Here's why hood vents should be placed at the front of the hood, not at the windshiel
02-04-2018, 12:05 AM
#16
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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.
Last edited by Martlor13; 02-04-2018 at 12:08 AM.
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Swlippelt (06-22-2019)
02-04-2018, 01:46 AM
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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.
Dumbass.
Last edited by bad_idea; 02-04-2018 at 05:05 AM.
02-04-2018, 05:44 PM
#20
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well im confused now, last summer out wheelin had some heat soak issue. so i got some nice napier hood vents sitting in my garage yet to be installed, dont know where i want to place them now. my jeep isnt a dd or a offroad only driver. but it does some of both.on & offroad use. what say ya'll now on placement ?
) For your purpose, placement isn't important. Placement in regards to aerodynamics only comes into account at high speed.
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JDB1937 (08-01-2022)
02-05-2018, 09:56 PM
#25
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Well, this is always fun to debate. But the best place is right where the air brakes over the front cowl because it draws a vacuum. I understand the "venting while crawling" aspect and will concede if that is all you want to do, then it doesn't matter. But by putting the vents in the front, you draw a vacuum on the engine compartment at speed. How many of you guys have ever had cooling issues at high speed? Vents in the front will help that at high speed or low speed. There is a reason why companies like GenRite start their vents in the front and then point them backwards at the rear of the full hood vent
02-06-2018, 11:30 AM
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I forgot I made this thread, lol.
And I'm not suprised it got a little heated!
I should have said this in the OP: I drive my Jeep on the highway and, after a few hours of highway driving in the summer, the temp can slowly creep up and it has actually overheated and coolant as boiled over a few times. That's why I installed hood vents (my time and money would have been better spent on electric cooling fans). People always say: "you have bigger problems" and they may be right but I have replaced the radiator, water pump, hoses, and fan clutch. It helped but the problem can still come up. The hood vents did not solve my problem but I'm sure that they help keep engine compartment temps down, and I'm sure that if they were placed on the back of the hood they would have hurt! I'm going to put a better radiator in and electric cooling fans. Yes I have tested to see if it's a blown head gasket, twice actually.
That is a legitimate concern. With the vents closer to the front, they're above many more electronic parts, like the ECU/PCM and all the throttle body sensors.
What I did, was I put dielectric grease on every single connector I could find, and I simply covered the ECU/PCM as best I could. It's been like that for about 8 months now and I haven't had a single problem. It's gone through huge downpours - one that dumped like 2" of rain in 30 minutes - and several snow events where the snow slowly melts and can drip into the hood through the vents. No problems!
But if all you do is crawl, then I'd put the vents in the back to avoid this potential problem all together.
Yes but the air is moving SLOWER through the radiator! At speeds 40+ mph, overall air pressure under the hood would be higher, this restricts air flowing through the radiator and passing over and under the engine, because air is also flowing through the vents by the cowl and under the engine. But with the vents up front, air is flowing through the radiator and then out through the vents. Much better!
I'm glad you brought this up! It would be very easy to show if it works or not, by taping little strips of yarn to the vents and see if air is blowing out of them or being sucked into them. You could tape the yard to the inside and see if it blows the other ends through the vent to the outside of the hood. I haven't done this though, but I would like to, however my hood is covered in snow at the moment. (BTW, this is called tuft testing).
This guy used tuft testing to show where's the best place to put the vents at what speed -
http://gojeep.willyshotrod.com/HowtoBonnetVents.htm
"I did some aero tests to see what would be the best place to put them. By sticking strips of wool on the bonnet and then going for a drive I found that the first half of the bonnet has a negative pressure area as I had surmised. If you look closely you will see the wool at the front being pulled forward and even straight up. But as the speed increased to around 40 - 50 mph, the wool was pulled around and kept flat like the rest of the bonnet indicating that the air stream was being forced flat at the higher speed. You can really get bogged down when you start to take into account things like Laminar flow, Random turbulent flow, Bernoulli's equation and Reynolds numbers etc. Basically having them in the front 1/3 will make it better at speed ( why Rally cars have them there ), and putting them in the rear 1/3 is better suited to slow 4wd speeds."
This is for a TJ but same concept: http://www.oman4x4.com/hoodvents.htm#theory
I wasn't trying to compare the XJ to racecars, I was just trying illustrate the concept. Most of us drive our Jeeps at 40+ mph speeds in which aerodynamics starts to make an impact.
Exactly!
And I'm not suprised it got a little heated!
I should have said this in the OP: I drive my Jeep on the highway and, after a few hours of highway driving in the summer, the temp can slowly creep up and it has actually overheated and coolant as boiled over a few times. That's why I installed hood vents (my time and money would have been better spent on electric cooling fans). People always say: "you have bigger problems" and they may be right but I have replaced the radiator, water pump, hoses, and fan clutch. It helped but the problem can still come up. The hood vents did not solve my problem but I'm sure that they help keep engine compartment temps down, and I'm sure that if they were placed on the back of the hood they would have hurt! I'm going to put a better radiator in and electric cooling fans. Yes I have tested to see if it's a blown head gasket, twice actually.
What I did, was I put dielectric grease on every single connector I could find, and I simply covered the ECU/PCM as best I could. It's been like that for about 8 months now and I haven't had a single problem. It's gone through huge downpours - one that dumped like 2" of rain in 30 minutes - and several snow events where the snow slowly melts and can drip into the hood through the vents. No problems!
But if all you do is crawl, then I'd put the vents in the back to avoid this potential problem all together.
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.
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.
This guy used tuft testing to show where's the best place to put the vents at what speed -
http://gojeep.willyshotrod.com/HowtoBonnetVents.htm
"I did some aero tests to see what would be the best place to put them. By sticking strips of wool on the bonnet and then going for a drive I found that the first half of the bonnet has a negative pressure area as I had surmised. If you look closely you will see the wool at the front being pulled forward and even straight up. But as the speed increased to around 40 - 50 mph, the wool was pulled around and kept flat like the rest of the bonnet indicating that the air stream was being forced flat at the higher speed. You can really get bogged down when you start to take into account things like Laminar flow, Random turbulent flow, Bernoulli's equation and Reynolds numbers etc. Basically having them in the front 1/3 will make it better at speed ( why Rally cars have them there ), and putting them in the rear 1/3 is better suited to slow 4wd speeds."
This is for a TJ but same concept: http://www.oman4x4.com/hoodvents.htm#theory
Well, this is always fun to debate. But the best place is right where the air brakes over the front cowl because it draws a vacuum. I understand the "venting while crawling" aspect and will concede if that is all you want to do, then it doesn't matter. But by putting the vents in the front, you draw a vacuum on the engine compartment at speed. How many of you guys have ever had cooling issues at high speed? Vents in the front will help that at high speed or low speed. There is a reason why companies like GenRite start their vents in the front and then point them backwards at the rear of the full hood vent
02-06-2018, 11:35 AM
#27
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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
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
02-06-2018, 11:49 AM
#28
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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.
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.
02-06-2018, 12:05 PM
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Yes but the air is moving SLOWER through the radiator! At speeds 40+ mph, overall air pressure under the hood would be higher, this restricts air flowing through the radiator and passing over and under the engine, because air is also flowing through the vents by the cowl and under the engine. But with the vents up front, air is flowing through the radiator and then out through the vents. Much better!
And I really doubt the airfow will go thru radiator, and then out those vents up front at speed, as there is simply to sharp a turn to be made to do so at speed.
Mot of the air from the fan goes around the engine and out the wheel wells anyway.
02-06-2018, 01:02 PM
#30
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I don't know if that's true about the race cars having sealed engine compartments. They have belly pans, yes, for aerodynamics/speed reasons. But the hot air needs to go somewhere, so they make vents on the fenders behind the front tires. It's #5 in that article I linked to in the original post.
There is a C6R here at the shop right now. A real race car. I will try and get some pictures of its engine compartment and vent systems. I am pretty sure the engine is out of it still.