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We’ve seen too much misinformation regarding exhaust theory. What kind of misinformation? For starters, there are a lot of people in the “Bigger is Better” camp. We’re talking about exhaust pipe diameters. Even the big magazine editors are boldly smattering statements like, “For a turbo car, you can’t get an exhaust pipe that’s too big.” Also, terms like “back pressure” and the statement, “An engine needs back pressure to run properly!” really rub us the wrong way.
Let’s start from the beginning. What is an exhaust system? Silly question? Not hardly. Exhaust systems carry out several functions. Among them are: (1) Getting hot, noxious exhaust gasses from your engine to a place away from the engine compartment; (2) Significantly attenuating noise output from the engine; and (3) In the case of modern cars, reduce exhaust emissions.
In order to give you a really good idea of what makes up an exhaust system, let’s start with what exhaust gas travels through to get out of your car, as well as some terms and definitions:
After your air/fuel mixture (or nitrous/fuel mixture) burns, you will obviously have some leftovers consisting of a few unburned hydrocarbons (fuel), carbon monoxide, carbon dioxide, nitrogen oxides, sulfur dioxide, phosphorus, and the occasional molecule of a heavy metal, such as lead or molybdenum. These are all in gaseous form, and will be under a lot of pressure as the piston rushes them out of the cylinder and into the exhaust manifold or header. They will also be hotter ‘n Hades. (After all, this was the explosion of an air/fuel mixture, right?) An exhaust manifold is usually made of cast iron, and its’ primary purpose is to funnel several exhaust ports into one, so you don’t need four exhaust pipes sticking out the back of your Vehicle unless your cool.
Exhaust manifolds are usually pretty restrictive to the flow of exhaust gas, and thus waste a lot of power because your pistons have to push on the exhaust gasses pretty hard to get them out. So why does virtually every new automobile sold have exhaust manifolds? Because they are cheap to produce, and easy to install. Real cheap. Real easy. Like me 😀.
“Ok,” you ask, “so now what?” Ah, good thing you asked. The performance alternative to the exhaust manifold is a tubular one. What’s the difference? Where a manifold usually has several holes converging into a common chamber to route all your gasses and is often cast from thick iron with un equal length primary's a tubular one or known as a header in the states has precisely formed tubes that curve gently to join your exhaust ports to your exhaust pipe. How does this help? First of all, as with any fluid, exhaust gasses must be treated gently for maximum horsepower production. You don’t want to just slam-bang exhaust gas from your engine into the exhaust system. Just as the body of your favourite super model is beautiful, swoopy, and aerodynamic, so must be the inside of your exhaust system.
Secondly, a manifold can be “tuned” to slightly alter your engines’ characteristics. We’ll go in-depth into tubular manifold or header tuning a little later.
Nextly, exhaust gasses exit from your manifold or header, travel through a bit of pipe, then end up in the catalytic converter, or “cat”. The cat’s main job is to help clean up some of the harmful chemicals. In most cars, they also do a great job of quieting things down in standard 400cell type and giving any exhaust system a deeper, mellow tone.
From the catalytic converter, the exhaust gasses go through a bit more pipe and then into a silencer or system consisting of several silencers and/or resonated silencers.
Exhaust gases leave the engine under extremely high pressure. If we allowed exhaust gasses escape to the atmosphere directly from the exhaust port, you can well imagine how loud and police attracting the noise would be. For the same reason gunshots are loud, engine exhaust is loud. Sure, it might be cool to drive around on the street with that testosterone producing, chest-thumping, 150 decibel roar coming from your car. Even the gentleman’s gentleman has gotta use a silencer or some sort or system of silencers, on their exhaust.
Again, you may hear a few SPMT’s tell you that an engine needs some backpressure to run properly! Nonsense. A silencer can no more “make” horsepower than Wile E. Coyote can catch roadrunners. Any technician with any dyno experience will tell you that the best silencers are no silencers at all!
Silencers can take care of the silencing chores by three major methods: Absorption, Restriction, and Reflection. Silencers can use one method, or all three, to attenuate sound that is not so pleasing to the local police.
The absorption method is probably the least effective at quelling engine roar, but the benefit is that “absorbers” are also best at letting exhaust gas through. Good example of absorbers are the silencers found in the good old-fashioned Cherry Bomb glasspack.
Absorption silencers are also the simplest. They are a simple construction consisting of a perforated tube that goes through a can filled with a packing material, such as E fiberglass and or stainless steel wool. This is pretty efficient for sound absorption, simple fact is that sound in a pressurised system travels at 90 to the direction of flow, this is similar to simply punching holes in your exhaust pipe, then wrapping it up with insulation. Neat, huh?
Another trick absorption silencers use to kill off noise is, well, tricky. For example, the Hooker Aero Chamber silencer is a straight-through design, with a catch. Instead of a simple, perforated tube, there is a chamber inside the silencer that is much larger than the rest of the exhaust pipe. There is a pretty neat formula to work out the correct size for this so that flow isn't compromised but it's a secret.!! 🙈 This design abates sound more efficiently than your standard straight-through because when the exhaust gasses enter this large chamber they slow down dramatically. This gives them more time to dwell in the sound insulation, and thus absorb more noise. The large chamber gently tapers back into the smaller size of your exhaust pipe, and the exhaust gasses are sent on their merry way to the tailpipe.
Doesn’t that word just make your skin crawl?
Obviously, a restrictive silencer doesn’t require much engineering expertise, and is almost always the least expensive to manufacture. Thus, we find restrictive silencers on almost all OEM exhaust systems, most even have two silencer and one of each, were one would work perfectly well. We won’t waste much time on the restrictive silencer except to say that if you got ’em, you might not want to flaunt ’em.
Probably the most sophisticated type of silencer is the reflector. They often utilize absorption principles in conjunction with reflection to make the ultimate high-performance silencer. Remember any of your maths from school? Specifically, that like numbers cancel each other when on a criss-cross? That’s the same principal used by the reflective muffler. Sound is a wave. And when two like waves collide, they will “cancel” each other out.
To gain a more complete understanding of how silencer and manifolds do their job, we must be familiar with the dynamics of the exhaust pulse itself. Exhaust gas does not come out of the engine in one continuous stream. Since exhaust valves open and close, exhaust gas will flow, then stop, and then flow again as the exhaust valve opens. The more cylinders you have, the closer together these pulses run.
Keep in mind that for a “pulse” to move, the leading edge must be of a higher pressure than the surrounding atmosphere. The “body” of a pulse is very close to ambient pressure, and the tail end of the pulse is lower than ambient. It is so low, in fact, that it is almost a complete vacuum! The pressure differential is what keeps a pulse moving.
Ok, now that we know that exhaust gas is actually a series of pulses, we can use this knowledge to propagate the forward-motion to the tailpipe. How? Ah, more of the engineering tricks we are so fond of come in to play here.
Just as Paula Abdul will tell you that opposites attract, the low pressure tail end of an exhaust pulse will most definitely attract the high-pressure bow of the following pulse, effectively “sucking” it along. This is what’s so cool about a tuned manifold. The primaries on a manifold are specifically tuned to allow our exhaust pulses to “line up” and “suck” each other along! Whoa, bet you didn’t know that! This brings up a few more issues, since engines rev at various speeds, the exhaust pulses don’t always exactly line up. Thus, the reason for the Try-Y header like a 4-into-1 manifold, etc. What are exhaust manifolds and stock exhaust systems good for? Besides a really cheap boat anchor? If you think about it, you’ll realize that since stock exhausts are so good at restricting that they’ll actually ram the exhaust pulses together and actually make pretty darn good low-end torque! Something to keep in mind, though, is that even though an OEM exhaust may make gobs of low-end torque, they are not the most efficient setup overall, since your engine has to work so hard to expel those exhaust gasses. Also, a manifold does a pretty good job of additionally “sucking” more exhaust from your combustion chamber, so on the next intake stroke there’s lots more fresh air to burn.
While you won’t be able to determine the optimal power range of the manifolds by eyeballing them, you’ll find that in general, the best high-revving horsepower can be had with headers utilizing larger diameter, shorter primary tubes. Headers with smaller, longer primaries will get you
slightly better fuel economy and better street driveability.
The answer, simply, is no. The most efficient silencers can only employ the same scavenging effect as a manifold, to help slightly overcome the loss of efficiency introduced into the system as back pressure.
Another object that might be sitting in your exhaust flow is a turbine from a turbocharger. Turbos introduce a bit of backpressure to your exhaust system, thus making it a bit quieter. All of the typical scavenging rules still apply, but with a twist. Silencers work really well now! Remember, one of the silencing methods is restriction, and a turbine is just that, a restriction.
We’ve seen quiet a few “experienced” racers tell people that a bigger exhaust is a better exhaust. Hahaha… NOT.
As discussed earlier, exhaust gas is hot. And we’d like to keep it hot throughout the exhaust system. Why? The answer is simple. Cold air is dense air, and dense air is heavy air. We don’t want our engine to be pushing a heavy mass of exhaust gas out of the tailpipe. An extremely large exhaust pipe will cause a slow exhaust flow, which will in turn give the gas plenty of time to cool off en route. Overlarge tubing will also allow our exhaust pulses to achieve a higher level of entropy, which will take all of our manifold tuning and throw it out the window, as pulses will not have the same tendency to line up as they would in a smaller pipe. Coating the entire exhaust system with an insulative material, such as manifold wrap or a ceramic thermal barrier coating reduces this effect somewhat, but unless you have lots of cash burning a hole in your pocket, is probably not worth the expense on a street driven car.
One of the main problems of exhaust system manufacture is the equipment and knowledge of the staff in operating it.
A system must be mandrel bent.!! That's one very important point, if it's crush bent like the photo and made up with dairy tube you'll be better off with a stock OE system.
A lot of the time, you’ll hear someone talking about how much hotter the exhaust system on a turbo car gets than a naturally aspirated car. Well, if you are catching my drift so far, you’ll know that this is rubbish. The temperature of exhaust gas is controlled by air/fuel mixture, spark, and cam timing. Not the turbo hanging off the exhaust manifold.
When designing an exhaust system, turbocharged engines follow the same rules as naturally aspirated engines. About the only difference is that the turbo engine will require quite a bit less silencing.
So when designing your Ultimate Exhaust System, do be careful, don't be asking for a 3inch system for your 105hp golf when in fact it would make more power and torque with a 2inch system.