Exhaust System – Forward Flow
An exhaust system with low resistance, starting from the turbine on turbo engines, or starting from the exhaust manifold on the rest, is an indispensable basis for obtaining good power. In most cases, the construction of a high-flow exhaust should occur at the initial stages of modifications, since this is a relatively inexpensive procedure.
How does it work? The back pressure of the exhaust gases that occurs behind the exhaust manifold – prevents the increase in power and therefore should be minimized as much as possible.
Reducing back pressure to improve the emission of gases from the combustion chamber allows more fresh air and fuel to be driven into this chamber. And as you know, the more fuel and fresh air enters the cylinders, the more power you get.
Another advantage of exhaust systems with low resistance is a reduction in exhalation losses, in other words, less effort is required from the pistons to eject burnt gases.
The road potential of high-flow exhaust systems cannot be underestimated – it gives a noticeably better response to pressing the accelerator pedal, increases torque and power at high speeds. Fuel consumption is also usually ‘improved’ – but it depends more on your driving style..
However, the dimensions of these advantages vary greatly from machine to machine. Turbo engines typically achieve greater power increases from exhaust modifications than atmospheric ones. This is due to the fact that the impeller of the turbine starts to rotate faster, which leads to an increase in pressure in the intake manifold and this pressure increases more quickly. Be careful – some companies claim a significant increase in power only from installing the right exhaust – it is very important to ask how much the pressure in the intake manifold increases.
Atmospheric engines are very sensitive to the design of the exhaust manifold, but for starters we will concentrate purely on pipe work, behind the exhaust manifold. And the design of the collector itself will be discussed some other time..
Let’s talk about how to choose the right components for the exhaust.
Types of silencers Currently, there are two main types of silencers: these are straight-flow and reverse-flow. Once widespread options with refitting begin to disappear, as people get the fact that their exhaust stream is scanty.
As the name suggests, Straight-Flow silencer (straight-through) – has a straight perforated (i.e., all in holes) pipe between the inlet and outlet. This perforated pipe allows exhaust gases to expand up to the outer wall. Note that the term straight-through is also suitable for systems that have a silencer on the left and a silencer on the right side of the machine. Such systems are also called biased exhaust silencer systems..
From the point of view of low flow resistance, once-through silencers are just a godsend. Tests prove this – quality straight-through silencers have more than 90% of straight pipes of the total length. In other words, you lose only 10% of the flow compared to as if you were driving completely without a silencer 😉
Back-flow silencers – at all times they showed something in the region of 60-70 percent of the flow with a straight-through silencer. Which is not surprising if you look at the internal design of such a system – the exhaust gases are forced to move in the opposite direction from their original direction (hence the name of the reverse flow), and unfold again before leaving the back of the muffler. So it turns out that they make two 180-degree turns, which leads to a decrease in flow as a whole. It can be added that some manufacturers of reverse flow systems make design changes, but the essence of this does not change. A silencer design can create or ruin the operation of the entire system. No-go silencers are leaders. (on this occasion, special tests were conducted).
In case you are not in the know, a “heart” tube is just a tube that has been bent to the desired position with the core inside, which allowed it to maintain a “practically” full diameter. The biggest plus from this – the gas flow will be maintained at the highest level. Note that not all “heart” pipes are equally good, the best option is when the output of your exhaust manifold matches the input of the “heart” pipe and the entire system consists of 1 (one) pipe. However, in practice, the manufacture of various kinds of short transition sections with already formed bends is much more common. I must tell you that this is not cool, because welds – inevitably penetrating into such a pipe in this case contribute to additional resistance and the occurrence of turbulence. A talented welder can help the problem by using a grindstone (grinder) to minimize roughness inside the pipe..
‘More is better’ – this is undeniable for turbo cars. After the turbine, these engines should breathe as well as possible, and therefore a larger pipe diameter will allow a larger gas flow with less resistance. However, the generally accepted standard diameter is 3 inches, though 4 ‘is already practiced now. For atmospheric engines, there is little argument that for atmospheric engines a small pipe diameter is required to create back pressure for optimal performance. However, this is only a theory, it has not been proven in practice and correctly tuned ignition and fuel supply provide good work with large pipes.
High flow down pipe
The downward pipe that goes from the back of the turbine is a great place to pick up a few horses. Gases coming out of the turbine – must be able to do this very quickly (swiftly), otherwise turbulence will take away valuable horses. One of the methods for maintaining the gas flow rate is to have a separate pipe for exiting the turbine and a separate one for the wastegate. This method is often called ‘Screamer’ – since the outlet from the bypass valve is usually directed to the atmosphere, which makes a hell of a lot of noise. Another more common alternative is the use of a large smoothly bent pipe corresponding to the mounting at the outlet of the turbine. This works well too. The factory usually installs an iron descending pipe (with fairly decent resistance) and, accordingly, it can be successfully replaced.
High-flux modern catalysts practically do not slow down the gas flow (very slightly), but for some reason they are often underestimated, especially in terms of the advantages they give. Pros, you ask? Yes, – I was not mistaken, – precisely the pluses. Catalysts significantly reduce noise levels, help keep the atmosphere clean and keep you away from unnecessary communication with eagles (servants of the law). Catalysts are relatively cheap these days. If you are still skeptical about the throughput of modern catalysts, consider the fact that a modern 3-inch catalyst has better throughput than many 2-inch tuning mufflers.
Variable Exhaust Valves
Variable exhaust valves are becoming more common on stock cars. Many exhaust systems use a variable “butterfly” valve to reduce the noise level from the muffler at low speeds, while maintaining the possibility of high-flow exhaust at high speeds. The angle of the butterfly is determined by the computer (brain), depending on the position of the accelerator pedal and the load on the engine. A similar system is installed, for example, on the Subaru Liberty B4 delivered to Australia, however there it works depending on the speed.
The location of the valve may vary – it can be either at the outlet of the rear of the muffler, or at the second inlet to the rear of the muffler.
As a tuning, flow change systems are proposed, which consist of a large valve inserted inside the exhaust system and a control unit. At low engine speeds – the valve is in a virtually closed position, reducing noise. At high loads on the engine, the valve opens, opening up space for the exit of a large stream of gases.
Is it worth it to improve the exhaust system
Before changing the entire exhaust to the exhaust pipe, I thought about it for a long time. Nevertheless, replacing the entire system is a lot of money, will it be a profitable investment? There is a real way to check the effectiveness of the system – this is the measurement of back pressure. Back pressure can be measured with almost any sufficiently accurate pressure gauge connected through the wall of your exhaust pipe. With a heavy load on the engine, you will see how quickly this pressure rises. On my system, I decided to measure the pressure at two points: just behind the turbine and on the first bend of the exhaust pipe. My goal was to understand how bad the factory pipe.
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