CAI vs. Ram Air

[dreaded_hope]

Hey what's up guys, just a quick question as I contemplate the next move with my car. I have the ws6 style hood from slp, and was thinking to make it functional. The air box is 300 bucks, and is quoted to give up to 10 hp on the site. However, the CAI is listed around $200 or something like that, and is clamed to give 20hp. Isn't it weird that a CAI would yeild supposed better results than the ram air package? I think I would rather get the ram air kit, but do you guys actually think that there would be any difference between the 2? Thanks

[T-Punk]

they are both just as good. the ram air is a nice looking cold air that gets the air from the outside and the cold air (at least for your car) is under the hood off to the side of the engine bay. the ram air is slightly better but not 100 dollars worth better. the ram effect of the ram air is neglegable, its a marketing ploy. that should answer your question pretty well.

[dreaded_hope]

Thanks for your help!

[FloydSummerOf68]

I've heard from lots of people that the CAI is supposed to make more power...like SLP or FTRA

['98 BlackBird]

I have both the SLP CAI and the SLP Ram Air H.O. which is operational an hood. Does that mean I just gained an extra 30-40 hp?

[BlitzPuppet]

Ram Air is a myth, you will see the same gains with a CAI vs a Ram Air setup.

The Ram Air Myth Dave Rodabaugh
The Ram Air Myth is the most mythical of them all. It differs from the other myths, in that the other myths are misinterpretations of physical phenomena, whereas ram air simply does not exist.

MYTH: Use of a scoop on the front of the vehicle to collect intake air, or provide “ram air” can raise engine performance.

TRUTH: At automobile velocities, there is no ram air effect.

SIMPLE EXPLANATION

The "Truth" statement says it all. How much simpler can it be? The Ram Air effect is a total myth because it simply does not exist. “But Pontiac uses it on the Trans Am, and they know more than you do.” To those who offer this, tsk tsk. Careful reading of Pontiac’s statements on the matter reveal that the HP increase of the WS6 package are a result of a less restrictive intake, and a freer-flowing exhaust, NOT any ram air effect.

So why does Pontiac use Ram Air? Easy! To make people buy their cars! And they are quite effective with this strategy.

DEEPER 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.

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 such as the Holley scoop for the Fiero cannot provide a ram air effect.

[V6Firebird94]

i had the CAI and u could tell a difference between the Ram Air i now have...they both have bout the same gain from a stop....but when u punch if from like a 45 roll you can so tell the difference..i dont see how u accuse it of a myth...put it on ur car and hit it from a roll you'll see the difference

[BlitzPuppet]

The only difference that the Ram Air and CAI have in difference is the deliverance of the air. Ram air takes in outside air, CAI takes in air from the front of the engine bay. You could cut a hole in the bottom of your splashguard and then have the same deliverance of fresh air to your intake. It's just simple physics. Don't worry about how much air your engine will get from a CAI or Ram air intake unless your engine is really hungry for air (nitrous, Forced induction etc).

['98 BlackBird]

WOW man, deep. very deep.

[OneQuickV6]

Nice copy and paste! Now for something people can understand. Originally, "cold-air" was when you eliminated the factory style baffles on a car. These baffles circulate the air and make it warm, and thereby more quiet. This is what the general population wants from their vehicle, peace and quiet. A "cold-air" kit gets rid of these baffles, making the intake air cooler, and therefore producing more power.

Ram-air does not have a ram-air effect, that is true. BUT, instead it does give the cold-air kit you put on a larger quantity of air to suck in, and thereby making more power. And, if you Autotap a car with say the SLP kit, you will see at higher speeds you have more CFM of air coming into your motor. "Mild supercharging effect" or ram-air effect??? Yes, kind of. They both work great, and by and far every performance 4th gen F-body in the country will have both, because they are both proven. Time to quit taking someone else's word for everything, and simply acknowledging it for truth. There is a difference in the two....SLP just kind of screwed up when they named their Ram-air kit "Cold Air". Class is over;)

[Ttop34]

Everybody notice that the person that originally asked this question has a 1997 Firebird with the WS6 Ram Air Hood. Most of the "useful" information posted is worthless to him.
I don't know much about the pre-98's but the later year WS6's have a kit to make the ram air a "true" ram air. The design of those systems look like they would have the best benifit, the air isn't redirected as much. You can also just make that scoop to go under the cavity where the air filter goes for the CAI, there's instructions somewhere on the internet.