Pod Filters

[Alphamazing]

I made a post in some thread about pod filters taking off bottom end power. A few people came back and told me I was wrong but I didn't think so. I did some research and got some theory behind it, proving my point.

First things first, air is moved into the cylinder when the intake valve opens and the piston begins moving downwards in its path, creating a vacuum and drawing air into the cylinder. You can't force air more air than the cylinder can hold without turbocharging or superchargine, but you can change the rate at which it fills.

The airbox creates a small pressure chamber from which the cylinder draws air from. In the example above we assume equal pressure on both sides of the valve, so that when it opens the only thing that can cause air to move is the movement of the piston creating a vacuum. However, suppose that there is a slight positive pressure on the airbox's side. When the valve opens the air will start moving into the cylinder without the piston even moving due to differences in pressure. This allows you to fill the cylinder faster than you would without the pressure chamber.

Another way to move the air into the cylinder quicker is to have a mass in motion. Think about it this way. We've got a very long tube, and on one end we've got a simple engine. As the piston moves downwards there is a mass of air that begins to travel down the tube. Now suppose we stop the source of the vacuum; that mass of air is still traveling down the tube towards the engine. When that mass of air moves towards the engine it has a force associated with it, and when it gets to the engine and the intake valve opens again that mass of air is pushed into the cylinder, filling it faster than the first example. The cylinder also started another mass of air moving when it opened up that second time, thus allowing the process to be repeated.

You can also have the cylinder fill quicker at high RPMs by removing the breather restrictions. As the engine makes more RPMs it needs more air pulses (as the mass in motion example above) per minute as well. When you remove the restrictions (for example the airbox) it allows air to move through the intake track quicker, thus allowing the cylinder to fill with air quicker. This sacrifices the pressure chamber, which as described above doesn't allow you to fill the cylinder as quickly at lower RPMs.

You must be able to fill the cylinder quickly for low end torque.
You must be able to fill the cylinder quickly for high end power.

One has rather long (and therefore restrictive) intake and exhaust tracts (relative to the operating RPM) that produces excellent low end. The other has reduced restrictions for high end power, always with a loss in low end.

Consider low peak HP with very high torque (common in cruisers) to very high HP, low torque racing engines that depend heavily on high RPM to develop any power at all.

Yamaha has this thing called the EXUP valve in their exhaust system which creates a back pressure of air at low RPMs, pushing air into the cylinders faster, but it opens up at high RPMs to allow for freer flowing air, creating more power.

Pod filters remove the airbox, therefore removing the pressure chamber it creates as well as shortening the intake tract. That shorter intake tract allows for easier engine breathing at high RPMs, but poor performace at low RPMs. Jetting can correct the air/fuel ratio, but it cannot create airflow. Pod filters decrease your bottom end. Period. At the drag races no one cares about bottom end. We ride on the street where bottom end plays a major role in our daily commutes. Sketchy power delivery at the bottom end is typically not something you'd want when you're going about at parking lot speeds, or extremely tight turns in the twisties. That low end torque is why twins dominate the inline-4s in the twisties.

Summary: Pod filters DO take away your bottom end.

[Roadstergal]

Summary: Pod filters DO take away your bottom end.

Mine's still there.

See avatar.

[Alphamazing]

:lol:

And what a fine bottom end it be, lass!

[Alphamazing]

But seriously. Someone tried to give me VERY misleading information that was VERY wrong.

[Mk1inCali]

While I agree with your reasoning, I have ridden a pod filtered and jetted and full aftermarket exhaust'ed GS before, granted...it was setup for much higher elevation, and had the new style 01+ carbs...but that had just as much low-end as my rejetted and holes drilled in the baffle '00 does/did.  We drag raced, and while my shift speed and timing was much better, and I weighed 50lbs less than the other rider, I was only a car length or two ahead of him at about the 1/4 mile mark.

Rode it around in traffic, and it wasn't apparent that it had been effed with, it was tractable and smooth and wasn't abrupt or harsh or anything negative.

I know for the really high-end Nissan RB series motors, when they go past 5-600 HP, the tuners like to go to shorter and wider sheet-metal intake tracts, compared to the stock cast alum. pieces.

Very interesting phenomena (sp?) occur at those 8-10K engine rev. speeds.

[Roadstergal]

The only way to know for sure is to run the same bike with stocker and with pod on three per (for n) back-to-back runs on the same dyno on the same day.

The difference, up or down, is likely to be far too little for anyone other than Rossi to grok.  Butt dynos are notoriously inaccurate.

[JetSwing]

another futile argument...

provide a concrete evidence or shhh....  

[sprint_9]

I disagree, I think the bottom end stays the same, if you have things jetted right.  It feels weaker because you gain so much at the top.  You have to have it jetted right though.  I ask then, if it does take away then why do all the dyno charts show either the same or a slight increase through the low rpm range, if they do lose, then it is for like 1-2k rpms and its like 1 pt.

[Alphamazing]

While I agree with your reasoning, I have ridden a pod filtered and jetted and full aftermarket exhaust'ed GS before, granted...it was setup for much higher elevation, and had the new style 01+ carbs...but that had just as much low-end as my rejetted and holes drilled in the baffle '00 does/did.  We drag raced, and while my shift speed and timing was much better, and I weighed 50lbs less than the other rider, I was only a car length or two ahead of him at about the 1/4 mile mark.

In drag racing, as I noted in my original post, the bottom end is essentially useless. Your drag race test doesn't prove anything.

I disagree, I think the bottom end stays the same, if you have things jetted right. It feels weaker because you gain so much at the top. You have to have it jetted right though. I ask then, if it does take away then why do all the dyno charts show either the same or a slight increase through the low rpm range, if they do lose, then it is for like 1-2k rpms and its like 1 pt.

You can disagree, but the science is there. While you may be right that it is not a noticeable decrease, saying that it increases is wrong.

As I said in the first post, jetting can correct air/fuel ratio, but it cannot create air flow.

[Roadstergal]

All of this is meaningless.  Back-to-back dyno runs or nuttin', folks.

[JetSwing]

All of this is meaningless.  Back-to-back dyno runs or nuttin', folks.

+1,000,000,000

[The Buddha]

I made a post in some thread about pod filters taking off bottom end power. A few people came back and told me I was wrong but I didn't think so. I did some research and got some theory behind it, proving my point.

First things first, air is moved into the cylinder when the intake valve opens and the piston begins moving downwards in its path, creating a vacuum and drawing air into the cylinder. You can't force air more air than the cylinder can hold without turbocharging or superchargine, but you can change the rate at which it fills.

The airbox creates a small pressure chamber from which the cylinder draws air from. In the example above we assume equal pressure on both sides of the valve, so that when it opens the only thing that can cause air to move is the movement of the piston creating a vacuum. However, suppose that there is a slight positive pressure on the airbox's side. When the valve opens the air will start moving into the cylinder without the piston even moving due to differences in pressure. This allows you to fill the cylinder faster than you would without the pressure chamber.

Another way to move the air into the cylinder quicker is to have a mass in motion. Think about it this way. We've got a very long tube, and on one end we've got a simple engine. As the piston moves downwards there is a mass of air that begins to travel down the tube. Now suppose we stop the source of the vacuum; that mass of air is still traveling down the tube towards the engine. When that mass of air moves towards the engine it has a force associated with it, and when it gets to the engine and the intake valve opens again that mass of air is pushed into the cylinder, filling it faster than the first example. The cylinder also started another mass of air moving when it opened up that second time, thus allowing the process to be repeated.

This is an excellent argument for longer but straighter intake manifold tracts ... Doesn't quite translate to removing the airbox ... In fact removing the airbox creates a better positive pressure this alludes to ... Remember ... in the air box with the airfilter in ... there is negative pressure ...

You can also have the cylinder fill quicker at high RPMs by removing the breather restrictions. As the engine makes more RPMs it needs more air pulses (as the mass in motion example above) per minute as well. When you remove the restrictions (for example the airbox) it allows air to move through the intake track quicker, thus allowing the cylinder to fill with air quicker. This sacrifices the pressure chamber, which as described above doesn't allow you to fill the cylinder as quickly at lower RPMs.

You must be able to fill the cylinder quickly for low end torque.
You must be able to fill the cylinder quickly for high end power.

One has rather long (and therefore restrictive) intake and exhaust tracts (relative to the operating RPM) that produces excellent low end. The other has reduced restrictions for high end power, always with a loss in low end.

Long and restrictive dont necceraily mean the same thing ... Longer is better yes, but restricting the flow rate is just counter productive.

Consider low peak HP with very high torque (common in cruisers) to very high HP, low torque racing engines that depend heavily on high RPM to develop any power at all.

Yamaha has this thing called the EXUP valve in their exhaust system which creates a back pressure of air at low RPMs, pushing air into the cylinders faster, but it opens up at high RPMs to allow for freer flowing air, creating more power.

No exup doesn't do that ... exup makes the muffler more restrictive at low revs, and that prevents the massive mix loss they were having due to having 3 intake valves and 2 exhaust ... its an argument for a restrictive exhaust for bottom end, and a tenous link to a bike that got two small valves filling a chamber ...

Pod filters remove the airbox, therefore removing the pressure chamber it creates as well as shortening the intake tract. That shorter intake tract allows for easier engine breathing at high RPMs, but poor performace at low RPMs. Jetting can correct the air/fuel ratio, but it cannot create airflow. Pod filters decrease your bottom end. Period. At the drag races no one cares about bottom end. We ride on the street where bottom end plays a major role in our daily commutes. Sketchy power delivery at the bottom end is typically not something you'd want when you're going about at parking lot speeds, or extremely tight turns in the twisties. That low end torque is why twins dominate the inline-4s in the twisties.

Summary: Pod filters DO take away your bottom end.

Intake tract counts mainly between carbs and engine, not between carbs and air box ... pressure in the air box is negative and there will be lesser forward air flow with it ... Pod will make it rev up quicker ... making you not want the bottom end ... the top is so much better ... Jet it and see ... if still not right, fiddle to get it better ... eventually you'll know ... butt dyno is better as is real dyno ... twins dominate the twisties is low end, however ... pods will not create or destroy it much ...
Cool.
Srinath.

[juno]

Doesn't the older GS already have crap for low end anyway?

[Alphamazing]

This is an excellent argument for longer but straighter intake manifold tracts ... Doesn't quite translate to removing the airbox ... In fact removing the airbox creates a better positive pressure this alludes to ... Remember ... in the air box with the airfilter in ... there is negative pressure ...

Positive, negative, whatever. There is still a change in pressure that causes air to move without the piston moving. This is lost with the pod filters as the pressure chamber is removed.

Long and restrictive dont necceraily mean the same thing ... Longer is better yes, but restricting the flow rate is just counter productive.

Yes, restricting the flow rate up top is counter productive as the engine needs more air flow up top, but if you put pod filters on you SHORTEN the intake tract and DERESTRICT the airflow, thus allowing better high end power and breathing. This translates to a loss in low end power.

No exup doesn't do that ... exup makes the muffler more restrictive at low revs, and that prevents the massive mix loss they were having due to having 3 intake valves and 2 exhaust ... its an argument for a restrictive exhaust for bottom end, and a tenous link to a bike that got two small valves filling a chamber ...

Nope. The EXUP valve changes airflow through the exhaust tract in order to provide freer movement of gasses at high RPMs and restricts them at lower RPMs. In order to gain optimum exhaust efficiency you need a wider pipe at higher RPM compared to lower RPM. The EXUP valve essentially changes the diameter of the exhaust pipe.

Intake tract counts mainly between carbs and engine, not between carbs and air box ... pressure in the air box is negative and there will be lesser forward air flow with it ... Pod will make it rev up quicker ... making you not want the bottom end ... the top is so much better ... Jet it and see ... if still not right, fiddle to get it better ... eventually you'll know ... butt dyno is better as is real dyno ... twins dominate the twisties is low end, however ... pods will not create or destroy it much ...
Cool.
Srinath.

Intake tract counts EVERYWHERE. Even if the pod makes it rev up quicker there is still a loss of power in the bottom end. And when I'm puttering around at stop lights here on campus, or trying to weave my way through a parking lot I want my low end torque.

Butt dynos aren't as definative as a real dyno. If someone sends me a pod filter and jetting, I'll get dyno runs done.

[Blueknyt]

how is the airbox pressureized at the lower off idle and/or not moving?  i can see if fairings were ducted to act as a psudo ram air once your moving but thats not really the case with the GS.  the bottom end snappyness coming from a restricted airflow isnt so much from a pressurized airbox, but i think the fact the Air is moving faster do to restrictions. you can have a higher velocity aircharge with a restricted feed where as the open carbs/pods allow are larger quantity/volume of air to enter at a slower rate,  In effect the airbox acts kinda like a badly designed velocity stack.     velocity stack will take  a larger area of air and force it into a tighter area, when this happens the speed that aircharge travels increases to a point.(very crude discription is about all igot right now)

[The Buddha]

This is an excellent argument for longer but straighter intake manifold tracts ... Doesn't quite translate to removing the airbox ... In fact removing the airbox creates a better positive pressure this alludes to ... Remember ... in the air box with the airfilter in ... there is negative pressure ...

Positive, negative, whatever. There is still a change in pressure that causes air to move without the piston moving. This is lost with the pod filters as the pressure chamber is removed.

Long and restrictive dont necceraily mean the same thing ... Longer is better yes, but restricting the flow rate is just counter productive.

Yes, restricting the flow rate up top is counter productive as the engine needs more air flow up top, but if you put pod filters on you SHORTEN the intake tract and DERESTRICT the airflow, thus allowing better high end power and breathing. This translates to a loss in low end power.

No exup doesn't do that ... exup makes the muffler more restrictive at low revs, and that prevents the massive mix loss they were having due to having 3 intake valves and 2 exhaust ... its an argument for a restrictive exhaust for bottom end, and a tenous link to a bike that got two small valves filling a chamber ...

Nope. The EXUP valve changes airflow through the exhaust tract in order to provide freer movement of gasses at high RPMs and restricts them at lower RPMs. In order to gain optimum exhaust efficiency you need a wider pipe at higher RPM compared to lower RPM. The EXUP valve essentially changes the diameter of the exhaust pipe.

Intake tract counts mainly between carbs and engine, not between carbs and air box ... pressure in the air box is negative and there will be lesser forward air flow with it ... Pod will make it rev up quicker ... making you not want the bottom end ... the top is so much better ... Jet it and see ... if still not right, fiddle to get it better ... eventually you'll know ... butt dyno is better as is real dyno ... twins dominate the twisties is low end, however ... pods will not create or destroy it much ...
Cool.
Srinath.

Intake tract counts EVERYWHERE. Even if the pod makes it rev up quicker there is still a loss of power in the bottom end. And when I'm puttering around at stop lights here on campus, or trying to weave my way through a parking lot I want my low end torque.

Butt dynos aren't as definative as a real dyno. If someone sends me a pod filter and jetting, I'll get dyno runs done.

OK negative pressure is vacuum ... it prevents forward flow ... you want positive pressure ... too much negative and nothing is going in the chamber ... with Pod the motor loses that negative pressure making for a better fill ...
How would better high end automatically imply poorer low end ... filling it at high rpm is trickier than low rpm ...
Exup valve mainly helps the fuel loss situation at low rpm, and at high rpm you dont care ... besides, EPA testing is done at 3500 I believe ... and the exup was put in to help it clear all that ... BTW I have ridden a non exup 5 valver ... it runs fine, the catch was ... it had smaller pipe than the exup equipped one ...
Butt dyno thinks that the peaks in power delivery are good, and the bike makes more power, while the real dyno doesn't ... butt dyno may say the bottom end is great with Stock filter and air box, but real dyno will disagree ... anyway I can see beyond that from the bikes I have ridden and tuned ... I have dynoed with K&N and pipe as have others, and you dyno it with stock and we could compare ...
Cool.
Srinath.

[Alphamazing]

When the air is flowing (the air/mass in motion example) through the airbox, it defintely helps get the air moving into the next cylinder. They don't just put those airboxes there to hold the air filter.

People with modified airboxes or removed airboxes who jet to correct the air/fuel mix have found that it's extra rich at the bottom end in order for the bike to run correctly. It is best to set the fuel/air ratio for best power, then fill the cylinders quickly.

With pods the fuel/air ratio is set to about 11 - 11.5 so it will run okay at the bottom, but it will make the whole lower RPM range rich and weak. Good fuel/air power is 12 - 13.

[PatrickInNC]

Dude, I have pods on all my bikes, and get 13-14 dyno proven at all rpm's. Its about making sure your bike is running at its top efficiency. Removing the pods helps increase the efficiency as long as you have the jetting to go along with it. You are right that air box is not there to just hold in the air filter/ its there to prevent water penetration as well.  You can not say that removing an air box garauntees a loss in low end power. Most dyno's prove the contrary that removing the restrictions of air box, and exhaust along with the fuel mixture adjustments to match, will make an increase in efficiency, and an increase all across the power band. The sort of scenario you are speaking of happens alot in high power/high technology engines where you sacrifice low end for top end. Not really something that is noticeable on an engine as simple as a GS. How did you come up with all this science? Did you read about it? Did you actually conduct and experiment? You are not going to lose your low end putting around campus. I putt around campus on my GS and have plenty of oomph to get up and go. Its kinda funny that you make all these claims when you have no real world experience betwen with both setups. I just look at things objectively.
-Patrick

[The Buddha]

When the air is flowing (the air/mass in motion example) through the airbox, it defintely helps get the air moving into the next cylinder. They don't just put those airboxes there to hold the air filter.

The air mass in the air box should be moving forward ... yes however there is a vacuum in the airbox ... that negates any effect of air flow and more ... in fact airboxes are designed with a resonance sorta thing ... they reduce noise and have an appropriate volume for the engine and air filter that they dont kill performance too much ... BTW the biggest air box they can stuff in while still retaining removability and access around it is what they do ...

People with modified airboxes or removed airboxes who jet to correct the air/fuel mix have found that it's extra rich at the bottom end in order for the bike to run correctly. It is best to set the fuel/air ratio for best power, then fill the cylinders quickly.

They have found what ... and who is they ... I have done this scores of times, and never have felt that ... in fact we tune each segment of the rev range individually and no where is it significantly richer ...

With pods the fuel/air ratio is set to about 11 - 11.5 so it will run okay at the bottom, but it will make the whole lower RPM range rich and weak. Good fuel/air power is 12 - 13.

This is one of those urban legends too you set the AF ratio the same in all cases, more air and more fuel in case of pod ... in effect the bike will spin up quicker with pod ... you're putting around at low rpm ... and open the throttle and the thing revs up faster ... making you think its not powerful at lower revs ... butt dyno is misleading ... the stock bike will sit at lower revs longer and make much more spiky power at the 5K+ mark ... and you'd think it has better bottom and mid range typical of people that have V maxes ... and they do that servo bypass thing so its always in the V boost mode instead of at 3500 or whereever it comes on ... and they feel the bike has lost power ... on the dyno the V boosted one makes more power and its also faster lap times on the race track ... to your butt it feels like the bike lost something ... Put pods on and jet it right and it will in all rpm's make more power than a stock airbox bike that is jetted right.
Cool.
Srinath.

[treybrad]

Well, when we meet up, take my bike for a spin. I have the old style carbs, but when I put on my pod, I know I didn't lose anything on the low end. It'd be interesting to do back/back butt dynos between our two bikes... is your bike completely stock? Mine had exhaust on it when I bought it so I don't even really know what the baseline is...

trey