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Key Factors In Today's Header Designs

Provide knowledgeable recommendations for customers through a clear understanding of racing exhaust systems—including tuning, port-matching, materials and processes.

By Jeff Zurschmeide

The most common response you’ll hear when you ask experts about exhaust headers is, “Well, it depends.” They’re not being evasive and they’re not necessarily guarding any big secrets. They’re just being honest.

We all know that getting the right header onto an engine can make a huge difference in performance, but the hard part is figuring out what kind of performance the client wants, and matching the header’s performance to your client’s needs.

There is tremendous variety in the pre-made exhaust headers, header components and complete exhaust systems available on the market today. Some systems are strictly decorative—designed to look good and give a street engine a nice exhaust note. But while all exhaust builders claim performance gains, real-world, side-by-side dyno tests often show little or no benefit from kit systems. There are a lot of reasons for that, and most of those reasons are legitimate. If the exhaust system is not the limiting factor of engine performance, then changing the exhaust won’t affect engine power very much.

When it comes to the size and design of exhaust headers, that principle remains true. There’s no point in spending time and money on a custom header if it’s not going to help. But when the time arrives that a header is required, it can make a significant difference. At that point, you need to know how to make the best possible recommendation for your customer.

Header Basics

The most widespread types of headers commonly found in racing applications use 4-into-1 collectors or 4-2-1 designs, also known as Tri-Y. As the names indicate, 4-into-1 headers gather all four primary pipes together into a single collector; and 4-2-1 headers first combine two sets of two cylinders into collectors, then combine the two collectors into a third collector. Almost all headers are some variation of these two designs, including stepped headers, which use progressively larger diameters of tubing each time tubes come together.

There are exceptions to many of the accepted four- or eight-cylinder header rules for engines that use a “siamesed” central exhaust port, such as the venerable flathead Ford, or any number of vintage British engines. There are also special cases for V6 and inline six-cylinder engines, and for engines using flat-plane cranks, which are available for many V8 engines.

Finally, there are header designs such as “Zoomies” that are nothing more than four short exhaust pipes, and special designs for use with dynamometers and turbocharged applications. Bottom line: There’s much more to this topic than can be explained in one article.

Getting It Right

The first step to getting the headers right is to ask the right questions to the right people. Every engine is different, and the engine builder should be the primary source for header specifications to match the engine they created.

“Usually your engine builder is very knowledgeable and particular as to the exhaust system,” said Gary Donahoe of Coast Fabrication, Huntington Beach, California. “You need to go with what they say. Our real knowledge is how to package headers that will live under the constraints being asked of them. You bring us the car, and we discuss your needs and come up with a plan.”

Also, both the engine and the header must be optimized for the rpm range and the particular type of racing intended. It’s not that there are special headers just for stock car, drag, or road racing individually, but header design reflects the nature of the power one needs and the space available inside (or outside) of a given car.

“When you’re getting into road racing, you’re then talking about a broader speed range, as opposed to oval-style racing,” said Dr. Andy Randolph of ECR Engines, Welcome, North Carolina. “If you’re on a sharp flat oval, it might be a broad speed range. In superspeedway racing, your rpm only varies over a couple hundred rpm over the course of a lap.”

In addition to the engine builder, the racer must consider their fabricator as well. As Chris Hill of Specialty Product Design (SPD) in Rancho Cordova, California, noted, “A common problem is when chassis manufacturers make changes without a header manufacturer’s knowledge.” So, the manufacturers have to keep up to adjust as necessary.

If you know the build specs and details of the engine, you can get pretty close to an optimized design. One source to contact is Vince Roman of Burns Stainless in Costa Mesa, California. Burns offers a detailed online worksheet that takes every engine measurement into account.

“The customer goes onto our website and fills out the form,” Roman explained. “Then we run it through a computer program and we get an output. Then our engineers go through and make sure we’re taking all the different aspects of the application into account, and we make a recommendation on the header design. It has a broad range of applicability as long as you have the data on the engine.”

Go with the Flow

Right at the start, flow is the single most important value in a header, and in many cases you can discard options that don’t flow well.

“The first principle is that you want to minimize radiuses,” Randolph said. “You want to have nice gradual bends, you want no sharp turns, no 90-degree turns, and no pinch areas. You just want a nice gradual flow from the cylinder head out to the muffler or tail pipe.”

Sometimes using the smoothest bends is not possible due to the space available in an enclosed engine bay. At that point, compromise must come into the picture.

“Sometimes you get into a box, and our job is to get you out of that box,” said Jeremy Lehr of Heartthrob Exhaust and Accessories, Litchfield, Minnesota, which makes mandrel-bent exhaust components. “We have multiple radius bends, from sweeping 15-degree bends to really tight bends. We have a radius we call a 1D, which is super tight. Sometimes you have to work around the chassis.”

But also keep in mind that it’s advisable to start with the cylinder head. Tom Beco of Beco Headers in Bensalem, Pennsylvania, explained the interface between the head and the header. “The header tube for the first 12 inches or so is an extension of the [cylinder] head. So if you make the tube bigger than the port, it would be like taking a garden hose and putting it into a bucket; it’s just going all over the place. It disturbs the flow.”

Using header pipes that are too small is even worse than using pipes that are too large. It’s important to match the cylinder head port size and shape to the primary header tubes and the header flange.

“I weld the tubes on both sides of the flange so that exhaust gases don’t hit the edge of the tube,” he explained. “I use a real soft rod made of brass, and then I grind it so it’s smooth to make a continuation of the port. If you don’t weld on that side, then the exhaust gases try to get between the tube and the flange.”

Similarly, Beco teaches that it’s important to precisely match the header flange to the cylinder head so that the ports are aligned. Some engine builders request the headers with the engine so they can install locating pins to keep the header aligned properly.

“People don’t realize how critical it is to get the flanges lined up with the ports,” Beco insisted. “They’ll go on, but maybe they’ll be blocking the port a little. It’s the same with the tubing. If you don’t use the right size tubing, it restricts flow. That is the most critical area right there because it’s like hitting a brick wall. It makes the exhaust go back in.”

Size Considerations

The diameter and length of every part of the header makes a difference in the effect the header has on engine power. It’s tempting to think that a bigger header will make more power. After all, the objective is to get the exhaust gases out of the car as efficiently as possible, so why not just use the largest possible pipe in all cases?

“The diameter of the tube matters,” Randolph said, “because it establishes the velocity of the gas of the tube.”

Here’s an important tip: If you are tuning the header to a specific rpm, you can change the diameter of a header pipe and maintain that tune by adjusting the pipe’s length, and vice versa. “If you make the pipe longer, in order for it to tune the same you have to make it bigger in diameter,” Randolph added. “If you make the pipe smaller in diameter, you can make it shorter.”

One of the most common mistakes racers make when choosing a header, according to Hill, is assuming that any header will do. Of course, this assumption is wrong. He explained that there are many components to consider when choosing a header. “In simple terms,” he said, “the diameter of the tube used has mostly to do with exhaust gas speed and pressure. The smaller the tube, the more speed and pressure. The larger the tube, the less speed and pressure.”

He added, “With big-horsepower drag race cars, the tubes are usually larger to help achieve max horsepower at a desired rpm. In road racing or circle track, the tubes might be smaller to help achieve max torque for acceleration off corners and such.”

Most all run a stepped-style header to help broaden the power band.

In any header, all the primary tubes should be the same length, unless you have a Siamese-port engine. This is critical, because the timing of exhaust pulses helps the engine breathe.

“I have seen guys spend $3000 and the headers look nice, welded and everything,” Beco said, “but look at the headers! One tube is like 18 inches, the other tube is like 24 inches, and another tube is like 30 inches. The exhaust is going to get out of an 18-inch tube faster than a 30-inch tube.”

That’s significant because an exhaust system doesn’t just flow one way. Pressure waves bounce around the whole system. As each exhaust pulse travels down its primary tube, it sends a reverse pressure wave back up the entire system when it arrives at the collector and mixes with other moving gases there.

What you want is for that returning pulse to arrive back at the exhaust port at the right time, so it can help pull the next pulse out of the engine. And that’s where tuning the headers comes in.

Tuning Headers

The most important thing to know about tuning headers is that you can only tune to a single ideal rpm.

If you’re building an engine to run at the Bonneville Salt Flats or on a superspeedway, that’s perfect. If you’re building an engine that has to work across a broader range, then you’re looking at trade-offs, but you can still do a lot of good.

“At some engine speeds you will have an inverse pressure wave arrive at the exhaust valves that coincides with valve overlap, and it helps scatter out the contents of the cylinders,” Randolph explained. “That will happen only at one engine speed. There will then be engine speeds where you’re totally anti-tuned, where the pressure pulse arrives at the exhaust valve during overlap. That will cause a localized dip in torque.”

The peaks and valleys of torque that Randolph describes are based in part on the timing of exhaust pulses and the length of header primary tubes. That’s partly why the torque line on a dyno chart often looks shaky. “So what we tend to do is try to size the exhaust to fill in those peaks and valleys, and counterbalance them,” Randolph revealed.

While it’s one thing to understand the theory of how all these factors work together to create an optimum header, how does someone figure out what’s going to work best for their engine?

To properly tune a header, know precise exhaust temperatures, because they affect the speed of sound. The speed of sound determines how long it will take for the pressure wave of an exhaust pulse to travel down the primary and come back, and then match that primary length to the time it takes the engine’s crank and valvetrain to come around to the point where you want that return pulse to hit. And that’s just to tune for one rpm.

So the short version is, without a dyno and other specialized equipment, a header that’s perfectly tuned at a precise rpm can’t be easily made without much trial and error. That’s why the experts exist, and why there are very good and well-known compromise solutions for most commonly raced engines.

One header element that helps to create a broad powerband is the collector. Roman explained, “The way I like to help people visualize what’s going on is to think of a Zoomie header. That’s just an exhaust header with no collector. They can give you good power, but they give you peaky power because when you’re on the rpm of the pipe, you’ve got really good scavenging happening, really good supercharging occurring. But when you come off of that rpm, you lose power band.”

The collector on a header decreases the strength of the returning pressure wave by splitting it two or four ways, which reduces the benefit at the tuning rpm, but also reduces the detriment at all other rpm’s, and thereby broadens the power band.

“What we’re doing is further adjusting that outlet size to maximize that phenomenon to give us that broad power band,” Roman explained. “The secret is to make it small enough that we broaden the power band, but not so small that we’re losing power.”

Choosing Materials

We asked Jim Renella of Performance Tube Bending in Irwindale, California, about the choice of materials in headers, and several good options are available.

“The thing is that materials react differently,” he said. “People sometimes think that making a stainless steel header will be much better than making a mild steel header. But sometimes stainless will crystallize under the constant heating and cooling of the material and you’ll develop cracks, especially if there’s any sort of vibrations on that header.”

Mild steel is less expensive and more malleable than stainless, making it tougher for racing. Renella cautioned against using aluminum tubing in headers. He said, “Aluminum is definitely no good for headers because it’s way too soft and it won’t withstand the heat.”

According to Renella, ERW tubing of .065 wall thickness (16 gauge) is the most common material used for headers and exhaust systems.

Getting Bent

Another critical point in pursuit of a food header is to make sure that all bends have been made with a mandrel bender. This ensures that primary feature of good flow. Renella is an expert on the proper way to bend header tubes.

“There are two main methods of bending,” he said. “There’s production bending, and that means press bending. The drawback of the press bender is that it crushes the tube. The other thing is the tube has to be strong enough to support the crushing and bending, so they have to use thicker material. Now with a mandrel tube bender you could go as thin as .035. You have to get .083 thickness to bend on a press bender.”

Because a press bender tends to crush the pipe, it reduces the diameter of the tube for a given length, which impedes the pipe’s flow characteristics and alters the pipe’s individual tuning.

The final factor to consider is making a header that will fit in the space available in a race car. “Ninety percent of the time, I make the headers with the car because it is just impossible to get it right otherwise,” Beco declared. “My stuff is all custom built for the vehicle.”

 

Key Factors In Today's Header Designs

When it comes to header production, minimizing the radius is key. Gradual bends with no sharp turns, and no 90-degree turns or pinch areas are ideal to produce a steady flow from the cylinder head out to the muffler or tail pipe, one expert source explained.


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