Engine temp Redux

[MikeP]

I want to revisit something that's we've done to death but I have some opinions that differ a bit, I think - proper engine temp for these DDs.

The thermostat's job is to ensure that an engine maintains a minimum temp. For 71's I have seen tstats in 160, 170, and 180 ranges - there may be more. In theory, with a properly designed and clean cooling system, the T-stat should be cycling from closed, to partially open, to fully open, back down to partially open, etc to maintain the temp within the tstat range.

As Karl has pointed out many times, if the tstat is fully open and stays that way the cooling system is running at max capacity and there is no additional cooling available. In this condition, the temp rating of the tstat has NO bearing at all on how hot the engine will get IF there is not sufficient cooling to cause the tstat to "cycle." So whether a 160 or 180 tstats is installed, it has no effect on the max temp of the engine if the cooling system is compromised. The temps will rise to whatever level - maybe enough to damage the engine.

OK, it's obvious that the primary device for proper operation then, is a clean cooling system. A thermostat simply ensures (by opening/closing) that the engine maintains (doesn't get any cooler than) the proper temp.

But what is that temp? In my experience with GAS ENGINES - the minimum coolant temp for efficient engine operation is 180-185. Most cars now use at least 195. I don't see any reason why diesels would be any different. Yet the Tstats that DD says are correct for my engines (based on the engine serial numbers) are 160's.

The coolant temps in our boat's (53MY's) 8v71TIs (after complete cleaning of the RW/FW cooling system) does not exceed 180 running at full load WOT. But as soon as you come down off WOT, the temps will drop to/maintain 165. In my opinion, this is too cold. The fact that the temps will not rise above 180 at WOT demonstrates that these cooling systems, WHEN CLEAN, are adequate to cool the engines regardless of throttle setting. This also suggests to me, that 160 tstats are much too cold.

NOTE: my comment about the clean cooling sys capability applies to the 435HP 8v71TIs as installed on our 1980 53MY. Obviously, engines with higher HP need more cooling ability; whether they have it, I don't personally know.

Perhaps 170s might be a better choice because a thermostat is rated from approximately its rating to usually 15 degrees higher. So, a 170 Tstat would be expected to start opening at 170 and be fully open at around 185; a 180 would be fully open at around 195. Again, keep in mind that these tstats would be cycling between fully open and partially open to maintain the temp as long as the cooling system is clean.

When I bought our boat, one engine maintained a steady 194 on the three day trip. The other was 10 degrees cooler. I was very concerned re the temp and found that engine had 180 tstats - the other had 170s; I don't know why. HOWEVER, the PO had a log on the boat dating back to 1990 that included temp reading every time the boat was out. THe engs were rebuilt in 1995 and the temps since then showed the same 194 on that engine. Obviously the engine suffered no damage or any other ill affect from this temp. Even so, I replaced both with the 160's per DD and cleaned the cooling systems as I mentioned. (Why a rebuilder would have put 170's in one and 180's in the other, I have no idea)

I am now totally convinced that the engs should have at least the 170's in there, and maybe the 180's because overall engine efficiency will be improved at the higher coolant temp. Again, if the cooling system is not sufficiently clean, it will make no difference at all what stat is in there, it will overheat anyway - at least at higher throttle settings.

I know there has been a lot of concern re coolant temps above 185 but one of the engines in our boat ran at 195 for 10 years and DD states that 210 is the max coolant temp. This leads me to think that perhaps we are missing some efficiency, reduced smoke, and longer engine life by keeping these temps too low - at least when operating at normal cruise speeds.

I'd like to see my engines at 180-185 all the time but that's a tolerance that these tstats cannot provide. But IF the range of a 180 stat is too wide for owner/operator comfort (up to 195), it requires a decision whether to set the engs up for a more efficient temp at normal speeds or for WOT.

Heck, in my opinion, even my 180 WOT temps are too low.

[Avenger]

I think Detroit engineers designed a lot of margin into these things for neglect, improper operation and the fact that they expected these engines to be working hard all the time, not paddling a MY at 10 kts. That being said I would also consider that since the 71-series is a dry-sleeve engine it transfers heat less efficiently and there's probably some margin to allow for that. The other thing that would concern me is that, in theory (there's that word again) two-strokes run a higher piston crown temperature since there's no exhaust-intake cycle which would cool the pistons. My question would be that if you set up to run 180° at cruise and risk running 195° or so when pushing it. Is the efficiency gain significant? My pick-up had an aftermarket thermostat in it that was rated 15° lower than the factory t-stat to compensate for a design difference. The engine normally ran at about 170° unless I was working it really hard. I put the factory 'stat in it and it ran 190° - 195° all the time. Any improvement in fuel mileage? None that I could measure. Engine life? Really hard to say because the crankshaft fell out of it about six months later. Another "design difference". Not every GM diesel is a good one. Your theories may be more noticeable on a larger scale, so I guess you get to be the guinea-pig. I'll expect a full report on my desk ASAP.

[Genesis]

The problem with running higher temperatures is that DD designed these systems to run with 7psi caps. This cuts the boiling temperature WAY down.

The problem arises if you get hot spots in the cylinder head and flash boiling of the coolant. That will INSTANTLY crack the cylinder head.

This is how the damage occurs, and its why 195F is the "never exceed" temperature on a Detroit. It is not that higher temperatures would not run a bit more efficiently - it would - it is that you can't get there from here without running the risk of localized flash-boiling in the cylinder heads and liners with a hotter thermostat, and the margins are very thin on turbo engines in this regard due to the high heat load.

Realize that the key is the surface temperature at the interface between the coolant and the metal surface that is being cooled. If that exceeds the boiling temperature of the coolant you will get flash-boiling at the interface, which then leads to big temperature differentials across small areas of metal. The result is a cracked cylinder head.

You can get away with the higher thermostats in a road engine or a naturally-aspirated one, because these engines do not run at high, constant loads (compared to design limits) for long periods of time. Turbo marine engines, on the other hand, are running there most of the time.

I have inquired about running 15psi caps on Detroits to try to alleviate this and was told point-blank "don't do it!" Apparently the internal seals, especially on wet liner engines, are not designed for it and you run the risk of a coolant leak into the oil. While the warning was a bit less strenuous on the dry-liner engines the basic response view I got was extremely dim and negative on the idea.

[MikeP]

Well, I wouldn't mind putting a report on your desk as opposed to fried engine parts!

Your point is a good one re design issues and that might explain why DD says 160's. It might be that on these engines the actual cyl head/combustion chamber temps are inherently somewhat higher than other engines at the same coolant temp. But you still end up with the fact that the Tstat can only determine the low side of the operating temp; it can't control the high side if the cooling system capability is insufficient. So if the cooling system is dirty, putting even a 100 degree tstat (if such a thing existed) wouldn't prevent the engine from overheating at WOT.

Based on what I have seen on a clean cooling system, WOT, a 160 tstat, and 75 degree water, my 8v71tis will hold 180. But thats well above the opening range of the stat so it tells me that the tstat is totally out of the picture at that point - it is not cycling. That's why it seems to me that a 170 would be better because it would hold the cruise temps up to at least 10 degrees warmer while, at the most increasing the WOT setting to maybe 185 (based on the inherent variation range of a tstat). 180's might be iffy because they would raise the cruise to around 180 (Good!) but the WOT to possibly around 195.

As far as any real efficiency gain, heck I don't know. Probably be best to apply the "if it ain't broke" philosophy and not worry about it at all.

But the 160 range seems too cold for routine operation...

[MikeP]

Karl, after posting the above I saw your response; makes good sense. As I noted here some time ago, the PO had 15psi caps on the boat and they were there for at least a year after I got it before I noticed it when I pulled the intercoolers/HEs for cleaning. I changed them back to 7s. Maybe the 15PSI explains why they were comfortable with 180s in one(?) engine. Obviously they were not aware of, or not concerned with, the other issue re 15psi.

[Genesis]

The "160s" are in fact not toally open until about 180ish.

At least not when I've tested them in a water pan. "160" is when they START to open.

The 195s in modern cars result in "operating temperatures" around 200-210 for the same reason.

The reason you see "165" with 160s at low load is that once they start to open even a very small amount of cooling capacity is enough, as diesels are a lot more efficient than gas engines and reject less waste heat.

This becomes obvious in my little Jetta in the winter; sitting waiting for a train or some other thing like that and it will stop heating the passenger compartment as there's not enough waste heat to tap!

[krush]

This becomes obvious in my little Jetta in the winter; sitting waiting for a train or some other thing like that and it will stop heating the passenger compartment as there's not enough waste heat to tap!

Yeah, have u ever tried to wait for a diesel to heat up in the cold weather with no load at idle? hahah.

Diesel naturally run very lean at no load unlike gassers that need to maintain a nominal 14.7:1 air:fuel ratio.

Also, the efficiency increase in 10-15 degrees is most likely negligible.

[Avenger]

In my Jetta TDI, VW actually had to put electric heating elements in the cooling system to work the defroster in winter.

[Trojan]

The DD manual states that the tubes in the inner cooler should not exceed 13 PSI when testing for leaks. I don't think 15 PSI cap would be a good idea. Block cooling temps and cylinder temps of course are not the same. It just may be that the dry liners require the lower temps in cooling liquid in order to dissipate the higher heat from the hotter cylinders.

BILL

[doc g]

I think the other rationale for the 7psi caps is that if you have an oil cooler or fuel cooler begin to leak it will not allow cooling water to go into the oil or fuel as the pressure in the water jacket is significantly lower than either the fuel or oil pressure. I think the DD engineers thought this one out .You can of course end up with water jacket contamination ,though .............Pat