Ram Air a Myth...?
04-16-2003, 04:46 PM
#1
TECH Regular
Thread Starter
Join Date: May 2002
Location: Lubbock
Posts: 434
Likes: 0
Received 0 Likes
on
0 Posts
Ram Air a Myth...? " http://www.vetteguru.com/ramair/
Dispelling the Myth about "Ram Air" in Automobiles
EXPLANATION
Of all of the applied sciences, fluid mechanics is among the most difficult for many people to comprehend. It is a relatively youthful applied science as well, meaning that it has not had two or three centuries of work to mature into an applied science on par, with say, chemical combustion. To make matters worse, it is mathematically defined almost entirely by experimentally-determined mathematics.
This last point is the true differentiator between those who only understand concepts, and those who can quantify what they are discussing. Truly, quantification is the real skill of the engineer. It is one thing to speak about qualitative issues (the ?what? of the physical sciences); it is entirely another to quantify them (the ?how much? and ?to what extent? of the same). In grade school , students are first taught about ?closed form mathematics? and then that these mathematics are typical of scientific expression. A good example of this is Newton?s famed ?law of action and reaction?, the mathematical expression of which is a succinct F=MA. So straightforward. So simple. Three variables in perfectly-defined harmony. Given any two of them, the third is easy to nail down.
Unfortunately, a vast, vast majority of the mathematics used in engineering are NOT closed form. Instead, they are multi-variable correlations valid only for a narrow set of circumstances. Deviate from those narrow circumstances, and a new expression must be experimentally derived. Fluid mechanics is almost entirely defined by these experimentally-determined expressions, further muddying an applied science not well understood.
And if there ever were an applied science for which common sense is wholly inappropriate, it is fluid mechanics. Virtually nothing obeys the ?common sense? rules of observation, explaining why those who believe in ram air have extreme difficulty in believing that is simply does not exist.
The Deeper Explanation begins with a basic explanation of engine principles. Air and fuel must be combusted at a specific ratio, namely, 14.7 parts air to 1 part fuel (this is a chemical ratio). Stuffing more fuel into the cylinders without increasing the amount of air they also swallow will get no gain whatsoever. So the hot rodder?s adage ?more air = more power? is proven correct. Figure out a way to stuff more air into the cylinder at any given RPM and throttle setting, and you can burn more fuel. Since burning fuel is what makes power, more air truly does create more power.
The amount of air which is inducted into a cylinder is a function of the air?s density. As the air flows through the intake tract, it loses pressure, and as the pressure decreases, so does the air?s density. (Denisty is mass divided by volume. Since cylinders are a fixed volume, increasing the density will also increase the mass of the air in the cylinder.) There are two ways to increase the pressure and density of the air inducted into the cylinders:
- Decrease the pressure drop from the throttle plate to the cylinders
- Increase the starting pressure at the throttle plate.
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.
copyright 2002 www.vetteguru.com"" target="_blank">http://www.vetteguru.com"]www.vetteguru.com"[/url]</a>
I thought this would be an interesting thread sense we are all the ones being marketed these "Ram Air" systems by GM, and proudly display that on our hoods. A member on a local forum I post on, started a thread with this article in it ( www.anythingcars.com ) that lead to a 20 page debate. I am by no means for or against this guy's theory, I just want to know what you guys think about it.
<small>[ April 16, 2003, 05:49 PM: Message edited by: 2000LS1 ]</small>
Dispelling the Myth about "Ram Air" in Automobiles
EXPLANATION
Of all of the applied sciences, fluid mechanics is among the most difficult for many people to comprehend. It is a relatively youthful applied science as well, meaning that it has not had two or three centuries of work to mature into an applied science on par, with say, chemical combustion. To make matters worse, it is mathematically defined almost entirely by experimentally-determined mathematics.
This last point is the true differentiator between those who only understand concepts, and those who can quantify what they are discussing. Truly, quantification is the real skill of the engineer. It is one thing to speak about qualitative issues (the ?what? of the physical sciences); it is entirely another to quantify them (the ?how much? and ?to what extent? of the same). In grade school , students are first taught about ?closed form mathematics? and then that these mathematics are typical of scientific expression. A good example of this is Newton?s famed ?law of action and reaction?, the mathematical expression of which is a succinct F=MA. So straightforward. So simple. Three variables in perfectly-defined harmony. Given any two of them, the third is easy to nail down.
Unfortunately, a vast, vast majority of the mathematics used in engineering are NOT closed form. Instead, they are multi-variable correlations valid only for a narrow set of circumstances. Deviate from those narrow circumstances, and a new expression must be experimentally derived. Fluid mechanics is almost entirely defined by these experimentally-determined expressions, further muddying an applied science not well understood.
And if there ever were an applied science for which common sense is wholly inappropriate, it is fluid mechanics. Virtually nothing obeys the ?common sense? rules of observation, explaining why those who believe in ram air have extreme difficulty in believing that is simply does not exist.
The Deeper Explanation begins with a basic explanation of engine principles. Air and fuel must be combusted at a specific ratio, namely, 14.7 parts air to 1 part fuel (this is a chemical ratio). Stuffing more fuel into the cylinders without increasing the amount of air they also swallow will get no gain whatsoever. So the hot rodder?s adage ?more air = more power? is proven correct. Figure out a way to stuff more air into the cylinder at any given RPM and throttle setting, and you can burn more fuel. Since burning fuel is what makes power, more air truly does create more power.
The amount of air which is inducted into a cylinder is a function of the air?s density. As the air flows through the intake tract, it loses pressure, and as the pressure decreases, so does the air?s density. (Denisty is mass divided by volume. Since cylinders are a fixed volume, increasing the density will also increase the mass of the air in the cylinder.) There are two ways to increase the pressure and density of the air inducted into the cylinders:
- Decrease the pressure drop from the throttle plate to the cylinders
- Increase the starting pressure at the throttle plate.
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.
copyright 2002 www.vetteguru.com"" target="_blank">http://www.vetteguru.com"]www.vetteguru.com"[/url]</a>
I thought this would be an interesting thread sense we are all the ones being marketed these "Ram Air" systems by GM, and proudly display that on our hoods. A member on a local forum I post on, started a thread with this article in it ( www.anythingcars.com ) that lead to a 20 page debate. I am by no means for or against this guy's theory, I just want to know what you guys think about it.
<small>[ April 16, 2003, 05:49 PM: Message edited by: 2000LS1 ]</small>
04-16-2003, 05:28 PM
#4
Launching!
Join Date: Nov 2001
Location: Westminster, CO
Posts: 219
Likes: 0
Received 0 Likes
on
0 Posts
Re: Ram Air a Myth...? Overall I do agree, Ram Air is a bad name. Now calling it cold or cooler air is very true. Get outside air into the engine and you have more density and can make more power.
04-16-2003, 05:48 PM
#5
TECH Fanatic
Join Date: Aug 2002
Location: Objects in mirror no longer matter.
Posts: 1,566
Likes: 0
Received 0 Likes
on
0 Posts
Re: Ram Air a Myth...? I am not a master of fluid dynamics (just majored in it <img border="0" title="" alt="[Wink]" src="gr_images/icons/wink.gif" /> ) and here is my opinion:
An engine is a positive displacement airpump. It takes energy and power to pull the piston down during the induction stroke to fill the cylinder. An engine's ability to make power is directly dependant on the mass airflow through it. Mass airflow is equal to (air density)*(cross-sectional area)*(velocity). There is a quite high velocity of air in the intake tract of an LS1, and a ram air setup reduces the total energy required to pull the same mass airflow rate through the intake.
Actually the assumption of the incompressibility of air is really only applicable for Mach numbers less than .3. This would correspond to a velocity of 342 ft/s at sea level and 80* F, or 233 mph. If that article is true, why do so many drag cars use scoops and ducts that don't trap anywhere near Mach .5??
An engine is a positive displacement airpump. It takes energy and power to pull the piston down during the induction stroke to fill the cylinder. An engine's ability to make power is directly dependant on the mass airflow through it. Mass airflow is equal to (air density)*(cross-sectional area)*(velocity). There is a quite high velocity of air in the intake tract of an LS1, and a ram air setup reduces the total energy required to pull the same mass airflow rate through the intake.
Actually the assumption of the incompressibility of air is really only applicable for Mach numbers less than .3. This would correspond to a velocity of 342 ft/s at sea level and 80* F, or 233 mph. If that article is true, why do so many drag cars use scoops and ducts that don't trap anywhere near Mach .5??
04-16-2003, 05:58 PM
#6
TECH Regular
Thread Starter
Join Date: May 2002
Location: Lubbock
Posts: 434
Likes: 0
Received 0 Likes
on
0 Posts
Re: Ram Air a Myth...? </font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif">Originally posted by Fenris Ulf:
<strong>
Actually the assumption of the incompressibility of air is really only applicable for Mach numbers less than .3. This would correspond to a velocity of 342 ft/s at sea level and 80* F, or 233 mph. If that article is true, why do so many drag cars use scoops and ducts that don't trap anywhere near Mach .5?? </strong></font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">Ok, I don't know much about this, but here are some things that were said about scoops on drag cars in the first debate on this subject that I read:
"Beau is right, the article is right. Think of an Aircraft...the intake nozzle on a turbojet is MUCH smaller than the output conical after the rotors... the air is compressed before it enters the nozzle and actually SLOWS down, that is why the rotors spin when the air hits them and is once again compressed to SPEED up the air exiting the output conical therefore creating thrust... and the fact that an aircraft can reach speeds MUCH faster than Mach 0.5 Air can ACTUALLY be "Rammed" or "Compressed" through the intake nozzle... a "Ram Air T/A" is just a "Cold Air T/A" a T/A will never go uber Mach 0.5, so all you get is a "Cold Air" outlet supply. All the article states is that the term "Ram Air" doesn't exist in the manner most believe. If anything a Turbo/Super Charger should be called a "Ram Air" device. Next time you say an Article is BS why don't you read it before you bash it. I'm sure the person who wrote the article has a much higher degree of eductaion and understanding than you might think. It wasn't written to bash it was written to clarify, sometimes the truth hurts..." -Orian231
" http://media.gm.com/gmracing/drag_ra...technology.pdf "A Pro Stock’s trademark boundary layer hood scoop pressurizes the engine’s intake tract. At 200 mph, the theoretical pressure increase is enough to produce approximately 56 "free" horsepower. penalty paid for this increase in inlet air pressure is the additional aerodynamic drag produced by the scoop itself." it works....to an extent. don't give me crap about the quote saying theoretical, your entire article made a good point about how it's all based on experiments...." -GrandDamn
"GrandDamn: If you read through the article you'll see where he describes how proper pressurizing scoops should be shaped. A smaller opening with a larger chamber. That way the air is forced into the scoop by movement, and it expands in that bigger space even as it hits backed up air(this expansion is what can raise the relative ambient pressure in the scoop), and that prevents it from escaping back out of the intake of the scoop. A Ram Air Trans Am (or SS)...does not follow this shape and the air will back up in the scoops, and then just stay backed up and not let any more air truly get rammed in. Make sense?" -95Thunderbird (thread starter on www.anythingcars.com )
<small>[ April 16, 2003, 05:59 PM: Message edited by: 2000LS1 ]</small>
<strong>
Actually the assumption of the incompressibility of air is really only applicable for Mach numbers less than .3. This would correspond to a velocity of 342 ft/s at sea level and 80* F, or 233 mph. If that article is true, why do so many drag cars use scoops and ducts that don't trap anywhere near Mach .5?? </strong></font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">Ok, I don't know much about this, but here are some things that were said about scoops on drag cars in the first debate on this subject that I read:
"Beau is right, the article is right. Think of an Aircraft...the intake nozzle on a turbojet is MUCH smaller than the output conical after the rotors... the air is compressed before it enters the nozzle and actually SLOWS down, that is why the rotors spin when the air hits them and is once again compressed to SPEED up the air exiting the output conical therefore creating thrust... and the fact that an aircraft can reach speeds MUCH faster than Mach 0.5 Air can ACTUALLY be "Rammed" or "Compressed" through the intake nozzle... a "Ram Air T/A" is just a "Cold Air T/A" a T/A will never go uber Mach 0.5, so all you get is a "Cold Air" outlet supply. All the article states is that the term "Ram Air" doesn't exist in the manner most believe. If anything a Turbo/Super Charger should be called a "Ram Air" device. Next time you say an Article is BS why don't you read it before you bash it. I'm sure the person who wrote the article has a much higher degree of eductaion and understanding than you might think. It wasn't written to bash it was written to clarify, sometimes the truth hurts..." -Orian231
" http://media.gm.com/gmracing/drag_ra...technology.pdf "A Pro Stock’s trademark boundary layer hood scoop pressurizes the engine’s intake tract. At 200 mph, the theoretical pressure increase is enough to produce approximately 56 "free" horsepower. penalty paid for this increase in inlet air pressure is the additional aerodynamic drag produced by the scoop itself." it works....to an extent. don't give me crap about the quote saying theoretical, your entire article made a good point about how it's all based on experiments...." -GrandDamn
"GrandDamn: If you read through the article you'll see where he describes how proper pressurizing scoops should be shaped. A smaller opening with a larger chamber. That way the air is forced into the scoop by movement, and it expands in that bigger space even as it hits backed up air(this expansion is what can raise the relative ambient pressure in the scoop), and that prevents it from escaping back out of the intake of the scoop. A Ram Air Trans Am (or SS)...does not follow this shape and the air will back up in the scoops, and then just stay backed up and not let any more air truly get rammed in. Make sense?" -95Thunderbird (thread starter on www.anythingcars.com )
<small>[ April 16, 2003, 05:59 PM: Message edited by: 2000LS1 ]</small>
04-16-2003, 06:10 PM
#7
TECH Fanatic
Re: Ram Air a Myth...? The writer of the original book...er, I mean article has some valid points, but he missed a few. If you got thru that, here's some rebuttal.
Yep, pressure is inversely proportional to velocity of a compressible fluid. Mr. Bernoulli described that relationship fairly well. So, higher velocity = lower pressure; slower velocity = higher pressure.(I might take exception to the .5 Mach limit of compressibility, but not now or here). A carb venturi (smaller area) is a good example: when the air speeds up through the venturi it creates a lower than atmospheric pressure and the fuel flows into that low pressure area to mix with the air. (We think of it as being sucked in, but it's the atmospheric pressure on the fuel in the float bowl that pushing the fuel into the low pressure area)
Here's another example of the opposite: if air enters a small grille opening (or chin opening in a "bottom breather" like F-cars) and then the passageway gets bigger, the air slows down and the pressure risesjust in front of the radiator. Folks, that's how cooling air gets thru the radiator without help from the fans when you are moving.
Ram air can be made to work. Does it help a lot? Not much at sensible highway speeds. Actually without even using a nozzle there is some dynamic pressure effect which can be quite easily calculated. It's something like 1/2 psi or so at 100 mph so it's not really significant, especially when with proper intake manifold, ports and cam timing, an NA SBC can achieve more than 5 psi of positive tuning pressure at the intake valve even with the negative static pressure (vacuum) in the intake manifold of -.1 to -.5 psi.
Does WS6 Ram Air really 'ram' any extra air into the engine. Very unlikely. Does it perhaps collect cooler air than is available around the underhood area near the engine? Probably yes.
Are there places on the outside of the car where the static pressure is above atmospheric, and picking up air there would actually give a slight pressurization when the car is moving (and we could really call it "ram air")?
Sure! The base of the windshield is the best example. Coincidently, that where your car gets air for the passenger compartment (vent and a/c), and where Winston Cup cars gather their engine inlet air. Remember the old 70's Ram Air 'Birds with the reversed hood scoop? The inlet was very near the highest pressure area on the moving car. Hmmmm.
I'm saying that ram air isn't as mythical as some folks would have you believe.
My highly-opinionated $.02.
Yep, pressure is inversely proportional to velocity of a compressible fluid. Mr. Bernoulli described that relationship fairly well. So, higher velocity = lower pressure; slower velocity = higher pressure.(I might take exception to the .5 Mach limit of compressibility, but not now or here). A carb venturi (smaller area) is a good example: when the air speeds up through the venturi it creates a lower than atmospheric pressure and the fuel flows into that low pressure area to mix with the air. (We think of it as being sucked in, but it's the atmospheric pressure on the fuel in the float bowl that pushing the fuel into the low pressure area)
Here's another example of the opposite: if air enters a small grille opening (or chin opening in a "bottom breather" like F-cars) and then the passageway gets bigger, the air slows down and the pressure risesjust in front of the radiator. Folks, that's how cooling air gets thru the radiator without help from the fans when you are moving.
Ram air can be made to work. Does it help a lot? Not much at sensible highway speeds. Actually without even using a nozzle there is some dynamic pressure effect which can be quite easily calculated. It's something like 1/2 psi or so at 100 mph so it's not really significant, especially when with proper intake manifold, ports and cam timing, an NA SBC can achieve more than 5 psi of positive tuning pressure at the intake valve even with the negative static pressure (vacuum) in the intake manifold of -.1 to -.5 psi.
Does WS6 Ram Air really 'ram' any extra air into the engine. Very unlikely. Does it perhaps collect cooler air than is available around the underhood area near the engine? Probably yes.
Are there places on the outside of the car where the static pressure is above atmospheric, and picking up air there would actually give a slight pressurization when the car is moving (and we could really call it "ram air")?
Sure! The base of the windshield is the best example. Coincidently, that where your car gets air for the passenger compartment (vent and a/c), and where Winston Cup cars gather their engine inlet air. Remember the old 70's Ram Air 'Birds with the reversed hood scoop? The inlet was very near the highest pressure area on the moving car. Hmmmm.
I'm saying that ram air isn't as mythical as some folks would have you believe.
My highly-opinionated $.02.
Trending Topics
04-16-2003, 06:28 PM
#8
TECH Fanatic
Join Date: Aug 2002
Location: Objects in mirror no longer matter.
Posts: 1,566
Likes: 0
Received 0 Likes
on
0 Posts
Re: Ram Air a Myth...? Those are some great points SStroker.
Even more confusing is that you cannot use supersonic jet engines as a basis for the arument since air actually increases in speed as the duct area increases for Mach numbers greater than 1.
One of these days I will hook up some pressure transducers in the intake tract and collect some data, but you can easily monitor the benefit of ram air by looking at the MAF readings with the hood blocked off and open.
Even more confusing is that you cannot use supersonic jet engines as a basis for the arument since air actually increases in speed as the duct area increases for Mach numbers greater than 1.
One of these days I will hook up some pressure transducers in the intake tract and collect some data, but you can easily monitor the benefit of ram air by looking at the MAF readings with the hood blocked off and open.
04-16-2003, 06:46 PM
#9
TECH Fanatic
Re: Ram Air a Myth...? </font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif">Originally posted by 2000LS1:
<strong>
Think of an Aircraft...the intake nozzle on a turbojet is MUCH smaller than the output conical after the rotors... the air is compressed before it enters the nozzle and actually SLOWS down, that is why the rotors spin when the air hits them and is once again compressed to SPEED up the air exiting the output conical therefore creating thrust... and the fact that an aircraft can reach speeds MUCH faster than Mach 0.5 Air can ACTUALLY be "Rammed" or "Compressed" through the intake nozzle</strong></font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">Jet engines do not injest supersonic air, even if the aircraft is going 3+ mach like the SR-71/A11 birds. The air slows down from the time enters the inlet until it gets to the engine compressor. The faster the aircraft the more difficult this job becomes. If you want to get into generating thrust with just inlet air, study the SR-71 engine system; it's as close to getting something for nothing as I've ever seen. It's not simple, either.
<strong>
Think of an Aircraft...the intake nozzle on a turbojet is MUCH smaller than the output conical after the rotors... the air is compressed before it enters the nozzle and actually SLOWS down, that is why the rotors spin when the air hits them and is once again compressed to SPEED up the air exiting the output conical therefore creating thrust... and the fact that an aircraft can reach speeds MUCH faster than Mach 0.5 Air can ACTUALLY be "Rammed" or "Compressed" through the intake nozzle</strong></font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">Jet engines do not injest supersonic air, even if the aircraft is going 3+ mach like the SR-71/A11 birds. The air slows down from the time enters the inlet until it gets to the engine compressor. The faster the aircraft the more difficult this job becomes. If you want to get into generating thrust with just inlet air, study the SR-71 engine system; it's as close to getting something for nothing as I've ever seen. It's not simple, either.
04-16-2003, 06:54 PM
#10
9 Second Club
iTrader: (2)
Join Date: Jul 2002
Location: on the dyno tuning in MD
Posts: 2,583
Likes: 0
Received 1 Like
on
1 Post
Re: Ram Air a Myth...? Call them whatever the heck you want! I'm glad my hood doesn't say "cold air" on the side of it and I'm glad it has those scoops.
I don't think they are trying to make us believe that those scoops are tiny super chargers or anything! I think they are saying that the scoops will ram some better (cooler) air into your engine though - and that is exactly what they do! I've never seen an ad for a WS6 saying the scoops increase the pressure.
Besides, they look mean too!
I don't think they are trying to make us believe that those scoops are tiny super chargers or anything! I think they are saying that the scoops will ram some better (cooler) air into your engine though - and that is exactly what they do! I've never seen an ad for a WS6 saying the scoops increase the pressure.
Besides, they look mean too!
04-16-2003, 07:08 PM
#11
TECH Regular
Thread Starter
Join Date: May 2002
Location: Lubbock
Posts: 434
Likes: 0
Received 0 Likes
on
0 Posts
Re: Ram Air a Myth...? </font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif">Originally posted by BLK02WS6:
<strong> I think they are saying that the scoops will ram some better (cooler) air into your engine though - and that is exactly what they do! </strong></font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">Do they? If so, how does it work? So far I have read no proof of air actually being "rammed" into the engine.
<small>[ April 16, 2003, 07:11 PM: Message edited by: 2000LS1 ]</small>
<strong> I think they are saying that the scoops will ram some better (cooler) air into your engine though - and that is exactly what they do! </strong></font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">Do they? If so, how does it work? So far I have read no proof of air actually being "rammed" into the engine.
<small>[ April 16, 2003, 07:11 PM: Message edited by: 2000LS1 ]</small>
04-16-2003, 07:47 PM
#12
12 Second Club
Join Date: Dec 2001
Location: Santa Clarita, CA
Posts: 931
Likes: 0
Received 0 Likes
on
0 Posts
Re: Ram Air a Myth...? Maybe there is room for improvement in the mathematics. I'm a novice, but my instincts tell me that air velocity at any speed above zero should increase static pressure even without specially configured nozzles or at these +mach speeds mentioned. If static pressure in a vessel is the work of molecules bouncing off the sides, why wouldn't the kinetic energy of those molecules tending to move in a certain direction increase the static pressure near an obstruction in the air stream. I read that this transfer of energy simply doesn't happen at slow speeds, but I resist, and say maybe its small and hasn't been measured properly. The way I understand the ram air WS6 now after reading the posts is that the air molecules are simply backing up and out of the nozzle(inlets) and not increasing the static pressure in the throttle body at all. The only benefit is outside air which is normally cooler and denser than underhood air. Very interesting, thanks for the info!
04-16-2003, 08:53 PM
#13
TECH Enthusiast
Join Date: Jan 2002
Location: Orange Park, FL
Posts: 597
Likes: 0
Received 0 Likes
on
0 Posts
Re: Ram Air a Myth...? Its quite simple actually. You got a hole (the hood scoops), you got an object (air), you 'ram' one into the other.
This dumbass Navy Jet Mech with a 12th grade education calls that "Ram Air". <img border="0" title="" alt="[Big Grin]" src="gr_grin.gif" />
This dumbass Navy Jet Mech with a 12th grade education calls that "Ram Air". <img border="0" title="" alt="[Big Grin]" src="gr_grin.gif" />
04-16-2003, 09:19 PM
#14
On The Tree
Join Date: Aug 2002
Location: Nashville, TN
Posts: 129
Likes: 0
Received 0 Likes
on
0 Posts
Re: Ram Air a Myth...? <img border="0" alt="[bang head]" title="" src="graemlins/gr_banghead.gif" />
<img border="0" alt="[chug]" title="" src="graemlins/gr_chug.gif" />
<img border="0" alt="[chug]" title="" src="graemlins/gr_chug.gif" />
04-16-2003, 11:17 PM
#15
Launching!
iTrader: (35)
Join Date: Nov 2002
Location: Tallapoosa, GA
Posts: 239
Likes: 0
Received 0 Likes
on
0 Posts
Re: Ram Air a Myth...? Maybe we're just looking at the whole "ram-air" idea the wrong way. If I'm understanding the explination of why ram air may be a myth it is this: "According to the laws of fluid dynamics, you can't funnel static air into an engine and pressurize that air simply by driving the car down the road at normal (attainable) speeds.
In the original post, the author discount's ram air in saying tht you can't place a scoop in an airstream flowing around the vehicle, and use the VELOCITY of the air to ram the air into the scoop thus increasing PRESSURE. A lesson in hydraulics tells us that if PRESSURE increases, VELOCITY (flow) must decrease and vice versa. No one should expect an increase in PRESSURE from ram air (that's what we use superchargers and turbo's for) but rather an increase in air VELOCITY. These ram air system's are on naturally asperated engines where we don't expect to see +pressure but we do like to see high flow or velocity #'s. So is it pressure or air velocity that we should be trying to increase?
In the original post, the author discount's ram air in saying tht you can't place a scoop in an airstream flowing around the vehicle, and use the VELOCITY of the air to ram the air into the scoop thus increasing PRESSURE. A lesson in hydraulics tells us that if PRESSURE increases, VELOCITY (flow) must decrease and vice versa. No one should expect an increase in PRESSURE from ram air (that's what we use superchargers and turbo's for) but rather an increase in air VELOCITY. These ram air system's are on naturally asperated engines where we don't expect to see +pressure but we do like to see high flow or velocity #'s. So is it pressure or air velocity that we should be trying to increase?
04-17-2003, 08:02 AM
#16
Restricted User
iTrader: (43)
Join Date: Nov 2001
Location: Pittsburgh, PA
Posts: 2,669
Likes: 0
Received 0 Likes
on
0 Posts
Re: Ram Air a Myth...? I say block off your scoops and make a run monitoring the volume of air through the MAF. Next unblock the scoops and monitor the amount of air. If there is an increase then it must be "Ramming Air" in. I think they are implying that Ram Air is Supercharging to an extent... There has been way too much thought given on this...
04-17-2003, 10:36 AM
#17
Launching!
iTrader: (4)
Join Date: Aug 2002
Location: Alongwayfromhome
Posts: 224
Likes: 0
Received 0 Likes
on
0 Posts
Re: Ram Air a Myth...? "No one should expect an increase in PRESSURE from ram air (that's what we use superchargers and turbo's for) but rather an increase in air VELOCITY. These ram air system's are on naturally asperated engines where we don't expect to see +pressure but we do like to see high flow or velocity #'s. So is it pressure or air velocity that we should be trying to increase?"
DING DING DING!!!!!
WE HAVE A WINNER!!!!!
DING DING DING!!!!!
WE HAVE A WINNER!!!!!
04-17-2003, 10:42 AM
#18
TECH Enthusiast
Join Date: May 2002
Location: rochester,ny
Posts: 508
Likes: 0
Received 0 Likes
on
0 Posts
Re: Ram Air a Myth...? so then just remove your air deflector on the bottom of your car and replace it with a custom fiberglass scoop about 3"s high the full length of the deflector.(now for the hard part) the exit for the scope make a outlet of lets say 2"h by 8" wide. then before you hit your air box increase the ducting to as big as you can go to the air box.
now you have a large area of collection. the smaller size output to the larger final duct size now acts like a diverter. done! that was easy enough huh? oh ya.....get off the pipe man!
now you have a large area of collection. the smaller size output to the larger final duct size now acts like a diverter. done! that was easy enough huh? oh ya.....get off the pipe man!
04-17-2003, 01:43 PM
#20
Launching!
Join Date: Nov 2001
Location: Indianapolis
Posts: 294
Likes: 0
Received 0 Likes
on
0 Posts
Re: Ram Air a Myth...? Didn't the "ram air" vents on the sides of old cars (mustangs I think) actually "ram air" to the braking system, helping them to keep cool? If so, why shouldn't the "ram air" hoods collect at least some outside (colder) air?