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| CARiD |
The exhaust system diverts exhaust fumes away from the engine and decreases their harmfulness. If those exhaust fumes are not being expelled smoothly, it causes a major deterioration within the engine compartment. To optimize your engine, you can adjust the velocity of exhaust gas. The exhaust gases run faster and allow themselves to go out quicker.
Before I compare the 4-2-1 and the 4-1 headers, let's study how the diameter of the primary pipe affects the engine. To understand the fluid dynamics easier, no megaphone and collector are installed. The results are shown in Fig 1-3. Using a smaller diameter increases the velocity of exhaust gas. The data in Fig 1 is measured at 6000 RPM
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| Measurement data: Fig 1 |
On the other hand, back pressure is an obstacle in the exhaust system. The exhaust gases cannot be expelled effectively to the atmosphere under a large back pressure. A small tube may build up a strong back pressure. The exhaust gas in a narrow tube is more difficult to be sent to the atmosphere. This increases the back pressure that pales the performance of engine. Fig 2 demonstrates that the dyno performance is optimized if the diameter of primary tube is tuned to 1.5 inches. If the diameter is too small, the strong back pressure resists the flow of exhaust gases even though the gases travel faster. If the diameter is too large, the exhaust gases travel slowly even though the back pressure is negligible. Optimization of these two parameters is important in the exhaust system.
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| Measurement data: Fig 2 |
In addition, you can upgrade your engine via the use of a shorter exhaust pipe as shown in Fig 3. The use of a short exhaust pipe favors high-rpm performance. Why? Assume the exhaust pipe is extremely long and what will happen in red-zone? The crank has been rotated from 0 to 720 degrees and meanwhile the burnt-out gases are travelling along the extremely long pipe. If the exhaust pipes are long enough to give an non-ideal scavenging, the previously burnt-out gases still remain in the pipes even the crank is rotated from 720 to 1440 degree. That's why decreasing the length of the exhaust pipe enhances the high-rpm performance.
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| Measurement data: Fig 3 |
What is "4-2-1" and "4-1" exhaust system?
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| redline360 |
4-2-1 system: four primary pipes come off the cylinder head, and merge into two secondary pipes, which in turn finally link up to form a collector pipe.
4-1 system: four primary pipes come off the cylinder head, and merge into a collector pipe.
Figure 4 shows the difference between these two designs. The exhaust valves in both setups are 1.1 inches. In the '4-1' setup, the length of primary pipe is 15 inches with a diameter of 1.5 inches. The length of collector is 5 inches where the inlet and exit diameters are 2 inches and 3 inches, respectively. In the '4-2-1' setup, the secondary pipe with the diameter of 1.75 inches is inserted between the primary pipe and the collector. For the cars equipped with the 4-2-1 exhaust pipes, the engine produces more mid range power with the sacrifice of top end power. All cylinders do not expel the burnt-out gases simultaneously because a typical firing sequence (e.g. 1-3-4-2) is registered. When the engine runs slowly, the 4-2-1 pipes guide the burnt-out gases to leave the engine more effectively. In contrast, the use of the 4-1 exhaust pipes produces the most top end power because the geometry offers better flow characteristics. This design usually minimizes the effect of back pressure. The 4-1 header is not good at boosting the dyno performance at low-rpm-range. Why? When the engine runs slowly, you don't expect that a lot of exhaust gases are required to escape from the engine. If the 4-1 pipes are installed, the large diameter of collector usually decreases the speed of exhaust gas and hence the scavenging is not effective at low-rpm range.
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| Fig 4 |