• NAXJA is having its 18th annual March Membership Drive!!!
    Everyone who joins or renews during March will be entered into a drawing!
    More Information - Join/Renew
  • Welcome to the new NAXJA Forum! If your password does not work, please use "Forgot your password?" link on the log-in page. Please feel free to reach out to [email protected] if we can provide any assistance.

Hood vents, scoops etc. Heat extraction

I doubt anyone is going to warranty my stock exhaust manifold from 1998!

so you're saying it keeps too much heat in that it will compromise the welds and structure?

It gets wet/holds water & moisture against the manifold until the manifold gets hot enough to cook it off.

That could be days if you parked after driving it in the rain or snow.

It also holds heat in which won't help with stress cracks or heat related corrosion.
 
I live in Az which gets much hotter! Although I've never had any heat soak issues, when building my stroker I went with the DEI rail kit.
http://designengineering.com/fuel-rail-injector-cover-kit/

I made my own version of this kit. I think I ran it down to the LCA mount area. I probably didn't need to go that far down but between the pre-cats and plenty on the roll.

For hood vents I went the easy route and ordered a set from one our NAXJA sponsors; Napier Precision Products. They are a little taller than some of the junkyard options but they do their job
 
Thanks for all the mentions! If it helps, here's a photo of our hood vents and a direct link to our page is below my sig.
Thanks, Doug

35077809_2128620550705652_1167248360160100352_n.jpg
 
Lookup the 1" rear hood spacers. They go on the hinges and raise the rear of the hood almost like a cowl. Independent tests shows a 20* temp drop.

Got mine off ebay for $15. 5 min install

Rear of the hood is high pressure area:

fe_139_02_021105_f005.png


This means that at speed, you will be pressurizing the engine bay (bad). It will help at lower speed.

I've had problems with temps for quite some time with my XJ with the winch and extra coolers (trans/PS) in front of the radiator. I'm running a new stock radiator, new hoses, 33% coolant/DI water mix (antifreeze has less heat capacity than pure water) with water wetter, ZJ clutch fan, new 97+ auxiliary fan, rubber hose to fill the gaps between the fans and radiator, etc, etc, etc. On hot days at elevation (all summer for me in the San Diego County mountains), my XJ will regularly run at about 215-220*F if under load, and I've had to turn the AC off and run the heater on the trail to keep temps in check. Since this is a family hobby, I'd like to keep things as comfortable as possible, so running the heater when it's over 90*F outside is unacceptable.

Having had excellent experiences with hood venting on racecars, I bought a set of these:

http://rodlouvers.com/Pair-of-Angled-11-Louvered-panels-4-P1036801.aspx

Installed them 6" from the header panel (closest to the front I could get to keep out of the hood supports and also look right), 1/2" away from the crease line running diagonally on either side. Vents were installed so they point straight backwards for better airflow at speed.

A quick day of testing showed air conditioning output on a ~85* day is 5* cooler, engine coolant temps on the freeway and idling around town are more than 10* cooler (I'm now actually using the thermostat, so can't go any lower), radiator temps are WAY down now (approx 130* on an ~85* day), and the vents allow a surprising amount of air to escape both on the freeway (tuft testing) and stopped.

Other things I've noticed:

1. Less air coming out from underneath and wrapping around the front of the Jeep when stopped (kinda like VRS on a helicopter).
2. Less heat on the floorboards.
3. Less stupid hot air blowing under the jeep into the open windows.
4. There's a lot of heat under the hood of an XJ. Despite 130* radiator temps (again, thermostat is closed), air leaving the vents is 160-180*.
5. Placing the vents where I did put them over the battery and the airbox, so a lot less water on engine sensors.


I'll try to get some videos of tuft testing, etc when I get a chance and post it up. Overall, I think hood vents are a worthy mod if you run in hot temps.
 
Rear of the hood is high pressure area:

fe_139_02_021105_f005.png


This means that at speed, you will be pressurizing the engine bay (bad). It will help at lower speed.


Having had excellent experiences with hood venting on racecars, I bought a set of these:

http://rodlouvers.com/Pair-of-Angled-11-Louvered-panels-4-P1036801.aspx


I'll try to get some videos of tuft testing, etc when I get a chance and post it up. Overall, I think hood vents are a worthy mod if you run in hot temps.

I don't quite understand the color coding of the chart you attached. What does the various colors represent? (generalize)

I like where you installed the vents. IMHO, I always err in the side of the low or negative pressure at the front of the hood to create a venturi affect to draw the hot air out especially just after it come off/out of the radiator when driving at speed. The rear of the hood is good when crawling or stopped, but in reality, how often do most of us crawl or remain stationary with the engine running?

I have seen plenty of jeeps that were sold in Europe with hood vents installed as factory option. They are all installed in the front 25% of the hood with the opening towards the rear. I am certain there is a scientific reason they were installed that way.
 
I don't quite understand the color coding of the chart you attached. What does the various colors represent? (generalize)

I like where you installed the vents. IMHO, I always err in the side of the low or negative pressure at the front of the hood to create a venturi affect to draw the hot air out especially just after it come off/out of the radiator when driving at speed. The rear of the hood is good when crawling or stopped, but in reality, how often do most of us crawl or remain stationary with the engine running?

I have seen plenty of jeeps that were sold in Europe with hood vents installed as factory option. They are all installed in the front 25% of the hood with the opening towards the rear. I am certain there is a scientific reason they were installed that way.
It’s an air velocity graphic, but generally speaking the higher on the scale it is, the higher the pressure.

Simpler graphic:

aero1.gif
 
higher velocity = higher pressure ??

not really true....that how a spray nozzle, among other things, works....velocity increases, so the pressure has to drop....conservation of energy type thing.


.
 
Last edited:
I have an '01 and also had the heat soak issue. Installed hood louvers and insulated the fuel rails and injectors all for no change.



Then I replaced the fuel injectors and the problem was instantly fixed. At least one of the injectors must have been leaking out after shutdown. It can now be parked for a half hour on a hot summer day and starts instantly every time.
 
It’s an air velocity graphic, but generally speaking the higher on the scale it is, the higher the pressure.

Simpler graphic:

aero1.gif

This illustration is very clear,...it is what I expected to see. Thank you.
 
higher velocity = higher pressure ??

not really true....that how a spray nozzle, among other things, works....velocity increases, so the pressure has to drop....conservation of energy type thing.


.

Agree with your statement, but not it’s applicability here as your example is backwards. Air isn’t leaving the vehicle. Air is hitting it. High relative velocity air is hitting the vehicle with nowhere to go, creating high pressure. Go stick your hand in front of the windshield, then stick it out the open hatch and tell me which air feels faster.
 
Yes, it will feel faster, due to more velocity, NOT pressure.

Conservation of energy...............if the front of the car & windshield have high pressure, then the back windshield and back of the car have to have low pressure.

The velocity of the air don't matter much, if it is moving, it is happening. The higher the velocity of the air that makes high pressure at the front windshield will also make a lower pressure at the rear windshield.

Just because you have high flow/velocity doesn't mean you will have high pressure. The car example has high velocity air across the back deck, but it has low pressure there.

THAT is what I was saying.
 
Yes, it will feel faster, due to more velocity, NOT pressure.

Conservation of energy...............if the front of the car & windshield have high pressure, then the back windshield and back of the car have to have low pressure.

The velocity of the air don't matter much, if it is moving, it is happening. The higher the velocity of the air that makes high pressure at the front windshield will also make a lower pressure at the rear windshield.

Just because you have high flow/velocity doesn't mean you will have high pressure. The car example has high velocity air across the back deck, but it has low pressure there.

THAT is what I was saying.

Not sure what you’re trying to say here. There isn’t any relative airspeed behind the rear hatch. The low pressure at the rear isn’t because there’s high pressure up front, it’s due to flow separation because the vehicle is shaped like a brick front and rear.

In any case: why bring this up? For the effects of that diagram (context), higher on the velocity scale at left generally meant higher pressure. Exception to this is the blue are in front of the grille, which appears to be a stagnation problem with that car (hood vents would help :D).

I will admit it was a poor choice in diagram for this discussion. I found it browsing with my phone and didn’t read the text of the scale. In general, the areas on that diagram that are blue are low pressure, and darker green to red are high pressure.
 
I brought it up because you said the higher on the scale (wind velocity) the higher the pressure, which is not true most of the time.

There is airflow past the roof of the vehicle, and this flow has to widen to cover the rear windshield, which means the velocity will drop and the pressure will drop more. This causes the swirling effect and drag. It is compounded even more as the vehicle body passes, and even more space has to be filled with the same air flow, so the pressure drops even more, and even more drag is produced.

I just didn't want people who read it to think high flow always meant high pressure....
 
I brought it up because you said the higher on the scale (wind velocity) the higher the pressure, which is not true most of the time.

I just didn't want people who read it to think high flow always meant high pressure....

Splitting this to keep discussion points in order... :D

Agree. Was unrelated so it threw me for a loop.

There is airflow past the roof of the vehicle, and this flow has to widen to cover the rear windshield, which means the velocity will drop and the pressure will drop more. This causes the swirling effect and drag. It is compounded even more as the vehicle body passes, and even more space has to be filled with the same air flow, so the pressure drops even more, and even more drag is produced.

I see what you're saying here, but I think a more accurate way of stating it is airflow over the roof is attached and laminar except at the front behind the windshield since it's in plane and has stabilized. Behind the windshield and the rear hatch is a low pressure zone because the change in angle between the windshield and the roof (or roof and rear hatch) is far too great (generally over 17*) to keep airflow attached, but does not have a clean break, causing turbulent airflow and a low pressure wake, both of which cause drag.

In any case, we've gone so far off into the weeds we're no longer treading lightly. Fact is: the best place for hood venting is towards the front of the hood, not at the back. :D
 
Back
Top