true dual or cat back

Disclaimer: I am not an expert in regards to exhaust theory, design, or even installation for that matter. Exhaust is a very complex part of total engine design and there are many well written books that deal specifically with this subject. One in particular that I highly recommend for folks who like to dig into advanced theoretical writings is: Scientific Design of Exhaust and Intake Systems by Philip Smith and John Morrison, 3rd Edition. Don’t let the 1972 publication date fool you. It is theory and physics and they do not change with age. It does not cover EFI, boost, turbo, or crossovers, but again, theory is theory and this book has it. A second book that should be in every hot rodder’s library that is primarily centered on mathematical equations with great text explaining them is: Internal Combustion Engine Fundamentals by John Heywood. This book is also used as a standard text book at MIT. By writing this paper, I am attempting to bridge a gap that I see between the overly complex, and overly simple exhaust theory in a way that I hope is understandable to anyone reading it.

Before one can understand exhaust theory, you must first understand camshaft theory and how valve overlap and exhaust scavenging interact. This particular exhaust theory discussion will take place downstream of the cylinder head exhaust port and will not go any further into the engine. The key point to keep in mind is that an internal combustion engine can not function properly without a negative pressure area (vacuum) on the downstream side of the exhaust port. The exhaust system needs to be designed to pull exhaust away from the exhaust port. It also needs to continue pulling exhaust until it exits the exhaust tip.

Exhaust scavenging (or vacuum) is created by three basic principles and design characteristics that emphasis those principles. 1) velocity 2) thermal dynamics 3) pressure differentials

An exhaust system needs to be sized and built in a manor that will support those three principles for any given engine. Engine max rpm, cubic inch displacement and cylinder pressure dictate what those requirements are. An engine that spends the majority of time between 1500 and 3000 rpm will need a system sized differently than one that spends all of it’s time above 6000 rpm. The same consideration is for a 350 CID engine over a 454 CID engine. It’s all about the amount of exhaust gasses that the cylinder swept volume will generate. Cylinder pressure plays into this as an engine running a dynamic compression ratio of 8/1 does not generate the same amount of exhaust gasses as a boosted engine developing 10/1.

Starting at the primary tube, the cylinder swept volume and engine rpm needs to be considered. For example, a V8 engine with a cubic inch displacement of 350 has 43.7 cubic inches of volume per cylinder; while a 500 CID V8 has 62.5 CID. At 6000 rpm each cylinder will have fired and expelled exhaust gasses produced through combustion 3000 times, or 50 times per second. That equates to a gaseous volume of 2185 cubic inches for the 350 and 3125 for the 500. Conversely, at 8000 rpm the 350 will generate a gaseous volume of 2913 cubic inches and the 500 will generate 4166 cubic inches. With some simple math it’s easy to demonstrate why engine size and operating speed need to be considered when designing a system. To further complicate the design, velocity, thermal dynamics and pressure all play key rolls.

Starting at the primary, velocity is the primary consideration. The primary diameter and length are sized to where the exhaust pulse will dump into the collector at just the right time and speed to create a vacuum on the next cylinder that will be firing. The cross sectional area of a primary tube that is 1 5/8” in diameter and 32” long has a volume of approximately 66 cubic inches, and the same length tube with a 1 ¾” diameter has 77 cubic inches. (Bent radiuses require some calculus to figure and will affect the total calculation to some degree, hence the “approximate”). To demonstrate the effects that velocity has, remember that the gasses expelled from the cylinder are under pressure and are much denser than air. You can experiment with an unopened can of soda (or beer if you prefer) and piece of plastic pipe just large enough that the can will fit into and about three feet in length. Stuff the can in the pipe while holding it horizontally with your hand covering the end you slid the can in. Slowly lower the other end of the pipe and take notice to what degree of vacuum you feel pulling on your hand. Repeat this again, but this time quickly lower the pipe. You’ll notice a significant increase in vacuum generated by the can dropping through the pipe faster. This is the same thing that the exhaust pulses do as they travel through the primary tube to the collector. If you were to repeat this experiment but with a larger diameter pipe, vacuum is lost. This is the same effect that will occur if primary tubes are oversized. By over sizing the tubes, the density of the exhaust pulse is reduced. To think about that differently, hold your finger over a garden hose. The stream that comes out the end, although restricted, sprays out as a stream. Take your thumb off and the water just pours out.

Getting back to the length of the primary and timing of exhaust pulses, V8 IC engines fire each cylinder every two engine revolutions (four stroke engines). Every 90 degrees of rotation, a cylinder will fire and another cylinder will be expelling exhaust gasses. This is where primary length comes into play. When the pulse dumps into the collector, velocity is lost (coke can in the bigger pipe). Although velocity is lost, there is still an opportunity to extract energy from the pulse. To demonstrate this, blow over the top of a straw in a glass of water and watch the liquid rise up the straw. This occurs in the collector. As the pulse travels through the collector into the exhaust pipe, it pulls on the other primary tubes. To even further take advantage of energy in the collector, primary orientation into the collector, the length of the collector and the collector design in itself can assist in maintaining vacuum on opposing primary tubes. As mentioned earlier, a cylinder will expel gasses ever 90 degrees of crank rotation. The third generation GM small blocks now use a true bank to bank firing order (side to side). All previous versions used a different firing order as well other manufactures to where two cylinders would fire 90 deg apart on the same bank. In these engines, equal length primary tube headers would cause pulses dumping into the collect simultaneously, hindering the ability to pull vacuum on the collector if they were of equal length. That isn’t the case with the 3 rd gen engines. Primary tubes should be equal in length. Not only equal length, but how they are arranged into the collector. They should be arranged so that as each pulse enters the collector, the pulses dump in sequential rotation. This will create a venturi effect that will in turn create a negative pressure area within the collector. There’s one more way to extract energy in the collector by increasing velocity once again. Merge collectors are designed in a way that the primaries are blended into the collector emphasizing the effects of a nozzle. They also neck down in the middle and expand back out to their original size where the primaries dump into them. By doing this, the collector effectively becomes a nozzle which increases velocity, in turn pulling more vacuum on the primaries.

Continuing the exhaust pulse’s travel past the collector, it then enters the exhaust tubing. In years past, the rule of thumb was to run as short and straight a length of pipe that you could get away with. Most race cars will have just enough pipe to get the exhaust behind the driver’s side door to prevent the exhaust gasses from collecting under the car and migrating into the cab. The pipes would be the same size as the collector in an attempt to maintain velocity. There was no real way to generate additional vacuum on the system at this point, so the next best thing to do, was to let it flow unrestricted and exit the pipe as soon as possible. Between emission and noise control laws, catalytic converters and mufflers are required on vehicles that will be driven on public roads. Therefore, further design considerations must be made to prevent excessive back pressure in the collector.

One way to promote this was to join the exhaust pipes in a Y configuration to where the pulses from each bank would merge into the Y and pull a slight vacuum on the opposing bank’s pipe. The Y would then transition to a larger pipe that would then be routed to the back of the vehicle. The increase pipe size was to accommodate both banks and double the amount of exhaust gasses. Another innovation was to run dual pipes into an X connector that would then dump into dual pipes of the same diameter. The X configuration takes advantage of exhaust flow from the opposing cylinder banks (the straw in the glass of water). An H pipe system is another technique that has questionable merits. It does nothing other than equalize exhaust pressure between each pipe. I really do not know what else it can offer but maybe some sound characteristics to where each side of the exhaust will have a ‘fuller’, more pronounced tone.

There’s an abundance of different mufflers available with difference sound cancellation principles used. One type of muffler than can aid in generating a vacuum is a true turbine muffler. They will have internal baffling or dimples arranged in a way that will create a swirling effect on the gasses flowing through them. This swirling of gasses will create a low pressure area that will help pull the gasses along their way through the pipe leading to the muffler. In general, find the muffler that has the least amount of restriction you can find.

Thermal dynamics is rarely discussed when speaking about exhaust systems, but it also plays a critical roll in exhaust system design. Combustion gasses will reach temperatures around 1300 deg F. By the time the exhaust gasses reach the tailpipes and tips, it’s cool enough to hold your hand on the tip without burning it. 1300 deg gasses occupy more area than 100 deg gasses. For roughly every 320 deg F (77K), a gas will expand four fold. In other words, by the time 1300 deg F gasses reach 100 deg F, they will be taking up 16 times less space. Take a plastic milk jug and seal the lid at room temperature. Place it in the freezer and see what happens to the jug. A vacuum is now present in the jug and it is deformed. This also happens in the exhaust system as the gasses travel through the pipe and get cooler. As they cool, they contract which in turn creates a slight reduction in system pressure.

I’ve been speaking directly with headers to help clarify the effects of velocity, thermal dynamics and pressure differentials. The same principles apply to exhaust manifolds, but the effects of each principle are reduced due to inefficient design. However, they still apply. Each cylinder will still dump into a ‘mini’ primary tube that in turn dumps into a bastardized collector. By merely looking at the manifold on your vehicle you should be able to tell with little doubt how ineffective they are.

Without writing another ten pages or so, that sums up basic exhaust theory. I’ve intentionally left many details out of this to keep it as simple as possible, but yet retain some level of theory for you to ponder over. There are numerous header designs including Y, Tri, and stepped primary headers. They are all designed to take advantage of velocity for a given engine design and its intended application.

Exhaust tubing, (at least in my experience as others will surely differ in opinion) should be ran as straight as possible with long sweeps whenever possible where they need to be bent (mandrel bends are the best, but not very practical) to reduce restriction. The size of the pipe is governed by the size of the collector. If the pipe size is increased beyond collector size, velocity will be reduced. Remember that as the gasses cool, they require less pipe volume to maintain the same velocity. The only time it pays to increase pipe diameter is when running a single piped exhaust system where both banks are merged together. Size the pipe to have the same cross sectional volume as both pipes do from the collector / manifold.

Again, I’m not an expert on this topic and welcome discussion. Especially if you disagree with anything I’ve wrote. It is when there is disagreement that the real learning takes place. -mdrew