LC4-640 Air Filter Math

[laramie LC4]

Ok, let’s do some math !!!! let me first start though by prefacing this with a short story…….

this all began after speaking with my engine builder about concerns I had with the woefully small, OEM air filter on the 640’s. anyone who owns one of these machines is aware of what I’m talking about. somehow, some geniuses in Austria thought that we could feed a 625cc bike with the same air filter they use on their 65cc race bikes. well, they are right, you can do it, but just barely. But more on that later…..

many people over the years have attempted to deal with this problem is many ways. most involved replacing the stock air box with one from a kick-start only model, or one from a duke II. success with these methods has been mixed. issues being that you have to relocate everything, the battery, the CDI, wiring, …..and it turns into a mess. I wanted to avoid this issue at ALL costs. I do not want to “rig” something up, I want it to work. so, for the past few months I have been staring at my air box, a 620 air box, my battery, electronics, and wonder how I was going to make it work. I think I finally got it figured out, but more on that later as well. I don’t want to spoil the secret…

back to the math.

so, as we have established, the air box is small, the filter even smaller, but is it big enough?

in order to figure that out, we need to know a few things.

1) amount of air being drawn into the engine
2) size of the air filter in cubic inches

with this info we can begin to calculate cfm – cubic feet per. minute

let’s start with the engine and its flow rate. we all know that our “640” is actually 625 cc’s in displacement. we can convert this to cubic inches and end up with roughly 38 in³

now let’s discuss volumetric efficiency. VE is defined at the percentage of actual volume to total volume. as we established, our 625cc engine should displace 38 in³ of air at 100% VE. problem is, not all engines run at 100% VE. normally aspirated engines (no boost) usually run 90%, a slightly modified engine can achieve 95%, and a highly modified engine can achieve 100% or more. if we know the VE and the RPM’s then we can calculate cfm. I use 9,000 RPM as an extreme example, to figure top end numbers for possible flow. most people, or engines, will never see those kind of RPM’s.

so, for the following VE’s, we have these calculations.

90% VE @ 9,000 RPM = 89 cfm
100% VE@ 9,000 RPM = 99 cfm
110% VE @ 9,000 RPM = 119 cfm

so, in a worst-case scenario, your engine will suck in 119 cfm

now lets talk about the filters.

I am using 3 different filters as reference for this experiment, the first is the OEM 640 filter that we all complain about, the second is the filter from a kick-start only 620 (same as the dukes), the third is a pod filter that I think I’m going to end up using.

to calculate the cubic inches of the filter I imagined them as a flat circle, measured their diameter, and used that as my number to calculate area. this is not precise, and if anything I feel I was more than generous to the filter and its actual “usable” surface area. so all of the numbers you are about to see assume a couple things,

1) my measurements are correct
2) the filter is clean
3) the filter has been properly oiled

to figure area you need the following equation.

A= Ƥ x R2

so, lets start with the small one….

when measured out, I came to an average of 6.5” diameter across the filter. Radius is ½ the diameter, so we end up with 3.25” = R

3.14 x 3.25² = 33 in²

next is the 620 kick-start only…

when measured out, I came up with an average of 8” diameter across the filter. So R= 4”

3.14 x 4² = 50 in²

Finally is the Pod Filter….

when measured, I came up with….

(12.5” x 6” body = 75 in² ) , + 3.14 x 2² = 12.5 in², for a grand total of roughly 87 in²


so, to clean that up a bit, here are the numbers…

33 in² for the OEM
50 in² for the 620 filter
87 in² for the Pod filter

now that we have these numbers we can figure total flow for the various filters, once again assuming they are perfectly clean, and well oiled. I did a lot of searching around the net to determine what is the flow rate of a dual-stage foam filter and had some difficulty. I finally used the flow numbers provided to me by my engine builder. he claimed that a properly maintained foam filter could flow roughly 3 cfm/ in²

if you use that number, then we end up with the following calculations;

OEM filter = 33in² x 3 = 99 cfm
620 filter = 50 in² x 3 = 150 cfm
Pod filter = 87 in² x 3 = 261 cfm

so, now back to the original question. does that small air filter provide enough flow to feed our engine. the answer is, just barely. this also assumes everything in perfect, no dust, no dirt, and 100% efficiency. I don’t know about you guys, but when I get back from a ride, my filter does not look brand new. granted I know, if your not thrashing on your bike and never see anything above 5,000 RPM the numbers don’t look so dire, but they still aren’t pretty.

so what have we learned form all this math?

make sure you clean those filters. next time you look at it and say, “oh, it will be fine for another ride.” go and wrap a rope around your neck and start choking off your air while running around the block. I bet it won’t take long to illustrate my point.

laters,

laramie

[meat popsicle]

[anotherguy]

Calculations are unreliable when it comes to flow. Use a flowbench.

[markjenn]

Interesting analysis. Quick comment that with foam-oil filters, pressure loss across the filter doesn't go up as the filter loads - in fact, it may tend to go down. It's counter-intuitive, but a foam-oil filter does not work like a sieve where dirt makes the passages smaller - it works like a maze where dirt impinges on the oiled fibers and gets caught. As the oil is used up, the filter continues to flow about the same, but the filtering efficiency goes down as the particles bounce through the filter rather than being caught.

Net, net, the issue with a too-small oil-foam filter is not pressure loss with loading and deteriorating engine performance - it is that the filter, with a finite amount of oil, too quickly reaches capacity and stops protecting the engine. Which also means you can address this by more frequent filter cleanings, obviously not ideal.

- Mark

[fastdadio]

I have greatly reduced the frequency of my required air filter maintenance intervals by riding way out in front.

[laramie LC4]

Originally Posted by anotherguy

Calculations are unreliable when it comes to flow. Use a flowbench.

my calculations are unreliable at best anyways, so pick your poison... yes, a bench flow would be the best way to measure.

Originally Posted by markjenn

Interesting analysis. Quick comment that with foam-oil filters, pressure loss across the filter doesn't go up as the filter loads - in fact, it may tend to go down. It's counter-intuitive, but a foam-oil filter does not work like a sieve where dirt makes the passages smaller - it works like a maze where dirt impinges on the oiled fibers and gets caught. As the oil is used up, the filter continues to flow about the same, but the filtering efficiency goes down as the particles bounce through the filter rather than being caught.

Net, net, the issue with a too-small oil-foam filter is not pressure loss with loading and deteriorating engine performance - it is that the filter, with a finite amount of oil, too quickly reaches capacity and stops protecting the engine. Which also means you can address this by more frequent filter cleanings, obviously not ideal.

- Mark

yes, foam filters do are highly dependent upon the oil and surface area. the loss of filtration as the filter becomes saturated is a major concern and i should have stated that as one of the many reasons i want a bigger filter. more surface area equals more oil, which equals more effective filtration for longer periods.

Originally Posted by fastdadio

I have greatly reduced the frequency of my required air filter maintenance intervals by riding way out in front.

that is no bullshit. i am constantly amazed at how much difference there is between days i'm following, and days i'm out front. maybe i can blame all my engine woes on the guy up front ?????

laramie

[Bullskin]

Maybe I am thinking about this all wrong, but does it really matter how big the surface area of the filter is when the rest of the intake tract after the filter is smaller anyhow? Wouldn't that be your smallest and most limiting point? I know the pipe between the airbox and the carb is far smaller than even the small air filter on the 640.

[juames]

Very interesting reading!

Question: From my limited understanding of the subject, can't too much air be bad thing too? What I mean is, with the BST and other CV carbs aren't the airboxes 'tuned' to a certain degree? You can't just hack big holes in the airbox and expect the motor to run right.

I'm I way off base?

[fastdadio]

Originally Posted by juames

Very interesting reading!

Question: From my limited understanding of the subject, can't too much air be bad thing too? What I mean is, with the BST and other CV carbs aren't the airboxes 'tuned' to a certain degree? You can't just hack big holes in the airbox and expect the motor to run right.

I'm I way off base?

Yes indeed. The air box volume is carefully considered by the engineers when designing the intake track, and the air box is part of the intake. When guys like us modify the boxes we head straight down a slippery slope of cause and effect. Especially when coupled to the finnicky POS carb that the BST40 is known to be. Which then leads us all to the 205 page thread on BST tuning in the LC4 index. That thread alone has alienated my children and caused undo stress upon my last two failed relationships. As soon as the budget allows, the BST40 will become a relic of the past.

[markjenn]

Originally Posted by Bullskin

Maybe I am thinking about this all wrong, but does it really matter how big the surface area of the filter is when the rest of the intake tract after the filter is smaller anyhow? Wouldn't that be your smallest and most limiting point? I know the pipe between the airbox and the carb is far smaller than even the small air filter on the 640.

Both matter, but for the filter to filter, it has to have a pressure drop across it, and it generally helps to have the filter be bigger to manage the pressure drop. An open intact tract obviously has a lot less resistance to airflow than a filter. And as we've been discussing, tiny filters may be acceptable in pressure drop but not in cleaning capacity.

IOW, I wouldn't expect much performance increase fitting a larger filter to the 640, but I would expect better filtering for a longer time between cleanings.

BTW, your reasoning is precisely the reason why fitting aftermarket high-performance filters to engines don't have nearly the performance benefit that the filter mfgs would like you to believe. The idea that engines are effectively "choked" by their air filters just doesn't hold water in most instances. The fact the that filter/airbox is, on most engines, part of a tuned intake system further limits any possible performance benefit.

- Mark

[dhally]

If K&N makes a filter it ought to have less pressure drop than a foam one (at least they say it does).

My solution was to carry a spare oiled filter or the liquids to clean the filter on the trail.

[dlh62c]

Originally Posted by fastdadio

Yes indeed. The air box volume is carefully considered by the engineers when designing the intake track, and the air box is part of the intake. When guys like us modify the boxes we head straight down a slippery slope of cause and effect. .

Scientifically speaking air-boxes can be thought of as Helmholtz Resonators.
http://en.wikipedia.org/wiki/Helmholtz_resonance
http://www.thunderproducts.com/AirboxesDynotech.htm

daryl

[wrk2surf]

I change mine (I have extras) almost every dirt ride.. just a good practice

[meat popsicle]

Originally Posted by fastdadio

Yes indeed. The air box volume is carefully considered by the engineers when designing the intake track, and the air box is part of the intake. When guys like us modify the boxes we head straight down a slippery slope of cause and effect. Especially when coupled to the finnicky POS carb that the BST40 is known to be. Which then leads us all to the 205 page thread on BST tuning in the LC4 index. That thread alone has alienated my children and caused undo stress upon my last two failed relationships. As soon as the budget allows, the BST40 will become a relic of the past.

Your humor is not lost on me - presentation is everything! - but your content is muddled at best and I am shooting for clarity for La Familia. By most accounts the BST40 is less finnicky than the FCR (limiting acronyms to the two most common); altitude changes are a good example of how the CV carb excels in some areas that the pumper becomes... ah, finicky.

Most people drool over the pumper for throttle response (often comparing a new FCR to a used and questionably maintained BST). Do they want more snap from their wrist? You bet. Do they need more snap? Probably not, but we are getting into personal preference territory, which is not terribly useful material.

Good reading:

which carb is best on the LC4?
(editor's note: I HATE the use of the term best, even though I catch myself using it from time to time, because it leads people to get things that they don't need. Better terms are "well built" or "

Creeper's LC4 setup thread
(is this the one you found difficult?)

[fastdadio]

Originally Posted by meat popsicle

Your humor is not lost on me - presentation is everything! - but your content is muddled at best and I am shooting for clarity for La Familia. By most accounts the BST40 is less finnicky than the FCR (limiting acronyms to the two most common); altitude changes are a good example of how the CV carb excels in some areas that the pumper becomes... ah, finicky.

Most people drool over the pumper for throttle response (often comparing a new FCR to a used and questionably maintained BST). Do they want more snap from their wrist? You bet. Do they need more snap? Probably not, but we are getting into personal preference territory, which is not terribly useful material.

Good reading:

which carb is best on the LC4?
(editor's note: I HATE the use of the term best, even though I catch myself using it from time to time, because it leads people to get things that they don't need. Better terms are "well built" or "

Creeper's LC4 setup thread
(is this the one you found difficult?)

I guess my content may be seen as muddled. I wasn't complaining about anything specific. I used your links to go back to the old posts. Most of which do not pertain to my bike as it is a 2000, LC4e, 400. I decided not to mess with mine too much. I'm happy with CV performance. Just not the BST40. I read in the JD Jetting web site that the BST40 should be rebuilt about every 5000 miles. Plastic slide and guide? Mine were grooved and badly worn at 5500 mi. Just not acceptable to me.