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Discussion Starter · #1 · (Edited)
A basic primer on Ram-Air Induction Part 1

There is a lot of misunderstanding about how Ram-Air works, and a whole lot of misinformation floating around. Most of the things you see on the internet calling themselves "ram air induction" are in fact, just cold air intakes. While there's nothing wrong with a cold air intake, and it can in fact increase performance, it is not in any way forced induction.

Some definitions:

Forced Air Induction: Any method where the intake system of an engine is pressurized. This can be done with a compressor of some kind such as a turbocharger or a supercharger, or with a diffuser. In this case, "induction" means "intake."

Diffuser: A diffuser is a static mechanical device similar to a venturi, that converts fast moving low pressure air into slower moving air at a higher pressure.

Cold Air Intake: A tube or manifold that moves the point of intake to where it can take in the coldest air possible. Usually as far forward as possible and in front of any obstacle.

Turbulence: Violent swirling and chaotic movement of air.

Laminar flow: Air flow in nearly straight lines. (Opposite of turbulent) (Laminar means "layers.")

Pressurized air box: I've used this term throughout this article, even though pretty much no Ram Induction will actually pressurize the airbox to above ambient pressure (with the throttle open) until you reach nearly 200 MPH. That said, it is possible to lower the vacuum inside the airbox, which is a good thing. The lower the vacuum inside the airbox or rather, the more air there is in there at any given time, the easier it is for the piston to draw a full charge into the cylinder.



The basic idea of Ram-Air induction is to force more air through the throttle bodies by using the forward motion of the bike. This makes all sorts of sense, since it is essentially free horsepower. The more air/fuel charge you can get into the cylinder the more horsepower you can get. Anyone who has ever held their hand out a window on the freeway can feel the pressure the movement of the vehicle makes through the air. The question is how do we convert that pressure into more air in the cylinder.

The first common misconception of Ram-Air is that just pointing a big hole forward will do the trick. This is completely wrong, even though it seems like common sense. Common sense tells us the more air we can direct into the airbox the better. But, just like running a marathon with too many people on the street, everyone ends up running slower. The actual forward-facing hole must be about the same area as the total area of the throttles or intake ports.

(It's not exactly, but the math is rather esoteric and I don't understand it well myself. But Area=Area is a good rule of thumb.)

Actually, the faster you go the smaller the diffuser needs to be! This is why high-speed fighter jets even back into the 50's had variable size diffusers on their intakes. As their speed increases their intake hole gets smaller. Sometimes this is done with a cone that moves forward and back on the intake (such as on the Blackbird) or sometimes it's a sort of flap that moves (such as that on an F-15.) This is the purpose of the "spike" you see in old fighter jets on their intake.

In the factory intake, the hole is much like figure 1. It's usually attached to some kind of rubber intake horn, but this has more to do with sound deadening than anything else. The hole is optimized to the acoustical qualities of the airbox. It may not be ideal, or it may produce higher horsepower numbers by being modified slightly, but probably it will do so at the expense of other engine qualities. It's a good compromise.

However, just pointing this hole forward doesn't give you any advantage at all. Even if you attach a cold air pipe to it and run it out into the breeze. The turbulence around the intake will stop any extra air from entering the airbox, and what air does come in will be little different from ambient air at a stop.






RamAir.png
 

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Discussion Starter · #2 ·
Part 2



The second misconception is that a funnel shaped intake with the bell facing forward will increase pressure. This is also completely wrong. In fact, this is the worst thing you can do to an intake system.

See figure 2 in the accompanying illustration. When air goes into a funnel from the bell end, it becomes turbulent. This has the effect of knocking streams of air out of the bell, but also of plugging the hole with turbulent air. That turbulence causes increased air pressure in front of the intake! It actually stops air from entering the airbox.

Turbulence anywhere but inside the combustion chamber is a bad thing. (With one exception that we'll get into at the end of this article.) Attaching a funnel to your intake tract such as the illustration shows can and will actually cause a slight increase in airbox vacuum. It's more of a restriction than the factory hole, even if you make the small end of the funnel larger!

A third misconception is that cutting the front off the airbox will "increase flow" and therefore increase horsepower. In some cases enlarging the intake hole can indeed increase power, but this is a matter of knowing exactly how big the intake hole needs to be. Go too big, and you get a situation like figure 3. To begin with, most modern airboxes are acoustically tuned to enhance the characteristics of the particular engine they are designed for. Sometimes cutting a larger hole helps, but more often than not it degrades performance. However, removing too much has another problem.

If you remove too much of the airbox in search of better "flow" (which most people don't properly understand) you start getting the Figure 3 problem: moving your turbulent airflow inside your airbox. This is almost as bad as attaching a funnel to the intake. The turbulent flow in front of the throttle (or velocity stacks) impedes the airflow into the cylinder. Again, in search of "more flow" you are in fact losing horsepower. The turbulent flow plugs the airbox and even though it's more open, less air gets in.

Flow is important in the intake of a motorcycle, but it has to be the right kind of flow. Ideally, it should be laminar flow. Laminar flow is airflow that is in nice straight lines and layers, with little or no turbulence. Turbulence is an energy waster, and it tends to fight itself at restrictions.

Air has mass, just the same as water does. That mass is what you feel pushing on your hand when you hold it out the window of your car. That mass has inertia. The inertia means that it wants to keep moving in a particular direction unless it is effected by another force. If you place an obstacle in its way, it tends to start swirling and it loses energy and speed. That energy and speed is transferred (first law of thermodynamics) to the following air coming in, slowing it down. Therefore, the less turbulence you have the more air you can pack into the airbox and the higher pressure you can achieve.

Now take a look at figure 4. This is a properly designed diffuser attached to the airbox. Notice it is similar to a funnel, but it's reversed. The bell end is towards the throttle. This seems backwards, but it's not. The whole point of a diffuser is to take fast moving air, and turn it into slower moving but higher pressure air. Notice that the intake end of the diffuser is still about the same area as the throttle. However, as it moves towards the throttle it gets bigger. Mathematically, it should ideally get 2%-3% bigger for every foot in length.

What this does is slow down the air flow, and allows it to pack up inside the airbox. Bingo! Pressure. Which was what we wanted all along. We're converting high speed air into pressure. Before, with the funnel intake, we were producing that pressure outside the airbox and plugging it up. Now we have it inside the airbox where we want it.

But this diffuser has another advantage. Notice in figure 4 the length. It sticks out in front of the bike. This is very important. Any time air hits a surface of a bike at speed, it produces turbulence. Remember turbulence is a bad thing. If that turbulence happens anywhere near the intake hole, it tends to plug it up and slow down the airflow into the airbox.

However the intake of this diffuser is out in front of the turbulence caused by the bike. It only gets laminar air flow as the bike moves through the air. It is properly designed. A good example of this kind of ram-air diffuser is the Triumph Speed Four. It's the only bike I know of that actually gets it completely right.

Finally there's one more kind of ram-air intake that you sometimes see, mostly on race bikes. It's a little different from a typical diffuser style. This is because it works on an entirely different principal. Instead of using the flow of air into a diffuser to pressurize the airbox, it actually uses the [i[turbulence itself[/i] to pressurize the airbox. This is a very technical process, and it only works on fully faired bikes. The Daytona 675 uses a form of it, as well as a lot of the other race replicas. It has to do with the shape of the intake on the fairing, as well as the position. The energy in the turbulence caused by the fairing itself is converted into laminar flow by the shape of the intake, thus producing increased airbox pressure.


If anyone finds anything wrong with this article, or anything isn't explained quite right, please let me know and I'll be happy to alter it.

Later we can take up velocity stacks, and discuss why they differ from this concept.

Cheers!

Crash
 

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Discussion Starter · #3 ·
Speed Four Diffuser:



F-15 Variable intake:



Center spike variable jet intake



A drag racer with a well designed diffuser type ram induction cowl. Note how it gets larger in volume the further away from the intake hole it gets.



This drag racer, on the other hand, has nothing more than a cold air intake.



The business end of an F86 Saber fighter jet. Note the small intake that gets larger as it moves towards the engine.



Classic MiG21 with variable spike intake. If you look at various pictures of the thing in flight, you can see how the spike is further in or out depending on speed.

 

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Crash i feel like your concept is pretty good but i dont feel like it accounts for the fact that airbox itself is under vacuum. Wouldn't the airflow find the path of least resistance. The turbulent parts of the airbox would just create air pockets reducing the effective volume of the airbox. kind of like how airflow works over a pickup truck bed. pickup-taildown.img_assist_custom-320x240.png
I'm not sure about the older triples, but i do know the 2011+ speed airbox and intake are quite turbulent on their own. i was under the impression this was for the purpose of intake noise reduction.
 

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Discussion Starter · #6 ·
I wonder what S4 style piping would do to the Triple's intake noise.

I'd imagine if the trumpets are no longer facing each other like in stock config, some intake sound goodness must be able to escape.

Ram Air Speed Triple - Sport Evolution

View attachment 17193
View attachment 17194
Those could be considered cold intakes rather than ram-air, although they may work slightly. As for sound, it's possible they could decrease intake noise but I suspect they wouldn't change it much. Dunno for sure, but I know the 11+ Speed Triples and my Street Triple have forward facing intakes (behind the headlights) and they both still have a lot of intake noise. If you really wanted to reduce intake noise, I suspect you'd have to put some noise insulation around the airbox itself and maybe the throttle bodies. That's my guess anyway.


Crash i feel like your concept is pretty good but i dont feel like it accounts for the fact that airbox itself is under vacuum. Wouldn't the airflow find the path of least resistance. The turbulent parts of the airbox would just create air pockets reducing the effective volume of the airbox. kind of like how airflow works over a pickup truck bed. View attachment 17192
I'm not sure about the older triples, but i do know the 2011+ speed airbox and intake are quite turbulent on their own. i was under the impression this was for the purpose of intake noise reduction.

I know less about airbox design, so I'm not going to comment. Yes, the airbox is under slight vacuum when the throttles are open, as I mentioned in the Definitions section. I do know that this is part of how the Daytona style ram induction works, but that it doesn't work on naked bikes. But that's about all I know about it.
 

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All very interesting to read about. Out of curiosity, are you actually trying to make adjustments to your bike or is this all pure informative? Those intakes on the Speed Four are kind of cool.......
 

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Discussion Starter · #8 ·
Nah, no changes. It's just that every once in a while someone makes some bogus claims about "ram induction" or some such and I get tired of explaining it. So I figured since I had time yesterday I'd just write it all up and post it so I could just link to it.
 

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BMW engineers stated that the ram air on the s1000rr actually didn't create any more pressure until 156mph. And the daytona 675 has a flapper right in the hole of its intake.

good write-up
 

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Discussion Starter · #10 ·
BMW engineers stated that the ram air on the s1000rr actually didn't create any more pressure until 156mph. And the daytona 675 has a flapper right in the hole of its intake.

good write-up
Thank you.

Back in the mid 90's the Kawasaki ZX11 was ostensibly the first street bike to have Ram Air Induction. Until it got up to ludicrous speed it didn't do anything. (I think they said 180MPH) They also found that due to the placement of the intakes, one on each side of the fairing, the engine would surge at speed in any kind of cross wind. These days most things with Ram Induction have a single front mounted intake like the D675.
 

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:sign5: I rather enjoy the extra intake snort when compared to my last bike.
 
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