I've read that cowl hoods used to be the method for air induction for older vehicles with carburetors and that the only purpose they serve for vehicles with fuel injection is for looks.
However, I hear quite a few people (especially Cherokee owners) say they help keep the engine cool, too. Is this true or does a cowl hood make any significant difference in engine temp?
While I won't say a cowl was never used for cooling the engine bay (I'm sure someone has done that), it really won't do much for cooling (as in to draw air out from and engine bay) and here's why:
As your vehicle is travelling down the road the air travelling around different parts of the vehicle creates high pressure areas. The two biggest of these are the front end (grill area) and the windshield. The base of the windshield especially, because this is where the air starts to make its turn upward to go over the top of the car. Because of this, there is basically no way for there to be any heat extraction from this area. You can, however, draw air into the engine from there. This high pressure area not only provides a source of denser air, it also provides cooler air, all of which helps produce horsepower. This was GM's theory behind the Chevelle Cowl Induction hood.
There are three main purposes for a cowl hood:
If a manufacturer has wanted cooling effects from the hood area, they will usually use an extractor to do this. Heat extractors work by using the venturi principle, where the air is sped up over a lip and when it gets over the lip, it slows down and creates an slight vacuum at the port, which pulls air out from under the hood.
Hood cowls can help engine cooling when properly designed. But this is not something most drivers will ever need.
In most vehicles, cold air is ingested into the radiator at the front of the vehicle. The now warmer air is then passed along the engine block (not a great design, by the way), and then exits mostly low down underneath the chassis.
This is fine in most operating conditions, but not in two specific cases:
The first case concerns sports cars, or cars that are used in sporting events. The solution is to give the hot air inside the engine compartment somewhere to go, so here is where side vents come into use. Hood cowls, facing incoming airflow, also give the engine block access to cool air that (a) bypasses the radiator and (b) is forced inside by dynamic pressure.
Other solutions include siting the radiator in some more sensible position than just in front of the engine, for example in the rear of the vehicle (which is then not covered by roofing).
The second case -going slow- is often found in four-wheel drives, for example when going rock-crawling. The combination of slow speed (low airflow) and increased heat production (revving the engine to get over multiple obstacles) make overheating a possibility. This is where hood cowls, louvers, or in fact any opening in the hood is a definite advantage. Since hot air likes to go straight up, so let it. This explains your Cherokee drivers' comments.
It also explains why there are after-market replacement hoods for vehicles that need to go slowly in hot conditions, containing extraction louvers. As @Paulster2 correctly points out in his answer, these are located to the front of the hood, to avoid the local high air pressure at the base of the windscreen.
AEV Heat Reduction Hood, Photo credit: Quadratec, www.quadratec.com
So: two use cases, two different types of problem sets, and two different ways of solving it. However, it is safe to say that anything that helps get calories out of the engine compartment is welcome on a high-output engine.
On the other hand, a standard, non-tuned, engine has been designed with this in mind. Engineers have more surely set it up to cool correctly in a whole range of situations - those found in normal driving conditions. They will even have taken steps to ensure it does not immediately heat too far up when stuck in urban driving in hot weather conditions.
Any radiator needs air going through the radiator fins to exchange heat and cool it down and its fluids that runs in it.
If the cowl hood directs air to the radiator (engine radiator, oil radiator, intercooler, etc) or to a part with fins (ex: some oil pans has fins on it to help cool the oil) and are the main source of air flow, then the cowl hood helps cooling the system. If not, they are only for looks.
