you guys were getting close...answer is salt crystallize's at 150 deg, so the manufactures set the operating temps just below that to help keep the salt for killing them quicker than it does, and the fresh water cooler got higher temps as they did not have to worry about that issue..

Above ^^^^ is really somewhat correct only about the salt crystalizing but really technically not 100 % true either. EVERY STOCK OEM Marine engine uses a 160 degree t-stat even carb engines since 1997 to currently and ALL EFI / MPI engines use a 160 degree t-stat since 1993. A lot of closed cooling OEM Stock marine engines currently use a 170 degree t-stat.

The OEM engine builder has no idea what area of the world the boat was going to be used to determine salt apps get this temp - t-stat and freshwater gets this temp t-stat.

It was always taught in factory marine schools that the salt in saltwater crystalized around the 145 degree mark. Back in the day early 90's and back all carb engines had a 143 degree t-stat. Everything except EFI / MPI (160) from 1996 and BACK uses a 140 degree t-stat. Again everything after that uses a 160 and closed cooling a lot use 170 -t-stat.

coming up all marine I/O's and inboards with be closed cooled and most will a 170 degree t-stat. Also an engineer for GM told me in person that the marine engines run more efficient using a 170 and even a 180 degree t-stat but you are starting to increase temps inside the engine compartment in which can cause problems.

Looking on mercs parts site, the HP500 gen 6 and gen 6 540 bulldog still used a 140 stat

Correct and so did the 500 EFI use a 140 - the 600 SCI used 160 and the newer quads 1100, 1350 and 1650 use a 170 degree -

I will rephrase Merc black production engines in which sells 750 to 1 on the USA water ways and also are used in saltwater apps have used a 160 degree t-stat since end of 1996 / model year 1997 EVEN in CARB apps. .

Wonder why they would use a 140 stat in a high performance and a 160 in a non high performance (black engines)?

As far as the modern engines like the 1350, or even 600sci, I'd imagine there is alot more to the timing vs coolant vs Afr vs IAT etc , closed cooling, . basically a much more sophisticated engine management system over a carbed big block with a preset timing curve ....

Wonder if the alum heads play a part?

To add to the mix - the black production well actually the 8.2 EC - CAT engine from Merc is assembled in house uses a 170 degree t-stat - yes it is closed cooled but keep mind the heat exchanger water cooled internal portion and the exhaust system is not any part of the closed cooling areas. The CAT has to be at bare min of 600 degrees to even start working properly - yes there is water jacketed portions but that still internally is very hot - so keep in mind these engines are used in saltwater as well. Matter of fact drove a 8.2 apps in Miami.

Next the Mercruiser 5.7 / V-8 - 350 EC and FYI - EC = emissions control = CATALYST engine - if it is a closed cooled engine uses a 170 t-stat and if it is not closed cooled then a 160 degree t-stat. Again the CAT needs to heat at 600 to 800 degrees to even work correctly.

The Merc Racing HP 525 used a 160 t-stat if anyone cares. Probable Merc racing number 1 selling engine from early 2000's till 2012.

I care, and always wondered why my 525efi never goes above 150 if it has a 160 t-stat?

and then there is this 120 deg thermostat. Why would someone choose this?

http://www.hardin-marine.com/p-14889...5-engines.aspx

Maybe useful in really cold lake water temps

That is the stat spec'd for the AZSM kit for the 575's. That's what I use in my engines.