has 1,608 mm2 of surface area, most of which can let airflow into the cylinder. Yes, we left out the valve stem area, but it’s not important. The important part is that it can flow almost a third more air, which means a third more fuel. Which means more bang every time at wide-open throttle.

Putting two smaller valves instead of one uses up more of the space in the combustion chamber, and so it increases the flow. Well, what about more? What about the five-valve engine that VW and Audi used?

Because the exploded charge of air and what used to be fuel is significantly hotter than the outside air, it’s under higher pressure. It’s also being pushed by the piston rising. So when the exhaust valves open, the air is more than happy to get out of town. So happy, that a valve 85 percent smaller than the intake valve can move enough air.

That’s how you can get away with an odd number of valves. Go asymmetrical and you can fit three intake valves to feed the engine, wasting even less space. And you only need two exhaust valves, leaving more room for the intakes. So why didn’t it last? Well, it’s even more complicated, for a start. More cam lobes, more springs, more rockers, more of everything. And the gains are smaller. The three 26.9 mm intake valves on the 20v have a surface area of 1,704 mm2. Only about six percent more than the 16v. Now the extra valve stem is suddenly more impactful. The air flowing in from three valves instead of two is also turbulent in a different way that reduces power. Plus with a modern direct-injection fuel injector, there’s not enough room in the cylinder head. Don’t forget you still need a spark plug.

There’s one last benefit to multiple, smaller valves, and that’s when it comes to high rpm. With one big valve, the heavyweight needs a big spring to control. And when the engine starts spinning really quickly, the valve can still have a point where the spring no longer controls it. It closes when it wants, not when it’s supposed to. It’s called valve float. That’s bad because then the valve can hit the piston. It can also let all the air and fuel and exhaust go the wrong way. Smaller valves are lighter, so can move faster before they start to float.

So that’s why more is better, but not always when it comes to cylinder heads. Two valves per is fine, as many big beefy V8s can show, but four can definitely be better. As long as you’re ready for the extra cost and complexity that comes with them.