Air Flow Bonnet Buying Guide

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Air Flow Bonnet Buying Guide

Air flow bonnets are popular aftermarket add-ons for some cars, especially sporty models. These bonnets have openings, usually either flat vents or raised, vertical scoops. Some scoops face forward, others face backward. Installed correctly, they improve airflow in the engine compartment and therefore improve the engine's performance. Many people also really like the way air flow bonnets look, but it is important to install them correctly. If not installed properly, an air flow bonnet can still look good, but does nothing for engine performance. Learning how and why air flow bonnets work is a big part of making sure that they do work, and is the first step towards a wise purchase. Alternatively, there is always the option to buy a purely decorative bonnet; these are simple purchases that depend entirely on the buyer's interest.

Air Flow Bonnet Types and Functions

Getting an air flow bonnet begins with learning how these openings work. Just as important is learning why they sometimes do not work. There are advantages and disadvantages to installing air flow bonnets, and it comes down to a judgement call on the part of the buyer.

Air Flow Bonnet Types

Air flow bonnets can allow more air in, in order to cool the engine and provide it more oxygen, or they can allow more air out of the engine compartment, or they can be purely decorative. While some people get functional vents just for looks, there are also purely decorative vents that do not open the engine space at all. If properly installed, bonnet openings serve to increase engine performance by improving the flow of cool air through the engine compartment. Race cars sometimes use scoop-style bonnet openings to increase air pressure inside the engine compartment to further boost performance, but this only works at very high speeds. This function is called ram air, a phrase that has also been used as an advertising phrase for cars with vents that do not actually create extra pressure.

Air Flow Bonnets Functions

Airflow is a very complicated subject, and car companies put a lot of work into balancing airflow into and out of the engine compartment. Aftermarket alterations can improve the balance, but if done without forethought, they can easily change the airflow pattern for the worse instead. The issue is relative pressure; air cannot flow into a space unless the air pressure inside is less than the air pressure outside. If too much air is allowed in, the pressure equalises the airflow. The end result is that the engine gets less cool air rather than more. To be effective, an extra air inlet must be balanced by extra exhaust vents. Sometimes exhaust vents alone improve performance better than an air scoop could.

Balancing airflow is not just about the size of openings; the placement of openings is important as well. Different areas of the engine compartment, and different areas over the car's bonnet, develop different pressures at different speeds. Whether a given opening admits air, expels air, or has no function whatever, depends on where on the bonnet the opening is. Fortunately, it is possible to get detailed pressure differential readings using simple and inexpensive equipment, such as a Magnehelic gauge or a water manometer, in order to determine the best vent arrangement possible.

Pros and Cons of Air Flow Bonnets

This chart summarises the major advantages and disadvantages of air flow bonnets. Note that getting the advantages depends on installing the bonnet openings properly. Unfortunately, some of the disadvantages are true no matter how the openings are installed.



Helps cool the engine, improving performance

Allows debris, dirt, and water into the engine compartment

Looks good

Increases engine noise, sometimes violating local law

Sometimes delivers forced air to the engine

Changes emission profile and decreases fuel efficiency

There may be additional advantages and disadvantages out there. Which side wins out, whether an air flow bonnet is actually a good idea or not, probably depends on the goals of the driver. Note that getting debris in the engine compartment is not a major problem, but does increase the car's maintenance needs. Notably, the air filter must be changed more often.

Choosing an Air Flow Bonnet

Once the buyer knows how air flow bonnets do and do not work, the next step is to find the right one for the car. The bonnet has to match both the car and the way the car is to be driven. Buying aftermarket bonnets for muscle cars has the added complication of historical accuracy.

Matching the Air Flow Bonnet to the Car

Many aftermarket air flow bonnets are available. While some air scoops are designed to fit any car of a given style, such as any muscle car, most are specific to particular models. Other opening types are more generic and can be taken from one car model and installed in a completely different car.

Matching the Air Flow Bonnet to the Driving Style

On at least some cars, the air pressure over different areas of the bonnet depends on the speed of the car. This means that an opening that vents hot air at low speed might become an air intake at high speeds. This also means that where bonnet openings should go depends in part on how fast the car is going to go. A car bought for racing, or even for normal driving at highway speeds, needs a good airflow balance at speed. A vehicle used mostly off-road, on the other hand, needs better performance at low speeds. Once again, exact placement for any given car depends on a group of complicated factors, but the pressure differentials at various points can easily be measured before finalising plans to install anything. Just remember to measure the pressure differentials while driving in the style that is going to be typical for this car.

Getting Air Flow Bonnets for Muscle Cars

Air scoops have always been popular for muscle cars; some came with bonnet scoops originally, others had scoops added as aftermarket parts. Muscle cars are popular as restoration projects, and many people restoring a muscle car want to add a scoop if there is not one already. But because muscle cars are vintage vehicles, most people want to match the scoop to what would have been available when the car first came out, and that can be difficult. Muscle cars, including their bonnets, were made of solid steel, and steel bonnets have become hard to find. Composite air flow bonnets are sometimes considered acceptable, because they have strength and durability similar to steel, and they resemble steel, visually.

Buying Air Flow Bonnets on eBay

eBay is a good place to find both decorative and functional air flow bonnets. Prices are reasonable. The site is simple to search.

Finding Air Flow Bonnets on eBay

If you know exactly what bonnet you want, try the Advanced Search feature. If you prefer to browse, try a simple search instead. Use a keyword like'air flow bonnets' and see what is available. Or, try your luck with eBay Deals, where the best prices of the day are listed.

Buying Air Flow Bonnets on eBay with Confidence

An important step is to check the listing before buying, to make sure it is the right item. For example, buying a purely decorative bonnet scoop instead of a functional scoop by accident could be a problem. Use the contact link on the seller's profile page to ask any questions. The seller's policies and feedback score are also listed on his or her profile.


Adding an air flow bonnet could be a great way to improve vehicle performance. The idea is to let more air into the engine space to help cool and oxygenate the engine. Plus a bonnet scoop or bonnet vents make a car look stylish and powerful. Unfortunately, adding openings to the bonnet can be counterproductive if they are the wrong shape or size, are positioned wrong, or are not part of a well-thought-out airflow pattern. Fortunately, it is not difficult to test air pressure differentials across the engine space and bonnet. With this information, it is possible to redesign the car's airflow pattern so that added air intakes or vents can improve engine function. Car owners who are not interested in this kind of work but do want the look of an air flow bonnet can choose purely decorative bonnets.

Air Flow Bonnet|Air Scoop|Engine|Muscle Car|Air Filter

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