Exhaust Design

Exhaust Design, Turbo and Naturally Aspirated

Exhaust design is one of the best areas to get great gains if done properly. There are many factors to consider when designing an exhaust system. There are also different requirements for turbo and non-turbo engines. But, firstly we need to talk about backpressure.  "backpressure makes torque" is often quoted but not always true. The reality is that backpressure is the enemy. You want to keep it as low as possible but keeping the velocity up. However doing the things that keep velocity high involves slightly more backpressure under some conditions. You want to keep the gasses moving as quickly as possible to make both good torque and top end power. The perfect exhaust system would keep the gasses moving as fast as they did coming out of the cylinder and have zero backpressure. However this is impossible to achieve in the real world. 

Pre-turbo Exhaust
The exhaust manifold / system before the turbo and the turbo itself have a greater effect on backpressure than the exhaust behind it. You want the least restriction after the turbo as possible for both top end power and quick spool-up. Careful attention has to be paid to keep velocity high before the turbo and in the exhaust housing of the turbo to spool the turbo up as quickly as possible while not choking off the exhaust gasses on the top end.

The exhaust manifold can be simpler in some ways than a non-turbo header. Bigger dividends can be had by getting the exhaust gasses to the turbo with the least amount of restriction, highest velocity, and the most heat rather than worrying about a tuned equal length design. Like non-turbo systems, it is optimal to make an equal length header, but often the space requirements do not allow for a tuned equal length manifold. This helps explain why we usually get near identical results from a factory manifold when compared to the aftermarket ones. The factory manifold gets the gasses to the turbo as quickly as possible and goes a good job of keeping the heat in. Aftermarket manifolds tend to take a longer path and loose quite a bit of heat in the process (reduction in exhaust gas temperature = smaller gas volume = loss in velocity). In this case a good manifold collector does play a more important role than the length of the pipes. If all of the gasses ram together at a steep angle it causes a lot of turbulence, creates backpressure, slows velocity, and tends to make a mess of things before the turbo, which is the worst spot for inefficiency on a turbo charged car. A good manifold design would be something like a 4 into 1 design that uses castings or good thermal coatings as much as possible to keep heat in and would also get the gasses to the turbo as quickly as possible. The manifold collector would have to be longer and transitions nice and smooth.

Turbo Exhaust Housing
The size and design of the exhaust housing plays a major roll in the spool-up characteristics of the turbo and its ultimate power potential. There has to be a balance met if you want to have the quickest spooling turbo for your power goals. If you go with too large of an exhaust housing you greatly increase lag. Too small of exhaust housing and you severely limit the amount of boost and top end power you can make. You can only push so much gas volume through a small housing without having negative side effects. Adding to the complication is that each psi / bar of boost created makes a ratio of backpressure before the turbo. It is different for each turbo, the amount of boost you are running, the size of the motor, RPM, and load on the motor. Once you start trying to push too much through the exhaust section of a turbo (running too much boost for the turbo) you start making a huge ratio of backpressure, and it only gets higher the more boost you run. This not only limits the amount of power you can make, but makes EGT go up, hinders the motor's ability to get the burnt gasses out of the motor, and makes the car more prone to detonation. This is also a big cause for failed pre-cats in the downpipe. Choose too large of an exhaust housing for the application and it takes the turbo too long to spool, effecting torque production. The best way to make good torque on a turbo motor is to spool up the turbo as quickly as possible. Also, who cares how big your turbo is, or what power it can theoretically produce if you can never spool it up or if it falls out of the powerband every time you shift.

Adding another factor is the design of the exhaust wheel. It has to have good aerodynamic properties or it is inefficient. A more efficient wheel design means that you will make more power and/or less lag.

Flange w/Simple Pipe - The only advantages to this design are cost and simplicity. The pipe does not have to be formed and the flange is simple therefore reducing cost. The labour to weld the pipe to the flange is easy and therefore less costly as well. That is the main factor that make it desirable to the factory and why it is used on the stock exhaust. The wastegate gasses joining the turbo gasses right at the turbo outlet does create turbulence in the worst spot post turbo and reduces flow, thus not making it as desirable for performance as other designs.

Bell Mouth - This method is much closer to optimal for joining the gasses from the outlets. There is more room for them to join and if the transition is done properly it can flow very well into the main piping. It packages very well and does not have a lot of complexity, making for less to break. We have gotten the best results from this type of downpipe so far. Boost response has been the best out of the outlet designs we have tuned on, it is easy to put a wideband oxygen sensor bung into. We have also had the fewest problems with this design.

Split Bell Mouth - This design separates the gasses in the beginning of the turbo outlet and joins them at the rear of the bell mouth section. It works well and has some of the advantages of the bell mouth and some of the advantages of the divorced wastegate designs. The main deterrent for this is the cost and complexity of adding the splitter. I am a fan of keeping things as simple as possible while still making the product work well.

Divorced Wastegate - Keeping the gasses from the turbo outlet and wastegate separate until farther back in the system is an attempt to combine the advantages of not collecting the gasses and the real world. Combining them far back is closer to optimal than collecting them closer to the outlets. It is also critical to power production and spool-up to join the pipes smoothly and avoid turbulence. The disadvantages are that you add a lot of cost and complexity. You have big temperature differences on each pipe and that makes for a system that can crack. Putting in flex or expansion joints helps, but adds even further complexity and yet another part to fail. Between 4" and 6" is the best compromise before blending in the wastegate back into the system. 

Cast Outlets - Castings have the advantage of keeping a lot of heat in the exhaust as well as freedom with design. You can basically make it almost any shape you want. The disadvantages are more weight and cost. Cast iron pieces can weigh a ton and that is a valid concern for many people. The casting form that the piece is made in is also very expensive and depending on complexity can range from a couple of thousand dollars to well up in the tens of thousands.

Formed Piping -Forming pipe has almost as much design freedom as a casting with less expense and less weight. The only disadvantage lies in if it is not done properly. Poor forming can look bad and effect flow by having creases and crimped spots. You can also get the piping too thin if you try to stretch the metal too far. If done improperly you can also make the metal brittle and it will usually happen where the metal is the thinnest. This is why we use mandrel formed bends which have no ovality and no creases etc due to the forming of the bend. In addition, where possible, no bend angles over 60 deg are used as over this angle, backpressure starts to increase. 
Remember, you will only flow as well as the greatest restriction. If you have a poor cat or exhaust silencer box design, then it will choke the flow no matter how good the rest of the system is designed. Fortunately straight through exhaust silencers and newer high flow cats flow very well. Having a cat is not only good for the environment, but we have seen very little power difference in levels in excess of over 350 h.p. Also, a cat tends to quiet things down a little.
Pipe diameter does have an effect on flow rates as well, but again it is not the major factor in most cases. 2.5" may flow enough for 300-350 h.p. without being a restriction. 3" is usually capable of flowing 500-600 h.p. before becoming a restriction. This is assuming that you have designed the rest of the system up to par. There are also full 3.5" systems and those that start out at 4" and taper down. Unless you are making over 500-600 h.p. anything over 3" is a case of diminishing returns and in most cases has no advantage. There is more to gain going from 2.5" up to 3" than there is going from 3" to 3.5". A 3" system will not loose torque compared to a 2.5" system if designed properly. In fact if designed properly 3" may be capable of making better low end torque than 2.5". Again, since the way to make the most torque with a turbo exhaust is to get the turbo to spool-up as quickly as possible, it should be the main goal of the entire exhaust system and good flow after the turbo is one way to achieve it. We use 3" for 3.0T Supra, 2.0T Impreza etc as we want our system to flow enough to be capable of coping with a customer's changing goals ie there will always be more modifications to follow after a good flowing exhaust system. Properly designed we can offer it to the big power crowd while still appeasing the low end torque club.

The only reason to reduce the size towards the end of the pipe is for packaging, cost, and noise reasons. Tapering the diameter does not make more power, torque, or bring on boost faster. However having smaller pipe towards the end has less effect that having smaller piping at the beginning. In other words a system that has 3" pipe for the majority, and necks down to 2.5" at the end will flow enough for more power than a complete 2.5" system. The further downstream you neck down the exhaust the better……..if you decide to neck it down. Attracting unwanted attention and not hearing your stereo or you passenger would make for an exhaust system great for a racecar, but poor for the average Joe Public. Law enforcement and your neighbours do not appreciate loud exhausts either, even if you do.

Manifold
The manifold has the greatest effect on the power band and ultimate power production of a non-turbo car. There are MANY factors that go into a properly designed header. One factor is the way you join the pipes together. The two possible configurations for a 4 cylinder are 4-2-1 and 4-1. Basically a 4-2-1 design joins two primaries together into a secondary pipe, and then joins the two secondaries together. A 4-1 design joins all four at the same time. Both have advantages, but the 4-1 design allows the gas pulses to interact in a way that makes the best torque. Here is a simple drawing to help visualize the two different styles.

Primary Pipe Diameter -Smaller diameters keep velocity higher with smaller exhaust volumes. The more exhaust you are trying to push out the larger the primaries need be. The volume of gasses that you need to flow depends on displacement, RPM, and load. The more displacement you have per cylinder the larger the primaries need to be. The same is true for RPM, the more RPM you will be turning, the more diameter you will need as you will be pushing out a lot of volume over time. Higher loads on the motor also create a higher volume of gasses. As with every other variable there is a balance to be kept. If you are not flowing enough gasses for the pipe diameter (pipes are too big) the gasses will loose their velocity If the gasses get too slow you loose torque, and if you go way to large you can even loose top end power as well. Get it right and you get the best of both worlds, good low end torque and good top end power.

Primary Pipe Length -This has a huge effect on the powerband. Generally longer primaries make better low end while shorter lengths move the powerband up in the RPM range. The length affects the powerband by timing when pressure waves reach the cylinder. To put it as simply as possible, the pressure wave comes out of the cylinder and travels down the primary pipe until it hits the collector. There it gets reflected back down the primary pipe as a negative wave. When it hits the cylinder it helps pull more exhaust gasses out of the cylinder and pull more air in to the cylinder. Since power is made by mixing air and fuel and then exploding it, more air and fuel make more power. This effect is known as scavenging and is one of the main goals of a well designed manifold (scavenging is vitally important on 2-stroke engines  but plays a major influence in 4-stoke engines aswell). Equal length primaries help each exhaust pulse pull the one behind it. This helps create a suction in a sense. Instead of just relying on the pressure of the exhaust stroke of the motor to get the spent gasses out, the suction of the pulse in front of it helps pull it out. One factor some header designers forget when trying to design an equal length header is that the length of the exhaust port is effectively part of the header and needs to be accounted for. Complicating this is the fact that the exhaust ports are not the same lengths. Below is a simple drawing of the port layout. You can see that Cylinder A has a longer path than Cylinder B.

Collector Width -The width of the collector helps control how well the exhaust pulses interact with each other. Make it too big and one pulse cannot help pull the next very well and the gasses can stagnate hurting flow. Make it too small and you hinder flow by causing too much backpressure. Yet another area to test.

Taper Angles - Basically you want the least amount of abrupt changes as possible. This mostly applies to the collector where it necks down to the diameter the exhaust will be. You do not want an abrupt angle as it will hinder flow.

The entries into the primary pipes from the head also have to be as close to the diameter of the exhaust ports as possible. This is so that you do not get yet another area for turbulence to get in the way of things. Protrusions into the gas flow should be avoided here most of all, as they have a much larger effect than in any other point in the system. According to many experts that do not play the marketing game, the stepped header designs are an attempt to cure other problems inherent in the design. The steps also add complexity and cost.

Catalytic Converter
If you use a well designed cat there is very little power to be gained by not having a cat. The cat is a place where abrupt angles make a huge difference. Since inside the cat you are making drastic changes going from the diameter of the pipe, into a large diameter area inside the cat, and back to the diameter of the pipe having abrupt angles can really slow things down. This is as true for turbo as it is for non-turbo cars. You also want the gasses spreading out to flow across the complete area of the catalyst bricks of the cat. If the gasses are too concentrated on one part of the cat you will not be able to flow to the full potential of the catalyst bricks. That is why you see the gentle angle at the beginning of the "good" cat rather than at the end of the less optimal "better" cat.

Cat-back
Designing a good cat-back is fairly simple compared to the exhaust manifold. Keeping exhaust gas velocity high is still the goal. Pipes that are too large will loose low end torque as the gas starts moving slower. Pipes that are too small will loose top end power. So again there is a balance to be reached. The same rule applies to keeping the pipes smooth and using proper bending techniques. The exhaust silencer needs to be as free flowing as possible without being too loud. Besides that there is not a lot of complexity in a cat-back. It is definitely easier to design than a manifold.

Conclusion
We hope that you learned some new and useful information. Our goal is not only basic design education but to highlight the knowledge we have in exhaust design. It is not meant to be a full guide as a full detailed explanation of exhaust design would take a huge book to discuss completely. We do have the resources to design components that would be 100% optimal, but there has to be a balance reached with cost and complexity just as much as any other factors to consider. We try to keep a balance in all of our products.

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