Editing Setting up an exhaust system

Diving head first into one of the most misunderstood aspects of performance, we’re going to talk a lot about exhaust flow and a little about sound. How it works, why it works, what makes it better, and what myths we need to bust! Let’s start with a basic exhaust design and go from there. The engine is making its power and sending spent hydrocarbons pulsating into the exhaust manifolds, dumping into pipes leading to a converters (on most cars), exiting to pipes that lead to the mufflers, and exiting the mufflers into another pair of pipes at the rear of the vehicle to safely expel the spent gases into the atmosphere. The exhaust system has 2 primary jobs to do – direct exhaust flow away from passengers and mitigate engine sounds. On late model cars they also have to cleanse the exhaust gases of pollutants. Yes, this is obvious to most all here so let’s dig deeper… Manifolds - From the factory most engines come with either a cast iron manifold or sometimes a simplified short header. Each has good and bad points from a manufacturing and performance standpoints. Cast iron manifolds are so common because they are very cheap to produce, long lasting, they retain heat for faster catalytic converter light-off, and they have a quieting effect for noise reduction. The downside is they are very heavy, the thicker wall construction often has the exhaust flow making hard turns in small ports, and the engine loses power just trying to push the exhaust out making them fairly restrictive. Manufacturers have started using more of the simplified headers on higher performance engines and even garden variety cars over the last 20 years. The smoother exhaust flow is an obvious benefit for the high performance engines and the lighter weight helps more pedestrian cars meet weight targets for maximum mileage. Most are made of 409 stainless steel, have short primaries varying between 8-12” of length meant more for packaging efficiency that performance efficiency, and merge into a chamber that is often attached directly to a catalytic converter since the “headers” shed heat much faster than cast iron. To make the most efficient exhaust system you need to use engineered or “tuned” headers. An efficient exhaust system has to manage 2 things – Flow and Heat. Let’s forget about sound for a moment and focus strictly on performance To visualize exhaust flow you need to remember that the exhaust comes out in pulses, so think of marbles popping out of the exhaust port at supersonic speeds as each exhaust valve opens. In a stock system they have to make hard right angle turns and collect at the merge point without any specific timing. Picture the marbles bouncing off the walls and into each other as they travel the manifold and you can get a visual of how a factory system restricts exhaust flow. Now picture those same marbles traveling down smooth, equal-length tubes with fairly gentle bends and coming together in a collector that allows each to enter at a specific time and point to keep them all flowing in a steady line. That’s part of why an aftermarket header makes more power, it allows for more efficient evacuation of the spent gases from the combustion chamber. But flow is just half of the story, the other part is heat management. Think of each exhaust explosion and how the heat reacts to movement in a tube or passage. The exhaust heat is expanding as it leaves the head and for that 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. This is what drives the pulsations of the flow and maintains movement, along with the actual weight of the gases, but the exhaust gases also start cooling and contracting as they go through the header or manifold and that cooling contraction has to be managed for maximum exhaust efficiency. If you look at headers on a full-blown race car you’ll see “steps” or different sizes in the primary pipes, this is how they maintain exhaust velocity for effective scavenging in the collector. You’ll also see equal-length primaries that ensure each exhaust pulse has cooled the same amount to maintain consistent flow into the collector. The vacuum created by each pulse as it flows into the collector is what pulls the exhaust through consistently and to put that into perspective, a V8 engine turning 8,000 RPM is creating 560 exhaust pulses per second! Managing the flow and temperature creates velocity and that is the key to minimize pumping losses and maximize horsepower! Now that we’ve pulled the spent gases efficiently out of the engine, and assuming we don’t have a turbo impeller at the end of the header (we’ll touch on that soon) we need to maintain that flow and heat through the catalytic converter (If you have no catalytic converter skip ahead, unless you want to learn something!) Catalytic converters have been a part of the American automotive landscape since 1975 and are a great thing for air pollution. When they first came out they were horribly restrictive and just killed performance of cars built in the last half of the 70’s. A car passing at full-throttle sounded like it was pushing exhaust through a straw, the hissing sound of the engines trying to push exhaust through them was terrible! Fortunately engineers in the 80’s and 90’s created better designs that were much less restrictive for exhaust flow and out-performed the earlier versions by a significant margin. The catalytic converter removes nitrogen oxides, unburnt hydrocarbons, and carbon dioxide and removing a modern catalytic converter will not unleash horsepower like it did in the 70’s and early 80’s so just leave them be. If you have an older catalytic converter car you can legally upgrade them to aftermarket high-flowing units and maintain the performance. How can modern converters maintain exhaust flow? They maintain heat in the exhaust which helps overcome the restrictions they do create making them nearly a wash in terms of power loss. Keep your car legal, keep your converters. From the converters we head down some more piping so let’s stop here and discuss the sizes of pipes we use. Let’s kill one myth right of the bat – Bigger is definitely NOT always better! Exhaust pipes have 2 jobs to do – Maintain flow and heat! Sound familiar? Well, just as the headers maintain the flow of exhaust by controlling velocity through heat management the exhaust behind them has to do much the same. This is where the aftermarket and magazine editors often get carried away with the “bigger is better” theme. When a performance company, particularly an exhaust company, is doing tests that will appear in a magazine they are looking to maximize the output for the reader. The theory is the “hero number” they hit on a dyno at 7k RPM will translate into sales, even if the reader is typically driving street machine that will never rev past 6k RPM and rarely even approaches that! The tactic works as legions of gearheads are out there thinking the 5.0 Mustang they drive daily is really benefitting from a dual 3” exhaust system. Hardly, in fact it’s probably costing them low end torque and likely horsepower too! So what size pipes should you use? It all depends on the car, the engine, the intended use, and routing. Is it a heavy car that will need maximum torque or a lighter car that will benefit from higher rpm power? Is the engine built for a street car with a strong torque band, and what size is that engine? Are you planning to do any racing with the car and if so, how much? Are you routing the pipes to the side, under the car, or all the way out of the back? For an example let’s use my Firebird. It’s a relatively light car at 3300 pounds (We’re talking musclecars here, not Hondas), it will use a fairly high-reving but small displacement 5.3 LS engine with a 6-speed, it will be used for autocross a few times every year but 90% of its use will be cruising or road trips. It will have a full length exhaust system exiting straight out of the back and will use converters to maintain the emissions system of the late-model engine. Smaller engines need more help building torque, and as a regularly used road car comfort is important, and racing is not a priority for this car. A 2.25” exhaust system will work just fine maintaining the most torque, being a little quieter so it’s not intrusive on long drives, and be easier to route around all the suspension pieces that are going to be under it. That’s my car, how about yours? For a “rule of thumb” to start with on dual exhaust systems - If your car develops less than 300 horsepower a dual 2” system is just fine. If you make less than 450 horsepower a dual 2.25” system would be ideal. If your engine is in the 450-600 horsepower range a 2.50” system will work great. If you make between 600 and 750 horsepower you should look at a 2.75” system (though hard to find). If you make 750+ horsepower you should look at a 3.00” system. I won’t go further than that as we are talking about street cars and there are very few that make much more than that. Notice that engine displacement isn’t factored in? It really doesn’t matter if you have a 4.0 Ferrari V8 or a 572 Ray Barton Hemi, if it’s putting out the same horsepower it’s creating the same pressure and heat. These sizes may go against everything you’ve read or been told but reason is again, flow velocity as a function of heat. As the exhaust temperature cools the gases become denser, therefore heavier, and require more power to push in a column of air through a system that runs the length of the car. Exhaust pipes that are too large will also allow our exhaust pulses to create a higher level of entropy, destroying the header tuning since the pulses will not line up as consistently as they would in a smaller pipe. To maintain the velocity all the way through to the rear bumper it even makes sense to go to a slightly smaller diameter pipe after the muffler and many of the old musclecars did just that! Let’s talk turbos! If you are lucky enough to have an impeller somewhere in your system you may have been told to go BIG with your exhaust plumbing, but the only difference that turbo makes in an exhaust system is adding a small amount of backpressure restriction which quiets the exhaust sound. An all-out turbo race car will often have just enough pipe coming from the turbo to exit the cars body but in a street car all the rules still apply with a full-length, street driven exhaust system as that heavy column of cooler gases will slow your impeller speed too. The EGT readings are reflections of spark and cam timing and fuel/air ratio, not exhaust system size so throw that myth out the window too! Now that we’ve covered flow and heat management let’s talk mufflers and sound management… The whole reason mufflers are used on cars is sound dampening and often, tuning. The capability of sound reduction for each kind of muffler comes down to how each design handles different sound waves in the system. Before diving into the designs let’s look at sound waves and their behavior. Humans can hear sounds between 30Hz and 20kHz, a pretty wide range but most of us have trouble hearing much above 15k Hz. The low end will be booming sounds, like bass guitars, and they are non-directional sound waves that you can hear clearly without having the source aimed at you. A woofer in a car stereo is often placed in the trunk or under a seat with no loss in sound volume or quality because of the non-directional nature of the sound wave. The higher sound waves are what you hear from a clarinet, a cymbal, or a trumpet. These sounds are best heard when you are facing the source of the sounds and that’s why stereo tweeters are placed close to ear level and aimed at listeners for maximum clarity. While it bears little effect on exhaust system efficiency it’s an important to understand how the high and low frequency waves react in a system if you want to achieve a sound quality that is appealing. There are other things that affect exhaust sound levels other than mufflers. Remember, the sound coming out of the combustion chamber is that of an explosion, of course, but the shape, size and material of the exhaust port can have an impact on the sound as well. When you install tubular headers on an engine those explosions are heard in the engine compartment, and often in the passenger compartment, as a “ting” sound with each pulse. That’s the high-frequency sound waves bouncing around the walls of the port and the inside of the tube. With tubular headers the flange and wall tubing thickness, materials and coatings all have some effect on the sound and volume coming from the port too and some engines (particularly aluminum headed engines) are just more prone to loud exhaust sounds. Cast iron manifolds are quieter and this is where they shine in subduing this noise and why they remain a popular choice for OEM manufacturers. A catalytic converter helps dampen the sound volume and allows for freer flowing mufflers without excessive noise levels. Turbochargers are so effective at restricting exhaust noise the early turbo cars came with a new kind of muffler. The “Turbo” muffler was designed to be less restrictive and somewhat louder than a standard muffler to compensate for the quieter exhaust. Today the term “turbo muffler” has taken on an entirely different meaning and they are typically chambered and fairly restrictive. Beyond turbos , catalytic converters, and manifolds the primary shaper of sounds in the exhaust is the mufflers and there are three kinds – Restrictive, Reflective, and Absorbing. They each have their good points and bad points. Restrictive mufflers are usually what the OEM manufacturers use, mostly because they have to pass DOT “drive by” noise restrictions for production cars and trucks. They can be tubular, a flat oval shape, or “suitcases” that are found on several performance cars. To attenuate the exhaust sound waves they are run through sound insulation in one, two, or three passes, through chambers and through baffles. These are effective at attenuating most all exhaust sounds and can be tuned by an OEM company to provide a signature sound range for their cars but are the most flow restricting mufflers made. Reflective mufflers rely strictly on sound waves self-cancelling each other but none can diminish all frequencies so they often increase sound levels at certain frequencies. The lack of sound absorbing materials inside also allows frequency resonance to transfer through the case and into the passenger compartment. These mufflers were originally created to meet sound level requirements for race tracks that were near residential neighborhoods so the sound suppression is fairly minimal and it comes at the cost of exhaust velocity being reduced as it hits the walls inside the chambers. These are popular on limited use street machines and that popularity is almost exclusively due to racing roots and “close to legal limits” sound levels. Absorbing mufflers are those that are a straight-through design and use a layer of sound absorbing material around the core to dampen exhaust noise. The most obvious example is the glass-pack muffler but other designs use stainless steel wool as it lasts many times longer. The core is usually a perforated sheet metal tube, sometimes with louvers or paddles, but one company makes the core from a gapped-spring which also creates a reflective sound wave cancellation. These mufflers are the best compromise between sound suppression and exhaust flow among performance mufflers, particularly at cruising speed, simply because they have a consistent layer of sound absorbing material around the exhaust flow. They are also popular for the lack of restriction and nearly unmuffled sound when a car is at wide-open throttle that mellows at cruising speed for comfort. Designing a system that will meet your desire for power, efficiency, and sound quality is more involved than picking a system based on what you’ve heard. Using the information above will help you decide on what you want and need based on your car, what part of the power band you should focus on for power production, being realistic about how your car will be used, and recognize that the best system is one that will also be livable when you are on the highway.

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