Ram Air Fact Or Fiction: Part II
03-11-2004, 11:09 AM
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Ram Air Fact Or Fiction: Part II Ram air is an attempt to do the second. Under normal circumstances, the air at the throttle plate is at atmospheric pressure, and this pressure drops until the air reaches the cylinders. Ram air would start the process at some pressure higher than atmospheric, and even though the drop is the same, the cylinder pressure is higher because of the increase at the start.
Just how would this increase in pressure at the throttle plate occur? The oft-wrong “common sense” says, “If a scoop is placed in the airstream flowing around the vehicle, the velocity of the air ‘rams’ the air into the scoop, thus increasing the pressure.”
Why is this incorrect? There are two types of pressure: static and dynamic. Placing of one’s hand in front of a fan, or out of a moving car’s window, clearly exerts a force on the hand as the air diverts its path to flow around it. Most people would say “See? This is a clear indication that ram air works. Clearly there is pressure from the velocity of the air.” Well, this is correct, but only to a point. This is an example of dynamic pressure, or the force any moving fluid exerts upon obstacles in its path as the gas is diverted around the obstacle.
What an engine needs is static pressure. This is the pressure the same fluid exerts on any vessel containing it at rest. For those who were physics/chemistry geeks, it is the pressure caused by the force of the molecules bouncing off of the walls of the container. The key to understanding the difference between static and dynamic pressure lies in the velocity of the gas. Dynamic pressure is only a momentum effect due to the bulk motion of the fluid around an obstacle. Static pressure is an intrinsic property of a gas or fluid just because the molecules of the fluid are moving around. Any fluid which is moving can have BOTH dynamic and static pressure, but a fluid at rest only has static pressure.
The point of ram air would be to increase the static pressure, which would correspond to an increase in the in-cylinder air density, and of course, more air. Superchargers and turbochargers do what the mythical ram air purports to do. A supercharger trades the power of the belt and uses it to compress the air in the intake tract. This energy trade-off results in an increase in intake air pressure, more air in the cylinders, more fuel burned, and more power. A turbocharger trades the power of the hot gases and uses it to compress the air in the intake. The overall effect is the same – an increase in intake static pressure.
For ram air to work, it would have to trade the energy of the air’s velocity (as the vehicle moves through the air) for an increase in static pressure (since static pressure is a part of a gas’s internal energy, we see this is TRULY a trade in kinetic energy for an increase in internal energy). Now for the true reasons why ram air is a myth:
- The way for air velocity to be traded for an increase in static pressure is to actually SLOW IT DOWN in a nozzle of some sort. This is easily the MOST counterintuitive part of fluid mechanics for most people. The “common sense” mind says “In order to increase the pressure of the intake, the velocity of the air needs to be increased, just as increasing the speed of a fan exerts more force upon the hand.” Not only does this confuse dynamic with static pressure, but is also misses the point, which is to trade the kinetic energy of the gas for an increase in internal energy. How can this trade occur if the kinetic energy of the gas is increased? It cannot, and in fact, the only way to trade it is to use the velocity of the gas to compress itself – by slowing it down.
- Below about Mach 0.5 (or about half the speed of sound), air is considered “incompressible”. That is, even if the correct nozzle is selected, and the air is slowed down (the official term is “stagnated”) there will be zero trade. No kinetic energy will be traded in as work capable of compressing the air. The reasons for this are not discussed here; the reader may consult any reputable fluid mechanics textbook for confirmation of this fact. In plain English, a car is just too slow for ram air to work.
Still not enough evidence? Here is a little test. For ram air to work, the nozzle must be of a specific shape. The “Holley Scoop” for the Fiero is the wrong shape, by the way. The fact that it has no net shape at all immediately means it cannot effect any kind of energy trade off, so it cannot possibly create ram air. This is also true for the hood scoops on the Pontiac Firebird WS6 package as well, by the way.
What shape must it be? There are two kinds of nozzles. Pick one:
- Converging. This nozzle gets smaller as the air flows through it. It has a smaller exit than entrance. If the nozzle were a cone, the fat end is where the air would enter, and the narrow end is where it would exit.
- Diverging. This nozzle is opposite the other; it gets bigger as the air flows through it. With a larger exit than entrance, the narrow end of the cone is where the air would enter, and the fat end is where it would exit.
So, which is it?
Without hesitation, most of the “common sense” crowd will answer “Converging.” BZZZZT! Thank you for playing anyway! We have some lovely parting gifts for you! Bill, tell ‘em what they’ve won….
The answer is “divergent”. Yes, the nozzle would have to shaped so that the skinny end is pointed into the air stream, and the fat end connects to the throttle plate. How can this be right? Remember, to increase the static pressure of the intake air (which is the true “ram air” effect), the kinetic energy of the air must be traded to compress the air. This is done by slowing the air down, or stagnating it, and the only way to do this is with a diverging nozzle. Ah, but since air is incompressible at automobile speeds, it doesn’t matter any way.
Conclusion
Ram air is a myth because it does not exist, for the following reasons:
- Air is incompressible at any automobile speed., meaning that the kinetic energy of the air cannot be used to compress the air and raise the static pressure.
- The “ram air” nozzles commonly employed on automobiles tend to be the wrong shape. A divergent nozzle is required for ram air. Straight-profile scoops cannot provide a ram air effect.
Select one of the two types of intakes, warm air, or cold air. Beyond that its just about looks.
copyright 2002 www.vetteguru.com
Just how would this increase in pressure at the throttle plate occur? The oft-wrong “common sense” says, “If a scoop is placed in the airstream flowing around the vehicle, the velocity of the air ‘rams’ the air into the scoop, thus increasing the pressure.”
Why is this incorrect? There are two types of pressure: static and dynamic. Placing of one’s hand in front of a fan, or out of a moving car’s window, clearly exerts a force on the hand as the air diverts its path to flow around it. Most people would say “See? This is a clear indication that ram air works. Clearly there is pressure from the velocity of the air.” Well, this is correct, but only to a point. This is an example of dynamic pressure, or the force any moving fluid exerts upon obstacles in its path as the gas is diverted around the obstacle.
What an engine needs is static pressure. This is the pressure the same fluid exerts on any vessel containing it at rest. For those who were physics/chemistry geeks, it is the pressure caused by the force of the molecules bouncing off of the walls of the container. The key to understanding the difference between static and dynamic pressure lies in the velocity of the gas. Dynamic pressure is only a momentum effect due to the bulk motion of the fluid around an obstacle. Static pressure is an intrinsic property of a gas or fluid just because the molecules of the fluid are moving around. Any fluid which is moving can have BOTH dynamic and static pressure, but a fluid at rest only has static pressure.
The point of ram air would be to increase the static pressure, which would correspond to an increase in the in-cylinder air density, and of course, more air. Superchargers and turbochargers do what the mythical ram air purports to do. A supercharger trades the power of the belt and uses it to compress the air in the intake tract. This energy trade-off results in an increase in intake air pressure, more air in the cylinders, more fuel burned, and more power. A turbocharger trades the power of the hot gases and uses it to compress the air in the intake. The overall effect is the same – an increase in intake static pressure.
For ram air to work, it would have to trade the energy of the air’s velocity (as the vehicle moves through the air) for an increase in static pressure (since static pressure is a part of a gas’s internal energy, we see this is TRULY a trade in kinetic energy for an increase in internal energy). Now for the true reasons why ram air is a myth:
- The way for air velocity to be traded for an increase in static pressure is to actually SLOW IT DOWN in a nozzle of some sort. This is easily the MOST counterintuitive part of fluid mechanics for most people. The “common sense” mind says “In order to increase the pressure of the intake, the velocity of the air needs to be increased, just as increasing the speed of a fan exerts more force upon the hand.” Not only does this confuse dynamic with static pressure, but is also misses the point, which is to trade the kinetic energy of the gas for an increase in internal energy. How can this trade occur if the kinetic energy of the gas is increased? It cannot, and in fact, the only way to trade it is to use the velocity of the gas to compress itself – by slowing it down.
- Below about Mach 0.5 (or about half the speed of sound), air is considered “incompressible”. That is, even if the correct nozzle is selected, and the air is slowed down (the official term is “stagnated”) there will be zero trade. No kinetic energy will be traded in as work capable of compressing the air. The reasons for this are not discussed here; the reader may consult any reputable fluid mechanics textbook for confirmation of this fact. In plain English, a car is just too slow for ram air to work.
Still not enough evidence? Here is a little test. For ram air to work, the nozzle must be of a specific shape. The “Holley Scoop” for the Fiero is the wrong shape, by the way. The fact that it has no net shape at all immediately means it cannot effect any kind of energy trade off, so it cannot possibly create ram air. This is also true for the hood scoops on the Pontiac Firebird WS6 package as well, by the way.
What shape must it be? There are two kinds of nozzles. Pick one:
- Converging. This nozzle gets smaller as the air flows through it. It has a smaller exit than entrance. If the nozzle were a cone, the fat end is where the air would enter, and the narrow end is where it would exit.
- Diverging. This nozzle is opposite the other; it gets bigger as the air flows through it. With a larger exit than entrance, the narrow end of the cone is where the air would enter, and the fat end is where it would exit.
So, which is it?
Without hesitation, most of the “common sense” crowd will answer “Converging.” BZZZZT! Thank you for playing anyway! We have some lovely parting gifts for you! Bill, tell ‘em what they’ve won….
The answer is “divergent”. Yes, the nozzle would have to shaped so that the skinny end is pointed into the air stream, and the fat end connects to the throttle plate. How can this be right? Remember, to increase the static pressure of the intake air (which is the true “ram air” effect), the kinetic energy of the air must be traded to compress the air. This is done by slowing the air down, or stagnating it, and the only way to do this is with a diverging nozzle. Ah, but since air is incompressible at automobile speeds, it doesn’t matter any way.
Conclusion
Ram air is a myth because it does not exist, for the following reasons:
- Air is incompressible at any automobile speed., meaning that the kinetic energy of the air cannot be used to compress the air and raise the static pressure.
- The “ram air” nozzles commonly employed on automobiles tend to be the wrong shape. A divergent nozzle is required for ram air. Straight-profile scoops cannot provide a ram air effect.
Select one of the two types of intakes, warm air, or cold air. Beyond that its just about looks.
copyright 2002 www.vetteguru.com
03-11-2004, 11:44 AM
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This guy is wrong and I'll tell you why. The engine is not taking in its full capacity of UNCOMPRESSED air. Stock motors do not have 100% volumetric efficiency. Turbocharged and supercharged engines do, and can exceed 100% by compressing the air. A ram air system helps more air get into the engine to get closer to 100% volumetric efficiency. Also it draws in air which is farther away from the hot engine thereby providing colder air and more hp. This is all from stuff I read on FTRA's web site and my basic knowledge of physics. http://www.fasttoys.net/ramairsystem.html
If you still believe this guy, then why does the FTRA and SSRA give gains at the track?
If you still believe this guy, then why does the FTRA and SSRA give gains at the track?
03-11-2004, 11:47 AM
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The FTRA and SSRA and similair products work because of cooler air coming into the intake. Lower IAT's = more power. I agree that the actual concept of ram air acting as a mini boost is a crock. It does however ram air into the airbox so I guess it's not an outright lie. It is however misleading.
03-11-2004, 12:20 PM
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Regardless as to whether or not the ram air actually works on a ws6, I doubt most people bought one for the 15 extra advertised hp, most bought it for the looks. Thats the selling point for me, anyway.
03-11-2004, 12:33 PM
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If what he's saying is true. For all of you WS6 people out there. All of you that paid all of that extra money for it, would anyone be interested in giving it to me. Or better yet, I'll trade you my Mustang GT. PPPPLLLLEEEEAAAAAASSSSSSEEEEEEEE!!!!!!!!!!!!!!!
03-11-2004, 12:34 PM
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ram air works for indycars which aren't quite going mach 0.5 - just look at the airbox scoop on those cars. of course these intakes are shaped like the article says they should be - smaller at the entrance and larger closer to the engine. in order to reduce horsepower this year they actually mandated that a hole be cut in the back of the airbox to reduce the pressure (ram air effect).
another thing many people don't realize is that air passing rapidly over a surface actually reduces air pressure on that surface - this is how a wing works - but there are certain points on a car where air pressure is higher, for example the base of the windshield - which is why cowl induction works. if a scoop is just a hole on a flat hood surface it would create lower pressure, or actually tend to suck air out of the intake.
another thing many people don't realize is that air passing rapidly over a surface actually reduces air pressure on that surface - this is how a wing works - but there are certain points on a car where air pressure is higher, for example the base of the windshield - which is why cowl induction works. if a scoop is just a hole on a flat hood surface it would create lower pressure, or actually tend to suck air out of the intake.
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03-11-2004, 01:14 PM
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Read this too:
http://www.installuniversity.com/ins..._12.262000.htm
Test done using the FTRA showed a 1% increase in volumetric efficiency which gives 10-15 HP increase, that's not considering the lower IAT which they also demonstrate as being 10 degrees cooler.
So yes, it does "ram air".
http://www.installuniversity.com/ins..._12.262000.htm
Test done using the FTRA showed a 1% increase in volumetric efficiency which gives 10-15 HP increase, that's not considering the lower IAT which they also demonstrate as being 10 degrees cooler.
So yes, it does "ram air".
03-11-2004, 02:54 PM
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Originally Posted by L8BRAKR
another thing many people don't realize is that air passing rapidly over a surface actually reduces air pressure on that surface - this is how a wing works
The wing's shape is what causes lift. The top part is curved and the lower part is flat. The curved upper part creates a longer distance for the air to travel. This lowers the air pressure on the top of the wing. The air pressure under the wing is unchanged. The difference in air pressure causes lift. Basically there is more air under the wing instead of over it which pushes the wing up.
--VIP1
03-11-2004, 04:29 PM
#12
Whether any of the various ram air devices out there truly fit the bill of a scientifically defined ram air system is really completely irrelevant. it's been proven numerous times already that most of these devices DO provide an increase in power by increasing the overall volumetric efficiency of the system.
Not to mention, as was already said, they aren't performing any sort of compression anyways, just minimizing the vacuum being created by the engine. That whole uncompressable under half mach only applies when attempting to surpass atmospheric. Obviously low pressure areas can be affected in a number of ways at much lower speeds. If airplanes can fly, then automobiles can take advantage of todays conventional ram air systems, even if the name isn't technically appropriate.
Not to mention, as was already said, they aren't performing any sort of compression anyways, just minimizing the vacuum being created by the engine. That whole uncompressable under half mach only applies when attempting to surpass atmospheric. Obviously low pressure areas can be affected in a number of ways at much lower speeds. If airplanes can fly, then automobiles can take advantage of todays conventional ram air systems, even if the name isn't technically appropriate.
honestly the stock hoods have so many baffles common sense tells one that it is not getting a true ram air effect
03-13-2004, 01:01 AM
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Well to put it quite simply the novel written as the subject of this thread is almost completely WRONG. He's on the right track, he just has the explanation all wrong. Ram Air does work, just not in the advertised manner.
And no, i'm not writing a novel proving how it works. I'll do it in terms we can understand.
The Ram Air system used on the WS6 and SS CAN add the advertised 15hp...but only in a VERY small portion of the engine's powerband, and only at the right speed. There is an air speed at which the velocity of the air entering the air box creates a "balance" of pressure...it DOES NOT FORCE FEED THE ENGINE. However, it does negate, to a point, the engine's burden of sucking in it's own air. This is where the extra horsies come from. Again, it's only over a very limited range...thus the reason that you likely won't see it at the dyno, even if you are shooting air into the scoop. Also, he's wrong about the type of scoop to use. A divergent scoop creates way too much turbulence in the air stream to be of any aid. A convergent scoop is really the best way I can think of to do it. This design actually rids the airstream of turbulence, however the disadvantage is that at a point, the convergent scoop will become overwhelmed and start to bleed pressure, thus reversing the effect and robbing you of horsepower.
That's about as simple as I can put it. Sorry if it's still kinda hard to understand.
And no, i'm not writing a novel proving how it works. I'll do it in terms we can understand.
The Ram Air system used on the WS6 and SS CAN add the advertised 15hp...but only in a VERY small portion of the engine's powerband, and only at the right speed. There is an air speed at which the velocity of the air entering the air box creates a "balance" of pressure...it DOES NOT FORCE FEED THE ENGINE. However, it does negate, to a point, the engine's burden of sucking in it's own air. This is where the extra horsies come from. Again, it's only over a very limited range...thus the reason that you likely won't see it at the dyno, even if you are shooting air into the scoop. Also, he's wrong about the type of scoop to use. A divergent scoop creates way too much turbulence in the air stream to be of any aid. A convergent scoop is really the best way I can think of to do it. This design actually rids the airstream of turbulence, however the disadvantage is that at a point, the convergent scoop will become overwhelmed and start to bleed pressure, thus reversing the effect and robbing you of horsepower.
That's about as simple as I can put it. Sorry if it's still kinda hard to understand.
03-13-2004, 05:51 AM
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Originally Posted by Blakbird24
A divergent scoop creates way too much turbulence in the air stream to be of any aid. A convergent scoop is really the best way I can think of to do it. This design actually rids the airstream of turbulence, however the disadvantage is that at a point, the convergent scoop will become overwhelmed and start to bleed pressure, thus reversing the effect and robbing you of horsepower.
From what I've read about high-end (formula one) engines, they actually work on 'directing' airflow in the intake system in order to maximize the pattern of air in the combustion chambers and get better combustion. (I can't actually confirm this because I don't have 'in's with any teams, but I've read enough that I'm pretty sure this is the case.) In fact, it's generally accepted that the front wing and 'barge boards' on the formula one cars create vortexes of air. Some of this is for increased aerodynamic effect and things like brake cooling - but I feel some of these vortexes are directed into the air intakes in order to maximize engine air intake and / or cooling efficiency.
Dan
03-13-2004, 11:05 AM
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theres no way to tell if "ram air" actually works. you're standing still on a dyno to prove it, so there's no air being pushed through the opening. unless you put a dyno in a wind tunnel, their will never be any hard evidence. i think it works a little bit when you're going fast (like 70+) but its not going to give you 15 extra h.p.. its a proven fact that the extra 15 h.p. in a WS6 and a SS comes from the different exhaust system that they put on those cars compared to a T/A and Z28. just my .02 cents
03-13-2004, 03:40 PM
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Originally Posted by rschumacherfan1
I agree with everything you said in your post - even the quoted part. BUT are you sure that having turbulent air in the combustion chamber is a bad thing?
Originally Posted by rschumacherfan1
I think that given the same amount of air flowing into a combustion chamber, you will get more efficient combustion given turbulent flow over laminar. The reason is mixing. You are trying your mix the air and fuel as efficiently as possible. Laminar flow is going to want to cause swirl patterns in the chamber, whereas the turbulent flow is going to cause a huge jumble of air and fuel molecules - more evenly mixed for better combustion.
Dan
Dan
03-13-2004, 04:35 PM
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We can talk about this until we are blue in the face. The bottom line is that we have done scientific studies thousands of times and the results show that Ram Air does nothing. I personally have done around 75 tests myself...one every time I beat a stock WS6 or SS at the track.