When kicking off an engine project, the very first steps typically considered are an intake manifold, carburetor or EFI, and a free-flowing exhaust that begins with headers. It’s not just about choosing the best headers, it’s also learning the fundamentals of why headers create power. When you gain that knowledge, choosing the best headers to match any engine is easy. To help you guys do that, we visited with Don Lindfors, exhaust R&D manager at Pertronix Performance Parts to discuss his broad knowledge of different header applications. Don oversees the Pertronix brands of headers (JBA, Patriot, and Doug’s Headers), which are essentially designed for three different styles of exhaust applications.
While factory cast-iron exhaust manifolds are designed for durability and ease of installation, a comparatively lightweight tube header is all about increasing torque and horsepower. It does this by extracting the spent gases from the cylinder at a high velocity by means of the header collector. The collector uses fluid dynamics to scavenge the gasses as opposed to merely directing them. These choices are usually on the top of the list because any engine is essentially an air pump; getting the spent gasses out of the engine is equally as important to an engine’s efficiency as the entry of increased fuel and air. So, choosing the best headers is a must.
Varied Header Designs
“I’ll start with our JBA Performance Exhaust line. They are basically designed for late-model vehicles with catalytic converter-forward/EPA compliant applications,” says Lindfors. “For a guy who has a tight situation in a hot rod/street rod engine bay, he can make a good justification that he can still use this type of header and see some measurable improvement over a stock manifold.”
Lindfors continues, “Our other two header companies, Patriot Exhaust and Doug’s Headers, offer options where exhaust efficiency and performance are a priority. For some street rod and muscle car applications, Patriot offers tight fitting block hugger headers, while the predominant selections are mid-length or long tube headers.”
Mid-length headers primarily describe the shorter primary tube-length design. Lindfors calls these headers a good answer to improved ground clearance for lowered muscle cars and street rods.
Lindfors characterizes, “We see more and more mid-length headers sold for ground-hugging muscle cars with an LS swap. It’s a nice compromise between aesthetics, fit, and still not giving up a whole lot of horsepower compared to a conventional long tube four-into-one header.”
If you get the tube diameters and the lengths correct, it is going to make good torque and not hinder the horsepower. – Don Lindfors
In all-out performance or racing applications, Lindfors notes that the mid-length header can also do well in a drag racing application. He contends that using header collector extensions protruding 18 to 20 inches beyond the collector itself will help with a lot of the reversion—the flow of exhaust gasses back into the combustion chamber when the downward movement of the piston creates a vacuum in the cylinder.
Full-Length Or “Long Tube” Headers
The long-tube header is the most recognizable of the designs. “They have been the staple of the header industry since the ’50s,” Lindfors describes. “This four-into-one design is going to make the best overall power when compared to anything else on the market.”
The benefit of a long tube header is its ability to decrease backpressure on the exhaust side of the engine as the spent gasses flow down the individual tubes. During the exhaust stroke, spent gasses escape the cylinder very quickly. This high-speed exodus of exhaust creates an area of high pressure in front of it and an area of low pressure behind it. The low pressure behind the exhaust creates a sucking effect (vacuum). The vacuum helps pull burnt exhaust gasses from the cylinder which helps to pull in more fresh air.
We asked Lindfors if header designs have improved over time since long tube headers have been around the longest. He answers, “We are always examining our designs based on customer input, updates in current engine technology, and improvements we can make with new manufacturing technology.”
“The Tri-Y header was originally designed and made popular through efforts working with Carroll Shelby and header designer, Doug Thorley,” Lindfors continues. “What Shelby wanted and achieved in the GT 350 for road race competition was midrange to pull off the corners.”
What evolved from these Tri-Y designs became very popular for trucks, motorhomes, and heavy vehicles where torque is king. The Tri-Y header increases torque over a standard four-into-one type header because of the lengths and how they scavenge gasses from the cylinders.
“The Tri-Y is kind of a niche market. They are a good design if used properly in the right application,” clarifies Lindfors. “Our Doug’s Tri-Y designs yield maximum power from off-idle to 4,500 rpm. These are well suited for truck and large car applications demanding maximum mid-range torque with maximum fuel economy.”
Tubes And Collectors
From the header flange to the collector, the design of the individual tubes is based on length and diameter. The length of each primary pipe is related to the engine’s desired RPM range. For example, an engine operating mostly in a lower RPM range benefits from a longer primary tube. A high-RPM engine benefits from a shorter-length tube.
Each cylinder is firing out pulses of exhaust described as pressure waves. These waves converge at the collector, where you might think that the exhaust escapes, but that is not entirely the case.
“A portion of the waves will revert up the other primary tubes causing a negative pressure,” says Lindfors. “Primary-tube lengths are engineered to time that negative pressure wave in order to harmonize with the engine’s cam overlap. This timing creates a low pressure that can pull spent exhaust and draw an increased intake charge into each cylinder. This horsepower gain is defined as scavenging.”
Bigger Is Not Always Better
As you progress in your decision-making process, you’ll need to consider these sound theories as you step into a world of header options meant to add horsepower and torque.
All of a header’s bends and its routing can offer better flow. But a primary tube diameter has everything to do with controlling the velocity of the exhaust gas. Lindfors explains, “If you get the tube diameters and the lengths correct, it is going to make good torque and not hinder the horsepower. It’s going to kind of serve two masters.”
“Consider the exhaust coming out of the engine,” Lindfors continues. “If you have too large a tube diameter, the velocity goes down because it gets lost. It’s like it’s falling out into space. If you put a 2 1/4-, 2 1/8- or even 2-inch header on a relatively stock 350 Chevy, there will be no exhaust velocity at the collector. Therefore, no vacuum is created at the collector. This will cause the engine to fall on its face.”
Choosing a primary-tube diameter can follow these simple rules:
- For 200 to 325 horsepower, choose a 1 1/2-inch diameter primary tube
- For 275 to 425 horsepower, use a 1 5/8-inch diameter primary tube
- For 400 to 500 horsepower, use a 1 3/4- to 1 7/8-inch diameter primary tube
- For over 500 horsepower, use a 2-inches or larger diameter tube
You can skew these recommendations based on your performance demands. Gravitating towards a smaller or larger diameter can benefit if your engine has lower or higher peak torque than average. A 1/4-inch change in primary-tube diameter can move the torque peak up or down by about 700 to 800 rpm.
We also asked Lindfors if engine displacement plays a role in header tube selection? “With engines basically being air pumps, equivalent horsepower and torque outputs are going to be moving very similar amounts of air. A 454 big block making 400 horsepower and a 323-inch LS making 400 horsepower are going to do so at different RPM, the lower RPM of the big block will offset the cubic inch difference to keep the velocity at an optimum level. If you really want to play with the math, I like to use the formula; primary tube Area is equal to (peak torque RPM ÷ 88,200) times the cubic-inch displacement of one cylinder.”
The individual primary tubes can be “stepped” into a larger diameter at mid-length in extreme performance and racing applications. With big cubic-inch or high-RPM engines, these steps work with larger collectors to increase the scavenging process again. These headers typically have their performance parameters spelled out in great detail within their product description for extreme horsepower applications.
From functional off-the-shelf headers to carefully engineered, fabricated, and expensive headers, design basics remain constant. The only deciding factor about the best header is what level of engineering and performance gain you want.
As we describe, headers are all about shape, size, and cylinder scavenging. If Goldilocks was a gearhead, her story may well say, not too big, not too small, but performance is best achieved when your headers are just right.