How Headers Work

"Hey, look! Headers for a small-block. These'll work!" --Overheard at a swapmeet

How did you pick your headers? Did you spend more time deciding on the color of your plug wires than you did on this important component that can actually make your rod perform better? Headers, if selected correctly, can maximize the streetability of your hot rod.

The question is, how do you select the correct ones? Step one is to understand the different types of headers on the market and the pros and cons of each type. Step two is to weigh those pros and cons based on which are most important to you. This story is all about step one.

Headers do more than just funnel spent gases out of the engine compartment less restrictively than manifolds; they draw spent gases from the combustion chamber by a process called scavenging. During the exhaust stroke of the engine, the exhaust gas is forced out of the combustion chamber through the open exhaust port by a combination of lower pressure outside the chamber and pressure from the rising piston. The exhaust will continue to be drawn out as long as the exhaust valve stays open. If the timing is off and the valve closes with exhaust left in the cylinder, you lose efficiency. Headers draw maximum exhaust from the cylinder during the duration of the exhaust stroke.

The inertia from the escaping exhaust gases moving through the header pipes creates energy pulses that form vacuums, pulling additional exhaust out of the combustion chamber so that the maximum amount of fresh fuel and air can be drawn in. The more fresh fuel and air you can pack into the cylinder, the more power the engine can make. The less energy the piston has to use pushing exhaust out of the cylinder, the more it can use for something fun, like moving you down the road.

PRIMARY PIPE DIAMETER
The primary pipes are the main components of any style of header. The most important consideration when selecting headers is the diameter of the primaries. According to the many header company tech folks we talked to, one of the most common mistakes rodders make when buying headers is getting pipes that are too large in diameter. The theory that if it works on John Force's Funny Car, it'll work on your Model A roadster is wrong.

The diameter of the primary pipes directly affects the speed (or flow velocity) of the exhaust mixture traveling through them. Simply put, gas moves faster through a small tube than a large one. Change the pipe diameter and you change that speed. You also change the rpm where the torque peak occurs. The lower resistance of a large-diameter header moves the torque peak into higher rpm; a smaller-diameter pipe moves the torque peak to a lower rpm. Bolt on a set of headers that are just too big and your torque peak will move so far up the rpm range, you'll never see it again--and you'll wonder why your new headers screwed up your motor.

For a normal small-block street engine operating in the 1,500- to 3,500-rpm range, most of the manufacturers we talked to recommended 1 1/2- to 1 5/8-inch-diameter primaries, as a rule of thumb, for providing plenty of low- to mid-range torque.

PRIMARY PIPE LENGTH
The length of the primary pipes also affects torque. Where diameter affects the torque peak relative to rpm, the length affects the shape of the torque curve. Longer primaries provide more torque below the peak and reduce it past the peak. Shorter primaries provide more torque above the peak at the expense of below-peak torque. More torque in the low- to mid-rpm range is important to rodders who want that feeling of seat-of-your-pants performance on the street. Longer primaries also reduce the chance of escaping exhaust's being drawn back up another pipe.

In order to keep the torque curve the same for all eight cylinders, it is important that primary pipes be equal in length. Exactly how equal they have to be is more critical on uncorked race cars than for the vast majority of mild-engined street cars running through mufflers. In most applications, pipe length deviation of 2 to 3 inches on a set of full-length headers is not a problem.

HEADER STYLES
Walk through a rod run or flip through a header catalog and it's obvious that there are numerous styles of headers, which fall into these general categories: full-length headers, shorties, lakester style, and tri-Y. Any of these tubular header styles, if the size is correct, will offer noticeably better scavenging and less restriction than a cast-iron manifold.

FLANGES
The flange is the flat portion of the headers that mates the primary pipes to the cylinder heads. A low-quality flange (like a low-quality gasket) can hurt the overall performance by not creating an adequate seal between the heads and the pipes. The surface of the flange should be smooth and the flange ports should be the same size, or slightly larger, than the exhaust ports so as not to restrict the flow of the exhaust. The headers should not block the exhaust ports in any way. If you are running custom heads, or heads that have been machined, pay attention to make sure that the exhaust ports match up to the flange. Ideally, the exhaust should flow in as straight a direction as possible from the ports into the pipes. The gases should be able to travel a few inches before reaching any bends in the header pipes.

MAKING THEM FITAs we mentioned already, when it's all said and done, the most important consideration when selecting headers is whether or not they will fit. Before buying headers, be sure to accurately measure your engine compartment area. Many header company catalogs and Web sites list the dimensions of their products so that customers can tell ahead of time if they'll fit.

Customer service representatives can usually provide some help, but only if you can provide them with accurate information. Many of the leading manufacturers offer specific headers for specific engines--but if you fail to mention that your small-block Chevy has aftermarket heads with a different port configuration or plug angle, those small-block pipes you ordered may not fit. Take into consideration any component that could interfere with fitting headers, such as an aftermarket front clip, custom motor mounts, air conditioning, the starter, and power steering. Also, don't forget to allow room for access to the plugs and other parts that need to be reached.

COOL FINISHES
Maybe you've noticed that headers get very hot. This not only raises the temperature of the engine compartment, cooks plug boots, and causes damaging thermal fatigue to the headers, it can also spoil their appearance. Keeping heat inside the pipes improves performance by increasing the exhaust gas velocity and by keeping intake air cooler.

There are several products and methods for keeping pipes cool, but the manufacturers we asked recommend metallic/ceramic coatings. These coatings, developed years ago by the military, typically consist of an aluminum/ceramic composition that chemically bonds to the headers. They are extremely resistant to corrosion, discoloration, and chipping and can reduce the underhood temperature by as much as 40 degrees.

Many header manufacturers offer these coatings to their customers as an option at the time of purchase. Other companies, such as HPC or Jet-Hot, specialize in applying metallic/ceramic coatings to untreated headers. The metallic coatings offered by these various companies are based on essentially the same formula, so any reputable company would be a good choice.

BOTTOM LINE
There isn't one perfect header. Each style has its pros and cons. The good news for rodders with engines built for street use is that any style of header is going to provide a noticeable performance improvement over your old exhaust manifold. The three primary considerations facing you are performance, appearance, and fit. Your job now is to find the headers that meet your needs in all three areas. If those happen to be $50 swapmeet specials, then good for you. If not, read the catalogs and visit the Web sites. Your headers are out there.