A header and a manifold both serve the same purpose, and have their advantages and disadvantages. Both of these components are bolted to the engine and route spent exhaust gases out of the car. Most stock vehicles use manifolds, though some cars come with headers from the factory. Headers are generally considered a performance upgrade while manifolds are not.
The main advantage of headers over manifolds is that headers generally flow better than manifolds and thereby allow the engine to produce more power. Manifolds are cast, which means they have small exhaust passages in relation to the wall thickness of the manifold as well as rough cast surfaces, which disrupt the flow of exhaust gases and slow them down. On the other hand, headers are made from tubular steel, so the exhaust passages are larger in relation to the walls of the material, and the tubular steel is much smoother than in a manifold, which helps get the exhaust gases out of the engine more quickly and efficiently.
One benefit of manifolds over headers is that manifolds produce much less heat since the extra material in the manifold serves to absorb much of the heat produced by burned exhaust gases. Headers produce much more heat, which has a negative impact on power output (though headers generally still make more power than manifolds) and can negatively affect components around them, such as starters. Headers are frequently wrapped or coated to minimize heat soak.
Depending on where you live, headers may not be legal and may only be allowed on off-road or racing vehicles. Manifolds may be necessary to keep some vehicles street legal. It is wise to check your local laws before replacing manifolds with a set of headers.
Generally speaking, headers make an exhaust louder due to their thin-walled, tubular construction, which does not do a very good job of absorbing sound waves. A manifold, on the other hand, is of a relatively thick construction and does a far better job of muffling exhaust noise. It should be noted that a lot of people like headers because they are louder and can potentially sound better than manifolds.
Another reason that headers are popular on hot rods and modified cars is that they look better than manifolds which, with their cast appearance, don’t look very racy or performance-oriented. Tubular headers, however, have a unique appearance preferred by many.
