I will have to look into using the gap that Kjetil has identified.  I never turned my head around and looked the other way!  Maybe my coach will end up with a "smile" on its face! Barney, thank you for the update. I was wondering what the status was with your coach and was thinking of emailing you. As you are aware, adding thickness to a radiator doesn't porportionately add cooling capacity.

As air enters a radiator the maximum difference in temperature between the air and the coolant exists. As the air passes through the radiator and gathers heat, the temperature difference diminishes along with cooling capacity so that by the time the air exits it is gathering significantly less heat from the last row of tubes than it did from the first row.

A frequent cause of reduced cooling capacity of an aging radiator is not internal, but is the separation of the fins from the coolant tubes. If they are not properly bonded, the fins don't gather the heat from the coolant tubes and as a result don't transfer the proper amount of heat to the air that is passing across the fins.

Based on the input of others, I am feeling better and better about making this change. To be able to open the rear aluminum panel and see the engine will be great as that is the primary reason for making this change (followed by no fan noise!) 

When the time comes I wil be starting a new thread about ram ventilation of the engine compartment without external scoops. This will be independent of any radiator placement.

Just to ensure the fullest inculusion of thoughts, the following is a transfer of Louis Cruse's comments in the Yahoo Group site.  I trust he approves.  I do suggest that he contact the moderator to see about joining this site.

There is an interesting discussion going on at the other FMC site.
Not being a member of that elite group, I offer some comments here.

Experienced auto mechanics know that a heating problem on the highway
is a radiator issue, and a heating problem in street trafic is a
thermostat, fan, or pump issue. Front mounted radiators rely on ram
air for cooling at highway speeds, and on the fan to move air when at
slow speed and when sitting still at a light. Except for hill or
mountain climbing the highway speed power required and the
corresponding heat rejection requirement is less than the stop and go
acceleration power and heat load in traffic.

I am not convinced that the fan in the shroud ring setup blowing air
into a larger "plenum", the rest of the shroud, blows air any place
thru the radiator except a blade tip ring about 2 or 3 inches wide.
That is where the dirt collects and I can feel on my hand that
airflow and all the rest of the "dead" area, standing behind the
radiator with the engine at high idle. Just reversing the airflow is
not the same as when the fan sucks thru the radiator, then plenum,
then the fan/shroud ring. The high mounted exhaust manifolds
certainly raise the temp of the air entering the fan. I have fiddled
with plastic sheets placed over left, right, both, of the side
grills, and there seems only a slight airflow thru them at high idle.
Guess a lot of the air comes up around the engine from the open area
under the engine and transmission. I have no idea what the airflow is
like at highway speed. A box (typical truck trailer 18 wheeler,
delivery truck, etc) does have a low pressure area at the back at
speed. That is the "draft" that the Nascar guys try to run in when
they are very close together. But I don't know what the open side
grills do to that type of airflow in our FMCs. I might try some "yarn
strings" all over the back and try different speeds with pictures
from a following car. Got a feeling the stock "shroud" might
be a poor setup.

Now, the point of this! The stop and go "hot day and hot asphalt"
driving seems to me to be the toughest design condition for the front
(actually under the front) radiator(s) set up proposed. Recirculation
sitting at a light must be minimised. Suck thru electric fan(s) with
full shrouds similar to those used on dragsters
(see http://www.rondavisradiators.com/Radiators.htm)
might be more effective than other configurations. The "ram air" duct
(s), deflector(s), scoop(s), other, will be a compromise that still
give(s)suitable cooling in stop and go traffic. Have no ideas for a
stone screen that is not restrictive to airflow. Friend of mine wrote
the US ARMY Handbook for Liquid Cooled Vehicles. He often reminded me
that any "thing" with more than 10 fins per inch is FIRST a FILTER.
Other than the leaking fluid and dirt load, the stock FMC radiator is
pretty protected from FOD and other debris or clogging. Large
diameter copper tube to and from, with hose/other at the ends, would
also act as a radiator. Remember the surface radiators on the
Schnieder Cup seaplane racers from the 30s ? Fuselage sides, wings,
and finally the sponsons were covered with surface radiators.
Press on, Lou #120.

Quoting Leslie Hoagland:

Lou -

You should sign on the FMC Owners Club site and add your thoughts.

Here at MCR, some 15 or more years ago, we added 5 inch long ribbons every 4 to
6 inches across rear grille of an FMC Coach.

In the video, it was interesting to see the ribbons hang straight down at coach
speeds of 50 to 70 mile per hour except at the radiator grille where they stuck
straight out.

The airflow across the radiator is maxed when the fan is 2/3 in the shroud and
the engine belts are tightened so that fan can not be slipped by pulling from
top thru the passenger side grille door.

Electric fans on the side doors or radiator had little discernable indication
on the heat guage during my tests with the fan shroud, fan, A/F mixture, and 8
more factors all setup properly.

Thirty years and more ago, I went thru all of the above and more to lower the
coolant temp at my favorite cruise speeds of 80 to 100 mph.

Intially, my FMC's coolant temperature would rise to 230 degrees Farenheit at
advanced speed on 85 to 110 degree Farenheit ambient temperature days.

I finally got the coolant temp down to 210 to 215 on those hot summer days at
advanced speeds. The gas engine coaches run about 185 to 195 at 60 to 70 mph
with a 180 degree thermostat.

On one occasion many years ago, we were driving in Indiana with a white
Limestone shoulder on that highway. I noticed a white 20 to 30 foot vortex
rising fron the rear bumper which explains the strong stagnation point in the
airflow across the rear of the FMC.

Once the cooling system setup is completed, all one has to do is watch the
engine belt tension and wash the radiator on a regular basis with a good
cleaner like GUNK followed by a Kerosene wash and a rinse with clear water. The
Kerosene film allows one to wash the engine and radiator with much less effort.

HAPPY TRAILS - Leslie & FMC #0938,
 

I basically concur with the comments and observations of both Louis and Leslie.  I would like to see the string test done, not on the outside of the rear grill, but stationary on the radiator itself.  It would be interesting to see the pattern.  Like Louis, I have passed my hand behind the radiator and noted significant dead air area.  I have not done an actual test, but that is one reason that I feel the front radiator arrangement will provide an increase in cooling efficiency - with ram air the entire core area will have a similar air flow rate.

I would also anticipate either full or almost full shroud for the fans.  If that is not done the fans will not create the needed air flow across the entire core under high heat generation conditions.

I have thought of two possible rock guard designs: 1. 1/4 mesh hardware cloth with a few cross braces for strength (big rocks!).  This would be cheap and easy to install, but ugly.  2. My next option would be a ribbed design.  Imagine a number of ribs laser cut from 1/16" material that angle down in front of the radiators and curve to continue back underneath.  These ribs would present their edge to the front and down.  They would be spaced about 1/4 inch apart (slipped on 2-4 support bars with 1/4 spacers) to provide a good ratio of open area to solid surface and yet have significant strength.  Thin material presenting its edge can be quite strong. If a large rock is encountered, it would be easy to remove the ribs, straighten or replace and reassemble.  This is a modification of a very effective classification screen design used in some heavy duty agricultural equipment that I am familiar with. This would be effective and attractive, but significantly more costly.

The next question that needs to be addressed is the copper tubes that pass between the front and rear.  My initial thought is oval copper tubes about 2.5" high and 3.5" wide.  They would be attached to the bottom of the coach near the center.  Copper would allow impacts from rocks without breakage (would just dent) and some additional heat rejection.  The diameter would have to be large enough to ensure unrestricted coolant flow (this could be boosted with an electric coolant pump).

Your comments?

Stainless steel comes in many different blends.  The focus of the blends is to prevent rust, e.g. stainless!  Yes, a number of them are "stronger" in that they won't bend, but that also makes a number of them brittle.  In this case, brittle is not good.  Copper also comes in diferent blends, but all of them are more flexible than stainless.  I am tending to lean to copper for the heat transfer ability and also for the flexibility.  If a rock hits it, it will dent but not break.

If a metals person comes along with more information, I could be persuaded to change my mind. 

The following is from Jim Black of RVS:

At FMC we built a modified nose cap to accept a front cross flow
radiator. We used 1 1/2"copper tubing to route coolant to the
front. Cooled poorly. We added a 12 V boost pump to secure proper
flow. Worked much better but the boost pump could be weak link. FMC
canceled the project as not cost effective,as well as, having
questionable engineering area's. FMC added perimeter aluminum
shields aft of radiator to the inside of rear grill. BIG
IMPROVEMENT!!!! Also, added aluminum shield between aft engine
cross member to bottom of radiator. This provided protection from
gravel bouncing up into the engine fan and helped air flow. ALSO
ADJUSTED RADIATOR TILT TO POSITION ENGINE FAN 2/3 OF CROSS SECTION OF
BLADES INTO SHROUD. REPLACED oem SUPPLIED 180 THERMOISTAT WITH NOW
OBSOLETE 160 CHRYSLER THRMOSTAT. ALSO CHANGED RADIATOR CAP TO 7 LB.
END OF COOLING PROBLEMS UNLESS OIL LEAK PlUGGED AIR PASSAGE OR LOWER
HOSE WAS REPLACE with HOSE WITHOUT SPRING INSIDE OR MOULDED INTO HOSE
ALLOWING HOSE TO COLLAPSE. The next problem was OEM radiators
supplied approx 100 total had 90+ of the 190 total plugged with
solder. Subject problem was found and corrected. After FMC #646
Chrysler derated their engines due to Ca smog from 8.9 to l in 1973-
74 to 7.9 to l in 1976 and removed some head gasket holes to raise
cylinder head temp,.changed cam grind,changed distributor curve. All
of these modifications reduced power an caused the FMC motorhome to
enjoy elevated coolant temp. Nasa Ames asked RVS to assist with a
engine temp study by installing 9 heat sensor in the engine area
connected to a graph recorder. At 65 MPH on
i-5 w/l00 ambient temp
the engine compartment exceeded 350, above the carborator. Reduced
temp in engine area by replacing dondaldson muffler with dual exhaust
engine compartment temp down to 160 to 1780. The donaldson was a big
heater. RVS has also been usng 6-row radiator cores when replacing
bad cores with great success for many years as guided by FMC
engineering.
The correct FMC motorhome waterpump is not a shelf item (obsolete).
Factory remanufactured available. Correct impeller is 4 3/8 OD with
8 blade paddles. Also, note: Do not over lubricate water pump.
NEVER A POWER GREASE GUN, 1/2 pump mad 2 times year. Most engine
water pump failures are caused by OVER lubrication.
JIM

Thanks for the additional information.

For those that prefer a rear radiator and mechanical fan, take a look at this film clip of dual, mechnically driven, rear fans.