By Aaron Bonk
By the time the air and fuel mixture combusts and their waste product travels past the exhaust valves, the whole engine cycle is beginning to repeat itself once again. With this in mind, you might suspect the exhaust system is not really all that important, after all, it’s just letting out what the engine’s done with. It’s there to keep things quiet and help alleviate environmental concerns, but what else does it do? How important can it really be when it comes to horsepower?
Exhaust Manifolds: What They Do
The truth is the exhaust system is just as vital in dictating an engine’s performance potential as is the intake system. This is mainly due to the fact that the easier the engine can exhale, the easier it is going to be able to inhale the next time around. If burnt gases are unable to completely evacuate the combustion chambers, they may linger on, thus diluting the intake charge upon the next engine cycle. This will result in decreased power. If you understand how the intake system works, then figuring out the exhaust won’t be too difficult; it works in the opposite way. While the intake system relies on positive pressure to introduce air into the combustion chamber, the exhaust makes use of negative pressure to work most efficiently.
You may recall our discussion of pumping losses [chapter 1 link]. Since pumping losses have to do with the ability of an engine to flow, you have to be concerned with this when dealing with the exhaust system. The better the exhaust system, the better the chances you’ll have at decreased pumping losses, a good thing. The ill effects of pumping losses may be eliminated by decreasing the backpressure within the exhaust system. Backpressure refers to the positive pressure within the exhaust pipes caused by restrictive or small bends. Backpressure fights against the exiting gases preventing complete evacuation. A poorly designed stock exhaust system can exhibit as much positive pressure within its piping as a highly tuned turbocharged engine would within its intercooler piping – often times upward of 15 psi. The key is to reduce this pressure to more easily invite exiting gases outward. But despite the seemingly simple role the exhaust system plays, because of complex gas laws and thermodynamics properties, things can actually get quite complicated. A well-designed exhaust system is anything but simple.
There are really two types of exhaust manifolds: the traditional OEM-supplied cast iron pieces and high-performance aftermarket versions referred to as headers. While cast iron manifolds are designed for cost effectiveness and space conservation, headers are typically more concerned with power production. Headers are constructed of mandrel bent tubing and typically run individually from each exhaust port into a centralized collector, which we’ll discuss shortly. This is much more efficient when compared to many OEM manifolds that feature short, often unequal and log-shaped pathways, which often converge into one main tube instead of a merged, free flowing collector.
Exhaust Pulses and Energy Pulses
To understand how a typical exhaust system works, you first have to understand what exhaust pulses are. As for the exhaust manifold, its job is simple: to contain the gases that leave the cylinder head’s exhaust ports just after each power stroke. These gases exit the ports in the form of pulses, high-speed, high-pressure pulses created by the piston moving upward thus squishing everything out. The piston creates the pressure, but said pressure disappears once the exhaust valves close. Surrounding each exhaust pulse on a well-designed header is a large pressure differential with more pressure in front of the pulse than behind it – this is due to the valve closing. This pressure differential actually assists in evacuating the exhaust gases most efficiently. The process is as follows: once the high-speed, high-pressure exhaust gases exit past the exhaust valves, a low-pressure zone is left behind; in the best scenarios it not only leaves low pressure, but also leaves the negative pressure in the form of a vacuum. If you can control when this negative pressure referred to as the vacuum reaches the port then you can expect significant power gains. The process relies on the inertia created by the fast-moving gases. When strong enough, the inertia can actually help coerce gases through the open valves during overlap periods.
Aside from the exhaust gas pulses, there’s another kind of pulse going on here. Sound waves are also emitted into the manifold each time an exhaust valve opens up. Traveling at the speed of sound, these waves move at a quicker rate than the exhaust pulses do. Also in contrast to exhaust pulses, pressure waves bounce around within the pipes, and depending upon diameter changes and a number of other factors, they may often revert direction. The results are exhaust gas pulses traveling in one direction and low-pressure energy waves bouncing around all over the place. If the manifold can allow for these low-pressure energy waves to arrive back at the exhaust valves just as they once again open, the whole process can be used to the engine’s advantage. But this requires everything to be timed correctly and this is nothing short of difficult to figure out sometimes.
A collector’s job is to tie all of the cylinders’ pipes together in one common place and send them into a single exit pipe. A collector is generally a conglomeration of pipes all merged together, allowing for a smooth transition from the primaries or secondaries into the rest of the exhaust. When properly constructed, a good collector will take the low-pressure waves created earlier and send them back up the primaries, thus quickening the entire evacuation process. There are two scenarios in which exiting exhaust gases will encounter once they move past the valve: low-pressure or high-pressure. Low-pressure situations within the exhaust pipes help promote better flow by allowing for increased velocity through the exhaust ports while high-pressure situations do the opposite.
A properly designed collector will pair two cylinders together, which are opposite with one another in regards to firing order so as not to hinder one another’s ability to evacuate exhaust gases. The other cylinder’s pipes are kept separate so as not to contaminate one another. A good merge collector will pair the two opposite collectors together and not introduce them to the remaining cylinder’s pipes until a smooth taper has been achieved. It’s important to note here that in order for a collector to work properly, each header or manifold runner must be of the same length.
There are basically two different types of exhaust manifolds and they’re distinguished from one another by their piping configurations. The four-into-one exhaust manifold gets its name from its shape and design, four individual pipes exiting the cylinder head which all meet up to a single collector. This design makes use of extremely long piping initially exiting the cylinder head, otherwise referred to as primaries. Piping length on these manifolds is tuned specifically per each engine combination so that exhaust and energy pulses will be maximized. Additionally, by keeping each cylinder’s pulses separate from one another longer, each cylinder will be assured a low-pressure energy wave back at its respective exhaust valve. Four-into-one exhaust manifolds are generally restricted for high-RPM use and, as such, are commonly found on racing engines as well as street cars looking for the most they can get horsepower wise.
The other type of exhaust manifold is the four-into-two-into-one design, or more simply put the tri-Y. This design makes use of much shorter primaries, which then pair up and merge into a set of two secondary pipes. From there, the secondaries merge into a collector. The idea is to route two opposite cylinders in respect to firing order together and allow them to meet with the other cylinder’s pipes at a collector point. As far as length goes, these manifolds are similar to the four-into-one designs but feature shorter sections and additional transitions to get there. Tri-Y manifolds strive for the same low-pressure effect as the four-into-one manifolds do but in a different way. Since two cylinders share the same primary, the design allows for one of the two cylinders to make use of the other’s low-pressure wave produced by the previous cycle. In other words, a low-pressure area exists in the number two cylinder’s primary – which will ultimately speed up exhaust airflow – because of the number three cylinder’s earlier exhaust cycle. Tri-Y manifolds are best characterized for their mid-range responsiveness and modest horsepower gains in comparison to their four-into-one counterparts. This is due to the fact that the scavenging process, which we’ll discuss shortly, is spread out over a larger time period thus slightly reducing its potential peak.
While anti-reversion headers aren’t entirely different from a four-into-one design, what you’ll find inside of their primaries is enough to warrant their own discussion. These headers incorporate a step into each of their primaries just a few inches after the cylinder head flange. By increasing the piping diameter by as much as .25-inch, the process of exhaust gases flowing back toward the valves during valve overlap may be eliminated. A cool trick if it’s incorporated properly.
The Scavenging Effect
Before going any further, it’s important to discuss exhaust scavenging. Simply stated, exhaust scavenging creates a vacuum effect within the exhaust manifold by means of exhaust pulsation. The results can actually speed up the air and fuel mixture process thus creating a slight amount of positive pressure within the intake, of course resulting in more horsepower. The process begins once the exhaust valves close, and if air speed is kept high then the vacuum potential will increase by the time the exhaust valves open again. And if picking up a few horsepower is not enough, scavenging can also lower the temperatures in your cooling system. By evacuating the hot exhaust gases more rapidly, water temps often times inadvertently drop due to a cooler cylinder head.
Material Options and Coatings
Most OEM exhaust manifolds are constructed of cast iron. Generally speaking, these are heavy and prone to rusting and cracking. Aftermarket options include those constructed of mild steel and stainless steel tubing, which are both lighter than cast iron and offer their own advantages.
First, mild steel is cheap. These manifolds cost significantly less in comparison to stainless steel but are prone to rusting. It’s best to get one of these coated with a ceramic, heat-resistant coat. Stainless steel is the best choice, but is certainly the most expensive. This material won’t rust and doesn’t require any special coating. Accompanied with thick flanges, and provided thick 16 or 14 gauge tubing is used, these headers will last significantly longer than any OEM counterpart.
Speaking of coatings, they do more than simply offer rust protection and good looks; they also help keep heat inside the manifold, something that’s vital to proper scavenging. The hotter the exhaust gases remain, the more velocity they will have, which means a greater pressure drop and potentially increased horsepower. Aside from coatings, a number of header wrap cloths are available that are all designed to retain heat within the system. The bottom line is that the more heat you can keep inside the manifold, the cooler the engine bay and the cooling system will be.
Various Shapes, Sizes, Configurations and Their Effects
We’ve already established the differences between four-into-one and tri-Y manifolds, but there are still more manifold differences that should be covered, specifically, tubing diameter. As you may have learned by now, bigger is not always better. This statement holds true in regards to everything from ports, to valves, to exhaust manifold tubing. As we’ve seen with regards to intake manifolds, larger diameter pipes decrease the airflow velocity and vice versa. The same is true of exhaust manifolds. But that doesn’t always mean small is the way to go. If your main concern is to promote exhaust scavenging by means of maintaining a high gas velocity, then it’s best to lean toward the small side. But if high-RPM power is of top concern, then you may need to look toward larger diameter pipes. While it’s true that larger diameter exhaust manifolds will decrease the speed at which the air will exit the system, on the other hand it will allow for a greater volume of air to escape, thus allowing for the possibility of a greater volume of air to enter on the intake side.
The truth is exhaust manifolds are far more complex than they appear. What works well on one engine may prove disastrous on the next. To make matters worse, what works well on one engine only works well during a certain, specified and narrow RPM range. Yes, it’s possible to tailor pipe lengths, diameters, bends and shapes for one particular engine, but what works well for one engine speed may not be the best for the next. The key is constructing or selecting a manifold that works best for the type of driving you do the most. And don’t forget, like anything else, once other parts of the engine package are altered or modified, the whole dynamic changes and you may have to go back to the drawing board as far as the exhaust manifold is concerned.
Ideal scavenging may be realized only when a proper exhaust velocity is maintained. This is precisely why going with oversized exhaust piping is not always a good idea. Remember, the larger the area, the less velocity. Some may interpret this to mean that an engine requires backpressure; this couldn’t be further from the truth. Backpressure is bad in all cases when dealing with maximizing horsepower and this can be verified if you recall our discussion of the negative effects of pumping losses in chapter one. While a large set of exhaust pipes may decrease backpressure, they may also decrease the exhaust velocity to a point at which scavenging will be minimal. This all changes though when you throw turbochargers or nitrous oxide into the mix. Because of the increased exhaust gas volumes of these types of powerplants, it’s best to err on the larger end of the spectrum.
As is the case with tubing diameters, tubing length is equally as important. In order for the whole scavenging process to be timed correctly, it’s important that the exhaust manifold’s primaries be the proper length. Everything must correspond to the speed of the exhaust pulsed and the rate at which the exhaust valves open and close. Go with the wrong length pipes and the chances of the pulses being timed with the exhaust valve events decrease. Generally speaking, primary tubing diameters dictate where horsepower will peak while tubing length dictates at which point in the RPM range this peak will exist.
The bottom line is that you need to know what your goals are prior to selecting or building your own header. Generally, shorter and larger diameter primaries will yield more high-RPM benefits and longer and smaller diameter primaries will translate into a broader powerband.