When is a grille block opening too small/narrow?

[Piwoslaw]

I have an openable grille block, controlled by a power lock motor:

The motor moves the sliding part ~10mm (0.4 inch), so the vertical slits are 1cm wide with 1cm between slits.
I remember a discussion here about making a slitted grille block such that at higher speeds the air goes over the slits, but can still enter at lower speeds, or when the fan turns on. Something about pressure, turbulence, Reynolds number, etc., but I have no idea how to go about calculating this.

So are my 1cm slits too narrow to allow air to enter when driving fast? At what speed would the air start go around my grille block instead of through it?

[Daox]

Does your radiator fan come on often? If not I wouldn't even worry about it.

[NeilBlanchard]

My guess is that they are a bit too narrow to let in very much air at higher speeds. Though Tim makes the salient point...

[Piwoslaw]

Originally Posted by Daox

Does your radiator fan come on often? If not I wouldn't even worry about it.

Depends on how to define 'often'. Maybe less often than when completely closed, but it's not like it never turns on. When the block is open the coolant temperature very slowly starts to go down, but I have no comparison with the stock grille, because it has been more or less blocked since before I had a way to monitor temps.

[Weather Spotter]

The way I have my grill block it opens about 1.5 inches in each slot and seems to work as well as stock. I have to drive over 25 miles of back roads with the block open before the fan even thinks of coming on. Then I open the other half and the fan stays off. that with temps in the low 90s F.

So I would think your slots are too small. can you make a new cover with wider slots to test the idea?

[aerohead]

I haven't checked online for availability ,but you need to check out both of the above mentioned fluid dynamic flow loss phenomena.
The hard edges of each opening will effectively pinch off the air volume available to move through each of the slits.
Ideally,you wouldn't want anything in the airstream more restrictive than 1/4-inch hardware cloth.
One proper single opening will have the minimum drag compared to a series of smaller openings.
This is exactly why I chose the trashcan lid and bullet valve for the T-100.You'll see them used in hydroelectric plants where Pelton-wheel turbines are used,and if you've seen the recent movie 'Red Tails',you'll see therm on the tail end of the Me 262' Junkers Jumo 004 turbo jet engine's exhaust pipes.

[Piwoslaw]

I did a "test" during my vacation trip and it appears that the 1cm wide slits in my grille block are still good for something at highway speeds (~95 km/h): On the way out I had the lower grille block closed (upper grille and IC unblocked) and saw coolant temperatures around 91-92°C with outdoor temp ~21°C. On the return trip it was much hotter (32°C), but the engine was cooler (81-82°C) with the lower grille unblocked.
More details about the test here.

BTW My grille block now has more openings than on the old picture in the first post.

[some_other_dave]

Do you know what coolant temperatures your car "likes"? I know that some of my older cars have been set up to try to maintain the coolant at around 185F (85C) but my new MINI seems to try to maintain 220F (104C).

-soD

[rvpps2rocks]

when does the grill open?

[Piwoslaw]

Originally Posted by some_other_dave

Do you know what coolant temperatures your car "likes"?

Coolant temperature stabilizes at 78°C and it's hard to get it over 82°C without blocking the upper grille. If I block it (winter) the temperature will go up to 88-90°C, but often falls back to ~80°C when hypermiling. I believe that the engine "likes" to be closer to 90°C, consumption may be a tad bit lower then.

Originally Posted by rvpps2rocks

when does the grill open?

The upper grille is blocked November thru April. Both sides of the lower grille are closed when I start the car and I open the intercooler side when the coolant warms up to ~70°C. I open the radiator side only when the coolant temp stays above 90°C, to keep the fan from cycling on too often (turns on @ 97°C, off @ 89°C).