One characteristic of hypermiling is that you aren't asking your engine to produce large amounts of power. This being the case, my engine isn't putting out large amounts of heat and I shouldn't require the total area of my radiator, which would be sized for a worst case situation, to keep my engine cooled. Since the radiator is responsible for 5% to 9% of the car's total drag, I wanted to see how much of my car's radiator air intake I could block off without causing the cooling water's temps to increase under my normal driving conditions. Cutting down on the air flow through the grill would have two beneficial effects on reducing drag. It reduces the drag caused by the air flow through the radiator and engine compartment, and it reduces the turbulance under the car where the radiator air is exhausted. I currently have the grill blocked down to a 4" X 1" opening in the upper center of the grill, and the cooling gauge hasn't budged, even during a 35MPH, 800 foot climb in third gear up the mountain into North Carolina and while sitting in traffic. I'll continue to closely monitor the cooling temp as we move into summer to see if I'll need to open up the grill more as the outside temperatures rise. The result of this modification on my car, combined with some some other minor drag reduction mods and changes in driving style, has caused my MPG to rise from the usual low 50's I normally get at this time of year into the upper 60's (last tank was 69MPG). It'll be interesting to see where the mileage will go as we ease into summer.
On another subject, I haven't been able to log into the www.cleanMPG website, even though it says I am registered as a member. When I try to login, it says "Thank you for logging in" and then sends me back to a login page. On the home page there is a line that reads "If you have any problems with the registration process or your account login, please contact contact support", but clicking on this line takes me to another page where they ask me to login.
grille blocking benefits are probably a combination of both aero and higher engine/transaxle temps.
when testing last fall, FE increased *immediately* after installing the block on my car, which i would not expect to see if it were strictly a result of temperature (since temps don't increase immediately, and take some time to stabilize at a new higher level).
of course if all your driving is low-speed, then the only benefits you'll see are temp-related.
cooling system drag accounts for a significant amount of a vehicle's total drag. i've seen figures as high as 33% of total drag for a turbocharged volvo (with multiple radiators/intercoolers), but it's normally lower on regular cars - 5% to 9% as basjoos quoted in the first post in the thread.
Since engine idle-stop is SOP with hybrid cars and many of them have gone well over 100,000 miles with no engine problems, there must not be any adverse cooling or lubrication problems associated with stopping the engine for short periods of time. I have 200,000 miles on my current engine with no oil consumption problems.
I currently have clear tape over the openings in the grill, so you have to really close close to see that anything is different.
krousdb, I have been going to the www.cleanmpg.com site and haven't had any problems with accessing other boards. I'll keep working on the problem and see what I can work out.