Exhaust Headers

All of our cars have exhaust manifolds. Some of us are even using the stock manifold that came on the car when it was new. Others have spent thousands of dollars on the latest and greatest header system. Before you get your checkbook out you need to think about a few things. Am I spending all this money to go faster? Or, am I concerned about what people think when I open the hood? Maybe it’s a little bit of both.

Materials: We have a number of choies here. About eighty per cent of all aftermarket headers are coated mild steel. Stainless steel though is becoming a popular replacement material for exhaust headers. Just keep in mind that not all stainless is the same. The folks at Burns Stainless point out that technically, Stainless Steel is a trade name applied to what are known as corrosion-resistant steels. They believe that “It is a fabulous material that outperforms mild and alloy steels in so many different applications in racing that no other material can match it.”

A three-digit system is used to identify stainless steel but we need to focus on what is called the 300 series. Type 304 is a popular choice for exhaust systems. It’s also the least expensive. Another good choice is stainless 321. Jim Browning of Ultimate Headers (ultimateheaders.com) prefers the 321 because it’s about fifty per cent stronger than the 304. This means you can use a thinner sidewall thickness and save some weight. Weight is always a big deal with racecars.

A number of firms have recently started using 400 series stainless. You want to avoid any of the 400 series products.  These are simply not as durable or as resistant to corrosion as the 300 series. You can do a really quick test with a small magnet. A magnet will not stick to anything in the 300 series. It will though stick to the 400 series. You can practice this test on your barbecue grill.

One advantage that stainless has over mild steel is heat retention. Stainless steel will hold the temperature of the exhaust gases in the primaries much longer. We want to keep as much of the heat inside the exhaust manifold pipes as we can.

Inconel 625 is another popular choice for exhaust manifolds. Although the name "Inconel" is a registered trademark of Special Metals Corp., the term has become something of a generic reference to a family of austenitic nickel-chromium-based super alloys which have good strength at extreme temperatures and are resistant to oxidization and corrosion. Inconel's tensile strength and toughness at elevated temperatures allows for a very thin sidewall. This means your exhaust manifold is lighter.

  

All Formula 1 cars and a number of Sprint Cup teams use Inconel for their headers because of the weight savings. Some NASCAR teams are routinely using headers made from 0.020-inch wall Inconel tubing. One problem is that these header walls are so thin that you can easily damage them during installation. Given that Inconel is incredibly expensive I don’t think we’ll see a lot of Inconel header is vintage racing.

The Flange: This is the part of the exhaust manifold that goes right up against the cylinder head. We have some compromises to deal with here. The thicker the flange is the better the seal is. A thicker flange will simply be more durable over time. A thicker flange though means more weight. Even worse is that this extra weight is up high. Exactly where you don’t want extra weight.

Years ago everyone built header systems with a 1/4-inch flange. Today everyone is building headers with a 3/8-inch flange. The one exception is if you’re running a turbocharger. In that case a ½-flange is usually used. The best material seems to be an investment cast stainless steel flange.

Wraps: We want to keep all the heat inside the exhaust system as long as possible. Hot exhaust gases have greater velocity because they are not as dense. As the exhaust temperature cools the velocity slows. These wraps allow you to maintain the temperature of the exhaust gas.

Actually the wraps do the same thing as the high-tech ceramic coatings. They just do it a lot more cheaply. They do have one huge downside. They don’t look period correct.        Presentation is very important in vintage racing and a lot of people just don’t think that exhaust wraps look all that good. While we seldom worry that the inside of our engine is period correct we do spend a lot of time making sure that at least the engine compartment looks period correct.

Coatings: If you decide that a wrap doesn’t look all that period correct on your vintage racer you can coat the exhaust manifold to keep the heat inside the pipe. Remember though that there are two very different types of coatings.

First we have the cosmetic coatings. The only thing they do is make your exhaust header look look shiny. These low-tech coatings are nice looking but they do very little to insulate the exhaust or increase your power. You want a thermal barrier that keeps the energy in the header. Keeping the heat in the pipe lowers the temperature in your engine compartment and more importantly it keeps exhaust gas velocity high. These thermal coatings are usually about 0.15-inch thick as opposed to the cosmetic coating, which are seldom more than 0.002-inch. Swain Tech Coatings (swaintech.com) has a ceramic coating that normally reduces radiant heat by about fifty-five percent.

Most people feel you can skip the internal coatings. It’s virtually impossible to prepare the internal surfaces of a pipe to receive a thermal coating and these coatings can easily become detached. Even a thin paint can reduce the available flow area by up to 2% giving rise to reduced engine performance. You just need to worry about the exterior of you exhaust system.

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