While it seems like little more than a means to bring the four individual primary pipes into one tube that can attach to the rest of the exhaust system, the collector has a great deal of potential to positively affect the output of the engine. The obvious alterations are diameter and length, and those do make a difference, but there is actually so much more going on inside a collector due to the simple fact that essentially four single-cylinder engines are dumping exhaust into one tube. The key is handling both the exhaust waves and pressure waves that converge there.
A spike is a common choice to smooth flow as it transitions into the collector from the pr
Much like the primary tube diameter, the collector diameter is critical to power production. Ideally, the diameter needs to be large enough to support the amount of exhaust that will be entering it. Due to broad application considerations, most street car headers have a larger collector than they need, but that can be mitigated by how the collector handles the exhaust and pressure waves. The exhaust waves are somewhat self-explanatory; each cylinder sends a slug of exhaust down the primary tubes into the collector at around 350 feet/second. Standard headers will exit with a simple cutoff of the primary tubes into a canister-style collector. This design works and will make good horsepower in a relatively narrow rpm range, and is usually sized to maximize peak horsepower. One drawback, though, is that it tends to create a swirling of exhaust gases that will slow down their flow, making the collector diameter even more critical.
To counter that, several different designs have been created that work to more subtly merge the primaries into the collector and manage the pulses. The designs differ from cones to slash-cut pipes, or even multi-stage 4-2-1 collectors, but the goal is the same: control exhaust flow and maintain speed while not negatively affecting the exhaust pressure wave. These higher-end collectors are typically only found on more expensive headers since the labor involved to craft them is much higher. Nevertheless, companies such as Burns Stainless and Cone Engineering have kits that can be added to a header to improve the collector flow. If you want to get even more aggressive, Flowmaster and Stainless Headers have 4-2-1 merge collectors that can work remarkable well for broadening a power curve. Alternatively, many racers have found great success and power rewards simply by adjusting the length of their collectors. This one is definitely a "try it and see" approach best left to racers looking for the last bit of power under the curve.
What Do I Need For My Car?
All that theory is fun to know, allowing you to enter the decision process educated, but we know that most of you will likely stick to what is available off-the-shelf rather than have custom tubes bent. The good news is pretty much all standard combos are covered with a header that will work well on mild to super aggressive street cars. Could you be giving up some slight power under the curve somewhere versus an optimized custom header? Probably. Will you notice? Probably not.
With that in mind, the first consideration you’ll butt up against is physical fitment of the header. There’s more to consider than you probably realize, so Lee Robinson of Hedman Hedders helped us create a very practical list of things you need to know about your project before you even pick up the phone or start looking for headers. Check PopularHotRodding.com for our web-exclusive rundown on all the little stuff you may not have thought about!
Burns Stainless specializes in custom headers, but they are famous for their merged collec
Once the realm of custom race headers only, Hedman has introduced stepped diameter primary
At the end of the day, clearance and fitment are often the most important things to consid