I am usually in agreement but one part of his post stated "But, that is more to address oil survivability/extended service than the engine's survivability". We all know that our engines depend on the oil as the main factor, but not the only factor, for survivabilty. Oil coolers are designed to cool hot oil to more reasonable temps which protects oil from potential degradation which possibly extends oil life which hopefully extends engine life. The other main factor is the rider and the use/abuse of the bike. There is no replacement for proper maintenance of antique designs aka 40 year old technology of that period and we can only go so far with retrofitting more modern technology. Change yer oil boys!
I think you may have missed the point Jerry. But, I do agree with your ending statements.
The oil change interval for the SOHC4 during the 70's was 1500 miles. (No oil cooler installed from the factory.)
My 84 CB700SC has an oil cooler, and uses part of the frame tubing for cooling, as well. The oil change service interval is 4000 miles. (It has 72000 miles on it now.)
Oil formulation did not change that much from 78 to 84, yet the service interval WAS changed. One major variable was the incorporation of an oil cooler.
The 84 SC is a very low maintenance bike. Hydraulic valves, electronic ignition, shaft drive, leaves only oil changes as the most frequent and almost exclusive regular maintenance item to perform.
At the serious risk of turning this into yet another oil thread. I will state that oil does have a temperature where it begins to break down. The more frequent and the more intense the excursion beyond that limit, the faster the total volume of oil, and its quality, in the bike degrades. The degradation in the usual multi-vis oils is the breakdown of the additive package that makes oil behave like thicker oil when it is hot (within limits). When those long chain molecules breakdown, the oil reverts to properties of it's base stock, which is the lower number in the multi-vis rating. Lower viscosity oil does not present the film strength to bearing surfaces (keeping them separate and lowering friction) than a higher viscosity oil at operating temperature.
However, the reason why we don't just use a straight 40 or 50W oil is because the engine is not always at operating temp, and we need lubrication and oil flow when the oil is cold. Oil the consistency of honey or molasses is just not going to flow and lube the engine during start up. Pretty much the cam will starve and wear will accelerate there, and you can forget about splash oiling the cylinder walls and wrist pins. So, a base stock of 10W is recommended, so as not to wear out the engine before it gets warmed up. The challenge then, is to keep the oil from overtemp and breaking down the long chain additives that make the oil have better film strength when hot. Or, keeping the oil within the safe operating range that the engine designer specified or expected during the engine's service life.
There is more than one source of heat within the motor.
Combustion, and friction are fairly obvious. However, an often overlooked source is within the rod and main bearing journals. If you try to compress oil, it generates its own heat. The more pressure you exert on the oil in these places, the higher the heat spikes. Like many materials it is not just the absolute heat but also the time duration of the over range heat event that determines the breakdown characteristics. (Pass your finger though a flame quickly and it feels warm temporarily, hold you finger in the flame and will begin to destruct.) A study I found years ago measured heat spikes of oil in rod and main journals of over 1000 F. The heat spike tightly linked to the engine's power output. Low power produced very small heat increases. High power produced far higher heat spikes. This heated oil is ejected from the journal, and cooled by the next surface it touches. Which in the SOHC4 design is the engine casting. The engine case and cooling fins have already cooled the oil before it ever reaches the oil sump or scavenge pump at the
bottom. Still, the oil heat contributes to the overall temp of the engine cases, which is an important factor in heat transfer. If it is not obvious already, an engine that is modified to make more power, also produces a higher specific heat in all its production areas, the exhaust, friction, combustion chamber, and the journal oil heat.
To review the basics, heat is only transferred when a differential exists between source and destination. The rate of flow between two objects is governed by the quantum differential between them and the thermal transfer efficiency of the materials. To simplify, two hot objects will both remain hot when placed in contact, same is true with two cold objects. A hot and cold object will eventually equalize at a median temperature between the two extremes. Two important factors in the equalization, is the contact area and the thermal conductive efficiency of the materials in contact.
Back the SOHC4 engine. The engine cases absorb the heat from the oil. And, it does a better job of cooling the oil when the cases are much cooler than the oil. Keeping the engine cool is the best tactic for keeping the oil cool. This is where the engine's large external surface area becomes important. And, it is cooling fins, wherever they are placed, that add the most surface area to an air cooled motor. It is the surface area, the temperature differential between the cooling air and the heat source, and the constant replacement of heat saturated air in contact with the engine's external surface, that determines the engine case temperature and its ability to absorb heat from the oil. This is why, if you have an oil cooler, probably the worst place for it is in the upstream air flow for the engine cooling fins. Heating the air going to those fins reduces their efficiency. This is more critical at low air velocities than high velocities. (Which also explains why race bikes often put them there anyway.)
In summation I will reiterate, that the stock SOHC4 simply doesn't need an external oil cooler. Certainly not for normal street use. Perhaps if you are speeding through Death Valley heated air, using 100% of available engine power, it might save you from changing your oil 500 miles sooner.
Further, an improperly engineered oil cooler system added to these old bikes, will actually shorten the engine life by over cooling the oil to be circulated, and increasing internal friction. (There is also amino acid production within the oil supply to consider, btw.)
