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Here's why hood vents should be placed at the front of the hood, not at the windshiel

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Here's why hood vents should be placed at the front of the hood, not at the windshiel

Old 02-02-2018, 02:44 PM
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Default Here's why hood vents should be placed at the front of the hood, not at the windshiel

[edit: this does not apply to owners who only crawl their XJs. But if you drive it at 40+ mph speeds and you want to install hood vents to help the engine cool better, you may want to read this. This is why I put this in the stock tech forum and not the modified tech.]

I'm tired of seeing everyone putting their hood vents at the back of the hood, by the windshield where the air pressure is very high, instead of at the front of the hood where the air pressure is low.

https://www.carthrottle.com/post/eng...-of-race-cars/

#3:

Ventilated hoods allow for a clean exit for the air passing through the radiator, and they also help to maintain airflow through the engine bay, increasing cooling.


^^^ notice where the vents are.

Here's mine:



I'm not trying to criticize anybody who placed hood vents near the windshield. At crawling speeds, it probably doesn't matter at all and it may even help if that is where most of the heat builds up. But at 35+ mph vents will be ineffective at that location, and at 75+ mph, air may even travel into the hood through the vents because the pressure is so high there. Just like how air can flow into the cowl below the windshield and into the ac/heat and then through the cabin vents, even if the fan is turned off. If the air is going into the engine bay through the hood, this will cause air to travel slower through the radiator!

I just want to help everyone make the right choice.

Last edited by mannydantyla; 02-06-2018 at 11:39 AM.
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Old 02-02-2018, 03:55 PM
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^^^ This. I made similar points in a recent hood vent discussion thread on here.
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Old 02-02-2018, 04:01 PM
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Comparison of a Jeep Cherokee with a track car, I gotta think on this one fora while...
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Old 02-02-2018, 04:43 PM
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Some other things to note, most race cars have a very well sealed engine compartment. So any air that comes in the engine compartment has to have a way out. Some goes out the wheels but most of it goes through the hood. You see on that GTR how they have the louvers right in front of the main air release? They do that in order to create a low pressure zone for the main air release. The 2 work together. Notice how I said nothing about this being heat related, cause its not. Its about air flow.

Now on our Jeeps, the engine compartment is about 1% sealed. Any air that comes in gets sucked out the bottom. Even if you were to add hood vents, 99% of the air is still going to get sucked out from below. Its the path of least resistance. So the placement of the vents is not really that important while driving down the road in the case of our Jeeps.

What is more important is the size of the vents. The ones that you have on your Jeep are tiny. Yes they will extract some heat but not a noticeable amount. Placing the vents at the back of the hood would actually work out pretty good in a crawling situation (which is what a lot of us do). The mechanical fan is always pushing air to the back of the engine compartment even when sitting still. The firewall is exactly what it sounds like, a wall. Once the air hits is, it either goes up or down. So if you place your vents right above the firewall, hot air will constantly be getting forced through even when sitting still.

If you want to see what I mean, fire your Jeep up, get it nice and warm, have someone hold the rpms at 2000 and lay under the front of your Jeep (hood closed). Then you will be able to feel where all the air is going. You wont be looking for forward to back air movement but where the air is getting pushed down and out.
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Old 02-02-2018, 04:46 PM
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Originally Posted by Turbo X_J View Post
Comparison of a Jeep Cherokee with a track car, I gotta think on this one fora while...
Context is important. It's more of an issue at speed, though to be fair to your point, most underhood heat issues on Cherokees are low-speed problems.

Originally Posted by mannydantyla
I'm not trying to criticize anybody who placed hood vents near the windshield. At crawling speeds, it probably doesn't matter at all and it may even help if that is where most of the heat builds up. But at 35+ mph vents will be ineffective at that location, and at 75+ mph, air may even travel into the hood through the vents because the pressure is so high there. Just like how air can flow into the cowl below the windshield and into the ac/heat and then through the cabin vents, even if the fan is turned off. If the air is going into the engine bay through the hood, this will cause air to travel slower through the radiator!
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Old 02-02-2018, 10:09 PM
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Dont place them directly over the TPS, either. Sitting still at a light in a downpour and that water will make a B line right for the TPS if the vents arent setup for it. Plenty of airflow, but now the TPS doesnt work lol.
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Old 02-03-2018, 06:52 AM
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Most of us aren't building race cars. Looking to dump heat crawling. Dont think it makes much difference at 3 mph.
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Old 02-03-2018, 07:04 PM
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Bad comparison. As had been said, lots of differences in race cars and our cherokees....like better sealed engine compartments to begin. Airflow use to 'suck' the car to the pavement (and minimize/eliminate airflow under the car for handling) is another thing we don't care about.

So, the air pressure at the base of the windshield is higher....who cares ?, even IF air gets INTO the engine compartment from there, it has no choice but to flow out under the cherokee or via the wheel wells, cuz the pressure at the radiator will always be higher, which will prevent it from back flowing thru the engine compartment.

So you will have air coming in thru the radiator, and in thru your vents at the windshield base, and all of it has to go somewhere, the only place it can go is under the vehicle.

The vents are for low speed use anyway, in which case, ANY vent will work (high speed doesn't really matter, if you put vents on to help high speed cooling, you have worse problems to fix than air flow).

I'd even wager that if you had a way to measure it while driving at highway speeds, there will be very little airflow out any hood vents on our vehicles, as most of it is going under the vehicle. Once you slow down, the flow will start to get better.


.

Last edited by TRCM; 02-03-2018 at 07:08 PM.
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Old 02-03-2018, 07:19 PM
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Originally Posted by mannydantyla View Post
I just want to help everyone make the right choice.
Thanks, but a couple of points to consider. The "right choice" is personal preference as hood vents are mostly eye candy.

Secondly, have you ever seen a police officer that is tasked with watching over road construction all day? We all have. And SOME of those officers sit inside their car with the engine idling all day long. Which is about the same conditions when you are rock crawling. The point being a vehicle with a properly operating cooling system does not need hood vents.

Not saying I am against them. They do let hot air out. But they are not needed. So it's pointless to argue about it.
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Old 02-03-2018, 09:47 PM
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I firmly believe that the factory XJ cooling system was just adequate sized for on-road duties, but for oversized tires, towing, and slow speed crawling it is just not that great. A bit more headroom would have been nice, the radiator and AC condenser is really small compared to my other vehicles.

As such, you want to evacuate the heat as close to the source as possible - so placing them at the front of the hood is best, like how all race prepped cars have them. Increase downforce, probably decreases drag slightly by preventing a high pressure pocket at the nose of the vehicle, and helps heat vent more efficiently.

For slow speed crawling, that would be most ideal IMO. The rest of the heat will vent under the car as it normally does, and with no belly pan the XJ has plenty of flow for the cooling system.

Placing them closer to the cowl will help reduce heat-soak when the engine is off though, as heat from the header will more easily rise out of the engine compartment.

I will say, that vents placed closer to the cowl look better on the XJ
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Old 02-03-2018, 09:57 PM
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Originally Posted by investinwaffles View Post
I firmly believe that the factory XJ cooling system was just adequate sized for on-road duties, but for oversized tires, towing, and slow speed crawling it is just not that great. A bit more headroom would have been nice, the radiator and AC condenser is really small compared to my other vehicles.

As such, you want to evacuate the heat as close to the source as possible - so placing them at the front of the hood is best, like how all race prepped cars have them. Increase downforce, probably decreases drag slightly by preventing a high pressure pocket at the nose of the vehicle, and helps heat vent more efficiently.

For slow speed crawling, that would be most ideal IMO. The rest of the heat will vent under the car as it normally does, and with no belly pan the XJ has plenty of flow for the cooling system.

Placing them closer to the cowl will help reduce heat-soak when the engine is off though, as heat from the header will more easily rise out of the engine compartment.

I will say, that vents placed closer to the cowl look better on the XJ
No offense, but if one of your goals is to "increase downforce" on your Cherokee and you are not a stand up comedian, you have no business participating in this discussion. You can also increase downforce by having a monster truck run over it.
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Old 02-03-2018, 10:24 PM
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Originally Posted by StealthyMcStealth View Post
No offense, but if one of your goals is to "increase downforce" on your Cherokee and you are not a stand up comedian, you have no business participating in this discussion. You can also increase downforce by having a monster truck run over it.
It is physics, if you disagree go somewhere where physics doesn't apply (like another dimension)

Vents placed at the front of the hood (at speed) will decrease air pressure in the engine bay and at the very nose of the car, decreasing lift and increasing downforce. It will at the same time disrupt the airflow over the hood which will also decrease downforce to some degree. How much? A wind tunnel and some math will probably tell you

Decreasing lift will decrease aerodynamic resistance and increase gas mileage. A nice side effect is increasing downforce, but too much of either will hurt your drag coefficient.
If you don't believe that aero has an effect on gas mileage, put a roof rack and a 35" tire on your roof and drive at high speeds, then do the math on your fuel burn before/after.

Last edited by investinwaffles; 02-03-2018 at 10:27 PM.
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Old 02-03-2018, 10:31 PM
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Originally Posted by investinwaffles View Post
It is physics, if you disagree go somewhere where physics doesn't apply (like another dimension)

Vents placed at the front of the hood (at speed) will decrease air pressure in the engine bay and at the very nose of the car, decreasing lift and increasing downforce. It will at the same time disrupt the airflow over the hood which will also decrease downforce to some degree. How much? A wind tunnel and some math will probably tell you

Decreasing lift will decrease aerodynamic resistance and increase gas mileage. A nice side effect is increasing downforce, but too much of either will hurt your drag coefficient.
If you don't believe that aero has an effect on gas mileage, put a roof rack and a 35" tire on your roof and drive at high speeds, then do the math on your fuel burn before/after.
Let me state it as clearly as I can. DOWNFORCE DOES NOT MATTER IN A VEHICLE THAT NEVER GOES OVER 80MPH!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! !!!!!!!!!!!!!!!! Dumbass!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! Put that in your physics calculator and see what comes out.
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Old 02-03-2018, 10:39 PM
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I run in excess of 90 mph on i70 in the mountains. Not getting into an argument with someone who clearly doesn't embrace science.

I agree that they aren't necessary (I don't run them on my XJ), but if you are arguing that they are not effective - you clearly don't have anything meaningful to add to the discussion. Move along...
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Old 02-03-2018, 11:13 PM
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Originally Posted by StealthyMcStealth View Post
No offense, but if one of your goals is to "increase downforce" on your Cherokee and you are not a stand up comedian, you have no business participating in this discussion. You can also increase downforce by having a monster truck run over it.
Having a monster truck run over something will not increase downforce.....it will make the truck flatter, which, in all actuality, will likely decrease downforce, as it's no longer as aerodynamic (not that is was all that aerodynamic to begin with).

I'd say the person who is trying to use monster truck destruction to explain downforce is the one who should probably refrain from participating in this discussion.



Originally Posted by StealthyMcStealth View Post
Let me state it as clearly as I can. DOWNFORCE DOES NOT MATTER IN A VEHICLE THAT NEVER GOES OVER 80MPH!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! !!!!!!!!!!!!!!!! Dumbass!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! Put that in your physics calculator and see what comes out.


Downforce always matters. Cuz if it isn't downforce, it is lift, and that is not good either.

Now, is it enough to make a difference that is measurable, probably not, but it always matters.


.

Last edited by TRCM; 02-04-2018 at 01:34 AM.
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