reverse cowl or forward facing scoop


Hi all
I've just installed a new top mount intercooler and need more bonnet clearance. Already used spacers but need a bit more. I'm thinking of using a 4" reverse cowl to give it more clearance. However im not sure how a reverse cowl works. Does it suck in air from the high pressure created at the window or does it pull air out. Would it be suitable for a top mount cooler or a waste?
To answer your question I am not too sure. I would say there would be some purging of air out of the engine bay, air comes in through the grille, travels through air con, radiator and then back over the motor and down under the body, so there would be some escape of air up out of the rearward facing scoop. front facing it would be positive as in air into the engine bay. hope my 2 cents worth helps.


Well-Known Member
Most car with a top mount intercooler with have a forward facing cowl, not as effective at low speed as high speed, however, will still be getting cooler air flow in as opposed to not having a cowl at all, consdiering the air would have passed through the aircon and radiator before making it ways around the engine area and into the intercooler.


4x4 Earth Contributer
I reckon the reverse cowl would mainly draw hot air out of your engine bay, which is good. Forward facing cowl would force cool air in around your intercooler which is also good . The idea is to either reduce heat under the bonnet by extraction ( rear facing cowl) or create cooler air under the bonnet by induction ( forward facing cowl). I reckon the forward facing cowl, especially driving or towing at higher speed would be more effective. Either way, cowl must be at least 4" high.


New Member
Trosz18 I've been researching something similar with my GU Patrol but the principles are the same. As you suggested the high pressure area at the base of the windscreen forces air into the reversed cowl even at relatively low speeds. I've used a magnahelic gauge and at 60km/hr the pressure differential from the top of the bonnet to the engine bay is approximately 60 Pascals. To put this in perspective the pressure drop across a typical bathroom ceiling fan is about 1Pa. At idle there is no significant differential but of course convection would mean heat should flow out through the scoop. However a common modification is to fit a fan under the intercooler for low speed work
. As your speed increases the pressure differential increases substantially. At 80k on mine it is 100Pa and at 100k 160Pa so you can see that you will definitely not extract air with the cowl reversed at that position.
I'm not sure how you've fitted the spacers but if youve created a gap between the bonnet and body at any point this will influence and perhaps adversely affect what you are trying to achieve.
I hope this is of some help.

Aaron Schubert

I'm always wary of modifying airflow through an engine bay; a lot of engineering goes into making sure that the engine gets enough cold air under all conditions, and when you add spacers/scoops you modify that substantially, and often not for the better.

GU GT has provided some great information; I'd just do as much reading as possible and make the best decision based on what you find out. Obviously you need the extra clearance so a scoop isn't debatable. However, be aware that if you have to cut and supports in the bonnet your vehicle becomes un-roadworthy.

Another option is to install a thermo fan on top (although this may be hard with minimal room!).



Well-Known Member
Cutting a hole into the bonnet will have no effect on Roadworthiness in Vic as far as i know, however, the shape and size of the scoop fitted will.

Check out VSI26 and there is no mention of any of this except forthe shape and size of the scoop.