Typical original equipment (OEM) production manifolds are manufactured from cast iron (inexpensive, durable and tolerant of high-temperature gases). These manifolds are “optimized” to balance production costs, manufacturing simplicity, ease of assembly, durability and performance. They direct the hot exhaust gases away from the cylinder head/exhaust port and into the primary exhaust pipe. They are not generally capable of optimal flow, but they are quiet, durable and strong. In many cases, they incorporate a heat riser valve or pre-heater to aid in cold engine warm-up, emissions control and drivability.
Many high-performance applications in the 1960’s and 1970’s utilized long-tube manifold designs to improve output and engine breathing. The Pontiac HO/Ram Air manifolds, the MOPAR Max Wedge, Hi-Po & Hemi manifolds and the Ford Hi-Po manifolds are all examples of these higher-flow manifolds. These designs improved exhaust gas flow over standard manifolds by utilizing a design that more closely resembles a tuned-length header. Though not as free-flowing or effective at scavenging exhaust gas as tuned headers, these manifolds represented a superior alternative to the basic “log” that most production manifolds resemble.
Performance part suppliers jumped into the performance improvement aftermarket with tubular headers for nearly every engine and vehicle combination. By matching the length of the header tubes, the exhaust pulses from each cylinder can be tuned to reach the exhaust pipes in an orderly fashion. Well-designed manifolds or headers can actually improve the scavenging of spent exhaust gases from the cylinders by using the pulse from previous cylinder output to “pull” the gases from the next cylinder – almost as if the exhaust pulse is being vacuumed out of the cylinder by the previous pulse. A good set of headers can improve torque and horsepower by more than 10% compared to standard factory exhaust manifolds.
If you are using production, cast iron manifolds in your vehicle, follow these steps to help ensure a quiet, trouble-free installation. Make sure that the sealing surfaces are flat and that the heat riser valve (if originally present) is properly installed and operating freely. Treat the manifolds with a high-quality cast iron coating after media blasting them. Use bolt locks where they were originally installed and a high-quality gasket. Follow the bolt installation sequence that is specified in your service manual and tighten the attachments to the specified torque in the proper sequence. (Usually starting at the center and working alternately forward and rearward) This maintains the flat sealing interface between the manifold and the head and minimizes the potential for leaks and cracks.