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Affordable Exhaust Manifold Swap

When cast iron gives up the ghost

Historically, most cars and trucks have used cast-iron exhaust manifolds to scavenge spent gasses from the engine's cylinder heads. The rationale: Cast iron has good thermal properties, so such items such as spark plug wires can be routed closer to it than to other materials which expel more heat. Also, cast-iron manifolds are apparently cost-effective to produce in mass quantities.

Temperature fluctuations cause metal to fatigue over time. Exhaust manifolds aren't exempt from this fact. Engine vibration can cause the increasingly brittle metal to crack, and failure at the bolt locations is another common problem. Also, water is a byproduct of the combustion process, so exhaust manifolds can rust from the inside out. The fix seems simple: replace the dead part with a new one.

Aftermarket Solution

One potential drawback: cost. After pricing the cost of factory-replacement exhaust manifolds for a '93 Chevy 350 (about $560 at the local dealership), we looked for more-affordable options. Aftermarket headers (sometimes known as tubular exhaust manifolds) emerged as an attractive alternative. Header prices can vary significantly, based on the materials used to make them and the "performance engineering" that goes into their design. For example, stainless steel is more expensive than mild steel, thermal-barrier coatings add to the cost and factors such as making each primary tube the exact same length can present a manufacturing/engineering puzzle.

Our quest led to a set of direct-replacement "shorty" headers. Sourced from a large mail-order parts supplier, the header kit arrived at the door for a little more than $170. Aside from an approximate $400 savings compared to new cast-iron manifolds, these headers offer a few other advantages. First and foremost, they're designed to occupy the same space as the manifolds they replace: no additional exhaust modification, spark-plug wire routing problems and such. The drawbacks are that performance gains are often unnoticeable from the driver's seat (contrary to the manufacturer's claims), headers can radiate more heat into the engine compartment (causing plug-wire failure among other considerations), steel is more likely to leak than cast iron, and durability can be a concern on non-thermal-coated headers.

The $400 Question

In this case, the economic benefits outweigh any potential performance compromises. These headers replace the exhaust manifolds on a 200,000-mile daily driver, and we aren't expecting another 200,000 from the engine. Furthermore, these particular aftermarket headers have 3/8-inch-thick cylinder-head flanges, fat enough to stave off warping from the heat generated by a stock Chevy 350 V-8. However, they're coated by high-temp paint as opposed to a more-expensive ceramic coating.

Two other considerations in the manifold vs. headers debate: First, check local emissions laws if applicable. So-called "50-state" aftermarket headers might not be available for your application and/or might be as expensive as replacement manifolds for some less-popular cars. (Junkyard manifolds can often by revived by having them thermal-coated.) Also, verify that the header kit comes complete with all the necessary gaskets, hardware and plugs. (Many headers include oxygen-sensor bungs to comply with both pre- and post-1996 emissions requirements.)

Installation Tips

> Use penetrating oil or silicone spray on all applicable bolts the day before you intend to do the job.

> Wait until all exhaust parts are cool before beginning the swap.

> Label spark plug wires before removing them.

> Heat also affects mounting bolts. Always use new hardware to minimize the chances of exhaust leaks.

> Check the header flange surface with a straight edge. File or grind any welding slag or other surface irregularities smooth for the best possible seal.

> Scrape any residual gasket material off the cylinder head and exhaust pipe.

> To check for leaks, rev the engine and listen for irregularities. Also, a piece of paper can help reveal leaks by moving it around the flanges and watching for it to flutter.

> Retighten all bolts per the header manufacturer's recommendations.