Joe - More flow is a result of engine block design, rad, water pump size, and the likes. Unless you are going to put in a 1200 engine, I would surmise there is very little can be done regarding the cooling system on your bike.
Having said this, I've put together some information I gathered with the help of Dave in Abbotsford (forum moniker of SilverDave) regarding cooling of the 1200. It is about the 1200 but I think the information is relevant to the 1000/1100 models as well.
When I did my alt mod on my 1200, I had an issue with the cooling system operating hotter than I wanted so I did some research on it. Another fellow up here chimed in and between us we came up with a reasonable premise as to why the 1200s tend to "heat up" at low speed and while idling. The reader's digest version is that Honda designed the cooling system to "just" meet the requirements of cruising on the highway and expect that there will be minimal idling/slow speed operation to aggravate this. I had a new rad core put in and the older gentleman was very familiar with the old Hondas. He mentioned that Honda had designed athe rad as a high efficiency rad that was just big enough to do the job. It's not as large as the 1100.
Another issue I had was the aftermarket grill covering the rad. It reduced cooling area because of the design by 20%. It is now history.
Dave and I did some looking into the history of the 1200 and found:
If your antifreeze is new and topped up, your fan is clean and working properly, and your thermostat is a Honda OEM and installed with the hole right way up and so on, then the 1980 Honda engineers apparently did their job as required. The cooling system was originally designed so that at "speed" it worked perfectly, and at slow driving speeds, the fan was "just enough" to bring it down to 5 bars (on our digital readout). The cooling system design is just on the cooling cusp of doing just what it is supposed to do.
Dave sent me this information as reference material (I've edited it a bit).
Regardless of what engine it is is that there are three key issues that affect cooling efficiency: radiator surface area, coolant speed through the system, and the amount of airflow through the radiator. These three functions determine the efficiency of the system as expressed in Btu of heat rejection per minute."
So how do his many findings apply to cooling our GL1200’s?
A) Radiator surface area. We are sort stuck here. The rad size and surface area is limited by the frame, and the fairing. The article does mention double layer rads (very expensive), but with our bikes, the rad seems to be about as big as it can be . Its also the correct size, to do just what the 1985 engineers wanted it to do, that is keep it cool at higher speed.
B) Coolant speed - these bikes have a gear driven water pump which does move faster at higher rpm, but the machine is also producing more heat at higher rpm, so its sorta a wash. Older machines used to have a fan belt driven water pump, so you could change the pulley size/ratio, we cannot very easily, without tearing the engine completely apart. In any case, the water pump with its tiny blades does manage to keep a GL1200 cool enough that many of them last 300,000 or more, so I guess its also designed correctly .
C) Amount of airflow - This part, if you are building a hot rod, gets tricky. Firstly, the airflow at high speed appears to be about right. When you go at higher rpm, and faster, the airflow is correct, unless you are puttering thru Death Valley, or Osooyos in our case.
For slow speed cooling: it is recommended:
1. A back mounted, puller fan (which we have);
2. Electric as opposed to fan belt motor driven cooling fans (which we have) - a fan belt driven fan can use up to 30 hp. Big electric fans use just 1 hp;
3. Twin fans, with a good cowling (which we do not have);
4. An air dam to funnel more air up to the rad;
5. Good electrical connection to ground and to 12 volts on the fan circuit; and
6. A carefully tuned and shaped fan blade with approximately ¼ inch clearance to the cowling edge.
In other words, unless you want to create a special back shroud, and better fan blades, with two smaller twin fans - the 1200 has reached the engineering limit of cooling.
Since the 1200 is the last iteration of the 1000 cc block, with a few refinements of course, and that the design engineers at Honda knew that the block was at the limit of going larger, there is not much can be done to it or to the other sizes as well (JMHO).
If you had the available room, I wonder if an off engine water pump would be useful/practical? Possibly changing the rad fan thermo switch to one that has the rad fan operate at a lower temperature.
Just a few thoughts on the issue of cooling.
Cheers
