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Ram Air: What is it, does it work, and why? |
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Written by Devin Durham
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Tuesday, 11 September 2007 |
You see it written on the hoods of cars, you see it in marketing pamphlets and you see products that promise to make use of the "ram air" effect. It certainly seems like a simple enough idea: put a scoop on the front of your car and let the air pressure that builds up feed directly into the engine. But does it really do anything, or is it just hype?
To really understand why ram air and equivalent systems are installed on many modern sports cars, we need to travel back about 50 years, when a great amount of the pioneering efforts in aerodynamics and high performance piston engines happened because understanding these things were literally a matter of national survival.
NACA
Search around online for hood scoops and air ducts and you'll soon enough find references to NACA. NACA stands for National Advisory Council for Aeronautics, and was established by an act of Congress as the United States was concerned about the increasing use of aircraft in warfare. In 1958 NACA was folded into the recently formed NASA and faded into history, although the work they did still impacts modern aircraft to this day.
 Among the designs and ideas they originated were air cowlings that increased the performance of radial engines by 15%, wheel spats, the principals of laminar flow that was used in making the P-51 Mustang wing which gave that aircraft its amazing range and performance, scoop and duct designs and airfoil designs that are still being used on the ultra-modern F-22 Raptor.
In 1948 NACA did a considerable amount of research on something called a Ram Jet, which was an engine that is very useful in high speed applications and is extremely simple and rugged in that is has no moving parts. It uses the forward speed of the engine itself to collect and compress the air into the combustion chamber, and the faster it goes the more power it develops.
So is any of this sounding familiar? This is exactly what is promised with Ram Air intakes: the faster that you go, the more the forward momentum causes the air to be compressed which is then pushed into your engine. It's like a natural supercharger! It's free power!
Well the fact that Ram Air is a valid idea can't be disputed, after all that's what makes a ram jet work. There is a problem when it comes to cars though, and that is one of speed.
Ram jets work best at Mach 2 and faster. That's about 1400 miles per hour and upwards. While some ram jets have been made to operate (very poorly) at speeds as low as 100 mph, the extensive tests that NACA did in 1948 provided both the math and the test results to show that the Ram Air effect is nearly useless below about 320 mph. It seems that Ram Air is a dynamic function that is proportional to the square of the air velocity. As such, at 75 mph the gain in air density is about 7/10ths of one percent, or .007 percent. At sea level you'll be getting 14.8 PSI instead of the standard 14.7 PSI. But that's in a perfect world. Ram Air is generally considered to be only about 75% efficient, so in truth at 75mph you are gaining only about 1/2 of one percent.
This begins to ramp up as you get faster. At 150 mph the density increase is 2.75% (2.06% adjusted) which is probably helpful in some way but is still in the "nearly impossible to measure" category.
At 350 mph the increase in density is in the 15% range and this is where Ram Air becomes usable.
So unfortunately at normal highway speeds and even at drag strip speeds, Ram Air just doesn't add anything to the performance of your car.
So why does my car have all these scoops all over it?General Motors introduced the first Ram Air equipped muscle cars in the 1960s primarily as an avenue of air flow to the carb. The theory was that introducing cold(er) air from outside the car was better than the typical "right off the top of the engine" style intake that most cars had, and that's very true. Modern fuel injected cars are no different and actually benefit more from a direct intake system because the air can be more efficiently routed to each cylinder without the added complication/disruption of a carb. Many of the General Motors Ram Air equipped cars have intakes that are very short and feed outside air directly into the intake manifold and that can be nothing but good. So from the standpoint of a cold air intake, those scoops are a good thing.
The second reason that cars are scooped is because it looks cool. Looks don't make a car go faster but it certainly helps the sale.
And the third reason is marketing. It sounds cool to say that a car has Ram Air. It also sounds cool to say that a modern Chrysler has a Hemi, when the cylinder heads are pentagonal! Marketing makes the world go 'round, so you might as well get used to it.
 A few final words. NACA experimented with a great number of scoop and duct designs, some were intended to create the most airflow possible without regard to the impact of performance, others were designed to try create some positive airflow without adding drag. The most efficient design that they came up with was what is now called a NACA Duct, which can create positive airflow with hardly any impact on drag at all. It works because of the way it is shaped and the exact angle that it is ramped into the surrounding body, and it something you often see on both high performance cars and street cars as well.
There are also NACA Scoops that are intended to just grab as much air as possible. It's not really clear if most of the scoops for sale that claim to be NACA scoops actually meet the specifications, but then again that's marketing for you.
It's probably not a good idea to introduce a hood scoop into a conventional engine bay without it being directly ported and sealed to the intake because scoops create positive pressure, while behind the radiator we want negative pressure. Instead of cooling things off you will probably just make the engine hotter since you will be interfering with the functioning of the radiator.
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Last Updated ( Saturday, 29 September 2007 )
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