All engines benefit from an unrestrictive free flowing exhaust system; big-blocks in particular because they generate a large amount of exhaust volume. The traditional stumbling block of factory exhaust systems is the cast-iron exhaust manifold. In the case of bigblock Chevys, the manifolds are better than they seem. While there is no real exhaust tuning going on, some of Chevrolet’s big-block exhaust manifolds are relatively efficient at extracting spent combustion by-products. All big-blocks have used cast-iron exhaust manifolds in various configurations depending on the vehicle and the limitations of the chassis. Factory exhaust manifolds are fully interchangeable between Mark IV, Gen V, and Gen VI engines, but you encounter some fitment issues with raised-port heads. Raised exhaust ports place the manifolds higher in the chassis and may require slight relocation or extension of the down tubes to prevent contact with nearby components such as the starter wire harness, plug wires, fuel lines, etc.
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Big-block exhaust manifolds all have a casting number, a date code, and either LH or RH to indicate the left or right side of the engine. The letter code is superfluous because the correct side of the engine is generally obvious on big-block exhaust manifolds. Date codes are also irrelevant except for restoration applications. If you need to read a date code, they reveal the month, day, and year of manufacture. The letter indicates the month and the numbers indicate the day and the year.
For example, D167 decodes as D for April; 16 for the day of the month; 7 for the year of the decade, 1967, 1977, or 1987. High-performance exhaust manifolds usually have long sweeping runners that nearly emulate tubular headers. Their effectiveness stems from lessons learned with the NASCAR Mystery Motors. Restoration efforts have to use the correct manifold for their application, but any other applications where cast-iron exhaust manifolds are considered should gravitate toward the more streamlined long-runner highperformance manifolds if the intention is to run stock-type manifolds.
These manifolds are difficult to find, but are easily identified as they look more like cast-iron headers instead of the more typical log-style manifolds with very short runners feeding into one long tube that exits toward the rear. The longrunner manifolds have longer individual runners all cast together in one assembly.
Careful inspection is mandatory when shopping for cast-iron exhaust manifolds. Many of them crack from the heat with age and the cracks are often concealed by sheet-metal heat shields. Manifolds also warp and must sometimes be resurfaced to ensure a proper seal. Unlike blocks, intake manifolds, and cylinder heads, cast-iron exhaust manifolds don’t begin life with a nice coat of heat-resistant paint; hence they begin to rust almost immediately, particularly in damp climates. Then when the engine is scrapped or the manifolds are discarded for headers they lay around in scrap yards slowly returning to their base elements. The casting numbers often take a beating and become difficult to read.
To be sure, almost all big-block builds that are not restoration oriented are going to end up with headers for the exhaust system. Cast-iron manifolds are not very efficient, but they are sometimes useful in other applications, such as engine swaps in vehicles with tight clearances. This might include smaller cars and trucks, Jeeps, and a wide variety of engine swap candidates that just don’t have the room to accommodate headers. So cast-iron big-block manifolds are not much in demand except for restorations and most applications are covered by replacements from specialty suppliers such as Classic Industries.
Header compatibility is not just restricted to chassis fitment, but also to exhaust port shape and location. Raisedport heads often experience fitment problems with headers even though the bolt placement and flange shape match. In most cases a mismatch causes one or more tubes to contact a frame rail. The usual fix is to dimple the header tube or grind a notch in the frame rail; you never really know until you’re looking at it. In extreme cases some swappers have had to modify the header by replacing one or more tubes in the offending section. These are not difficult issues to fix, but you should be aware that you might run into them.
Some header manufacturers offer a supplemental part number that has been redesigned to accept raised-port heads. Check with your manufacturer directly as most suppliers do not always stock the specialty applications. For most applications using standard port height heads, it is just a matter of choosing the correct catalog header for your application. Catalog headers assume stock heads and do not contemplate aftermarket heads that may have raised ports and/or different and possibly incompatible port openings. Check this carefully before purchasing headers to ensure the best possible fit and a leak-free seal.
Written by John Baechtel and Posted with Permission of CarTechBooks