The Definitive 'Pod Filter vs. Open Airbox' thread

[brad black]

i think all the k&n have a velocity stack incorporated in them.

[supertjeduc]

Airbox lid was open?

No but air/fuel was with airbox and Pods about 13 to 13.5

[greenmonster]

OK.
I think opening the lid is one of the most substantial changes you can do.
OEM compared to open lid w KN & Dj kit, what a difference!

[psychochild]

I'm still not convinced when it comes to velocity stacks,  mostly because every carburetor out there uses the venturi effect to create vacume to add fuel, basically if you look at your carb, the tapered shape entering the carburetor is a velocity stack,  my filters are attached directly onto the carbs, I don't see the point of adding another stack above the one already cast into the carbs.  The only way I could see the velocity stack principal working significantly more than stock would be to continue the taper of the outer edge of the carb to create a bigger inlet into the carb itself.

[dlearl476]

I'm still not convinced when it comes to velocity stacks,  mostly because every carburetor out there uses the venturi effect to create vacume to add fuel, basically if you look at your carb, the tapered shape entering the carburetor is a velocity stack,  my filters are attached directly onto the carbs, I don't see the point of adding another stack above the one already cast into the carbs. 

And my "fresh from the Dyno" data bears that out.  Unfortunately, I don't have a strictly "apples vs. apples" comparison, as I've added FCR's and DynaCoils as well but here's the results. 

Pre-FCR/DynaCoil  71.8 hp and 57.8 ft/lbs of torque.


FCR's+Dynacoils with UNI pods with OEM velocity stacks inside: 70.4 hp and 54.4 ft/lbs.

FCR's + Dynacoils with UNI pods only 73.2 hp. and 56.9 ft/lbs

All these are with the Gallaseti 2-1 exhaust.  I had better torque, and less hp with the RoadRacing exhaust, which I'm going to put back on and re-test.



OEM carbs with Dynajet kit, open box/K&N, RoadRacing exhaust.

The only way I could see the velocity stack principal working significantly more than stock would be to continue the taper of the outer edge of the carb to create a bigger inlet into the carb itself.

On an FCR, that radius is machined into the airbox adaptor.  It is replaced by a velocity stack.  You don't have both, one on top of another.  And you're right, of course that wouldn't work. (Which is why, I think, the pod with the OEM velocity stack on top of the airbox adaptor, pardon the pun, sucked.)

[MotoCreations]

When you say "medium" what size do you mean.  AFAIK, they come in 15, 25, 30, 50 and 70mm lengths. (At least they used to.  Apparently, Sudco isn't making them anymore).
I've been contemplating the same, although I'm sure the moment I did I'd hit a friggin dust storm  (which happen all too frequently here in Las Vegas).

Machined my own -- 85mm with a very radius'd shape on the end.  Didn't dyno test as of yet as I will do a few more exhaust modifications first.  Seat of the pants testing tells me the shorter ones I had machined before made more power.  Again -- not validated on the dyno as of yet.

I talked with Alex O. awhile back in regard to trumpets after my experience.  He has run the Sudco ones before -- but got better #'s using the FCR very short ones.  The engine had Guy's MBP heads and a bunch of work on it and is not a normal engine. 

I've known of quite a few others that have done what we do.  Some use a pre-filter airsock ONLY to keep the big things out of the stacks.

[bullet boy]

I've been running no airbox -- no air filters either -- just medium velocity stacks wide open on FCR's for @4+ years.  Admittedly not the ideal situation for most people.  Loud as can be listening to them gulp the fuel while riding.

I've got REALLY BIG K&N's on my 620. I doubt they improve the performance of the machine in any way, but they are inexpensive, look really cool, and are louder than my cored exhaust when you wank the throttle. Makes just ridin' around a lot more fun...

[psychochild]

ok, I really want to see a picture of your bike from the side,  right now, Im almost sure that the top cylinder pod filter would look like a penis for you motorcycle   

[brad black]

if you were going to play with velocity stacks on a carb bike i think you'd need to mke some that go into the std carb opening, as the transition from carb mouth to the larger diameter will all but kill the effect of the stacks.  anytime you get a big change in cross section you get big flow speed changes and wave reflection.  you don't want any steps if you can help it.

the carbed 2v motors have a long inlet system downstream of the throttle, but most of the injected bikes have trumpets upstream of the throttle that would make the overall system nearly as long.  ST2 are one model that doesn't.

removing the trumpets from the throttle bodies and running pods will shorten the inlet tract on nearly every model.  usually shortening an inlet tract reduces the peak torque value, but widens the spread of torque across the rpm range.  so that might be part of the issue as far as dyno tests go, and they're not getting revved high enough to make an obvious difference.

[bullet boy]

ok, I really want to see a picture of your bike from the side,  right now, Im almost sure that the top cylinder pod filter would look like a penis for you motorcycle   

Well, if it does, at least its a healthy-sized penis:

[psychochild]

eh must have just been the angle,  killer looking bike btw, love the solo seat  with the shorty pipe and headlight,  thats a winning combo right there.

[dlearl476]

if you were going to play with velocity stacks on a carb bike i think you'd need to mke some that go into the std carb opening, as the transition from carb mouth to the larger diameter will all but kill the effect of the stacks.  anytime you get a big change in cross section you get big flow speed changes and wave reflection.  you don't want any steps if you can help it.

Brad, I don't know who you're addressing, but here's where I'm at.

The FCRs come with this type of airbox adaptor:



Using it with the rubber bit that apapts the OEM MIkuni's to the airbox (in the shape of a velocity stack) didn't work well, as you suggest.

Where I'm looking to go is replacing that entire set-up with velocity stacks, which replace the radius of the adpator with an entirely different one, in a variety of lengths.

[brad black]

Brad, I don't know who you're addressing, but here's where I'm at.

The FCRs come with this type of airbox adaptor:



Using it with the rubber bit that apapts the OEM MIkuni's to the airbox (in the shape of a velocity stack) didn't work well, as you suggest.

Where I'm looking to go is replacing that entire set-up with velocity stacks, which replace the radius of the adpator with an entirely different one, in a variety of lengths.

what yourself and psychochild were saying about carbs pretty much.  wonder what a small diameter trumpet inside a filter that ran from the 38mm carb entry out would do?  like those fcr ones you show - they're a true extension of the carb itself, not another bellmouth twice as big as the carb entry which has its own bellmouth.

[dlearl476]

wonder what a small diameter trumpet inside a filter that ran from the 38mm carb entry out would do? 

That's what I'm looking to find out, as soon as I can find a source for them.  According to Chris, Sudco no longer carries them.  I'm hoping another dealer, like the one I lifted the pics from, still has some in stock.
In the meantime, I'm trying to decide which length to go with.  They're kind of expensive to be doing some trial and error dyno tuning.

[brad black]

They're kind of expensive to be doing some trial and error dyno tuning.

welcome to my world.  expect there to be a lot of error in your trial.

you could make up a base section and a top bellmouth and use exhaust tubing for the stacks.  cut a diagonal slit section out of one side, then push it together and weld up the seam - cheap tapered tube.

or just run straight tubing and have multiple lengths on hand so you can do consecutive dyno runs, just swapping the middle bits.  don't stop to look at the results and it won't take too much time to get the results.  possibly a good way to get a feel for what length does.