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| Grimmer Motors |
Single-plane manifolds pass air into the plenum sharing with all chambers. This setup is good for internal combustion engine to run in high-power circuits because a larger area of plenum draws more air molecules into the cylinders. While the valve motion is not in-phase, the air-fuel mixture is always changed direction and meanwhile the turbulence is created (or called back-pressure pulse). On one hand, this turbulence causes the carburetor to misjudge the concentration of fuel. On the other hand, the tiny amount of the burnt-out gas owing to the turbulence may enter to the chambers unnecessarily. This issue becomes obvious at low-rpm range because the engine does not require a lot of fresh air to operate in the low-end region.
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| enginelabs, LHS: dual; RHS: single |
To remedy this situation, using dual-plane plenum is recommended. The dual-plane plenum separates airflow into two channels. Assume it is a V8 engine, each plenum and four intake runners are connected to every other cylinder under a programmable firing sequence. This minimizes the previously discussed turbulence inside the plenum. As the dual-plane manifold makes a better organization in airflow, the induction pulse only exist at every ~180 degrees of crankshaft rotation. That's why the dual plane manifold is always used to improve low-end torque. However, pushing the engine performance at high-rpm range via the use of dual-plane manifold is ineffective because the area of dual-plane manifold is smaller. The dyno data of Chevy 302 engine in different types of manifolds are consistent with what we have discussed.
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| Measurement data: The single and dual plenum hold the same total volume for a fair comparison. |
Here we study the intake airflow CFM of Chevy 302 engine. The CFM refers to the measurement of the velocity at which air flows into or out of a space. The larger area of single plenum allows more air to fill into the chambers which gives a higher CFM in red zone. In contrast, at low-rpm region, the CFM of the dual-plane enabled Chevy 302 is higher than that of the single-plane enabled Chevy 302. To explain this phenomenon, we need to recall a scientific fact that air molecules travel faster in a narrower tube (if the mass flow rate remains unchanged). A smaller area of the dual-plane system triggers a faster airflow so that its CFM values are higher at low-end region.
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| Measurement data |
In addition, we provide the airflow rate of the single-plane installed Chevy 302 in various engine speeds. A completed stroke cycle means the engine has been rotated 720 degrees and the following figure shows that the intake valves are opened between ~380 to ~580 degrees. As we expected, the intake velocity is proportional to the engine speed.
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| Measurement data |
