header/expansion tank size

[welshswampy]

I posted recently regarding getting some extra hp from my 6.2, thanks for all your replies, but I decided that I'll live with what I have
My latest dilemma is, when I bought this engine it came with an expansion tank which is approx 1/2 gal volume, this I then fitted to my boat (closed cooling via heat exchanger on a 24ft bayliner trophy 2460)
1, Is this tank big enough to take the rate of expansion within the water system
2, I currently run it without a thermostat (runs about 175 constant after warming up) because thats how it came when purchased, is this OK
3, seems to have a fair bit of water pressure after running/cruisng for a while within the header trank, is this OK
When I bought this engine the flow and return pipes to the tank were the wrong way around, so it wouldnt suprise me if there are other things that are not right!
Anyone shed any light?

[DmaxMaverick]

I would consider 1/2 gallon a bare minimum for normal operation. The volume of expansion may not require that, but you need enough overhead to prevent air intrusion. 1 gallon+ would be more ideal, to accommodate an occasional time more may be needed. Much may depend on the actual cooling system capacity and operating temperature range, which may vary greatly compared to automotive systems. Until you are sure what is required, I would recommend a catch-container attached to the overflow of the reservoir. This will allow you to "see" the amount of overflow, if there is any.

Also, you should be using a thermostat. If it was removed, there may have been a reason for it, such as an inadequate cooling system in the previous application. A regulated system is much more reliable than taking a chance that the system will always stabilize at the temperature you are currently seeing. Also, you'll have better economy and more power in the 195°F range, as long as the temperature can be maintained below 210-220° during the most extreme conditions. This is not usually a problem for liquid/liquid heat exchanger systems, and most are adjustable to accommodate varying conditions. Your heat exchanger may have a thermostat control. Either one will work, and both aren't required. An unrestricted internal engine coolant flow (no thermostat), with regulated exchanger temperature is more desirable, in most cases. In this case, you'd want to increase the operating temperature of the exchanger, to the 195-210°F range. 175° is too low for efficient operation, and may present other issues, such as increased sooting (which also shortens engine oil life).

[welshswampy]

Thanks Maverick, my reservoir hasnt got an overflow pipe, just an air tight screw top, should it have an overflow and not be pressureised?
Also what rating thermostat should I get and are these readily available as a standard gm thermostat, same for petrol or diesel engines?

[DmaxMaverick]

If the tank has a pressure cap, it's no more than an extension of the core header. You should still have an overflow, or vent on it. Overpressure has to go somewhere.

The thermostat type would depend on the housing type. If it's the OEM automotive crossover/housing, an OEM type thermostat is what you need. As I said before, it may not be needed, if your exchanger is regulated. If it isn't, I'd start with a 195° stat, and see how that works. If the exchanger is regulated, increase the temp until it will maintain about 195°. Either way, if you are able to maintain the temperature, it will optimize economy, power, and system reliability. Hotter is better, but system components (seals, etc.) begin to feel the heat above 210-220° (OEM "red line" temperature is 240-260°). Below 180°, only a 13 PSI cap is required. 220° requires a 16 PSI. Both at sea level, with automotive capacity cooling systems. If it were my choice, and the equipment allowed, I'd run a reliably regulated temperature of 210°, but that doesn't allow for much overheat. It depends on the capacity and ability of your actual cooling system.

Some other gains can be optimized, such as the use of NPG coolant, if the system is stable enough to use it. NPG (Non-aqueous Propylene Glycol) is unique, as it expands the least of any coolant (essentially zero expansion pressure to 300°+), requires no pressure cap, and is more efficient at heat transfer than any other coolant. NPG is also non-toxic, so automatically waterway friendly. It's expensive, but doesn't depreciate over time (has no service interval).