[Putershark]

I love it Nate, another "Motor Head" so this should be much easier for you to picture than some of our other members. Hopefully though, I can help more picture how this all comes into play

Well, the biggest difference between the 2 cooling systems (after the thermostats open up of course) is the ability or more important, speed at which one transfers heat away from the engine. As you well know the hotter the engine (cylinder heads and intake manifold mostly) are running, the higher your octane requirements can / will become with all other things being constant...

So if you take my engine and put it in my local Gulf waters that are running 80 to 85+ degrees at X RPM's and then move it into waters that are now (for example) 50 degrees. The amount (rate) of heat transfer from the engine while running in the cooler water water temps is going to be greater for the same amount of water flow. Thus in the hotter water what needs to run on 91 octane to run at its full timing curve map, can now run on 87 octane fuel and not suffer any real measurable performance loss in the cooler waters.

This is unlike a "closed loop" cooling system like your typical automotive engine has which uses 15 psi (or more) of radiator cap pressure to raise the boiling point of the coolant. Much less, a thermostat which helps it transfer more (and at a more constant rate) engine heat for X amount of coolant flow. We also have to remember that these cylinder head temps soar much, much, faster in any boat engine. This is due to the amount of work / load that engine has... Let off the gas pedal in your truck and it will coast for quite a distance. Drop the throttle on your boat and it stops very quickly due to all the drag. Outboards have to work on a constant basis to keep the boat hull moving much more than your standard automotive engine. This is also way I state that rule #1 for saving fuel is... "Never drive a boat where you can drag it, for it burns much more fuel to get it to the same area "off" the trailer then on it"

Another good example of this uncontrolled (faster build up) of heat and how it effects octane requirements is when you get a car engine real hot and when you go to shut it off how they can rap and carry on and sound like you need to take out a stick to it to beat them it to death This is because the engine is so hot that is it slowly becoming like a Diesel engine in nature, and "trying" (or is) to run on compression alone long after the spark plugs are shut down... The Air / Fuel mixture is still exploding due to heat alone, not because it has any controlled ignition source.

So Outboards (much like Automotive or Aircraft air cooled engines) have a harder time in controlling heat transfer due to the fact that the inlet water temperature (as in the case of the outboard) changes more as the season's change. For many users, their biggest issue is in getting a 4 stroke engine "warm" enough due to the lower inlet water temps to get the engine oil hot enough to "boil off" all the moisture it collects. That crankcase moisture becomes very acid in nature as you well know and can create some internal engine havoc in its own right...

Here in Southern Florida though, we only combat with that issue (running too cool) for a few (very few) winter weeks. We on the other hand, battle with higher temps both Air and Water thus, our engines can naturally have a higher octane requirement for at least 6 months or more out of the year. Even summer months up North are not as bad of an operating environment as here even when the air temps push the 90 degree mark outside because the water the engine is drawing from for cooling, (transfer rate) is usually 15 to 20 degrees cooler. Here though it may be 87 degree water and 90 degrees outside. That is just not that much of a temperature spread to draw off much heat. And we wonder why back in the late 1980's why carb Mercury Outboards (painted black now) the fuel would boil in the lines and they would vapor lock and not start after running 20 miles offshore? Never shocked me as to how it was happening Yea, that is real smart... Lets paint the engine black and see just how "hot" we can make it run! Ha-Ha...

Heat, Friction, Engine Load, Weather (both in temp and humidity levels) all effect "real time" octane requirements. Though an engine can be designed to run on X octane level (87 for example) those design elements cannot really factor in for engine carbon deposits that either "lean out" the incoming air / fuel mixture due to dirty intake valves or plugged fuel injectors OR (which many forget) carbon deposits on the crown of the pistons that is in effect, making the compression ratio increase due to the now smaller, combustion chamber size.

So you take a new clean engine that has a compression ratio (for example) of 9 to 1 and "can" run on 87 octane fuel and not knock, but give it a few millimeters of carbon build up and is now almost 10 to 1 (or higher) compression on a few if not all cylinders now will knock even if you don't hear it doing such... And even if you do not hear it rap (knock) the knock sensor has and "is" backing off the timing curve and power output (much less RPM's) all at the same time...


Now Nate I am quite sure you have tapped on a running engine with a small hammer while at idle to test the knock sensor operation and see the timing curve jump to know what I mean...
Heat though is the issue plane and simple, and even worse in Florida when it comes to octane needs. The closer you can keep the engine to new (clean) both inside and outside the better you can transfer heat and as such, help manage fuel octane requirements.

So as I stated before, one needs to really test, track, and compare how "your" engine reacts to find the "sweet spot" for your hull and engine / prop combo in your area. Don't be shocked to find though, that as the engine gets some hours on it and it is getting warmer outside... That you find your ROI (Return on Investment) by running a higher grade and higher octane of fuel even if it is just jump to 89 octane shows a lower fuel burn rate and better performance on these newer 4 and some HO 2 stroke engines

One last item... 91 octane fuel on average is what now 30 cents more a gallon? So a 190 bay holds (if empty) 29 gallons total. So for a extra $8.70 not even the cost of lunch, to get fuel that may set in your tank for 6 months on end is the TOTAL cost difference between it and the lower octane grade of fuel. This while maybe returning better short term gains much less, long term gains by helping keep engine deposits in check to me is pretty dang cheap engine performance insurance in my eyes

Hope this helps?

Dave