Thermostats for low speed operation

[bobk]

Wen I run the 48MY at 1000 rpm, the engine temperature comes up to 160-165F. A little marginal so I bump it up once a day to heat them up and clear the carbon. One of our HOF members who has the same boat and engines reports his 6V92's run 180 F at 1000 rpm, and that only increases 5 degrees at cruise. Does any one know know what that thermostat would be? Sounds like a good thing to have as we are all running shower.

Bob

 

[MikeP]

In theory, of course, a thermostat should hold the temp rating of the stat at any RPM. Auto Tstats/cooling systems do exactly this all the time. DD says that engines should not idle for long periods at less than 8 or 900RPM (can't remember which) because the temp will fall too low. So that indicates that at 1000+, the engine can generate enough heat for the temp to rise to whatever the Tstat can control.

So the Tstat, should be able to hold the eng temp in its appropriate range at any RPM from 1000 to WOT.

HOWEVER, the reality is that at least some of these marine DD cooling systems aren't that good. It's not a tstat limitation, it's a system limit. My 8V71TIs with 160 Tstats (oem spec) will hold around 165 at 1400RPM cruise and 185 at WOT IF the system is freshly cleaned. But that's not how it should work. If the cooling system was adequately efficient, the temps would not change to any great degree, the Tstat would just open/close as necessary to maintain the proper temp, like it does on your car.

Hotter stats would improve engine operation at the lower speeds we all seem to run now but could be a problem if you needed to run at high speed for any reason - WX, emergency, whatever. The PO had 180 stats in one engine (don't know why). At WOT it would run at 195; most folks think that's too hot for these engines. I discovered this when I was trying to figure out why one engine ran 10 degrees cooler than the other. When I was checking into this, DD had 160, 170, and 180 Tstats available for my 8V71Tis but per the engine serial number, 160s were originally speced so that's what I used.

I think often about going to 180's...

 

[Walter P]

I think that we are comparing apples to oranges by even thinking that our old technology DDs would operate like our car engines. As has been mentioned many times over the years on this and other forums, the cooling systems on our DDs seem to work well when everything is as per original spec. That includes a spotless cooling system, proper coolant mix, outside water temp and so on.

Bob, I think your engines are just fine the way you run them. As you noted on our recent trip for the NE gathering, my engine temps only went up to 180 when we ran at 1950 - 2000 from Sandy Hook to Manhatten. From the battery to Westbrook we kept the rpms at right around 1180 - 1200 and temps were right around 160.

Incidentally running slow isn't so bad. We saved a bunch of fuel and the ride was very relaxed. I'm a believer...

Walt

 

[REBrueckner]

Your friend likely has 180 degree thermostats.
Why not take out the 160 thermostats it sounds like you have and put in
170's...it's a small step in the right direction....Like Mike, I find 180's a bit high at higher RPM's in my same engines.

I had a similar experience with my 8V71TI's as Mike: After rebuild about ten years ago, my stb engine always ran about 6 to 8 degrees warmer than the port...and got almost to 190 at 2000 RPM when I occasionally ran them up. (via infrared thermometer and gauges) Port ran about 185 degrees at higher RPM, ok by me.

After checking EVERYTHING over several years, the final being an acid wash of raw water components that was clean as a whistle even after eight years, I finally pulled the stbd eng thermostats at New Bedford Mass during a few days of rain (nothing much else to do and lots of mechanics there in case I screwed up)...I found 180 degree stb vs the 170 in the port....I have been running 170's in both ever since....

My mechanic says all three thermostats are run in my engine model and thought nothing of it....