We’re going to approach the engine a little differently in this book. Most books approach engine building and the explanation of engine operation by starting with the induction side. But in numerous discussions with engine builders and designers, it’s clear that it’s best to start with the exhaust side. If you look at the camshaft degree diagrams and pressure versus crank angle diagrams, they all start with the exhaust event. So that’s what we will do here.
The reason for this is because much of what occurs in the cylinder is a direct result of pressure pulsations that occur while the exhaust valve is open. Again, we will start with quite a bit of exhaust system theory because that’s the only way to really understand what’s going on inside the engine.
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Let’s begin the exhaust side of things with the opening of the exhaust valve. Combustion has already occurred and somewhere around bottom dead center, the exhaust valve opens. At this point, there is tremendous pressure in the cylinder and evacuating or reducing the pressure is important so that the engine does not have to expend power to pump the exhaust gas out of the cylinder. When the exhaust valve first opens, there is a pressure pulse that is sent out the exhaust header pipe along with the particles of exhaust gas residue or remnants of the combustion process.
It is important to evacuate as much of this residual exhaust gas as possible during the exhaust cycle. While purging 100 percent of the exhaust gas is probably not realistic, the greater amount of exhaust that is scavenged from the cylinder will result in less residual exhaust mixing with the next incoming intake charge. This mixing of exhaust gas with the fresh incoming air and fuel will reduce the potential cylinder pressure since the exhaust component will not burn a second time. So you can see that no matter how well you design and execute an induction system, the engine won’t make good power unless you do an equally good job of evacuating the exhaust from the cylinder. That’s another reason why the exhaust side of the engine is so critical. Again, this supports the system approach engine building.
Early production engines did a poor job of addressing the exhaust side of the engine, using inexpensive, log-style castiron manifolds to duct the exhaust gas into the exhaust system, perhaps through a pair of mufflers and then out the tailpipes. This restrictive system allowed the exhaust gas to stack up, creating pressure in the system. This required the engine to use power from the other cylinders to push the exhaust gas out of the cylinder. Because of this residual exhaust pressure, more exhaust gas remained in the cylinder. Reducing back pressure is a great way to increase horsepower and torque, merely by creating a less restrictive path for the exhaust to escape.
We will concentrate most of our attention on headers, but it is critical to emphasize that the entire exhaust system is crucial to overall engine performance. Since we are focusing on street engines, mufflers and full exhaust systems are required on most cars. This means the exhaust system must be sized accordingly with the engine. Large diameter systems tend to be louder, attracting the wrong kind of attention from the local representatives of the law. Large diameter exhaust pipes are also difficult to route past suspension and body pieces as well. It is possible to construct an efficient exhaust system in the 2 1/2 to 3- inch range that is quiet, high flowing, and efficient.
PRIMARY PIPE DIAMETER
The best way to help understand how an engine creates its power curve is to think in terms of gas velocity. The engine is an air pump, ingesting air, mixing it with fuel, squeezing and combusting the mixture, and then dumping the residual exhaust gas out of the cylinder. The velocity at which this air moves through the engine is critical to performance. As a way of understanding this, take a deep breath and then try to exhale the entire contents of your lungs through a small drinking straw. Exhaling all that air through that small straw is difficult and requires significant effort on your part to do so. Now increase the diameter of that straw and you’ll notice that it is much easier to exhale. This is exactly what the engine experiences except on a much grander scale.
Header tube diameter dictates much of this exhaust velocity. Exhaling through that small straw created a certain maximum gas velocity that is dictated by several factors, but the most important is the diameter of the pipe. Smaller pipes create higher gas velocities, but are limited to reduced volume by their small size. Larger pipes increase the volume of airflow, but suffer from reduced velocity. Neither situation is ideal. Compound this dilemma by operating the engine over a wide RPM band, and no single header pipe diameter is ideal. But we can come up with a few compromises that can improve overall power. In essence, we are compromising velocity with mass flow to create an exhaust pipe combination that will do both adequately.
Engine RPMdirectly relates to maximum exhaust gas velocity. That’s why small primary pipe diameters improve low and mid-range torque but cannot support adequate mass flow at higher engine speeds. Larger diameter pipes do a better job of supporting good exhaust gas speed and mass flow at higher engine speeds, but suffer from slow exhaust gas speed at lower engine speeds, creating a loss of torque. The accompanying sidebar on the effect of primary pipe diameter on an engine’s torque curve will help you see how this works.
PRIMARY PIPE LENGTH
The second half of the header equation is primary pipe length. If timing is everything, then that is also true when it comes to header pipe length. If you’ve read the sidebar on wave tuning, then you know that a reflected wave occurs after the exhaust pulse has traveled out the end of the header tube. This reflected wave can be used to help induction tuning if it arrives at the proper time. One factor affecting the timing of this wave is the length of the header primary pipe. Longer primary pipe lengths require more time for the reflected wave to travel back up the pipe to arrive during the overlap cycle when both the intake and exhaust valves are open. This reflected wave can be used to increase power, but it only works within a very narrow RPM range. A shorter header pipe length tends to improve power at higher engine speeds because the reflected wave has a shorter distance to travel. Longer header pipes tend to improve power at lower engine speeds because there is more time to allow the reflected wave to travel the length of the pipe. This is where the term tuned-length headers originated.
One other point worth discussing is actual primary pipe length. Many header companies claim to offer headers with equal-length tubes, but the fact is that with a few exceptions, you need to go to a custom-fabricated set of headers in order to truly achieve equal length. The idea behind equal-length headers is simple. With various primary pipe lengths, the additive effect of each cylinder enhancing power within the same narrow RPM band should improve torque at that point. If the pipe lengths vary, this will diminish the cumulative effect, but also spread the torque out over a broader RPM range. Most racers will also choose a set of headers with true equal-length primaries, but this may not necessarily be the answer. For street engines, equal-length primary pipes are not nearly as important as the pipe diameter and overall average length.
What does appear to be important is that the primary pipes are somewhat equal. It’s not good when one tube is eight or ten inches shorter than it’s longest cousin. The better quality headers tend to minimize the difference in length, but again, they are rarely equal. As far as length is concerned, the best street headers even for big cubic-inch small blocks tend to be around 25 to 32 inches in length. Shorter headers, as you might expect, are easier to fit in the car, but generally give up torque in favor of slightly better horsepower numbers. Header primary pipe diameters for a big cubic inch small-block will likely start at 1 3/4-inches and perhaps go to 1 7/8-inch on some of the larger engines of over 430 cubic inches.
There is some discussion and there have been tests performed on engines equipped not only with various length tubes but also ones of varying diameters. The idea is based on creating a family of four, two-cylinder engines that merely combine their power over a greater RPM range. This can create a broader torque curve in which you might actually see two distinctly separate torque peaks with a slight dip in between. The idea is more power across a wider RPM range for a circle track application where the engine can pull off the corner better. This has never gained major acceptance in the racing community and perhaps never will, since this also requires a rather complex camshaft with four different intake lobes and four different induction and exhaust pipe cross-sectional areas! It’s an interesting idea, however, and might have possibilities in the street market.
The collector was originally developed just to connect the four pipes into a common single pipe. But racers soon learned that by changing the shape, diameter, and length of the header collector, there was hidden power to befound. The collector, it turns out, is also a collector of additional power, if you know how to reap its rewards.
For standard-diameter primary pipes arriving at the collector, this is the first large area increase that the pipe experiences. This area increase also sends an expansion wave back to the cylinder. Individual pipes create a strong but short returning expansion wave back to the cylinder. But add a collector around the four pipes, and the expansion wave becomes less intense but stretches out over a longer period of time. The net effect is generally a boost in low and mid-range torque, with no negative effects at higher engine speeds. The torque increase is directly attributed to both the diameter and the length of the collector itself. Length plays the biggest part in developing when the torque increase is felt. Racers have known this for years as based on the results of lengthening the collector. Most often, there is a sweet spot at which additional length does not increase torque gains.
The net result of idealizing the collector is that power is increased below peak torque with no negative affects to top-end horsepower. Most collector diameters come in around 3 to as much as 4 inches in diameter for large cubicinch big blocks, but the most common collectors are 3 and 3 1/2 inch.
There is also considerable work on what are called merge collectors. The Burns Stainless company has probably done the most amount of work in this area and claims significant success with most open-header configurations. What is unusual is that the diameters of these merge collectors are generally much smaller than conventional collector sizes, as small as 1 3/8-inch in diameter for a small block. The gains are attributable to the converging/diverging collector design that enhances exhaust gas velocity while also increasing the net effect of the reflected wave back into the cylinder.
What has not been documented is the overall power gain that could be realized by using these merge collectors in conjunction with a muffled exhaust system. The merge collector relies on a major area change at the end of the collector for open-header applications. In a muffled exhaust system, this does not occur in as dramatic a fashion, so the positive horsepower effects of the merge collector may not be present. A series of dedicated tests on muffled street engines would be necessary to underscore their importance.
The most common style of header is the four-into-one style header where each of the four primary pipes join at the collector. A variation of that theme is the 180-degree header where cylinders firing 180-degrees apart from each other are joined at each collector. This means that two pipes from each bank must cross over under (or over) the engine to join the collector from the opposite bank. As you can image, this creates a very complex header system that eats up a lot of room. For a street car, this is impractical and is generally used only on competition engines. These header designs tend to maximize power within a very narrow RPM band, which is probably another reason why they’re not as highly prized, except perhaps for their distinctive highpitched screaming exhaust note.
Another variation in the header routine is the Tri-Y design where the four primary pipes are joined roughly halfway down the header length into one larger secondary pipe. The two secondary pipes are then joined to a smaller collector creating a four-into-twointo- one system. This header design has some slight torque advantages over the four-into-one design, but sacrifices topend power in the process. Generally, for a big-inch street engine that enjoys a torque advantage through displacement, the Tri-Y would not appear to be a primary choice.
Stepped headers are another variable that became popular with race engines in the late 1980’s. The idea was to create a small exhaust pipe at the connection between the exhaust port size where it is an extension of the port. Then 8-10 inches later you step the pipe size up. Be careful here, because this is very application specific. While the idea has merit, and several companies offer these headers in street configurations, the advantages have never been proven consistent. Unusual gains in power could probably be duplicated with a larger primary pipe header with a larger collector.
Another consideration for large displacement small blocks, is the concern that a 454ci small blockmay need to go to a header with 1 7/8-inch primary pipes. This size header is not beyond the realm of race-style small blocks, but for a street car, it does mean that you will need to invest in a set of custom or race-style headers. Hooker, for example, does make a 1 7/8-inch and a 2-inch header for a small-block Camaro or Chevelle applications. These headers generally come with an adapter plate that fits between the head and the header. This is used to position new attachment bolts since the larger tubes do not allow using the stock bolt holes. These headers can be adapted to fit a full exhaust system without having to weld the collector directly to the exhaust lead-down pipes. Another advantage to these headers is that they often offer adjustments to the primary pipe length if you wish to experiment with longer primary lengths. These adapter flanges are also available from Hooker and other header companies to allow you to adapt headers to other specialty heads such as 18-degree heads.
H AND X-PIPES
Racers and enthusiasts are also inveterate tinkerers, creating some unusual designs that can lead to small power gains. Downstream of the header collector, H-pipes were created to increase the volume and equalize the pressure imbalance on each side of the engine during the entire 720-degree firing cycle. The closer you move theH-pipe to the collector, the greater gains are possible. These H-pipes merely connect the left and right-bank pipes with a short, equal, or larger-diameter pipe. Tests have shown this idea to be worth some additional torque, but not all H-pipes work as advertised. The gain may be in found in the additional volume in the cylinders or perhaps in some reflected wave excursions, but the results are not consistent.
Another popular revision of the Hpipe idea is the X-pipe, where the two exhaust systems are merged in the shape of an X sharing a common area at the convergence of the two pipes. This system enjoyed some notoriety when used on Terry La Bonte’s Daytona 500-win-ning NASCAR car for a couple of years, but this system has yet to really prove itself on the street, although in certain applications or combinations of parts it may be found to create power in a limited RPM band.
MAKING EVERYTHING FIT
Ensuring that whatever combination of parts you choose fit properly is more than just a small concern. One of the biggest problems with headers is fitting them in the chassis and making sure they will clear the spark plugs. The biggest problem is that each head manufacturer places his spark plug in slightly different locations and angles. This is a nightmare for header manufacturers who can’t possibly offer a header for each cylinder head and chassis combination. This leaves it up to the end user to ensure that the headers will clear the spark plugs and spark plug boots. Sometimes this requires some minor header tube dimpling in order to create the necessary clearance. The best way to dimple a header is to heat the tube first with an oxy-acetylene torch, or even a hand-held propane torch, until the pipe is red hot. Then place a mandrel or large socket on the tube and strike the mandrel with the hammer. Do not hit the tube directly with a hammer. This will most likely split the tube, which will require welding to repair. Generally, a few whacks with a hammer on the mandrel will create the clearance you need.
Another trick that will help create spark plug and boot clearance is to use one of several shorty spark plugs that are now on the market. Most aftermarket heads use a long reach, gasketed spark plug. ACCEL offers a shorter version of this spark plug whose overall length is reduced to help with clearance problems. Depending upon the model and the manufacturer, there are several companies that build spark plugs that are slightly shorter and could offer clearance advantages.
The overall goal of the exhaust system is to carefully match the headers to the engine combination so that the exhaust does not present a restriction and also does an excellent job of scavenging as much of the residual exhaust gas out of the cylinder as possible. Do that, and your engine will reward you not only with excellent peak power numbers, but will also make great torque. This creates an excellent street engine that does everything well. That’s when you know you’ve done it right.
Written by Graham Hansen and Posted with Permission of CarTechBooks