It's Not Forced Induction

[mother_russia]

Cold air intake. I am installing one in my 350z, but frankly I am baffled by the price differences I've come across on the web. Some of these no name brand intakes are like 50 BUCKS! Are the Injen or AEM intakes some how superior to the no name ones or are you paying for the name? Excluding the filter, I see no technology involved in a metal tube. Talk to me.

[Reed]

there is technology involved in the length of the metal tube. and also in the bends. these affect the amount of air they can flow and also the rpm band they are most effective in.

but i think its way too much bullshit to worry about for the 3hp difference (if that) between them all

[Wolf]

Originally Posted by mother_russia

Are the Injen or AEM intakes some how superior to the no name ones

No.

[Holyterror]

Originally Posted by mother_russia

Excluding the filter, I see no technology involved in a metal tube.

Deceptively simple, isn't it? In essence, a CAI is little more than a pipe that draws intake air from a better location. AFAIK, there is no harmonic tuning that goes into these mass-produced units such as AEM, etc. There are basically three important things to look for in a CAI (these are also requirements if you decide to go cheap and build one yourself):

1. The intake must draw air from a low temperature, low moisture area. A high pressure area (like the cowl) would also be preferable in most applications.
2. It must be unrestrictive. That means at least a certain diameter(varies by application, most commercial units properly matched) and full mandrel bends.
3. It must seal properly. This is one reason to be wary of cheap intakes. It doesn't matter how good the filter is if you're pulling dirt around it. If you make an intake yourself, make sure you get an airtight seal on both ends (filter and manifold).

When you buy an AEM, Injen, etc. intake, you're paying for the name as well as the pretty anodized finish. In the end, it is mostly just a streamlined metal tube. Even multiple filter units for turbo applications are just variations on a theme. You should be able to achieve similar results with one you made in the garage, if you put some thought into it.

[longlivetheZ]

Actually, some types of "Ram Air" induction setups can be considered a type of forced induction. At 70 MPH, the intake is actually having air rammed into it at ~2psi.

I would definately go 'do-it-yourself' on this one. Instead of buying the 'K&N fuel injected performance kit', or whatever that crap is, for...like...140 bucks...I ordered a 10" universal cone filter and put that on. Retained my stock resonator (which does nothing but change the intake sound) and everything, but took out the air box and brackets and all that crap. A bit less cluttered looking under the hood, lower restriction (that 10" will flow more air than my 3.0L engine could ever need), cooler air (cuz it's not drawing air from behind the radiator), and only cost a mere $60 shipped from Jegs. I would recommend one of those expensive, trendy ass kits under the very few circumstances where I know they serve a specific purpose.

[SaabJohan]

2 psi is a little less what a F1 car can get when it travels at speeds around 300 km/h, and it converts almost 100% of the energy in the high speed air.

For the pressure to mean anything the dynamic pressure must be converted into static pressure which is done by slowing the air down from full speed to almost zero by an intake that increases it's area in the duct that feeds air to the engine. I have never seen any aftermarket item having even close to the design that is needed for that and this only works in very high speeds.

[longlivetheZ]

..........................Umm..................... .....

[duplox]

Originally Posted by SaabJohan

2 psi is a little less what a F1 car can get when it travels at speeds around 300 km/h, and it converts almost 100% of the energy in the high speed air.

For the pressure to mean anything the dynamic pressure must be converted into static pressure which is done by slowing the air down from full speed to almost zero by an intake that increases it's area in the duct that feeds air to the engine. I have never seen any aftermarket item having even close to the design that is needed for that and this only works in very high speeds.

Agreed. I know a person who tested a system on a '69 mustang where he replaced the 2 center headlights with air inlets, and achieved a whopping .3psi at 70mph. Ram air is next to useless on a street car. Just figure out where you can get the coolest, dryest air and route your intake tubing there.

[longlivetheZ]

Originally Posted by duplox

Agreed. I know a person who tested a system on a '69 mustang where he replaced the 2 center headlights with air inlets, and achieved a whopping .3psi at 70mph. Ram air is next to useless on a street car. Just figure out where you can get the coolest, dryest air and route your intake tubing there.

True...you do want the coolest, driest air you can get for your intake, but I heard that the Ram Air system that's on the Firehawk Firebird gets about 2psi of boost at about 70 mph, as opposed to the 2 tenths of a psi that you quote. I may be wrong as I don't remember exactly where I heard this, so if you can site your sources, it's be cool.

[duplox]

I'll try to dig up the post. Its very likely that the system he built was not very well engineered and therefore didn't produce much pressure, this was a homemade setup. It was a long time ago I read this, so no guarentees I'll find it, but I'll do my best.

[Holyterror]

Technically, yes, ram-air is a form of forced induction. However, it is unlikely to produce any boost over atmospheric pressure. But, do not assume that this means ram-air is ineffective. Let me explain:

When we refer to a vehicle or a particular forced induction setup "making boost," we simply mean that our instruments register positive manifold pressure. Boost (measured in PSI or bar) is the "magic number" of forced induction. However, like peak HP, it is not nearly as important outside of drag racing. Burn the next sentence into your memory, and don't let anyone tell you otherwise:

Flow is more important than boost.

Flow (measured in CFM) is what the engine is designed to do, period. It's basically an air pump. So more flow is better. It's that simple. The purpose of forced induction is to help the engine flow more by giving it more air than it can pull it by itself. Ram-air, while not good at producing boost, fulfills this more important goal.

Let's make a hypothetical vehicle with a N/A engine. We'll give this vehicle an accurate vacuum/boost gauge, so we can see what happens to manifold pressure as we drive. Out on the road, we take the engine up to 3000 RPM in fifth gear, an overall speed of 65 MPH. At this speed, the v/b gauge reads 18 inches of mercury (vacuum). That means there's negative manifold pressure, i.e., the engine is sucking for dear life. Back at the garage, we bolt up a ram-air system. We take the car back out on the road, and repeat the conditions of the earlier drive. The vacuum gauge now reads 8 in./hg. What happened? The ram-air intake helped push more air into the engine than it was able to on its own, fulfilling the purpose of forced induction. That means better cylinder filling, more flow. And if you're hung up on psi, then consider how much negative pressure was removed from the system. 10 in./hg. = 4.9 psi. So, ram-air does create boost relative to negative manifold pressure. But, as I said earlier, what is really important is CFM, which obviously increased in our example.

Real world ram-air systems are not necessarily as efficient as our hypothetical one. In fact, they rarely are. For an example of one that actually works, check out the VaraRam C5 Corvette ram-air system. Note that some ram-air systems, with their upside-down reverse magic venturis, add a bit of restriction to the intake. An upspooled turbocharger, of course, does the same. But, in general, ram-air does not come with many penalties.

All things considered, I'd still prefer to keep a hairdryer under the hood.

[SaabJohan]

Now I think there is some confusement. If you get a reading of 8 in. mercury, that's .27 bar the engine is running with part throttle.

An otto engine is an engine which is lambda controlled, that means that for a certian air flow you need a certain fuel flow (this isn't the case with for example diesels). This means that if you want to adjust the power from the engine you must limit the air flow through the engine. This is done with the throttle, and thereby the reduced pressure, so even if you supercharge the engine you will still have that .27 bar pressure in the inlet manifold under the same conditions. You can however increase the pressure (and hence get a better fuel efficiency) if you for example have a smaller displacement as a certain mass flow is needed to produce the power but the engine at a certain rpm is pumping a certain volume flow.

To produce 1 kWh you typically need an airflow of 3.4 m^3 of air at atmoshperic pressure, that means around 4kg of air. At part throttle that amount will increase due to the loss of efficiency.

When the air has a certain velocity, there is a certain dynamic pressure. The dynamic pressure is what you can notice if you pull your hand out of the window in car at the highway. This dynamic pressure must however be converted into static pressure to get the density increase that we want as an engine can only consume a certain volume flow of air.
To get an increase in static pressure we must slow the air down, for 100% conversion (which is impossible) the air must be slown down to 0 velocity and without flow resistance (friction). In reality we can slow the air down to almost 0 and with a small flow restriction. Anyhow, the increase in pressure can be calculated by:

(V1-V2)^2/(2RT)

so for 70 mph at 20 degC that means

V1 = 70 mph = 31 m/s
V2 = 0 m/s
T = 20 degC = 293 K
R for air = 287 J/(kg*K)

(31-0)^2/(2*287*293) = 0,006 = .6%

.6% means that the maximum pressure increase is 6 mbar or .09 psi.

So at low speed ram air is worthless, but what you can do is decrease the flow restriction of the intake. There's a pressure loss before the manifold/throttle or turbocharger inlet of a car, on a Saab 9-5 the pressure loss is for example around 200 mbar which can be decreased with a less restrictional air filter and plumbing.

To use "ram air" with a scoop isn't either the only way to get a pressure increase, on a car there is high and low pressure zones when the car is moving, caused by the air velocity around the car. Typical high pressure zones are around the front bumper and at the rear of the hood close to the windshield. Typical low pressure zones can for example be found at the corners of the car and at the front part of the hood. Of those reasons you can usually find air intakes (both for engine and cooling) at the high pressure zones of a race car, while outlets (for cooling air as one example) are found at low pressure zones. For example NASCARs have air inlets close to the windshield and super touring cars often use the inlets in the middle of the front like the grill.

Note that a turbocharger, even at idle speed are producing a pressure which is slightly higher than in the intake. The only time the pressure in the intake after the turbocharger is lower than the outside pressure is when you increase the throttle fast.

Also note that you slow down the air by increasing the area of the duct the air is traveling through, so people that are trying to use some funnel like intake have got it all wrong.

[CBFryman]

WOW....Physics 1 came in handy....lol...2 years ago i never would have understood that...and i go t oa PUBLIC SCHOOL