POD Filters - Does Shape Matter?

[KillerGriller]

Hey Folks,

Not sure if I will be moving to pod filters or not when I tear my bike down this winter, but as I was doing some research I noticed that some brands offer pods of different shapes but same intake diameter.

My first assumption is that they offer different shapes to make your fitment options easier, but looking at them you start to notice a volume discrepancy. e.g. - a 42mm intake dia. cone filter would have less surface area and internal capacity than an oval filter of the same diameter. So...does anyone here know if the shape effects the performance of similarly booted pods, or is it strictly a form/fitment concern?

[Suzukian]

The bigger the filter area, the less velocity through it, and more filtering, as more area doing less work will let less particulates through. Personally, I don't like Pod Filters. They need constant lubrication to work properly, if not, they do let in particulates and they tend to wear down the cylinder wall linings and rings more. If maintained properly, it's just one more thing to upkeep.

If you are going for a performance increase, Pods will make your bike more "Peaky" in the power band. You will use more fuel, you have to re jet the carb for them, and if you have CV carbs, you have just opened a can of worms. If you want Pods for some other reason, I'd be interested to know. They do not add more power to the bike. They do change the power band, and some people misinterpret that as a power increase. If you have a good tuned aftermarket exhaust, and want more umpf, then going with bigger cars (flat slides) and pods, and seeing if you can find some mildly hotter cams (to keep it streetable) is what you could do. That starts to cost a lot of money and you need to set a price that your willing to spend, to have a real discussion about which way would give you want you want. I think a good header with the stock intake system will get your break breathing better. Some headers will require you to re jet, some say not necessary, but it usually is. IMHO

[zed1015]

The pod shape doesn't matter. ( velocity stacks are a different subject ).
It's the quality and air flow capability that does.
Using un-named or cheap chinese brands such as DNA,EMGO, HiLevel, etc etc is asking for trouble as most actually flow less than the stock airbox or mask the air passages in the mouth of the carb and cause restriction. rich running and power loss.
K&N, S&B, APE,Ram Air, Piper and UniFilter are the reliable choice.
The foam "sock" type generally flow the most and don't risk setting up intake pressure waves in the venturi like the solid end cap type which can cause fuel drop out in certain conditions but that's not usually a concern on the road.
For CV carbs dual pods are best as they help balance out the intake pulses which upsets the diaphragms and cause slide flutter.

[bwringer]

...They do change the power band, and some people misinterpret that as a power increase...

I swear, this thinking afflicts every model of every motorcycle ever. Make the bike peakier, twitchier, and/or louder, and people think they've magically discovered more power. Without some very deep and expensive engine work, you're only rearranging the same basic quantity of beans.


As for the OP, it sounds like you have an airbox that works, and you're thinking of getting rid of it for unknown reasons. There's some very good advice on the tradeoffs in the posts above. I'd also add that you should document your changes carefully (and if you ever manage to find a combination of exhaust, pods, and jetting that works well, PLEASE share it with the forum), and if at all possible, ensure that you have a path back to stock.

If the alteration is permanent, then don't discard the airbox, whatever you do; sell it to someone here!

[Brendan W]

In the 80s I recall reading that Honda devoted 40% of their research computer time to exhausts.
Trying to get the perfect, or at least optimal, timing of shockwave bounces to keep the charge in the cylinder until the exhaust valve was fully shut.
That was enough for me to decide that the stock system, from airbox to muffler, has a lot going on and is probably best left well alone.
Not that I haven't tried things out, I'm just realistic about what the possible outcomes are going to be.
But yeah, more area will have less pressure drop and pairing will smooth things out.
In fact all four intakes in one pod........ oh wait! That's an airbox

[KillerGriller]

Thanks for the responses. To be clear, I'm not trying to start another pods vs. airbox debate, I'm curious as to how the shape of these filters effects the performance of the filter and the engine.

There are a tonne of statements out there like these that contradict each other which leads me to believe none of us knows for sure.

The bigger the filter area, the less velocity through it, and more filtering, as more area doing less work will let less particulates through.

The pod shape doesn't matter. ( velocity stacks are a different subject ).
It's the quality and air flow capability that does

The larger surface area comment makes sense, but would that also mean the lower air velocity enables more fuel vapour to travel into the filter cavity? I don't know?

[zed1015]

Thanks for the responses. To be clear, I'm not trying to start another pods vs. airbox debate, I'm curious as to how the shape of these filters effects the performance of the filter and the engine.

It's not the shape but the air flow capabilities.
This type of filter can be any shape as long as it flows enough air to equal or exceed the demands of the engine ( which is just an air pump ) whilst providing adequate filtering without restriction.
Compared to a single airbox the combined area of four individual quality filters usually exceeds that of a single filter.
A larger surface area would mean you can use a finer element to filter out smaller particulates whilst maintaining adequate air flow.
The air velocity and fuel vapour corrolation is not an issue however the increase in atmospheric pressure over the needle jet that occurs at lower rpm's due to the drop in velocity through the venturi needs to be adressed to prevent the common off idle flat spot that is caused by a momentary lean spot on the transition from the pilot to jet needle.

[rphillips]

Fuel vapor travel into the filter cavity? Fuel vapor should be from center of carb throat toward intake ports and cyl. Wouldn't think it would be going backwards (upstream) into filter cavity.

[KillerGriller]

Fuel vapor travel into the filter cavity? Fuel vapor should be from center of carb throat toward intake ports and cyl. Wouldn't think it would be going backwards (upstream) into filter cavity.

Saw this somewhere else, I'm not sure how common it is but this idea is what I was referring to with my comment.

VELOCITY STACKS!!! Yeah those rubber boots that attach the airbox to the carb. Notice how they are flared at one end.. Those carbs were designed for the velocity stack to stop the pulsating of the gas mix bouncing back out of the carb as the valve closes . We all know velocity stacks improve performance

[KillerGriller]

It's not the shape but the air flow capabilities.
This type of filter can be any shape as long as it flows enough air to equal or exceed the demands of the engine ( which is just an air pump ) whilst providing adequate filtering without restriction.
Compared to a single airbox the combined area of four individual quality filters usually exceeds that of a single filter.
A larger surface area would mean you can use a finer element to filter out smaller particulates whilst maintaining adequate air flow.
The air velocity and fuel vapour corrolation is not an issue however the increase in atmospheric pressure over the needle jet that occurs at lower rpm's due to the drop in velocity through the venturi needs to be adressed to prevent the common off idle flat spot that is caused by a momentary lean spot on the transition from the pilot to jet needle.

Thank you, this is exactly what I was looking for.

CHEERS!

[zed1015]

Thank you, this is exactly what I was looking for.

CHEERS!

Also if considering pods with CV carbs you should look to fit the DUAL type pods if available.
CV carbs like still air and dual pods go someway to balancing out the individual intake pulses which upset the diaphragms and cause slide flutter.
As for the off idle lean spot i have a simple proven solution for that which does not involve any changes to pilot jets or needles and just a change of main jet from the 650's stock 110 to 147.5.

[Suzukian]

The pressure drop which does all the fuel sucking happens inside the carburetor (Bernoulli Principle), not the air filter. CV carbs with fuel pods, there's a whole 'nother thread on that.

[rphillips]

Doesn't sound right. If the filter system has no effect on the fuel being sucked through the carbs, why when installing pods do the jet size need to go up a lot.

[Suzukian]

The bike needs to be rejetted because the volume of air has increased into the Venturi chamber, which is now compressing that larger volume of air, but needs more gas to balance out the ratio. More air, greater pressure drop, in keeping with the "Laws of Conservation", and the Bernoulli Principle. You should read a book on how carburetors work. Very worthwhile, it will answer many of your questions, and with good graphics and photographs explain the process in greater detail. If you do the same thing to a fuel injected motorcycle, you still have to remap the system to inject more gas to compensate for the larger volume which has been compressed.