Performance Exhaust Systems: How to Design, Fabricate, and Install. Mike Mavrigian
fuel starvation for individual cylinders. For instance, if the engine displacement is 408 ci, the plenum volume should provide 408 ci. If plenum volume is too small, upper-RPM power can suffer. This becomes more of a critical issue for competition engines in contrast to street-driven engines.
Cylinder heads are a key component to the overall airflow system of an engine and should be one of the first hard parts to consider when creating a plan for a new engine build. They are responsible for conditioning the air/fuel intake charge into the cylinder, aid in the conversion of chemical and thermal energy into mechanical energy, and process spent exhaust gases into the engine exhaust system. Once the cylinder heads have been determined for an engine build, the selection of support components, such as intake manifolds, pistons, exhaust headers, and camshafts, becomes less cumbersome to choose the right combination of parts to achieve the desired outcome.
Each component of the engine airflow system must work together to achieve an efficient flow path, while minimizing flow restrictions, based on a given engine volume displacement. Most performance V-8 cylinder heads are of overhead-valve (OHV) design, where the intake and exhaust valves are positioned above the engine cylinder bore for maximum intake and exhaust efficiency.
Chamber Size for Engine Displacement
These are typical examples for GM LS–type engines where the cross-section area of the intake ports has been maximized for particular applications.
| Engine | Chamber (cc) |
| 4.8/5.3/5.7L | 195 to 205 |
| 5.7L | 205 to 220 |
| 6.0/6.2L | 225 to 255 |
As you increase the intake-side flow, you must also increase the exhaust-side flow to avoid exhaust restriction. As you pack increased airflow and feed more fuel into the engine, the exhaust system must be able to evacuate exhaust gases accordingly. The need for a less-restrictive exhaust system and the ability for the exhaust system to pull, or scavenge, the exhaust becomes increasingly important. This is where moving to an appropriate tubular-header system provides a distinct advantage in contrast with a set of OEM cast exhaust manifolds.
Selecting the Right Cylinder Heads
In the performance aftermarket, there is an overabundance of cylinder head choices for most popular engine applications. Consumers can purchase complete brand-new performance aftermarket cylinder heads or rework OEM cylinder head castings through hand porting or by high-tech CNC (computer numeric control) porting. Whatever the engine goal, cylinder heads must be sized properly to the volume displacement of an engine, operate in the intended RPM range of the vehicle application, physically fit into the desired chassis space, and function effectively with support components such as intake manifolds, exhaust headers, and camshafts.
Most aftermarket cylinder head manufacturers rate performance cylinder heads based on engine volume displacement, RPM, and application (on-road only, race only, or both).
Generally, a smaller intake port volume range creates torque at a lower RPM compared to a larger intake port volume for the same engine application. Always consult the cylinder head manufacturers’ product data for specific engine builds and support components for comparison between intake port volumes.
Intake Port
Cylinder head intake port shape, cross-section area, and volume can be considered limiting factors when discussing engine performance in naturally aspirated applications. Generally speaking, most engines that perform well in naturally aspirated form perform even better when forced-induction strategies or nitrous oxide is added.
Intake port shape is dictated by the envelope of space given by the overall design of an engine, valvetrain layout, and intended vehicle application. In terms of pushrod-type engines, the intake port width must not be much larger than the distance between the pushrods, generally called the “pinch point” of an intake port. There are several strategies for increasing the distance between pushrods, including offset lifters, offset rocker arms, and compound valve angles.
Valve angle is referenced relative to 90 degrees from the block deck surface. A smaller intake port volume generally creates torque at a lower engine RPM compared to a larger intake port volume on the same engine.
The length of a cylinder head intake port is established by its location relative to the centerline of the cylinder bore and intake manifold mating flange. Height of an intake port is determined by space constraints of matting components such as intake manifolds, valve covers, and valvetrain hardware. Generally speaking, intake ports with larger corner radii outperform port designs with smaller corner radii, based on the cross-section shape transition from the intake port opening to the valveseat sealing surface.
Intake port cross-section area is one of the most important dimensions when determining the cylinder head airflow requirements for a particular engine size. Equations derived from fluid dynamics science tells you that a given cross-section area is only going to flow so much fluid based on its speed, mass, temperature, and compressibility. Most computer engine analyzer software programs require an intake port minimum cross-section area input to calculate potential torque and horsepower estimates of an engine.
Intake port volume is basically a function of the length, width, and height of the intake port shape that fits into established space constraints. Marketing trends in the performance aftermarket can be misleading because cylinder heads are categorized in most cases by their intake port volume rather than minimum cross-section area. Technically, a cylinder head with an intake port volume of 180 cc could outperform a cylinder head with a 195-cc intake port if the cross-section area of the 180-cc port is larger.
As mentioned earlier, whenever you increase the engine’s intake flow, the exhaust flow must be increased to avoid exhaust stream restriction. High-performance aftermarket cylinder head designs often feature raised exhaust ports. Raising the exhaust port location on the cylinder head generally results in an increase of exhaust flow, since this provides a more-direct path. Aftermarket performance cylinder head manufacturers invest quite a bit of time and testing in an effort to develop improvements in power. Raised-port cylinder heads are offered for applications where this has provided a benefit based on their extensive testing.
Exhaust Port
Cylinder head exhaust ports function in reverse compared to intake ports in that the exhaust gas flow enters the exhaust port from the face side of the valve, rather than the stem side of the valve in an intake port. Generally, exhaust ports are smaller in volume and cross-section area compared to intake ports, flowing 15 to 40 percent less on average. This is because exhaust gases are pushed out of the cylinder by the piston, and also pulled out at the same time by the exhaust header system, referred to as exhaust scavenging.
Exhaust port shape is dictated by space constraints of exhaust headers, spark plugs, and vehicle/chassis clearance. Generally a longer exhaust port shape outperforms a shorter exhaust port, with both having the same cross-section area. This is because turning the exhaust gases out of the cylinder head and into the exhaust system is more efficient. In most cases, the diameter of the exhaust header primary tube needs to be at least the same or larger than the diameter of the exhaust valve; otherwise a flow restriction could be created. The bulk volume of flow in an exhaust port generally follows the longer side of an exhaust port, which is why there are trends to “D-shaped” exhaust ports, with the straight side toward the bottom of the port.
Exhaust