Size of hood vent
 

I'm thinking the next mod to my Focus Wagon is to vent the cooling air through the hood and then fit a belly pan. I've installed a air dam (here's the air dam) and modified my driving, with the help of a ScanGauge, to average above 30mpg. That includes time stuck in traffic, I hate seeing the 0.0 mpg on the ScanGauge.
This thread was close to answering my question but I don't have room to modify flow after the radiator since the exhaust manifold is only a couple of inches behind the radiator.
I've read the great articles at Autospeed.com and have done some testing with a Magnehelic gauge to find low pressure areas on the hood.
What size should the vent be? Is there a theoretical size based on the size of the grill opening?

I don't have a direct answer for you, but I do have some thoughts:

*Outlet area should roughly equal inlet area. If you know that value, it would probably make a decent baseline.

*You are thinking of chopping up a kind of expensive part of your car. Perhaps a junkyard hood could be obtained for your testing phase, then transferred to your "good" hood when you achieve satisfactory results.

*Or, make your own temporary hood out of coroplast or something and use tape and/or coroplast pieces to optimize the size and location of the outlet.

You could try what Ive seen some turbo cars do for cooling. Just use some washers as spacers to lift the back of the hood up near the windshield. Not sure if it would mess with aero or help it. Only a few bucks to try it though, and if nothing else its a windshield wiper block.

http://i112.photobucket.com/albums/n...l/DSC00644.jpg

FYI, the base of the windshield is a high pressure area. Not actually a good place to try to vent air from an engine compartment. That's exactly why cabin fresh air intakes are typically located there.

http://us1.webpublications.com.au/st...4/2455_4lo.jpg

Perhaps some is spilling out from the sides of the open hood in that Honda pic though. I don't know.

I am pretty sure you want the outlet to be around twice as large as the inlet (or maybe it's 50% larger, I've seen this somewhere). The exiting flow has less velocity, so it needs more room to escape at the same rate. You need the extra space so it can move the same volume of air as is entering.

That type of hood venting is primarily used for drag and drifting cars. It's great for allowing a lot of heat to leave the engine compartment... when you are not moving. Drag cars sit a lot and drifters are sideways at low speeds / high load most of the time, neither situation is helpful for moving air through the engine bay.

http://ecomodder.com/forum/showthrea...drag-2886.html

Probably better to go with what Phil submitted than what I said about area...

but I'd still go with my other two points.

Thanks for the information.
I looked into getting a hood from a wrecking yard before I start hacking at the perfectly good one on my car. I found one for $175! I wasn't expecting that. Makes the temporary hood seem like a good idea.
Based on the reading I've done, I understand that the windshield area is not the best place for a vent.

In order to make the vent large enough I'm going to have to cut through some of the reinforcing on the hood.
What do you think about a smaller hood opening along with venting out through the wheel wells?
I haven't found anyone post any results for this type of mod.
I'm beginning to wonder if this is worth the effort...

Search for posts by basjoos on wheelwell venting. He's done it successfully.

hood vent safety mention
 

I've driven cars old enough that,on occasion, they entertained me with exploding coolant hoses,and radiators.---------------------

Should you do the hood exit,in the event of a cooling system breach,you might find yourself blinded by a windshield covered in glycol and hot vapor attempting to condense on the glass if below it's dew-point.------------------

The mix won't wipe away like rainwater,and if,say you were right in the middle of a lane change in heavy and fast close-quarter freeway traffic,it might all end in a mishap.---------------

This may be a reason why auto makers have "neglected" hood-cited exits.--------------------

If you were to cite the exit closer to the A-Pillars,you'd be dumping air in to very fast-moving,lower static-pressure air than in the centerline,and should the system open up,center vision might be maintained.---------------------------

A kid threw a rock off an overpass in Los Angeles which shattered the windshield of my CRX once.I was in a situation as mentioned above,totally blinded,and it's a miracle that I escaped the situation in one piece.-------------------

Plan for the worst,hope for the best.

Hi,

My interpretation of the drawing you posted is the opposite -- the base of the windshield has some negative pressure, and so it is the best place to exhaust the cooling air.