Pods or Airbox--an Anthology

[oseaghdha]

.........

A tuned intake works in much the same way on the intake side as the pipe does on the exhaust side. In fact, the tuned intake can utilize exhaust frequencies. A tuned intake will be a specified length, complete with expansion and compression chambers, used to alleviate parasitic HP loss on the cylinder. None of these components exist on stock XS.

Slow down there, Sparky.

The XS intake is an individual runner (IR) design(YICS notwithstanding). From the valve to the tip of the velocity stacks is one continous path tuned for a specific RPM. When we start dikkin up the exhaust and intake lengths, then the problems set in, and we have to start tuning it outside of the design parameters.

Point being; it is absolutely a tuned intake design in the stock configuaration, within the technology available at the time, and the compromises made for the mass market.

[trbig]

I don't have pods for performance. I have pods because I have an XJ. Anyone who has taken the carbs off of an XJ will understand that this is enough said.

That being said, pod filters without any sort of velocity stack are going to perform worse than a stock air box or velocity stacked pod filters. The air/fuel jets on the carbs were designed to have air shoved directly straight down their throats. Look at the front bell of a carb and notice all those jets around it. Now imagine going down the road... your carbs on the outside (1 and 4) are going to draw more from one side... the outside... than in from between it and the other carb. Add crosswinds, swirling air over/around the motor, etc., while going down the road, and you have a formula for a tuner's nightmare.. constantly changing conditions. You are going to have a mess of air being drawn in from different directions around the bell. Sure, you'll get your 275.25cc per cylinder, but which side of that bell did it come from? Did it draw from the bottom side and let your slide drop some? How about from the top and made it raise up too much?

They all aren't created equal and less restriction isn't the deciding factor.


Tod

[T99Ford]

From the valve to the tip of the velocity stacks is one continous path tuned for a specific RPM.
These are velocity stacks, not a tuned intake. The velocity stacks go into a box. This is not a tuned intake. Maybe some here should look that up and learn about tuned intakes first. This is simply an optimized simple intake. If it was tuned for a specific RPM, it would not be very streetable, now would it?
Of course it was tuned for stock, since the Yamaha people wanted to sell something that would run. This does not mean optimum, or maximum power. It means useable and reliable power that does not need to be messed with much.

Tod-
I only use pods because my stock box was junk, and the box is ugly (IMHO). If everyone who has gotten so upset will look, they would realize that wehn I started this post, I said that there is very little HP, if anything, to be gained from running a bike one way or the other. I am not championing the causes of either. It is just an air filter! I have ended every post this way, and still people miss it! Evidently, the 'box and pod camps are too firmly entrenched to reason evenly. I have had enough, and will move on.
Thanks for all your time.

[Larrym]

De-NIED!!

Drat!

And I was hoping to come back to my computer after a couple hours and find that particle filtration (Microns...), durability, and maintenance were being discussed.

After using a POD on my dirt-street bike these issues reared their ugly head when I saw the scoring on the cylinder walls in a "tear-down."


That'll learn me to walk away from my laptop and ride my bike!

[Ivan]

Tuned intakes are pretty easy to figure out, there is a formula on how much volume the box needs for what RPM and how much air moves through it and such. Complicated for the layman but cake for the average engineer. I have seen the formula, but can't remember where for the life of me.

I do remember that it changes based on what you want the engine to do, produce maximum torque, or HP, or economy. This is why modern vehicles have huge plenums under the hood and a set of butterflies that control wether the engine uses the whole thing or the short part. Usually called Intake Manifold Runner Control. In a way it gives the best of all worlds.

On the XS, I am guessing the airbox is tuned somewhere around torque and economy. It certainly isn't about all out horsepower, since that configuration can make drivability suffer.

[Larrym]

Newton's apple

Ivan,

" since that configuration can make drivability suffer."

And by that do you mean the driver/rider needs velcro sewn into his pants in order to stay on the bike during acceleration?

[XJOK2PLAY]

It certainly isn't about all out horsepower, since that configuration can make drivability suffer.

Hey, Larry... along the same thought...

I recently read an article in the Aug. '09 issue of Cycle World where Hendrik von Kuenheim, general director of BMW motorcycles was discussing their new BMW S1000RR. He was quoted as saying, "We have seen the end of the horsepower race... the future is in driveability." Horsepower ratings on this one are 193 @ 13,000 rpm, @ the rear wheel.

Interesting article... they go into all the extreme engineering they've put into this bike , focusing on max power vs. rideability.

So, Ivan's got it right... IMHO... good call Ivan.

And... as for this particular bike, SUPER VELCRO'S probably the ticket...

[oseaghdha]

Of course it was tuned for stock, since the Yamaha people wanted to sell something that would run. This does not mean optimum, or maximum power. It means useable and reliable power that does not need to be messed with much.

Pretty much what I said.

[Larrym]

Twitchy?

Yep. For those that have operated any "uber-powered" vehicle the drive-ability is a factor. My first driving experience was in a Super Bee with three 2 barrel carbs sticking out of the hood. Letting up off the gas pedal between shifts seemed to be important because when I let out the clutch the wheels would just start spinning again but in the next gear.

I was anticipating those with pods to put forth that the throttle response would be "quicker". You know, a smaller twist of the grip would have a faster response? Maybe I'm just connecting the dots wrong but I would have thought that without the airbox to act as a "dampener" this would happen. Great for zooming up and back from the guy ahead of you but maybe a little too "twitchy" for comfort when you're trying to just maintain a steady speed.

I dunno.....I've got the stock airbox and thus far haven't ridden with another XS with Pods.

[DGXSER]

No. Just as Tod stated, it will be replaced by vacuum, unless it can come through the rings or other leak. Because time is a factor. The cylinder will not necessarily fill enough to replace the vacuum already present at a given RPM.

But the higher the vacuum the more it overcomes any restriction to the "air" getting into the cylinder and will draw the mix faster, therefor the higher RPM just means higher velocity through the carb, raising the slide, lessening the restriction slowing the air by giving it more volume to move through, thus the term Constant Velocity carb.

If this were water, and the restriction reduced a gallon per minute (to represent atmospheric pressure) to one drop per minute (extreme), and you had a displacement of one gallon, you would not fill the restricted cylinder as fast in RPM of one per minute as you would the cylinder with no restriction getting the full one gallon per minute.

You would be absolutely correct if the pressures remained the same at the changing RPMs. What happens though is that the higher vacuum overcomes the restriction to the water flwoing through the fixed opening so what was one drop per minute at the lower vacuum will increase back to the gallon per minute at the higher vacuum.

A tight cylinder would fill with mostly vacuum. It would not fill with its displacment in water, not matter how much horsepower was involved, since it can only go one RPM, and in that time it only gets one drop, not one gallon. This is because you cannot increase atmospheric pressure or vacuum!

If you completely enclose a system like say your air conditioner on your car, and let the pump run like they do the vacuum pump when they fill your system with freon, it will draw a vacuum. however, if you have ANY opening in it, it will draw air through it, the higher the vacuum, the faster it draws the air so you eventually can hear where it is leaking. As stated before that one drop rate at the changing pressure, lower vacuum, will increase to the one gallon rate at the higher RPM given the same opening to go through. If it did not the vacuum would continue to increase with each RPM until the engine could not provide the HP required to pull anymore and stall out.

Clearly you do not understand tuned intake theory. This is not something that is done by an operator, such as jetting. A tuned intake works in much the same way on the intake side as the pipe does on the exhaust side. In fact, the tuned intake can utilize exhaust frequencies. A tuned intake will be a specified length, complete with expansion and compression chambers, used to alleviate parasitic HP loss on the cylinder. None of these components exist on stock XS. Further, the technology was not implemented until the middle 1980's, after the XS was discontinued. That is pretty clearly documented in any history of sport bike development. The early airboxes were a place to store the filter, and were primarily introduced to reduce noise pollution. Taking a tuned intake off a bike will reduce midrange power, as it increases parasitic HP loss on the cylinder. While the physics still apply, they are much exaggerated when exhaust and intake tuning are applied to simple intake theory. The reason for this is that both the pipe and the tuned intake reduce (sometimes greatly) the amount of parasitic loss on the engine. The naturally occuring intake and exhaust frequencies are utilized to do this. I believe that the XS operates under simple intake, with the only benefits coming from the exhaust; that is, unless someone has undertaken to develop a tuned intake for this bike (haven't seen one).

I was merely asking you give the same level of proof you asked for, since you stated originally you did not want to talk opinion or assumption. As to the intake design. Again, it is about velocity and the pressures it creates, same with the exhaust. Pressure or the lack of it moves the air and gasses. The piston operating up and down creates those pressures. The more pressure it takes to move them, the more hp is consumed to do so. So I would suggest, that the engineers at Mother Yammie looked over the system and selected the pipes and tubes leading to the carbs, from the carbs and out the exhaust to match the engine and the selected carbs. That is tuned in my opinion.

I think in the end we are all arriving at the same conclusion, that the difference between the two is so minimal (maybe even undetectable) that further argument is unnecessary. If pods or filters made any significant change, we would all move to the one that did. Since there are devout followers of both, I still maintain that neither can outdo the other on a stock bike. I hope no one is getting mad here, since I am only trying to review closely the actual effects of running either.

I am not offended or upset by the statements you have made. You just struck an area of physics where I live and breath for a living and kicked in the Anal Retentive Engineer in me, so I HAD to repsond. I'm just enjoying thinking through the theory here. I have no opinion as to pods versus air box really. I like my air box cause it adds to the shineyness of my bike, its sparkly...and I do not have to fight to find the right jetting to make it work like i have seen others battle with when adding pods. I know several folks that run pods and love them, primarily because it is so simple to remove and replace the carbs. Of course it seems as though the jetting and rejetting caused by the pods is the reason they need to remove and replace the carbs os much. Then again, some ar enot as lucky as I am to have a stock exhaust that is in good shape, so they had to change and went to 4-1 that also effects the overall pressure relationships. Again, no books here, we are close enough with this thread already.

[T99Ford]

quote: But the higher the vacuum the more it overcomes any restriction to the "air" getting into the cylinder and will draw the mix faster, therefor the higher RPM just means higher velocity through the carb, raising the slide, lessening the restriction slowing the air by giving it more volume to move through, thus the term Constant Velocity carb.

quote: You would be absolutely correct if the pressures remained the same at the changing RPMs. What happens though is that the higher vacuum overcomes the restriction to the water flwoing through the fixed opening so what was one drop per minute at the lower vacuum will increase back to the gallon per minute at the higher vacuum.

One More:
This would be absolutely correct, except that vacuum tops out at around 29.92 inches of mercury at sea level (about 26 at my altitude, as measured on my Air Conditioning vacuum gauge while evacuating many A/C systems), you Anal Retentive Engineer! A restriction greater than this will not be affected by max vacuum. This is for the pure theory of it, though. A running engine will not achieve anywhere near max vacuum (more like 12-16 in, at my altitude, and depending upon engine condition). And I doubt that any marketable motorcycle air filter could achieve that serious of a restriction. Any problems are only going to be seen at a racing level of performance. Of course raising the slide (more throttle) reduces the throttle body restriction (for a time), and lowers post carb manifold vacuum. Anybody who ever had a car with vacuum wipers will remember what a joy trying to run down the highway in the rain AND accelerate was. I understand that the vacuum affect will remain the same to a point through the carb, (increased volume as an effect of higher RPM) but only until the slide is fully raised. The problem then is the lag in time needed to regain the effect of the vacuum. This lag is also a problem at lower midrange speeds, and can be affected by the shape of the slide, etc. This is why racers never used CV carbs. Too much time is wasted by the vacuum lost to operate the carb. Flat slide and other style carbs respond quicker, but are usually seen as too twitchy for street riding. But now we are to the effects of vacuum on the carb, and not the filter.

OK, now I am really done...for now...

[Ivan]

Ivan,

" since that configuration can make drivability suffer."

And by that do you mean the driver/rider needs velcro sewn into his pants in order to stay on the bike during acceleration?

No, usually that means the only thing that works is full throttle under load. Ever hear of a top fuel dragster blow the heads off the engine when the driver feathers the throttle? That is an extreme case, but is an example of trying to use "drivability" skills on a pure "horsepower" engine. Probably on these scoots, it would end up with a low end stumble that is almost impossible to tune out with the stock carbs..... (Twilight Zone theme song plays, fading away)

[cywelchjr]

And I doubt that any marketable motorcycle air filter could achieve that serious of a restriction. Any problems are only going to be seen at a racing level of performance. Of course raising the slide (more throttle) reduces the throttle body restriction (for a time), and lowers post carb manifold vacuum. Anybody who ever had a car with vacuum wipers will remember what a joy trying to run down the highway in the rain AND accelerate was. I understand that the vacuum affect will remain the same to a point through the carb, (increased volume as an effect of higher RPM) but only until the slide is fully raised. The problem then is the lag in time needed to regain the effect of the vacuum. This lag is also a problem at lower midrange speeds, and can be affected by the shape of the slide, etc. This is why racers never used CV carbs. Too much time is wasted by the vacuum lost to operate the carb. Flat slide and other style carbs respond quicker, but are usually seen as too twitchy for street riding. But now we are to the effects of vacuum on the carb, and not the filter.

Only, CV carbs don't raise the slides to increase throttle, they use butterflies just like automotive carbs, the slides are simply to maintain a constant air velocity through the venturies, and they are raised by an increase of vacuum above the slide diaphrams caused by increased air flow past the bleed hole in the bottom of the slide. There is a video on here where someone (TC I think?) did a video of the slides under different load levels while riding. It gives a better understanding of what the slides are actually doing.

[T99Ford]

Woops! Sorry about that, in all the side notes, I got a little confused and forgot which carb I was talking about!

[WMarshy]

Hi guys, very interesting topic you have started here. IMO the intake system on our XS’s have been “tuned”. Although its definitely no where near as sophisticated as modern tuned intakes I would be inclined to believe Yamaha had performance in mind when they decided what length to make the velocity stakes ect... Just me 2 cents, you see where I stand on the subject.

So… Im about to embark on a design project regarding this very topic. Think infinitely variable intake runner designed to optimize harmonic frequencies of the intake. To give you some technical information regarding this topic, look for Helmholtz’s acoustic theory. Here are a few sites I found during a quick search.

http://www.team-integra.net/sections...?ArticleID=471

http://www.eng.fsu.edu/~shih/eml4421...takeTheory.htm

To give you some idea of my project, Im on the SAE Formula Racing team at Rochester Institute of Technology in Rochester NY. Here is a picture of the car we built this past winter. We took second place in Michigan speedway out of 90+ teams, first place at California International Speedway again out of 80+ teams, and IIRC 23 place in Germany for our international competition in a field of 120+ teams (got DQ’d in our endurance race.. that’s another story in itself..) I hope to build a working prototype in 10 weeks… wish me luck!