Filter/intake flow testing (with results)

zrx1100

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I wanted to understand the differences in flow for various intake options. So I have setup a simple experiment to compare the stock XS650E airbox to UNI pods and a few other configurations. The “flow bench” is made using an old anemometer and a “wind tunnel” out of PVC pipes that closely matched the air filter/box outlets and the anemometer fan enclosures’ inner diameter. A shop vac was used for the source of suction with washers to change the air flow through the setup.
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The anemometer measured wind speed in miles per hour. “Tested” its accuracy hanging it out of the car window while driving on a clam day while setting cruise control for different mph settings. It reads as expected.

The cubic feet per minute (CFM) is calculated by multiplying the speed reading by mph to feet per minute conversion factor and then multiplying the product by the anemometer/pipe inner diameter cross sectional area in square feet:

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In the test I compared three settings for the air flow that would be equivalent to the engine speed of ~2500rpm, ~7000rpm and vacuum inlet wide open, a theoretical ~ 14000 rpm.

(To calculate intake CFM for one cylinder for a given engine speed use cylinder displacement value (325cc for a stock setup), convert to cubic inches( 21.36 ci) then intake CFM = (engine displacement * rpm)/(2*12inch^3)

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Here are the configurations tested:

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Here is the raw data and a bar chart for relative comparison of flow at different engine speeds:

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Some comments and observations:

The stock filter I had is something I received with a spare airbox, it is very used and I have questions about its integrity. That is why it may be flowing better than K&N at low engine speed, but one can see how its relative performance is dropping with the flow rate. The K&N has been in my bike for some time. It is fairly clean and well oiled, but I did not clean it specifically for this test.

The anemometer may not have enough resolution to capture fully the differences in flow as it had only one decimal in the mph reading. Where the first decimal would flip between adjacent reading back and forth, I would add a 0.05 to the lowest number.

It is no surprise that the open inlet does not provide the best flow as usually there is turbulence at inlet. The open box provides the best flow. That also makes sense as the construction of the box incorporates a velocity stack-type funnel at the inlet that smoothers out the flow.

Both UNI pods and the stock box without the filter covers give a 2-3% boost in the air flow. It is a bit strange that the relative improvement of flow of the UNI pods repeatably dropped off at the highest intake velocity (though not very relevant for the operating range of XS 650).

The 2-3% improvement in air flow would necessitate a 2, maybe 3 steps up in the main jet size as the jet sizes correspond to the flow rates of the jets. (follow the discussion here: http://rd500lc.free.fr/technics/jet_size_en.htm). This jet size increase is inline with expectations

Modifying/opening up the filter covers of the stock box seems like an attractive option (to me at least).
 
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Thank you for the lab report and concisely defining your methodology. Did you run a "cal curve" on the data? Another member here is building a LSR XS. Did you consider running the tests with velocity stacks to determine if they are actually beneficial? Kudos for sharing the data.
 
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Thank you for the lab report and concisely defining your methodology. Did you run a "cal curve" on the data? Another member here is building a LSR XS. Did you consider running the tests with velocity stacks to determine if they are actually beneficial? Kudos for sharing the data.
Kevin, the only calibration I did was my very scientific check of the anemometer accuracy by hanging it out of the car window at a set car speed and observing that it was reading the same number as the setting on the cruise control (tried different speeds and the day was calm).

I did consider including a velocity stack in the test, especially as I know that the simple pipe opening does not flow best. The open airbox effectively has a velocity stack in it, so one can expect similar flow from a dedicated velocity stack.

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Not the same thing but tuning my DRZ400 with a wideband I found the stock airbox on it highly restrictive. With an oem replacement K&N I ended up with the entire top and right side cut out. When I went to an aluminum tube with bellmouth and a uni pod I had to drop jet sizes to keep it from going rich. Ended up removing the back of the airbox. Switched the uni pod for a K&N pod and had to go up jet sizes to keep it from being lean. The uni pod was enough of a restriction above 5000rpm that it was creating vacuum and sucking extra fuel past the jets. The K&N doesn't create a restriction which meant the jets had to be larger to compensate for less vacuum.

This was with cams, port work, big bore and stroker with a FCR41 carb. I wouldn't have done so much with a CV carb as they tend to be more finicky with open air boxes.
 
Here is an interesting observation with respect to the stock filter and the K&N stock retrofit. The K&N unit is about 8mm shorter. This would translate roughly to 15% lower active area for K&N. A lost opportunity for K&N, but they may have been trying to match stock within the range of the stock jetting.
 

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I have plastic boxes stock 1980. I wrapped the metal filter frames with one layer of dish rag and glued the ends together. Sometimes I ride w the tube baffles out or cover off just plastic outside badged covers..now I gota get the nerve to free up my stock exhaust somehow. I was putting it off cause its so stock but I totally dulled the left muffler as a newbie scrubbing it w.scotch pad so. Now I want to drill in the muffle emd.to free it up
 
. I was putting it off cause its so stock but I totally dulled the left muffler as a newbie scrubbing it w.scotch pad so. Now I want to drill in the muffle emd.to free it up
Sell the bike to a fellow in England, traffic only sees the right side of the bike there...:laugh2:
 
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