thermostats

[takefive]

while changing out fresh water pump on 871TI had to remove thermostat housing and noticed thermostat is a 160 degree. Pulled left bank housing and that is also 160. Would not a 180 degree be appropriate? Have never verifyed actual running temp., but gauge does indicate 160-170 range. Boat is a 1979 46c.

[REBrueckner]

You know there are two thermostats per engine. one for each bank of cylinders.

My mechanic told me those engines might have 160,170 or 180 degree thermostats. I thought I selected 170's in my 8V71TI's for NY to Maine waters and found cooling just fine. I originally figured if there was a range or thermostats, being in the middle would be safest.

What actually happened was the starboard engine ended up with 180 thermostats instead; that was fine in cool Maine waters, but when I switched for two summers to the Nantucket area running at 1800 or 1900 from time to time resulted in uncomfortably warm engine temps on the one...about 185 to just under 190 degrees. When I finally figured out the one engine had different thermostats and replaced the 180's with 170's both engines stayed reliably at about 180-182 degrees at higher RPMs...even WOT.

180 degree thermostats in southern waters would likely get you too close to 190 or even above temperatures in 80 or 85 degree ambient waters if you run at higher RPMS. On the the hand if you run at slow RPMS for extended periods and don't need top end performance for extended run times, warmer thermostats might give you a bit of cleaner fuel burn....reducing the chances of cylinder washdown from unburned fuel....

[takefive]

I am going to leave in the 160's and moniter temps. with IR gun, it's not that big a deal to change them out if need be, thanks.

[Brian Degulis]

I would always use the lowest unless your running cold. The lower rating gives you more time in the event of a cooling system failure sometimes seconds matter. If you run pyrometers to mearure exhaust/combustion temp there is no diffrence between 150 degree water temp and 190 degree water temp. The water jacket temp has little effect on combustion temp in that range.

Brian

[REBrueckner]

I'm not a fan of fixing things that ain't broke unless the is a clear reason for making such a change.

One would think that from a mechanical engineering perspective, there would be some minor pros and cons of engine operation and performance with different temperature thermostats...yet I have NOT ever seen nor heard such commentary so if there are some variations I assume they are insignificant.

For example, the hotter the thermostat, the warmer the likely intake air for combustion and hence a tiny bit of combustion efficiency would be lost....but you might think that would be offset by higher combustion temperatures and maybe more complete combustion...likely it's pretty much a wash....but a competent review would be interesting.

Maybe somebody who is a member of more technical diesel forum could submit a post there. Be interesting to know what factors apply.

[Brian Degulis]

Maybe if you were testing with some sophisticated device you would see a difference but not in real world. A gas engine with a carburetor runs bad when cold because you need some heat for good atomization. In a diesel engine your shooting fuel into chamber heated by compression so jacket temp has little effect. Just take a look at a DD in perfect condition. If you started it in say 70 degree temps you would get less than a minute of cold smoke then it would clear. At that point the jacket temp is still cold maybe 100 degrees or so. If that boat had pyrometers and you cranked it up under load without waiting for the jacket temp to come up you would see your ex temp shoots right up even though the jacket temp is still cold. Actual firing temps run about 14x jacket temps so the effect is minimal. The only reason you get cold smoke from a diesel is because the pistons need to heat and expand to proper clearances to make good compression. That happens very quickly from the heat of combustion. There are some books that explain all this better than I can one is Diesel engineering or something like that. When I get to work I'll find it and see if I can post a link to it.

Brian