Re: risers and manifolds-iron or alum...
Expansion is a worse problem when you are using different metals on the exhaust vs riser - because there you have a narrow sealing area (most exhaust parts attempt to have a 3/8" wall), which can be a problem if the sealing areas are not moving at the same rate under heat.<br /><br />For instance, if my numbers are correct, a 5" section of aluminum riser would grow by 1.6 thousanths of an inch (.00156) at 600*F, while the same section of iron underneath would only be growing by 1/2 of that (.00072 in). It may not seem like much, but it is often enough to keep a gasket from sealing to both units.<br /><br />A scarier thought is the melting point of Aluminum. Cast iron melts at 2150*F. Aluminum has already hit its melting point (1220*F) around the time cast iron turns cherry red. <br /><br />This may not seem like a problem, your exhaust components are not _supposed_ to get that hot. But every once in a while they do - a bit of bad timing, some pinging perhaps, and / or maybe add some clogged water passages. Whatever the cause, sometimes things heat up in there.<br /><br />If the Aluminum fails, it fails BIG TIME. Bam whoosh, no inner wall on the manifold and a boat load of water in your engine. Or, perhaps worse, into your bilge - ever wanna bet on a race between a bilge pump and a seawater pump? The bilge pump may win, but I would not want my money on that horse.<br /><br />If cast iron fails from heat and stress it merely cracks. Often letting a fairly large amount of water into your engine, but nothing catastrophic.<br /><br />The odds on failure are perhaps not enough to scare most folks (after all, we have been running aluminum heads for years now), and I agree that those odds may be long enough to live with. But you are paying exta money for that? <br /><br />If I was paying extra money for exhaust parts I would look to a performance stainless header system, especially if my sig line was "In thrust we trust". Other than that I would stick with the iron. It is cheaper, lasts long enough, and has a greater tolerance against extreme failure.<br /><br />Sorry for the soapbox - getting down now.
