I'm looking at purchasing a boat with a fresh, mildly build 500 EFI w/aluminum heads. My usage would be approx 1/3 saltwater & 2/3 freshwater.

Something about dunking an aluminum head motor in the salt just gives me a bad feeling...dissimilar metals and all of that. Even though aluminum is more corrosion resistant on its own, I'd almost feel better dunking an all iron engine....but maybe I'm off base, since i have no experience with aluminum heads in salt.

- How would you feel about running an aluminum head engine in saltwater, knowing your were going to give it a good long flush afterwards with fresh water and salt-away?

- How effective are aftermarket closed loop cooling systems like this? - https://www.hardin-marine.com/c-1527-complete-closed-cooling-kits.aspx
Anything to watch out for? On one hand it's not rocket science....on the other it would be nice to know it was a proven set-up with proper thermal capacity, flow rate and thermostat temperature, etc. (i.e wouldn't want the thermostat to restrict flow rate when engine was cold, increasing chance of detention, etc.

- If the engine was tuned with raw water cooling, is switching to closed loop going to present any problems?

Thx.

There are no negatives to FWC . As long as you have an appropriate sized heat exchanger you're fine. As far as 'tuned for raw water cooling' goes the only real difference is that the thermostat is generally set to open a few degrees warmer on a FWC engine. Of course, understand the difference between full FWC and partial/half FWC.
Even if you had an engine with no dissimilar metals in its build , all iron or (heaven forbid) all aluminum , and you boated in fresh water there's still nothing but positives .
And don't forget , if you're in a location that falls below 32f in the winter winterizing is much easier.
If you poke around on here you'll see stories of people who had , as an example , stock 502s (that have alloy intakes) that they flushed religiously that still had intake manifold corrosion issues.

Here is an aluminum intake that was on my 525SC HELP!!! Need Merc 525SC Intake

without antifreeze(w/rust inhibiter) in the cooling system aluminum parts will corrode badly...salt water just speeds that up.

Don't think you are in the clear just cuz you have iron heads and intake. #2-7 exhaust guide like to rot through on iron GM heads.

Would it be possible to do a full FWC system if the engine has and EMI thunder exhaust, extended to dry tails?

I just put aluminum intake and heads on my 383. I installed three of these. One in the intake and one in each head. My boat is a fresh water only.

https://www.summitracing.com/parts/f...waAj-vEALw_wcB

Hard coat anodizing does a great job of resisting corrosion.
Especially done right.
Edelbrock sells some heads you could buy with coating.
https://www.summitracing.com/parts/EDL-61459/
You have to prep if taking existing ones and coating them.
Need to apply block off to keep it out of tapped holes or tap oversize before hand.
Do any machining before.
Don't let it get applied to dowel holes.
Always been surprised not more done for marine applications.

It is good but does not cover everything! Do not depend on it