Standard cast iron manifolds

Pic 1.
Standard cast iron manifolds - Mini on left, Metro ‘LCB’ type right. One major benefit of cast iron is noise absorption. Tubular steel amplifies resonance caused by exhaust gas phenomena. Stainless steel being THE worst.

Pic 2.
Cooper Freeflow. Careful, exquisite forming of primary pipes and neat welding ensures un-interrupted flow of exhaust gases. Medium length primaries promote performance where mild cams are used.

Pic 3.
Ubiquitous LCB. The two outer ports are connected by short primaries joined to a larger secondary pipe, the centre port getting one pipe all to it’s self. These equally sized pipes continue down under the car, ending in a ‘Y’ piece to join them to the system.

Pic 4.
‘Race’ 3-into-1. Three individual long primaries meet at a collector to join them to the system. Centre pipe’s bigger than outer two to compensate for having to deal with exhaust from two cylinders - despite the fact they don’t exhaust at the same time....

Pic 5.
Small-bore 3 into 1 demonstrating the nightmare sealing such a manifold can be, and why it’s almost impossible to use the larger bores on road cars. And probably why the LCB is preferred!

Pic 6.
Budget LCB. Ease and cheapness of fabrication means ineffective shape....

Pic 7.
....where form optimises function.

Diagram 1.
Exhaust gases don’t flow evenly throughout the port. The upper, areas are ‘busiest’ as illustrated. This can allow flow reversal at certain gas speeds - slowing down caused by low engine speed and over-sized exhaust pipes.

Diagram 2.
Clever arrangement of both exhaust port positioning in the head, and positioning of exhaust primary pipes can very beneficial in controlling/preventing this. Standard manifolds and ports often achieve this on their own. Unfortunately, many head modifiers tend to match the ports to the gasket size, almost eradicating this. Not doing so is far more beneficial than a bigger exhaust port - especially in an area that’s not ‘busy’.

Importation rules regarding vehicles are very specific: Only those over 25 years in age may be brought in to the USA without compilance to Federal Motor Vehicle Safety Standards (FMVSS).