Fresh Water Cooled Engine Question

I've stared at the engine diagrams in the Mercruiser parts manuals enough that I'm confused.

My 2003 320 Sundancer has the 5.7L 350 MAG MPI Horizon engines. They are Fresh Water Cooled, but from what I've been reading, some FWC engines are only half FWC systems while others are 100% FWC. I have heat exchangers on the rear of my engines, near the mufflers. And based on a conversation with my local service department, this seems to imply that I have a full FWC system. Is that a correct assumption? What's the actual difference between a half and fully FWC Mercruiser system?

I know that even fresh-water cooled systems use raw water to a degree. But in a fully FWC system, are any engine components DIRECTLY exposed to raw water? Or, is it just the indirect exposure to raw water and it's vapors via the underwater exhaust that presents the danger to a FWC system?

Finally, what components of the exhaust are really in jeopardy in a FWC system? It seems like the manifolds and risers should be pretty "far away" from the raw water to be subjected to any corrosion. What am I missing?

Thanks,

1/2 vs full system generally is the difference between the manifolds being part of the closed system or not.....if your manifolds are part of the closed system all you have to worry about are the risers, heat exchanger and any oil, trans or fuel coolers seeing sea water...

Thanks. Are all of these components directly seeing sea water? That is, do they have raw water flowing through them? I would assume most of these do, except maybe the risers?

On a fully closed system raw water is pulled through the bottom of you boat through an intake, through the raw water pump, through the heat exchanger and introduced in the exhaust flow after the risers. At least that's how mine is set up.
Your maintenance will consist of changing raw water impellers annually and checking the exhaust manifolds every 3-5 years depending on use. Plus visual inspections before every outing as part of your normal routine prior to cruising.

Great, thanks. I do change the impellers annually. And the risers were last inspected in 2013 - so it's time to check them again. I guess they don't get raw water directly, but could get a fair amount of vapor coming upstream from the combined raw water/exhaust flow. Especially after shut down. How often should I check/replace the heat exchangers? Seems like they could be subject to corrosion worse than the risers? Especially with raw water getting trapped on there after the engines are shut off?

Horizon Engines are only half systems.
Theres raw water cooling the manifolds and exiting the risers.
The heat exchangers are copper and hold up well in salt water.
Unless you're losing antifreeze or your temps are rising the exchangers are good.
You can pop off the end caps of the heat exchanger and inspect the cooling tubes for clogging at the end of the season.
The end cap gaskets are cheap for the exchangers.
The risers and manifolds are same replacement guidelines as a raw water cooled engine.
One thing I can recommend is that the gaskets for the spacers between the risers and manifolds fail quickly and i would replace them every 2 years.
This happened to my engines last year and were leaking internally into the cylinders.