Hood louvers aid in cooling

Thinking about dropping under hood temps by adding some louvers.  So before getting after it and hacking a hole in my hood, I figured it's best to get some opinions.  Would this kill aero, or maybe even not work at all for its purpose? I was going with a section of precut ebay louvers, and riveting/bolting it into a hole iny hood.  It would go in the grey box .  It's slightly to the passenger side.  But the hood has a raised section that I would use.  It would also keep it somewhat away from electrical stuff on the far driver side, in case rain managed to get it.

For sure they help. Made a big difference in my car.

There are some good threads in the aerodynamics forum on this board. Might want to check them out before buying those. 

In reply to amg_rx7 (Forum Supporter) :

I'll look through it again.  I was originally going to post this there.  But it isnt really aero minded.  And it seems this would get a little more traffic than the aero section.

In reply to 79rex :

Depends on placement. It needs to be in a negative pressure zone under the hood. 
 

If there is no delta between the pressure under the hood and the pressure over the hood where you cut the hole, there will be no benefit. 

In reply to SVreX (Forum Supporter) :

is there a good way to eyeball this?

In reply to TurboFocus :

If google doesn't have an answer, tape and yarn should do the trick.

SnowMongoose said:

In reply to TurboFocus :

If google doesn't have an answer, tape and yarn should do the trick.

Tape and yarn will let you eyeball the pressure on one side, not the other :)

Blobs of oil can show you the flow as well. Helpful if you don't have a way of watching the yarn at speed. Its just messier.

In reply to 79rex :

Location is everything too close to the windshield is. A high pressure area as air piles up in front of the windshield. Too close to the front is also a high pressure area.  
 

The simple way to pick the perfect location is to tape 3-4 inch lengths of yarn in various locations across the hood and drive down the freeway noting which area of your hood the yarn raises off the hood the highest. That's the location. If they are all relatively similar then put them over the exhaust manifold where the heat is the highest. 

It's my belief that underhood pressures are far more consistent than, umm, "overhood" due to a relative lack of airflow. So it's more important to find a low pressure zone on top than trying to figure out where there's a high pressure zone underneath.

Best way to measure hood pressures is with a magnehelic gauge. I did a video on this recently.

In reply to Keith Tanner :

ive been keepoing up on your videos, but missed the hood vent one.  Ill have to check it out tonight.  

In reply to frenchyd :

The header is on the back of the head, so pretty much under the windshield in the neon configuration.  

frenchyd said:

In reply to 79rex :

The simple way to pick the perfect location is to tape 3-4 inch lengths of yarn in various locations across the hood and drive down the freeway noting which area of your hood the yarn raises off the hood the highest. That's the location. If they are all relatively similar then put them over the exhaust manifold where the heat is the highest. 

I disagree with this. Bernoulli tells us that (basically) faster air is lower pressure air. If your yarn is standing up, that means you've got turbulent and/or separated airflow. That's not low pressure, it's usually the opposite. 

Here's a pressure map of a Miata hood I developed a few years back based on pressure testing with a magenhelic gauge. Note how there's basically a diagonal band of low pressure where the air can move quickly and smoothly across the hood. If you tuft that hood, you'll see your tufts laid out flat in that area due to the nice smooth airflow. At the base of the windshield, you're more likely to get your dancing tufts.

Appleseed said:

Blobs of oil can show you the flow as well. Helpful if you don't have a way of watching the yarn at speed. Its just messier.

reasonable to think that i could dye oil to make it more visable, and slather it under my hood to check to see if I have air to pull from the spot I want to vent?  Im assuming that the more the oil moves would indicate that I have a spot that would be benifical to add vents.  

didn't GRM do a hoodvent thingy on youtube last year?

The usual best place for a vent is right behind the radiator level, but people want to move them back into the high pressure area frenchy mentioned in front of the windshield.  If you want cooling, don't locate them for best looks, put them where they will do some good.

One comment - once the car gets going, they don't do much for cooling. Sitting static idling in the sun, the vents are pretty effective and driving at slow speeds they give decent effect, but at higher speeds the cars without them cool just as well as those with them.

Downside of the vents is that they let water in on top of things you wouldn't want to get wet. In some cases I have seen rust on the underside of the hood where some sound deadening material caught and held the water against the metal.

We've been through all this with the Jensen Interceptors - big block Chrysler engine pumping out heat in a smaller engine bay than they were born to fill.  We found that the cars with no louvers cooled just as well once you were off an running, but if you want to hold a contest where both types were allowed to idle in the sun until one gave up, the vented hood model would win.

Hood vents are very useful to cool at speed IF they are helping to pull air across the radiator. If you don't have any ducting forcing air coming into the engine bay to go through the heat exchangers instead of around them, they won't do much. But they're very beneficial if you have that ducting. They're more useful at speed than at idle if implemented properly.

Modern engine bays are reasonably watertight. We've come a long way from the Interceptor.

In reply to Keith Tanner :

I went and closed alot of the big gaps around the front of the radiator.  So it should be getting a good amount of air through it.  

You can build a simple manometer with som plastic tubing and light fluid . ATF works.

I think this has been covered in the archives.

I built one yeas ago.

As I recall, the 96 didn't have a low pressure area.

wspohn said:

The usual best place for a vent is right behind the radiator level, but people want to move them back into the high pressure area frenchy mentioned in front of the windshield.  If you want cooling, don't locate them for best looks, put them where they will do some good.

One comment - once the car gets going, they don't do much for cooling. Sitting static idling in the sun, the vents are pretty effective and driving at slow speeds they give decent effect, but at higher speeds the cars without them cool just as well as those with them.

Downside of the vents is that they let water in on top of things you wouldn't want to get wet. In some cases I have seen rust on the underside of the hood where some sound deadening material caught and held the water against the metal.

We've been through all this with the Jensen Interceptors - big block Chrysler engine pumping out heat in a smaller engine bay than they were born to fill.  We found that the cars with no louvers cooled just as well once you were off an running, but if you want to hold a contest where both types were allowed to idle in the sun until one gave up, the vented hood model would win.

 

If you look at Jaguar XKE Series3 V12 you'll see exactly what you are talking about.  The vents are right over the carbs and can splash into the distributor (center of engine)  and plugs 

to deal with that Jaguar put a shiny stainless steel shield and let water harmlessly fall where it won't hurt. 

Keith Tanner said:

frenchyd said:

In reply to 79rex :

The simple way to pick the perfect location is to tape 3-4 inch lengths of yarn in various locations across the hood and drive down the freeway noting which area of your hood the yarn raises off the hood the highest. That's the location. If they are all relatively similar then put them over the exhaust manifold where the heat is the highest. 

I disagree with this. Bernoulli tells us that (basically) faster air is lower pressure air. If your yarn is standing up, that means you've got turbulent and/or separated airflow. That's not low pressure, it's usually the opposite. 

Here's a pressure map of a Miata hood I developed a few years back based on pressure testing with a magenhelic gauge. Note how there's basically a diagonal band of low pressure where the air can move quickly and smoothly across the hood. If you tuft that hood, you'll see your tufts laid out flat in that area due to the nice smooth airflow. At the base of the windshield, you're more likely to get your dancing tufts.

We may be saying the same thing.  Yarn taped to the top of the wing at the center point of lift will be the most raised up.  Yarn in high pressure area will be closer to the surface.  

We are not saying the same thing, it's pretty much the opposite. If you have yarn raised up on the low pressure side of a wing, I think you're stalled. 

If you want to feed the engine compartment with high pressure air- take it from the cowal area.  That's where NASCAR feeds their engines, and that's where vitually all cars get their interior air from.  Which is the opposite of how to cool an engine.

And +1 to the idea that vents are great if you are sitting still- marginal vs. forcing air going into the radiator from driving.  The static pressure into the engine compartment is pretty high at 50mph, and a lot higher than the low pressure from venting.

If a car is overheating at nominal speed, something is wrong with the cooling system.  WOT for a long time is a different problem, as is idle and slow speed.

But airflow is all about going from high pressure to low. If you ram air into your engine bay, you have to give it somewhere to go or you'll just end up with high pressure and no airflow. The purpose of venting is to give that high pressure air somewhere to go so your hot stagnant air is replaced with cooler, moving air and thus you can dump more heat out of the exchangers. You put the vents at the low pressure spots on the top of the hood because that will promote the most airflow from the high pressure underhood area to the outside.

Sitting still, you're really just dumping underhood temperature. What matters then is your fans.