For Those of You With Hood Vents...

Just picked up some Lebaron hood vents at the JY for $2.50 each. Will vents actually make a measurable difference in coolant temps when idling or just moving slow?

I've searched and searched and there doesn't seem to be a definitive answer on whether they affect coolant temps when lumbering along a trail (I know they positively affect underhood temps). Before I cut some holes in my hood I figured it would be worth it to ask.

Disclaimer: My cooling system is working perfectly, not trying to solve any problems here...

Yes, cooler at all speeds including 0 mph.

There's a thread in Street & Performance where CobraMarty is doing some temp measurement:

http://www.naxja.org/forum/showthread.php?t=1083426

With vents, my engine temp dropped 5-10 deg when idling for extended periods and the hood temp down 20-30 deg. Mine are at the back of the hood and seem to make the biggest difference at idle and low speeds.
I don't have any cooling problems either, my goal was to reduce underhood temps. I was really surprised the first time I drove it in town and then touched the hood. I could actually leave my hand on it.

The vents will, by thier very nature allow the heat to escape the engine bay. Where you put them will determine at which speed(s) they will operated best. For on the highway cooling, you would want tem right behing the radiator. For low speed usage, near the back of the hood works best.

Easiest way to determine where the low pressure area is on the hood (determines the efficiency of the vents) is to tape on 2" pieces of yarn in a grid pattern. Go drive the Heep and the yarn will tend to stand up in the low pressure areas. This is an old idea that has been used for, well, decades.

Side vents (through the fenders) work as well and do not require drip pans to keep the water away from things like the TPS...

In a word, no.

I very highly doubt you will see coolant temps drop because of hood vents alone. I have runcool louvers which let quite a bit of heat out and it didn't affect the coolant temps at all. Underhood temps will probably be a bit cooler because the heat has somewhere to go, but that doesn't directly translate to your cooling system.

With hood vents, coolant temps are about 5* lower on the 4x4 trails and on the highway. The e-fan runs less often, and for shorter periods of time.

Hood vents will not fix a cooling system issue, but they do lower coolant temperatures slightly and lower under hood temperatures.

YMMV

Mine didn't lower the coolant temps by much, but they do let a lot of heat out.

I'd like to add some hood vents because I end up sitting / idling on slow technical trails a lot (while I'm out spotting or waiting for somebody). I think for my application vents towards the back of the hood would be best - correct?

For whatever it is worth, I saw a 5 degree Celcius drop in average engine temps after cutting the hood.

Voteban you are, IMO, correct sir in that placement at the rear of the hood is better for slow operations. The very front is best for highway operation. Just remember to install drip pans under them to reroute the water away from thing that need to stay dry. Like the TPS for instance. Pans can be made, easily, from the thin aluminium stock you can get from the local Home Depot or equivalent store. Thinner is actually better when bending by hand.

I put some aluminium angle stock onto the jaws of the bench vise to bend the tins. A very small hammer will help get the bends crisp. A leather/rubber mallet works even better.

A forum member sent me this link. I think it will be helpful as you look at hood vents and the cooling affects.

http://www.go.jeep-xj.info/HowtoBonnetVents.htm