Had 2-foot (or rather, 5-foot)-itis and upgraded to a 96 Powerquest 34 Vyper recently that had spent its entire life on a freshwater lake. Boat was originally equipped with twin 454s, but was repowered in 2003 with 496 HOs. My understanding is that in 03, Merc was putting aluminum exhaust manifolds on these engines. Now that the boat will be used exclusively in salt, that is obviously unacceptable for any length of time. Looking at the manifolds and risers currently equipped, they appear outwardly to be in excellent shape. There are no external signs of corrosion whatsoever around any joints or gaskets. The boat shows pretty meticulous maintenance, but the previous owner of 3 years did not replace the manifolds and does not know if they were ever done. Is there a way to tell by looking at the exterior of the manifolds whether or not they are the newer cast iron variety?

Use a magnet, it will be attracted to iron but not aluminum

Cast iron models will have core plugs on the exhaust port side. Aluminum models do not.

Oops, someone forgot his freshman physics. Thanks for reminding me lol.

Thank you all for the responses. If I have the water rail version, I will need a conversion kit as well, correct? I wasn't paying attention to whether or not I had the water rail, but I will have to take a look at that as well. I will probably get around to taking a harder look in the next day or two.

I've been running aluminum manifolds in brackish water for decades. I garden hose flush after use but nothing else.

Nearly all aftermarket performance exhaust manifolds are aluminum.