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Hood airflow - sucking vs blowing

smcdonaldaz

smcdonaldaz

NAXJA Forum User
Location
Litchfield Park, AZ
I live in the Arizona desert so I understand the need to keep the temperature down under the hood. I see people add ventsand am colse to doing in on my ne rig, but wonder if a small scoop would work better. Because of the air speed I assume there is a low pressure buildup thus sucking hot air up through the vents. I guess my question is this, is there enough airspeed to make a small scoop like this:

http://cgi.ebay.com/ebaymotors/Euro...008QQitemZ180227573521QQrdZ1QQsspagenameZWDVW

as or more effective as the vents?

You thoughts please?
 
A vent, if set up properly will vent heat directly up durring low speed due to convection. At road speed it will actaly suck the heat out of your engine bay. If you want a cold air intake and a cooling hood, go to go-jeep and check out the cowl induction hood.
 
Use your dictionary and look up thermoplastic. Can you say, melted blob of cr@p. Fiberglass or metal will last much longer.

At idle and slow speeds, convection will do all the work. At road speeds, suction will do the work. Check the article by GoJeep. http://go.jeep-xj.info/HowtoBonnetVents.htm

These are Pontiac Sunbird Turbo aluminum hood vents,
.
standard.jpg
 
Last edited:
Tim_MN said:
Use your dictionary and look up thermoplastic. Can you say, melted blob of cr@p. Fiberglass or metal will last much longer.

At idle and slow speeds, convection will do all the work. At road speeds, suction will do the work.
Click to expand...

Good call on the melted blob-o-plastic! Guess I'll start looking for metal vents. By the way, have you seen any numbers regarding under hood temps with and without vents?
 
Just seat of the pants observations. Before the vents, my summer temps were always a whisker above the 210* mark and the electric fan would run often. After the vents were installed, the temp is always a whisker below the 210* mark and the electric fan seldom runs. When the fan does run, it runs for 1/2 the time it used to.
 
Tim_MN said:
Just seat of the pants observations. Before the vents, my summer temps were always a whisker above the 210* mark and the electric fan would run often. After the vents were installed, the temp is always a whisker below the 210* mark and the electric fan seldom runs. When the fan does run, it runs for 1/2 the time it used to.
Click to expand...

I'd say vents are a no-brainer then, thanks!
 
I was going to try to work out an engineering/scientific explanation using pitot tubes and stagnation points, but really people have already answered the question (and I was never really good at fluids anyways). Rearward facing vents will help suck the heat out from under your hood, especially placed up closer towards the windshield. I've had some vents off a Grand Prix for a couple years now and although I can't say I've seen any changes in gauge temps, you can see the heat rising out of the vents and if there is snow on the hood (obviously not a big problem in Arizona) it is the first place to clear because of all the heat coming out.

As for the comments about plastic vents, I haven't had a single problem with mine and assuming you pull them off an automotive application then you shouldn't have any problems using plastic hood vents. The main benefit of metal ones is paint adhesion as far as I'm concerned, but then again, I don't have to worry about mine rusting.

401189_25_full.jpg
 
broqenjeep said:
havent cut-in vents yet but last summer I put 1" spacers between the hood and hinge
Click to expand...

Same, I eventually do plan on putting cut-in vents though as the 1" spacer makes the jeep look funny... like the hood doesn't fit properly etc..
 
Tim_MN said:
Just seat of the pants observations. Before the vents, my summer temps were always a whisker above the 210* mark and the electric fan would run often. After the vents were installed, the temp is always a whisker below the 210* mark and the electric fan seldom runs. When the fan does run, it runs for 1/2 the time it used to.
Click to expand...

I noticed the same. But I put my vents closer to the front of the hood. When the E-fan kicks on it forces a lot of air out.

HPIM0181.jpg


JIM.
 
smcdonaldaz said:
I understand the benefits of cold air intake, my question was just about getting the hot air out of the engine compartment and whether pulling up with a vent or pushing it down with a scoop would be more beneficial.

For cold air I plan on doing this: http://www.madxj.com/MADXJ/technical/technicalfiles/JJcustomSnorkle/CustomSnorkle.htm
Click to expand...


Vent it to the rear, getting the air out is the objective. Not letting rain water in, is also a consideration. Many guys have put in vents and have the rain leaking in get into bad places, like the TPS.
The windshield builds pressure at speed and vents too near the windshield can actually reverse flow.
Air doesn't flow out the bottom of the engine bay well. When my fan clutch was weak and the electric fan would come on, it would actually push air out the front through the mechanical fan, at idle.
If it was warmer here I'd put in a set of side facing vents (or scopes). If set up well the venturi effect would suck air at speed and the fan would have an easier time of moving air at low speeds.
 
ADVNTURR said:
As for the comments about plastic vents, I haven't had a single problem with mine and assuming you pull them off an automotive application then you shouldn't have any problems using plastic hood vents.
Click to expand...

The OP was looking at some cheap JC Whitney junk on eBay. I agree that any factory original plastic vents will hold up just fine. I have pulled a couple sets of GM vents and they are heavy duty plastic.
 
If you search, someone did some fab work on their original stock hood and made a cowl on it (with full write-up), which vented to the rear. I think gojeep also has a write-up on his site with a hood he bought and installed. IIRC quadratec has one, but it is about $400.
 
the base of the windshield is actually a high pressure area so putting vents near here is likely to blow air into the engine bay at speed. to test this tape some paper strips or string to vents.
 
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