Over the past week I have been mapping air pressure in various places on the KJ in an effort to identify whether hood venting will actually deliver a benefit in our hot climate. I used a Dwyer Magnahelic differential air pressure gauge to compare pressures hitting the KJ from the front grill, various locations on the hood, and into the engine bay in various places. I am posting up my data in a jpg file here in hopes that someone else might find it useful.

Here is my Disclaimer: Obviously, vehicle configurations vary significantly; and mine has a TJM bar, snorkel, front mounted aux tranny cooler, hood deflector, driving lights, is lifted 3", has the full bash plate set, and runs 245/70R16 ATs so other configurations may return signficantly different figures undre the same test points.

My initial conclusions are that while there is a good pressure differential between the frontal air pressure and the fan area, there appears to be a pressure bubble on top of the engine which is forcing flow down the side and out the back and bottom. As the pressures on the outside skin of the hood are significantly lower than those inside over the engine (in that bubble), venting the hood in the correct spots should release that pressure bubble and increase flow. That's the theory anyway.



Anyone wanting to read a good set of articles on the use of differential pressure measurements to determine airflow behaviour could have a look at these three that I found very informative:

http://autospeed.com.au/A_2159/cms/article.html
http://autospeed.com/cms/A_2160/article.html
http://autospeed.com/cms/A_2162/article.html

Wow... you'll fit in well around here!

So, you think that the increase across the top wouldn't come at the expense of losing too much along the sides?

You can see why some cars have that reverse scoop top/center of the hood, hmm?

Great stuff, thx.

_________________
--
2005 Liberty CRD
2004.5 Cummins 5.9L TD, F1s, NV5600 6-speed in a '93 GMC Suburban

Glend

Do you have a engine cover on your CRD
And what if any difference might the cover make

_________________
Atlantic Blue 06 CRD Limited (his)
Joined by a 2000 XJ Classic (hers)

Joe, no I removed the engine cover years ago. I should add that I also have removed the hood insulation blanket. Both of those devices, while suppressing noise seem to restrict air circulation in the engine bay. Until I see some indication that Jeep planned those devices to aid, or direct airflow I will assume they did it to reduce NVH for the customers that didn't want it to sound like a diesel. I am passing my diff air gauge on to a friend who still has both the cover and the blanket on his KJ CRD and he will do these measurements with that configuration - then we will compare notes.