Ever wondered how performance exhausts work and if its worth paying for high performance exhaust systems and when or if they will give any real performance upgrades or just a nice look and sporty sound? This content should explain the majority of the ideas that go in to making a sports exhaust system.
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There are many opinions on this, but from reading in to this from good sources (rather than just removed a silencer on their Civic, found it slower than instantly concluded that you need back pressure) I can tell you, back pressure is not needed.
Back pressure is created from restrictions in the exhaust, whether that be from certain silencers or kinks/bends in the pipe or the exhaust pipe size itself.
As the piston is pushing out exhaust gasses from the combustion chamber, the restrictions build pressure with the gasses in front of them, this makes it harder to exhaust all the gas from the cylinder and do it quickly.
The size of the exhaust pipe can change this by the diameter used for the piping. If the exhaust pipe diameter is too small then it will struggle to get all the gas through it, especially at high engine speeds and load. This does not mean that a massive diameter exhaust will work though.
By removing this back pressure you will make the engine more efficient and create more power as it doesn’t need to work against itself to push the exhaust gas out and with more gas out it can create more room for the fresh air/fuel mixture on the intake stroke.
Exhaust scavenging shows a problem that comes up when you use an exhaust pipe diameter that is too large.
As you increase the diameter of the piping, you allow more gas to travel through in the same space of time. This creates a problem by…
The exhaust gas comes out the engine in pulses, each after the exhaust stroke. At the front of each pulse is a high pressure area which decreases behind it. This decrease in pressure behind the pulse is useful as it can be used as a vacuum to suck out any remaining gas.
Using a large pipe here will slow down these pulses, whereas a smaller pipe would mean the pulse would have to travel at a higher velocity and have a higher pressure differential behind the pulse and suck out more remaining gas in comparison to a very large pipe.
This idea can be taken advantage of in multiple cylinder engines as a pulse from one cylinder can be used to suck out the gas from the next firing cylinder with clever manifold design.
It’s a bit of a balance trying to get the ideal size of exhaust to achieve high velocity gasses and low restrictions.
The idea behind anti-reversion is that as the exhaust stroke is nearing an end and the intake valve begins to open, fresh charge will only enter the cylinder if the pressure is lower than in the intake manifold, and if the case is opposite then exhaust gas could flow from cylinder to the intake manifold in the reverse direction as intended.
The exhaust system can be designed to reduce this effect by using a taper to go from a smaller diameter to a larger diameter exhaust soon after the valve as this makes gas easier to let out but creates a restriction on coming back in, same with the intake but in the opposite way, it will create a restriction on gases flowing in the reverse direction to reduce them going out the intake valve but not a restriction to the gasses going from the intake to the cylinder.
Resonators and Silencers
There are two main types of exhaust silencers, these are baffles and straight through types.
The baffle type is a box the contains a number of baffles, this interrupts the exhaust flow as well as the sound carried with it. As most aftermarket exhausts are built for performance, usually only stock exhaust systems use these types of silencer as they have a desire to emit the exhaust gas as quickly as possible.
The straight through type uses is a larger pipe around a perforated “straight-through” pipe. As the sound waves travel in to it, they can travel through the holes in the internal pipe and in to the outside pipe containing some sound deadening material which reduces the sound level and can distort it to cancel itself out. These are usually favored in the performance exhaust systems as the main exhaust stream is basically uninterrupted by the silencer. On some examples this type is actually less restrictive than a normal piece of pipe as the perforated pipe allows the air to flow over it with less friction than a solid pipe.
Some systems do use as combination of both types and changing this greatly affects the sound of the exhaust.
Heat Wrap & Ceramic Coating
These both try to accomplish the same benefits of each other, just the main difference with heat wrap is it doesn’t really look as nice as a ceramic coating does and needs replacing every so often.
Heat wrap is usually placed round the manifold section of the exhaust, these two modifications help by keeping the temperature high inside the manifold which keeps the gas hotter. This means less energy is taken from the gas and allows it to travel faster through the exhaust, making it easier for the engine to work.
The second benefit of this is it can lower than temperature around the engine bay as most of the heat comes from the exhaust, this will mostly benefit anything nearby the manifold as it won’t be exposed to the high temperatures caused by the exhaust gas.
I really hope this helped people with understanding how an exhaust system can increase and decrease power in your car. Any questions or improvements I can make? Please leave me a comment 🙂