Performance Exhaust Systems: How to Design, Fabricate, and Install. Mike Mavrigian
as the remainder of the exhaust system moves places increased stress on the cast iron. From a performance standpoint, as a general rule, cast-iron exhaust manifolds can act as bottlenecks, with abrupt exhaust passage angles restricting flow.
In essence, the purpose of a single-piece cast-iron exhaust manifold is to allow the exit of the spent exhaust gases, with little or no regard to engine performance. With a performance-built engine that features a camshaft with more overlap, it’s important to separate the primary exhaust paths, which is yet another reason to choose individual tubes featured in exhaust headers.
With this said, cast-iron exhaust manifolds still have their place. As already mentioned, for those who desire an original appearance, the use of original-style exhaust manifolds is mandatory. If your goal is to obtain function and original appearance rather than maximizing engine performance, the use of cast-iron exhaust manifolds is acceptable.
If you’re trying to salvage a set of original cast-iron exhaust manifolds that are cracked or have pinholes, be aware that welding cast iron can be tricky. Cracks may be addressed either by pinning or welding. Pinning involves drilling a small hole at each end of the crack in order to provide stopping points to prevent the crack from increasing in length. This is followed by drilling a series of additional holes along the crack, with each hole then fitted with a screw-in pin that is designed to pull the crack back together. Depending on the location of the crack, pinning can prove to be very successful. An excellent source for crack-pinning repair materials is from Lock N’ Stitch.
Another method of repair involves an actual cast-iron welding process. This involves careful V-grinding the crack(s), pre-heating the manifold to a specified high temperature, then spraying a specialized cast-iron powder into the crack, allowing the material to fuse together. An excellent source for this is Cast Welding Technologies.
There’s usually an exception to any general rule, and with regard to cast-iron exhaust manifolds, this involves a turbocharger installation. The use of cast-iron exhaust manifolds is often preferred in a turbo setup because the turbocharger is often directly mounted to the cast-iron manifold. The size of these manifolds must be considered as well. In a confined area, the compact cast-iron manifold simply better accommodates a turbo because it takes up much less space than a tubular header. In addition, the supportive strength of a cast-iron manifold may better suit the weight of a turbocharger. Moreover, the thicker and heavier cast-iron material better captures the exhaust heat.
Pumping loss in a gasoline internal combustion engine essentially refers to anything that resists crankshaft rotation. Pumping losses refer to the work, or energy, required to move air into and out of the cylinders. These are losses other than normal frictional factors, such as piston ring drag, timing chain drag, and the effort required to compress the valvesprings. This also includes driving accessories, such as water pump, alternator, power steering pump pulleys, etc. Pumping losses can also be viewed as “negative torque” effects that try to resist crankshaft rotation.
The engine suffers a pumping loss when the air intake is restricted while the intake valve is open. This occurs during the intake cycle when the piston is moving down the cylinder bore. Anything along the path of the incoming air charge can restrict airflow, such as the air ducting, air filter, intake manifold runners, and cylinder head intake ports. The throttle blade and assembly may be the primary restriction. When the throttle, whether this involves a carburetor or fuel injection throttle body, is partially open or closed, it creates a flow restriction against which the piston is trying to pull air. This is why the engine produces more vacuum at idle than open throttle.
Such pumping loss is unavoidable, but selecting the appropriate size carburetor or throttle body can minimize it. Because a carburetor needs to be sized for both air passage and fuel delivery, size selection must be based on the displacement and demands of the particular engine. With a computer-managed EFI system, moving to a larger throttle body reduces this air charge restriction because the ECU only delivers the amount of fuel needed to maintain the correct air/fuel ratio, regardless of the throttle body size. In other words, you can get away with a larger throttle body without overfueling.
Intake system pumping loss can be drastically reduced or completely eliminated with the use of forced induction because air is being delivered under positive pressure as the throttle is opened to wide-open position. This added cylinder pressure provides added force to help the piston move downward. Of course, since the camshaft expends energy to drive the supercharger, there is a degree of parasitic loss. Although a turbocharger takes advantage of exhaust gases in order to generate the forced air charge and requires no crankshaft energy, the turbocharger itself poses an exhaust restriction pumping loss. There is no way to completely eliminate pumping losses, but you can reduce this wasted energy by careful selection of the intake system, cylinder head design, camshaft profile, crankcase ventilation, and exhaust flow.
In order to improve engine vacuum when using a high-lift long-duration cam, an external belt-driven vacuum pump is often used. This can reduce excess crankcase pressure, which reduces piston ring blowby and crank windage.
Here’s an example of four-stroke engine events. On the intake stroke, the piston moves downward, the intake valve opens, and the exhaust valve is closed. During the compression stroke, the piston moves upward, with both intake and exhaust valves closed. During the power stroke, the spark plug fires, the piston moves downward, and both valves are closed. During the exhaust stroke, the piston moves upward, the intake valve is closed, and the exhaust valve is open.
It’s obvious that a piston generates cylinder pressure as it rises during the combustion stroke, pushing and compacting air upward. Less obvious is the pressure that the underside of the piston creates in the crankcase as it moves downward, acting like a cup, pushing air down into the crankcase. A positive crankcase ventilation (PCV) valve allows intake vacuum to help pull this pressure out of the crankcase. An external vacuum pump accomplishes much the same effect, in scavenging pressure out of the crankcase, reducing the parasitic energy loss.
Each stroke of the engine cycle has a different effect on the exhaust system. To better understand intake and exhaust events in the engine, you need to understand the four-cycle event, which includes the intake stroke, compression stroke, power stroke, and exhaust stroke.
Intake Stroke
The intake stroke begins at the end of the previous exhaust stroke. Before the piston reaches intake TDC, the exhaust valve is still open while the intake valve starts to open. This is referred to as valve overlap.
During overlap, both valves are open, allowing a small amount of intake charge to be pulled into the combustion chamber by the closing of the exhaust valve. This is referred to as the intake scavenging effect.
Furthermore, as the piston moves down it draws the bulk of the air/fuel charge. Pumping losses are created by any restrictions in the air intake path, including the position of the throttle plate. The piston is trying to draw air through, by “pulling” against any restrictions along the way, which increases manifold vacuum.
As manifold vacuum increases, pumping loss increases as the