There's not really much we can do about oil flow rate, short of not restricting it.  Does the oil typically flow at double the rate of the coolant in the stock configuration?  I've never really thought about that, but I can't see how it could. 

 

If the oil doesn't flow that much faster than water, and it has half the heat rejection of water, then do we need to size the oil cooler the same as the radiator?  I've been amazed at how little radiator I need, compared to how hard it is to cool the oil, so this is starting to make sense. 

 

 

Don’t forget that the amount of heat rejected to the oil is about 40% of what goes into the water.  The specific heat of oil is actually about 70% that of a 50/50 mix of water/glycol (.6 vs .84 in metric) My dyno test showed the flow rate of water just a bit more than double that of the oil if you have a thermostat in the coolant circuit (triple without the thermostat).  Now you put in the factor of typically allowing the oil about 20 F higher the coolant, and you have the air flow right, it works out that the heat transfer area (core volume, if you wish) of the oil cooler can be just a bit less than half the radiator.  And Mazda had things figured correctly.

 

This, of course, assumes that you have the same incoming air temp to both.  Putting the oil cooler behind the radiator throws a big wrench into optimizing the cooling systems; and is a bad idea from the git-go.  Heat rejected is pretty much proportional to the temp difference between the air and the coolant/or oil. But if that’s your only configuration option, and the air temp increase through the rad is, say, 35F; and you want to climb out on a 90F day, you can expect to need an oil cooler core volume of roughly 1 ½ times normal; or about ¾ the size of the rad.

 

This is rough estimating and assumes equal effectiveness of rad and cooler; but you get the idea.

 

Al

 

All Good points except that the oil cooler behind the rad *does* get maximum use out of every CFM of cooling air which is the ideal situation in aircraft.  If it takes a bigger oil cooler, then so be it.  Ever take a look at that 11" stack of heat exchangers in a P51?   That is the biggest reason the plane could escort bombers all the way to & from Germany.  

 

Sorry, I'm going to bore everyone again with my favorite saying on this subject - "Remember, we are trying to build efficient *airplanes*, not efficient radiators."

 

Tracy