One of the disadvantages of a plastic manifold, however, is the risk of breakage if the engine backfires which can be a danger when an engine is fitted with a nitrous oxide system. Some manifolds raise the intake plenum and runners away from the engine so air can flow under the manifold to help keep it cool. So many aftermarket performance manifolds eliminate the heat crossover passage. On a performance engine, you don’t want heat in the intake manifold because heat decreases air density and power. An exhaust crossover passage is often incorporated into the stock manifold to redirect exhaust under the plenum so the manifold will warm up quickly. When a cold engine is started, additional heat is needed to help vaporize the fuel. On carbureted engines, the intake manifold is “wet” and contains fuel droplets. The fuel is sprayed directly into the intake ports by the injectors that are mounted in the intake manifold just above the ports. This is not as critical in fuel injected engines because only air flows through the manifold. In an engine with a carburetor, the intake manifold should not have any sharp turns because it can cause the heavier droplets of fuel to separate from the air/fuel mixture. This provides good idle quality and throttle response, but also limits how much air the manifold can flow at higher engine speeds.Īnother important factor that influences airflow and how much torque and power an engine can make is the angle and curvature of the intake runners. Likewise, the cross-section of the runners is also small to keep the air moving at maximum speed into the cylinder ports. The plenum in a stock manifold is typically smaller to keep air velocity high.
Even so, a well-designed single-plane manifold will almost always outperform a split-plenum dual-plane manifold from 2,500 rpm and up. This makes a significant improvement in mid-range and high rpm power, but may sacrifice some low-end throttle response and torque. Opening up the plenum allows all of the cylinders to pull from all the barrels in the carburetor or throttle body for more airflow at higher engine speeds. These types of manifolds can be a good choice for heavier vehicles with automatic transmissions, stock gearing and engines that won’t rev much beyond 5,500 to 6,500 rpm.īut if an engine has a longer duration camshaft, stiffer valve springs, bigger cylinder heads and gearing for revving to 7,500 to 8,500 rpm or higher, an intake manifold with an open plenum, single-plane or “360 degree” configuration is usually the best choice. A performance manifold that is designed for a street application will often retain the split-plenum or dual-plane design for this same reason. This design, though wearing the “stock” label, isn’t necessarily bad.
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