Correct Engine Temperature

[ron6785]

Cruising this summer and while pushing up the TENN-TOM noticed my water temps at hull speed 9KNTS were up around 185/187 seems a little warm to me. Current about 1.5 knts. RAW WATER temp about 90 degrees. What do you think TOO WARM?????

 

[MikeP]

There are a variety of opinions on this...engines are most efficient in general with water jacket temps of around 185-200. HOWEVER, water jacket temps can be the same on two different engines while internal design results in the actual temps in each engine being radically different.

So the definitive temp should be whatever the manufacturer states. However, this again, can be confusing. For example, according to the DD service guys on Long Island who checked the computer using the serial numbers, the original Tstat for my 8v71tis is 160. Frankly, that's too cold a running temp for any engine. Interestingly, the DD service manual lists several Tstats for the engine, NONE of them as low as 160.

Karl has mentioned numerous times here that the DD cooling system on these boats is marginal. I agree, finding that although our boat will maintain 185 or thereabouts at WOT in 75 degree water with freshly cleaned HEs and intercoolers, it will not a year after cleaning.

Holding 185 with a 160 Tstat brings up another point. The max temp an engine SHOULD reach with a 160 t-stat is approx 175. The standard full open temp of a tstat is approx 15 degrees above opening temp, which is the rating of the tstat. So a 180 Tstat is supposed to START opening at 180 and is expected to be fully open by 195. IMHO, THIS would be the ideal temp for these engines.

However, because the system is marginal, the cooling capability of the wide-open system is usually insufficient to maintain that temp under high loads and it will tend to climb to some point that may be too high and cause major engine expenses.

If you compare it to your car, you can clearly see the difference in relative cooling capability. Most cars will maintain virtually the same temp whether idling or at fairly high throttle settings. It is true, of course, that a car seldom actually uses a high throttle setting for more that a few seconds at a time. But even on a track or pulling a trailer up a hill, most cars still hold the appropriate temp. I drove an 1989 sporty car from Nevada to MD last year and in 108 degree temps in bumper to bumper traffic in Las Vegas or at 140MPH on closed roads, the temp gauge stayed at 195. So cars do it all the time, why not these DDs.

It's not the fault of the engine, it's the adaptation to the boat and, apparently the concern about space required for a larger HE that could actually keep the engine at temp.

So anyway, my recommendation would be twofold...first, if you are CONFIDENT that you will only run the boat at some reduced power setting - hull speed, for example - then you could install a Tstat that keeps the boat at a comfortable temp doing that. But if you want to run at planing speeds, you will likely have to install a colder Tstat. I suspect that's why 160 was installed in the oem configuration in our 53. Frankly, the only way you can figure this out is to experiment AND ensure that the system is clean especially if you want to plane. Of course, running at hull speeds is not too tough and our boat maintains 164 on both engines at around 10K.

So the issue is that you want the engine in the 185-195 degree range for best efficiency...the problem is trying to figure out how to keep it there under the different throttle settings with a shaky, at best, cooling system.

 

[ron6785]

There are a variety of opinions on this...engines are most efficient in general with water jacket temps of around 185-200. HOWEVER, water jacket temps can be the same on two different engines while internal design results in the actual temps in each engine being radically different.

So the definitive temp should be whatever the manufacturer states. However, this again, can be confusing. For example, according to the DD service guys on Long Island who checked the computer using the serial numbers, the original Tstat for my 8v71tis is 160. Frankly, that's too cold a running temp for any engine. Interestingly, the DD service manual lists several Tstats for the engine, NONE of them as low as 160.

Karl has mentioned numerous times here that the DD cooling system on these boats is marginal. I agree, finding that although our boat will maintain 185 or thereabouts at WOT in 75 degree water with freshly cleaned HEs and intercoolers, it will not a year after cleaning.

Holding 185 with a 160 Tstat brings up another point. The max temp an engine SHOULD reach with a 160 t-stat is approx 175. The standard full open temp of a tstat is approx 15 degrees above opening temp, which is the rating of the tstat. So a 180 Tstat is supposed to START opening at 180 and is expected to be fully open by 195. IMHO, THIS would be the ideal temp for these engines.

However, because the system is marginal, the cooling capability of the wide-open system is usually insufficient to maintain that temp under high loads and it will tend to climb to some point that may be too high and cause major engine expenses.

If you compare it to your car, you can clearly see the difference in relative cooling capability. Most cars will maintain virtually the same temp whether idling or at fairly high throttle settings. It is true, of course, that a car seldom actually uses a high throttle setting for more that a few seconds at a time. But even on a track or pulling a trailer up a hill, most cars still hold the appropriate temp. I drove an 1989 sporty car from Nevada to MD last year and in 108 degree temps in bumper to bumper traffic in Las Vegas or at 140MPH on closed roads, the temp gauge stayed at 195. So cars do it all the time, why not these DDs.

It's not the fault of the engine, it's the adaptation to the boat and, apparently the concern about space required for a larger HE that could actually keep the engine at temp.

So anyway, my recommendation would be twofold...first, if you are CONFIDENT that you will only run the boat at some reduced power setting - hull speed, for example - then you could install a Tstat that keeps the boat at a comfortable temp doing that. But if you want to run at planing speeds, you will likely have to install a colder Tstat. I suspect that's why 160 was installed in the oem configuration in our 53. Frankly, the only way you can figure this out is to experiment AND ensure that the system is clean especially if you want to plane. Of course, running at hull speeds is not too tough and our boat maintains 164 on both engines at around 10K.

So the issue is that you want the engine in the 185-195 degree range for best efficiency...the problem is trying to figure out how to keep it there under the different throttle settings with a shaky, at best, cooling system.

Thanks for the info Mike, I have 8V71 naturals and the boat is loaded and I've been pushing up stream with raw water at close to 90 so I'm not too concerned. When a WOT I run up about 195 however there's no reason to do so cause the boat won't plane anyway and I'm just pushing water and fuel out the back end. Frankly I wish they had put 671's in the 53 better fuel economy.

 

[REBrueckner]

I agree with the above Mike P post, but I do NOT like being close to 195 degrees....that's just too close a margin of error for me. But it's not unusual in ninety degree ambient water.

My only additional comment would be if your raw water cooling side has not been cleaned in a while,say five years or more to pick an example, maybe in the off season you could inspect the intake (pressure) side of the heat exchanger to be sure there are not impeller pieces and zinc pieces blocking raw water water flow...that's easy to do with the remove of a hose clamp and four bolts to the expansion tank....there can be other sources of blockage but for naturals that's the best place to take a look. And of course you could consider acid cleaning the raw water system side, also not difficult.

I noticed a difference between Maine maybe 50 or 55 degree water and LI and Cape Cod at about 75 or 80 degrees. When I noticed one engine running warmer, 1972 8V71TI, after about nine or ten years since raw water system cleaning, I took everything apart, it was clean as a whistle(!!) , acid cleaned anyway...no change(??): turned out that engine had 180 instead of 170 thermostats and one of the two was not quite opening all the way.

 

[Genesis]

185ish, if on mechanicals, is about where you're going to be with 90F raw water and a reasonably-clean system.

Watch it CAREFULLY - 195F is "never exceed" on a Detroit due to the 7psi cap. You WILL get localized boilover not far from there (the internal temperatures are higher than those at the elbow by a few degrees, and its enough.)

Remember that you need GRADIENT to move heat - that is, it has to be hotter inside than at the heat exchanger (obviously) for heat to be moved from the engine to the raw water.

 

[Captned]

So when operating in warmer waters, you should put in 165 degree thermostats instead of 180 degree ones????


Captned
MBMM

 

[MikeP]

I would say yes but remember, the TSTAT does not limit the high end of the temp. It's purpose is to limit the low end. That is, to keep the temp at the minimum level for best efficiency. IF the cooling system is of sufficient capacity to provide proper cooling at any throttle setting, the stat will cycle open/closed/partially open to keep the coolant at the proper temp. But if the system is not more powerful than the heat-producing capability of the engine, the tstat temp rating is not meaningful because even with the tstat wide open, the engine cannot release the heat as fast as it is being produced.

So the temp of an engine at WOT with a cooling system that can't really cool it within the Tstats temp rating will end up at the same temp reading whether there is a 160 or a 180 (or whatever) Tstat in the engine. THat temp MIGHT be below the danger temp for the engine or it MIGHT NOT.

 

[REBrueckner]

"So when operating in warmer waters, you should put in 165 degree thermostats instead of 180 degree ones????"

Not really. That's the last step to correct a high temp problem, not a first step. If your cooling system is dirty, that's likely only going to help at moderate RPM if at all..
If your temps are ok, why bother...if they are not, then all the usual checks must be made...after all else, removing and testing olds T stats in a pan of water with an infrared is the way to go....that's how I found one of my older thermostats was NOT opening all the way...so while I was at it I changed from 180 to 170 degree because I liked the way 170's worked in my other engine.

 

[MikeP]

I agree with REB...the key is to have the cooling system as clean as possible and go from there. But IF the system can't adequately cool at WOT, the temp rating of the Tstat has no effect on the engine temp except at low speeds.

That's why, IMHO, if you are sure you are ONLY going to run at low power levels - displacement speeds - I would put in a 180 Tstat to keep engine temps up to a decent level. Actually, you are no worse off at the WOT power setting with a 180. EITHER the system will adequately cool the motor or it won't..the Tstat rating doesn't matter.

The only thing a low temp stat - like a 160 - does for you is that if you are cruising along at displacement speed and, say 165 degrees, and you go to WOT, you have more time before the eng temp gets too high if the coolings system is inadequate. With a 180 Tstat the temp may be around 185 or thereabouts at cruise. That's fine. but now there is only 10-15 degrees more or less available before a possible problem. With a proper cooling system, like a car has, that doesn't matter. It will still hold that temp because there is plenty of cooling capacity available.