Working with Hooker’s Adjustable Race Headers – Part 1

Wayne Scraba

24 Apr 2019

Click Here to Begin Slideshow Perhaps 20 years ago, maybe longer, it wasn’t uncommon to see dedicated drag racers at a test and tune working with adjustable headers. Included in that header parts mix were several sets of adjustable primary tubes and more than likely a set or two of adjustable collectors. What they’d do was “A-B-A” test each combination of the components, carefully monitoring conditions until the car showed which setup it liked the best. The key here is “the car.” The tests treated the headers as part of the entire vehicle package, not just a torque measurement and horsepower calculation on the dyno. Today, working with adjustable headers is much less common. In fact, racers renting tracks for private sessions or open test and tunes aren’t that common either. But that doesn’t mean having a set of adjustable headers isn’t an advantage. Obviously, we can’t tell you the length and size of the header primary tubes or the collector size for your specific combination, but if you’re prepared to test and tune, there’s definitely power to be found with adjustable headers. With that in mind, let’s start at the beginning: Header Tube Diameter: Header tube diameters vary from engine combination to engine combination. According to Reher-Morrison Racing Engines, there is a direct relationship between the diameter of the primary header tube and the exhaust velocity: “The key when selecting a tube diameter is to find a happy medium between the free-flowing characteristics of large tubes and the superior scavenging of small, high-velocity tubes. Header tube diameters normally range from 1-3/4-inches to 1-7/8-inches for smaller, low performance engines up to big 2-3/8-inches tubes for large displacement, high-horsepower applications.” Tuning The Primary Tube: Inertial scavenging helps pull the exhaust out of the combustion chamber. It also helps draw the air-fuel charge into the chamber. Hooker tells us that by varying the length of the primary tube, you physically change the time it takes for the vacuum pulse (low pressure area) to reach the header collector. Fundamentally, that’s what “tuning” header tubes is all about. Tuning the header primary tube length can compliment the components you’ve selected for the entire car (for example, the camshaft, engine compression ratio, torque converter stall speed, transmission gear ratios, rear axle ratio, tire diameter, etc.). Furthermore, working with the primary tube length and collector size can also affect how the car works at a specific track. That’s not the end of it, either. As the engine RPM increases, the need for a long primary tube is actually reduced. Hooker Headers tells us this is due to the fact that there is less time between cylinder firing (on each cylinder). The vacuum pulse or low-pressure area we mentioned previously has less time to work its way to the collector. Obviously, the size of the primary header tube(s) has an effect upon inertial scavenging. With all other factors being equal, when the exhaust speed increases, the engine sees more of a scavenging effect (typically with a smaller header tube). Something else to ponder is this: It’s been suggested by some people that the exhaust valve diameter determines the header tube diameter. This isn’t exactly correct. To a point, a company like Hooker Headers can build a header to control the speed of the exhaust gas by changing the actual diameter of the primary tube. The smaller the diameter of the tube, the faster the exhaust will flow (bear in mind, the speed of the exhaust gas slows as it cools). A skillful header builder understands that by varying both the diameter and the length of the header primary tubes, the largest amount of inertial scavenging can be established. It’s easy to see that header primary tube length and primary are intertwined. Where do you begin? Reher-Morrison offers a suggestion: “At lower engine RPM, long tubes help maintain good exhaust scavenging and increase torque output. As engine speed increases, exhaust gas velocities increase and a shorter tube length tends to work better. “The best you can do is find a header tube length that offers the best compromise between low and high end power. This is also why header length can be an effective aid in ‘tuning’ the RPM range of a racing engine. Most racing engines will work best with (primary) tubes between 28 and 30-inches long. “It is also very important that all header tubes are as close as possible to the same length. The tubes from the rear cylinders are closer to the collectors and need a few extra twists and bends to be as long as those (tubes) at the front of the engine.” Headers with significantly unequal tube lengths can create all sorts of tuning problems, too: According to Reher-Morrison, headers with unequal tube lengths actual mandate a different “tune” for each cylinder. Additionally, the folks from Hooker Headers claim there can be as much as 50 HP difference between 5500 and 6500 RPM when compared to a set of headers having close to equal tubing lengths. With that in mind, it’s clear why primary tube length is important. This leads us to zoomie headers, which (no secret, we’re sure) have made a re-appearance on many fast door cars. Sure, they’re used on blown alcohol cars and fuelers, but do they work for the little guy? In truth, they run pretty well in a couple of different power bands (for example from 3500 to 4500 and then again from 7000 to 7500 RPM). But there’s a catch: Hooker Headers informs us they can give up something like 25 to 35 HP or more between those two ranges (4500 to 7000 RPM for our examples). If you’re searching for strong average horsepower over a wide range, zoomies aren’t the answer. That’s the reason collectors are important. In our next segment, we’ll take a closer look at the collector. There’s actually much more here than you might first think. In the meantime, check out the accompanying photos.

Working with Hooker’s Adjustable Race Headers - Part 1 — Click Here to Begin Slideshow Perhaps 20 years ago, maybe longer, it wasn’t uncommon to see dedicated drag racers at a test and tune working with adjustable headers. Included in that header parts mix were several sets of adjustable primary tubes and more than likely a set or two of adjustable collectors. What they’d do was “A-B-A” test each combination of the components, carefully monitoring conditions until the car showed which setup it liked the best. The key here is “the car.” The tests treated the headers as part of the entire vehicle package, not just a torque measurement and horsepower calculation on the dyno. Today, working with adjustable headers is much less common. In fact, racers renting tracks for private sessions or open test and tunes aren’t that common either. But that doesn’t mean having a set of adjustable headers isn’t an advantage. Obviously, we can’t tell you the length and size of the header primary tubes or the collector size for your specific combination, but if you’re prepared to test and tune, there’s definitely power to be found with adjustable headers. With that in mind, let’s start at the beginning: Header Tube Diameter: Header tube diameters vary from engine combination to engine combination. According to Reher-Morrison Racing Engines, there is a direct relationship between the diameter of the primary header tube and the exhaust velocity: “The key when selecting a tube diameter is to find a happy medium between the free-flowing characteristics of large tubes and the superior scavenging of small, high-velocity tubes. Header tube diameters normally range from 1-3/4-inches to 1-7/8-inches for smaller, low performance engines up to big 2-3/8-inches tubes for large displacement, high-horsepower applications.” Tuning The Primary Tube: Inertial scavenging helps pull the exhaust out of the combustion chamber. It also helps draw the air-fuel charge into the chamber. Hooker tells us that by varying the length of the primary tube, you physically change the time it takes for the vacuum pulse (low pressure area) to reach the header collector. Fundamentally, that’s what “tuning” header tubes is all about. Tuning the header primary tube length can compliment the components you’ve selected for the entire car (for example, the camshaft, engine compression ratio, torque converter stall speed, transmission gear ratios, rear axle ratio, tire diameter, etc.). Furthermore, working with the primary tube length and collector size can also affect how the car works at a specific track. That’s not the end of it, either. As the engine RPM increases, the need for a long primary tube is actually reduced. Hooker Headers tells us this is due to the fact that there is less time between cylinder firing (on each cylinder). The vacuum pulse or low-pressure area we mentioned previously has less time to work its way to the collector. Obviously, the size of the primary header tube(s) has an effect upon inertial scavenging. With all other factors being equal, when the exhaust speed increases, the engine sees more of a scavenging effect (typically with a smaller header tube). Something else to ponder is this: It’s been suggested by some people that the exhaust valve diameter determines the header tube diameter. This isn’t exactly correct. To a point, a company like Hooker Headers can build a header to control the speed of the exhaust gas by changing the actual diameter of the primary tube. The smaller the diameter of the tube, the faster the exhaust will flow (bear in mind, the speed of the exhaust gas slows as it cools). A skillful header builder understands that by varying both the diameter and the length of the header primary tubes, the largest amount of inertial scavenging can be established. It’s easy to see that header primary tube length and primary are intertwined. Where do you begin? Reher-Morrison offers a suggestion: “At lower engine RPM, long tubes help maintain good exhaust scavenging and increase torque output. As engine speed increases, exhaust gas velocities increase and a shorter tube length tends to work better. “The best you can do is find a header tube length that offers the best compromise between low and high end power. This is also why header length can be an effective aid in ‘tuning’ the RPM range of a racing engine. Most racing engines will work best with (primary) tubes between 28 and 30-inches long. “It is also very important that all header tubes are as close as possible to the same length. The tubes from the rear cylinders are closer to the collectors and need a few extra twists and bends to be as long as those (tubes) at the front of the engine.” Headers with significantly unequal tube lengths can create all sorts of tuning problems, too: According to Reher-Morrison, headers with unequal tube lengths actual mandate a different “tune” for each cylinder. Additionally, the folks from Hooker Headers claim there can be as much as 50 HP difference between 5500 and 6500 RPM when compared to a set of headers having close to equal tubing lengths. With that in mind, it’s clear why primary tube length is important. This leads us to zoomie headers, which (no secret, we’re sure) have made a re-appearance on many fast door cars. Sure, they’re used on blown alcohol cars and fuelers, but do they work for the little guy? In truth, they run pretty well in a couple of different power bands (for example from 3500 to 4500 and then again from 7000 to 7500 RPM). But there’s a catch: Hooker Headers informs us they can give up something like 25 to 35 HP or more between those two ranges (4500 to 7000 RPM for our examples). If you’re searching for strong average horsepower over a wide range, zoomies aren’t the answer. That’s the reason collectors are important. In our next segment, we’ll take a closer look at the collector. There’s actually much more here than you might first think. In the meantime, check out the accompanying photos.

This is a set of Hooker Super Competition Race Headers. They’re designed with close-to-equal length tubes and, of course, they’re adjustable.

The diameter of the header has an effect upon inertial scavenging. When exhaust speed is increased, so is the scavenging effect. The smaller the header tube diameter, the faster the exhaust will flow.

On this Hooker’s race header, the primary is “adjustable.” What this means is you can add length to the primary tubes once the collector is removed. Adding tube length improves torque down below in the power band, and tends to sacrifice it up top. Hooker’s kit includes 2-inch and 4-inch primary extensions.

Look closely at this set of headers and you can see how the collector physically bolts to the primaries. Obviously, this is the key ingredient that allows for adding primary tube length.

Here’s an example of a “zoomie” header, as mentioned in the text. This header design simply does not work well in many (or any for that matter) naturally aspirated applications. The text offers more information.

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