When it comes to engine design and tuning, there is a lot more to it than just picking parts and tweaking carbs or fuel maps. Just about every gearhead, from the most experienced engine builder to the average weekend hobbyist knows that headers help you make power, but far too many simply don’t understand the science behind the tubes.
Headers can help you make more power, but they can also bring it down. That twisting, snarling mess of tubing is the last function in a complicated equation that makes an engine work. If you want to get the most out of it, you need the right pipes.
Most of the modern header technology has come from years of collecting empirical data from trial-and-error experience
“Most of the modern header technology has come from years of collecting empirical data from trial-and-error experience,” said Todd O’Neil, Exhaust Engineer for Hooker Headers. Over time, these designs have been refined with the aid of computer software. The big header companies have more resources when it comes to designing headers and exhaust components in the virtual world.
This means less cut and weld development, which takes considerably longer. This doesn’t mean that the traditional trial-and-error development is eliminated; it is however compacted, reducing product development time. This speed enhances the market for mass-production designs, but don’t count out the niche manufacturers. While the big boys have bigger budgets, they also tend to focus on the bigger markets, leaving more on the table for the smaller production shops.
How They Work
Headers can provide more than just sending the exhaust to the back. A properly designed header will actually increase the efficiency of the engine, helping to draw more air and fuel into the combustion chamber through a process called scavenging. This process is very similar to a stove pipe or chimney.
Under the right circumstances, the chimney draws air from the room and releases it out the top, this is called drafting. As wind blows across the top of the chimney, a low-pressure area is created inside, drawing air from the room below, which feeds the fire, generating more heat production.
If the chimney is too small, the effect will be reduced and smoke fills the room. If the chimney is too large, smoke fills the chimney, but the lack of drafting reduces the amount of air feeding the fire.
Engine exhaust works on a similar principle, with mechanical pressure coming from the engine side. As the combustion process ends, the exhaust valve opens, releasing the pressure into the exhaust system.
The air moves from the high pressure chamber to the low pressure exhaust. The rate at which the exhaust gasses move is directly related to the initial pressure inside the header (and exhaust system as a whole).
This is called “blowdown” and is the difference between cylinder pressure and exhaust system pressure. Excessive blowdown pressure means less gas moving into the header on its own, requiring mechanical expulsion (pump action, i.e. piston movement). Reducing blowdown increases the rate of gas movement in the initial stage of the process.
Once past the initial blowdown expulsion, when the pressure between the combustion chamber and exhaust system have equalized, the piston takes over, providing the pump action needed to expel the rest of the exhaust and push it along its way through the exhaust until the piston reaches TDC (top dead center) and the pump action ceases.
The exhaust gasses do not stop when the pump action is finished. The inertia of the hot gasses keeps it moving through the pipe. The exhaust pulse is a not a single action, one blast and it’s over, the repeated pulsing functions like a waveform, just like sound waves. If it were blasted into open space (as in no header or manifold) it would have no positive effect on the engine.
Instead, that waveform pulses through the tubing, pushing the gasses in front of it, while pulling the gasses behind it. This creates a vacuum behind each pulse. Overlap between the intake and exhaust lobes of the camshaft means that as the intake valve is opening, the exhaust valve is still open, allowing that newly-created exhaust vacuum to draw fresh air and fuel into the combustion chamber, beyond what is possible through normal atmospheric pressure in the top side of the engine. This process, called scavenging, is one of the biggest benefits of headers over manifolds.
The basic principles of exhaust flow are fairly simple; getting the maximum exhaust from point A to point B is where things get interesting. There are many factors at work in the scavenging and blowdown process. Primary tube length and diameter are the main components in the fight for horsepower and torque. From here, we will discuss the mechanics of headers in terms of short, mid-length and long primary tubes.
“Effective header primary tube and component geometry attempts to take advantage of two distinct forces occurring inside the header to increase performance” said O’Neil, “One is the kinetic energy of the gas stream and the resultant low pressure area behind it, and the other is the considerably greater energy of sonic finite amplitude waves that originate upon the opening of the exhaust valve.”
In terms of primary length, amplitude wave tuning is limited to long tube headers. Short and mid-length headers are simply too short to take advantage of the length of the waveform. The most common use for shorty and mid-length headers are cost and clearance.
These two factors will override the desire for a long-tube header, as those two factors can be deal-breakers. If it doesn’t fit, it doesn’t fit. That doesn’t mean they are useless. Shorty headers are designed to be as effective as possible in terms of getting the flow moving by removing restrictions.
Choosing The Right Tubes
Velocity is the key to the shorty and mid-length header design. The faster the pulse moves through the tubes, the less pressure builds up inside. This is a function of blowdown.
Reducing the blowdown pressure increases the amount of gasses that are expelled without mechanical assistance, which in turn reduces the amount of effort required to pump the remaining gasses out of the chamber. So you would think that a large-tube shorty header would be optimum in this instance, well, not exactly.
“Smaller tubes acts like longer tubes and bigger tubes act like shorter tubes,” says Dan Lemons owner of Lemons Headers. There is a fine line between too big, just right, and too small.
The key is matching the header to the engine, George Kook Jr., president of Kooks Custom Headers told us, “There is such a thing as over scavenging, particularly with merged collectors. This leans out the intake track, resulting in a loss of horsepower.” If it is not sized correctly, a negative loss of mid-range and top end horsepower will be the result.
The biggest advantage for shorty headers is the fitment. Short tubes provide the easiest installation of all the tube designs. “The short tube lengths keep the header collector up high and closer to the engine compartment. This reduces the risk of component interference, and ensures the maximum amount of ground clearance for your exhaust. Short tube headers offer respectable power gains over stock exhaust manifolds, but still leave a lot of horsepower and torque inside the combustion chambers, compared to long-tube header designs” says, Marc Lewis, V.P. of Marketing for the Hedman Performance Group.
“Hedman Hedders manufactures the entire gamut of tube length options. With nearly 60 years of design and manufacturing under our belt, Hedman’s selection of short tube header applications that provide maximum ground clearance is unrivaled in the industry.”
For the best performance, a full-length header is better. “Long-tube headers are a design exercise in unsteady gas flow dynamics and attempt to utilize all the potential energy present in the exhaust to evacuate the cylinders,” says O’Neil.
“This means maximizing the kinetic energy effects of short and mid-length headers and combining them with the tremendous tuning advantages of finite amplitude waves.”
The length and diameter of the tubes must match the application. Just because it works on a 1,000 horsepower drag car does not mean it will work on a 400-hp street machine, in fact, it will likely kill a lot of power. Master builders and tuners use this to their advantage.
“We may use a bigger tube to kill some of the engine’s torque to help manage blow to the tire especially if there is nitrous or a blower,” said Dan Lemons. “Nitrous and blower motors like bigger tubes.”
This is where the builder’s experience really comes into play, there is no set formula or rule that dictates the exact design and size of a high-performance header. The biggest drawback for most hobbyists is the fitment. Long-tube headers, at least off-the-shelf units, are designed for a stock chassis. Engine swaps, steering components and other modifications can seriously alter how a header fits.
Coming Together With Collectors
Optimized header collector geometry is the key to tuning the area that holds performance potential for specific applications. In cars with full exhaust systems, the collectors have much less effect than on open collectors systems.
“Collectors are generally ineffective beyond peak torque, so max HP applications such as drag racing are addressed by ensuring that the outlet diameter is not restrictive at mass flow and that the collector length is appropriate for providing effective reflected wave duration,” said Hooker’s O’Neil.
When you need low RPM torque, such as street and road race applications, a merge style collector provides a tuned choked point that propagates pulse velocity throughout the remainder of the exhaust system, as long as it is properly designed. Merge collectors can sometimes create that over-scavenging effect discussed earlier.
A perfect header would feature a single bend and out to the back of the car. This of course does not work in just about every street car chassis. “Large radius bends, along with keeping the tubing square to the exhaust port without back-cutting the bend at the flange limit constrictions while allowing the header to fit into to the chassis,” Dan Lemons explained.
Building Better Build Quality
While they don’t necessarily provide a finite performance boost, what the headers are made of does matter. “Stainless in the same gauge steel is approximately 1.5% heavier due to the dense nickel-chromium content” George Kook Jr. told us.
Stainless has a lower thermal conductivity coefficient, so it helps retain more heat, which helps propagate the exhaust pulse. Stainless steel often doubles the price of a header, but it will also last longer too.
If mild steel is required to meet the budget, adding a ceramic coating to the inside of the header helps. “Heat retention does make horsepower according to the dyno guys,” Dan Lemons said.
“Adding an additional coating on the inside helps control rust and carbon build up inside the tube which would disturb the gas flow. Coating will also help these expensive headers last a long time.”
Gaskets & Hardware
Little tricks like making the header flange a touch larger than the exhaust port to increase scavenging requires the right gaskets. “The collector gaskets we use are SCE copper. Since the copper can’t crush like a fiber gasket we use a thin layer of copper silicone on each side of the port opening.
As for our flange gaskets at the head, again we use the SCE flat annealed copper. Any gasket that is flat will seal at the header flange. I never use the embossed copper type. They’re made for the flange to match the port opening and won’t work with our headers,” said Dan Lemons.
The gasket must match the header opening more than the exhaust port. Any overhang covering the exhaust will cause turbulence, reducing flow and efficiency. Another importance aspect is locking header bolts. Over time, flanges swell and shrink with the heat, particularly with different metals, such as aluminum heads and steel headers. It is also a good idea to re-tighten the bolts after the engine has been warmed up.
There are a few installations tips that you should keep in mind to get the most out of your install. It is highly recommended that you use a flat flange gasket, either the fiber type or an annealed copper gasket. The embossed copper gaskets will fail.
Lemons Headers suggests using Permatex Ultra Copper high-temp silicone on both sides of the gasket. Heat and differentiating metals are the enemy of threads and bolts, breaking off a header bolt is a massive pain. Fixing a broken header bolt requires removing the head from the engine or the engine from car in most cases.
You can avoid that by applying a generous amount of anti-seize to each header flange bolt. You can also use it with slip-on collectors. Sometime you run into an interference point with the header tubes.
It is OK to add a slight dimple in the tube, that won’t affect the flow any appreciable amount. However, if you need to really knock a section flat, you either need to move the offending component on the chassis or alter the headers. so that they clear without flattening the tubing.
There are many options when it comes to headers, finding the right one requires a little research on your application. Whether your engine makes 400 horsepower or 4,000, there is a header that will get the most out of it, and many others that won’t.