How much do grill openings effect Cd?
 

I have heard that between 1/4 and 1/3 of the aerodynamic drag on a car is from the cooling openings. That sounds like a LOT of the drag on a car is from air going through the radiator, and I have a hard time believing that cooling causes 1/4 to 1/3 of the aerodynamic drag on a car. If that were the case, wouldn't I have lost a measurable amount of gas mileage when the front grill fell off my 2005 Civic? I was sure that having a big hole in the front of my car would hurt highway mileage at least a little, but the effect on gas mileage even at 60-70 MPH on the highway was not measurable. Also, if drag from cooling was so significant, why do people only get 1-3 percent MPG improvements, presumably mostly from faster warm up, from a full grill block? nPlease help me understand!

Enough to where electric vehicles have closed off grill openings.
If you block off the grill you may have the electric fans kinking on more, eating up some of the savings.
Newer vehicles must have spent time in the wind tunnel working on minimizing the grill opening effects compared to vehicles from years ago. That's the only thing I can think of.

That's a ridiculous % of drag. Where did you hear that?

Aerodynamic Investigation of Vehicle Cooling-Drag ...It is estimated that about 10% of the overall aerodynamic drag originates from the cooling...

https://www.slideshare.net/mobile/fr...g-aerodynamics ...cooling and ventilation system ~ 10%...

3% or 1/3 of the 10% seems reasonable for a DIY modification to a still functioning cooling system

I can't remember for sure since it's been a while, bit I THINK I actually heard that on Ecomodder. Not certain though.

Well, why would an EV have openings on the front for cooling? They have no combustion engine to cool, although I am sure that the electronics and motors require some cooling. Even a 1 percent drag reduction would be enough for the manufacturers to block off the openings on EVs.

Also, good point about the radiator fan running more. Just curious though- Do you mean that the fan would run more on the highway, or in traffic? On my Civic, the fan doesn't turn on until 207 degrees unless the AC is on, and it only runs at around 185 or so on the highway. So the fan will run longer in traffic from simply blocking off the upper grill? Won't the fan just pull more air in from the lower grill opening?

Batteries have to be babied to get the most range out of them. Too warm or too cool and the don't give their best. Cooling is therefore necessary depending on duty.

That makes sense, but they obviously would not require nearly as much cooling as an internal combustion engine.

Not so much. Somebody calculated that I am putting only 500 watts into my cooling battery system during the 10 kwh daily commute.

They do prefer being warmer than 80 * f.

Ok, so I duct taped over the upper grill on my 2005 Civic but left the bottom open since that is where I put my transmission oil cooling radiator, and I don't want to overheat my transmission. I will reply with an update soon.

Also, while many newer cars have huge grills, much of that is fake. People want to believe they have a mouth-breathing monster, but there is no reason for it.

how much
 

I've never seen more than 16% of overall drag reported as a worse-case scenario.
2% is credited to good racing design.
1976 Ford Lameller Grille was associated with a 12% drag reduction
1938 Adler-Jaray saw 15.78% with a 100% block
2013 Dodge Dart,active shutters 3-to-5%
1982 Pontiac Trans AM bottom-breather 3.08%
1967 Corvette EV with 100% block 6.5%
1986 FIAT R&D 100% block 10.2%
1978 Chevette 4-DR with 100% block 7.56%
2009 Audi A2 100% block 9.7%
2007 Ford Fusion 999 hybrid LSR 100% block 7.35%
By the old metric,you'd see a 1% improvement in HWY mpg for every 2% drag reduction,based only upon the original mpg.