Closed Cooling efficiency

Hi all,

Question on recommended means to increasing the cooling efficiency of a standard gas engine inboard closed cooling system. My Mercruiser 7.4L inboard was a transplant from a stern drive boat. The motor was originally raw water cooled in it's previous life. The transplant into the inboard involved a rebuilt short block as well as the addition of closed cooling. She doesn't overheat per se, however I don't appreciate the creeping temperature gauge after running for a wile over 3200 RPM. The current system is the standard Mercruiser raw water pump supplying raw water through transmission cooler, engine oil cooler, heat exchanger (4 inch), then out though the risers. currently she'll sit at 165 degrees as long as I keep the revs at or below 3200. If I run above 3200, the temperature gauge slowly increases. I don't let it get above 180 before I'll slow it down. It then hangs there for a while. When I run outside in the cooler Atlantic the temp maintains at 160-165 for a much longer period of time. The warmer raw water source in the bay seems to have a significant impact to the efficiency of the cooling system. the raw water pump impeller & housing are new this year. The heat exchanger is clear with no scaling. Standard 50-50(roughly) coolant/water in engine.
Any recommendations?

 

Some items to consider
1) If the engine heat only goes to 165 at lower speed, it suggests that the thermostat is opening at 165, or the engine would continue to heat up even below 3200. Do you know what temp the thermostat is? If it is 165, throw it in hot water on a stove, use a meat thermometer and see if it opens all the way at 165.


2) You say that you have a new impeller. Did the old one disintegrate before you closed the system or did it just get old?
If the old one broke up or one did at any time before the installation of the closed system , there is a chance that there are pieces of the impeller within the engine, hoses, thermostat or other cooling heat exchangers that are restricting flow at higher speeds.

3) Is the new engine heat exchanger sized for the 7.4?

4) Will the engine temp rise above 180 with sustained high RPM? ie it sounds like you are shutting it down at 180. 180 is not out of the ordinary depending on the temp rating of the thermostat.

 

The thermostat is a fresh 160 degree unit. I haven't allowed it to go above 180 due to the following: While over 165-170 degrees, the oil pressure starts a slow decline. Honestly, the 180 degrees isn't all that bothersome to me, it's the impact on the oil pressure or what the gauge is reading anyway. Once the motor cools down back under 180, the oil pressure will return. At it's normal temperature, the pressure is around 50 PSI at 3200 RPM. I've seen it drop to around 35 or so with the temperature at 180 with RPMs constant. Might be signs of other internal wear perhaps?

The impeller did in fact grenade on me last year which lead to the demise of the housing. Last night, I removed the output hose from the pump. There was some debris in the inlet of the transmission cooler. 1-2 blades of the previous rubber impeller we jammed up in the ridge around the outer circumference of the cooler. HOPEFULLY this was my restriction and the reason for the temp creeping. However, I was concerned with the heat exchanger as well. It appears to be a correct Big Block unit as identified by some research. As the motor was not originally a closed cooling system, it is not the unit that would have been there had this been a Gen IV MIE Blue Water inboard. Due to an unknown reason, the exchanger is remotely mounted on a bracket bolted to the stringers as opposed mounted on the engine. I am guessing this was done due to height issues. I bought the boat with this motor in it as configured, so not 100% certain of the conditions that lead to the conversion.

 

Oil pressure drop at moderate engine temperature is an indication of worn bearings and/or other parts that turn in them. You should get at least 10PSI per 1000RPM on these engines. You mention that the water goes out through the risers. Does this means that the exhaust manifolds are fresh water (coolant) cooled? If so, the heat exchanger may not be adequate if it was sized for cooling the engine only.

 

correct Gonzo, the manifolds themselves are fresh water cooled. Do you have any idea as to what the correct heat exchanger would be for that application? As far as the oil pressure is concerned, it is definitely lower then I'd prefer at 180 degrees. I run SAE40 wt with a qt of Lucas... 180 degrees should be just fine for maintaining viscosity. It probably is due for a bottom end job in the non too distant future. It maintains 45-50 lbs under most days other then at idle after a good run in from the wrecks. I feel comfortable with the pressure for now assuming it stays on the high side of the 40 line. I was also wondering if an increased capacity oil cooler was in order.

 

These guys may be able to help you on sizing the heat exchanger. However, you can always add another to the system and plumb them in parallel.
http://www.cpperformance.com/t-marine-closed-cooling-systems.aspx

 

Thanks as always. I'm going out tomorrow, I'll see how she does with the cleaned out raw water passages.

 

http://www.boatfix.com/merc/Servmanl/16/16B1R2.PDF

Attached is some info from Merc which contains some oil pressure specs
At idle 4 psi minimum
at 2000 rpm, 30-70 psi

20w40 as a preferred multi grade and even 20w50 as a second multi grade

A statement that might draw some discussion is that the oil pressure does not provide the lubrication boundary between the journal and the bearing. Ie the pressure does not float the crankshaft. Rather, the evidence of pressure ensures that there is an oil supply at the port into the bearing/journal interface. Viscous drag of the oil, at the boundary layer draws the oil into the race. The crank actually runs slightly eccentric (the centerline of the crank journal and the bearing is not the same) with the centerline of the bearings and the boundary layer drag pulls the oil into the smallest clearance area. The strength of the film, the temperature of the oil, the viscosity of the oil resists this this film breakdown.
So for normal rpm ranges, say up to 6,000 rpm, you do not need high oil pressures to provide this oil the bearings. In fact in some applications, a higher volume pump with the relief set at say 40 pounds is desirable.

I would not want to defend the following comment as it was told to me several years ago by a Caterpiller engineer/salesman. (1970"s) Cat thought that if they increased the pressure up to 100 psi for operating, that they would have less wear. But they did not find this to be the case and a 100 psi pump consumed considerably more hp than one set at say 60 ish.

Certainly, with more wear in the crank bearing interface, more oil bleeds out axially to the crank and eventually, you will see this as a reduction in the pressure gauge.

 

Thanks Barry.
I had previously seen that data as well. As the ambient temperature is over 50 degrees for the majority of the season, I opt for the straight 40 weight as also recommended.

Very interesting description regarding the function of the pressure. It's a different way of looking at it for sure. It makes sense though.

Took the boat out today for the first time since cleaning out the debris that had eluded me when I replaced the pump.. A pretty significant difference was realized. I could not get the temperature to get to 180 until I ran it up at 4300 for about 5 minutes or so. I did not experience anywhere near as significant of an oil pressure drop. It ran 45- 50 lbs all day at anything over 1500 RPM. I'm assuming that the increased raw water flow is appreciated by the oil cooler as well as the heat exchanger. I do still feel that Gonzo may be on to something regarding the heat exchanger's ability to handle the full system I have as opposed to a half system. More research is still needed.

 

Transom pickups to push water at high speed, you will sacrifice several knots.

The pickups from Fountain.

Wherever it was not possible to through-bolt the pickups due to stringer placement I epoxied and countersank bronze inserts.

Heat exchangers are costly but you can add them; try to find them on ebay but ensure they are bonded and better if they have pencil zincs to slow corrosion.

Keeping the cooling loop clean is also a big help. I have screened bilge pickups and ball valved to divert supply from sea to bilge. After using the boat I wash the the engine room with salt away (sulfamic acid is the active ingredient) then use the treated water in the bilge to flush the engine.

But I learned I also need to also flush the salt-away

 

exhaust risers blocked?
does the water pressure go up as rpm goes up?

 

Exhaust risers are not blocked (?) I flush the engine with a form of inhibited sulfamic acid after each run.

There is increasing water pressure with speed; up to 15 PSI, working impellers when on plane are not required.

I have a plumbing system that taps from the main engine raw water induction to an accessory strainer then manifold. A pressure relief dump is connected so accesories (like genset) do not have any positive pressure. Engine and accessory manifold pressures are monitored at the helm.

 

If the heat exchanger is mounted on a stringer how does the air in the system get to the high point which is where the filling cap would normally be?

 

Make sure that at speed this pressure relief does not create a loop in the raw water and your engine is not circulating raw water instead of sucking cold water from the chest.

 

Doesn't the pressure relief dump overboard?