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Discussion Starter · #1 · (Edited)
A while back there where several threads about BOV that had a lot of misinformation in them. I will try to clear up some of the misconceptions show how to setup a inexpensive BOV for use on the XPT.


1) All Blow Off Valves / Diverter Valves work the same way.
2) A BOV and a Diverter Valve are one in the same. The ONLY difference is a BOV may not have a second hose port but instead have a series of holes for pressure relief.
3) All BOVs require a vacuum reference except electronic BOVs (another discussion). Vacuum reference is post throttle body.
4) The SOLE function of a BOV is to relief boost pressure in the intake tract when the throttle is lifted. It is necessary to relief the manifold pressure when the throttle body closes because if not a shock wave will be sent the compressor. Compressor surge can cause damage to the turbo. This also allows for quick respooling of the turbo when you are on -off-on the throttle quickly.
5) A BOV functions (opens / closes) based on the differential pressure across the piston. At idle the spring keeps the piston closed while under vacuum. As intake pressure (boost) builds in the intake both sides of the piston see the same pressure and the valve remains closed. When the throttle body closes vacuum develops post throttle body. This vacuum is applied to the top of the piston while the bottom remains positively pressurized. This difference in pressure allows the piston to overcome the spring tension and open. Once the pressure is relived the valve closes.
5) The stock XPT develops about 6-7psi of vacuum at idle.
6) The stock XPT diverter valve is open at idle and closes at -1.3psi.

Those are just random facts as I thought of them. An inexpensive BOV works just as good as a high dollar BOV. Matter of fact there are a lot of companies buying generic BOVs and relabeling them as their own and charging big dollars. RKX is a good inexpensive BOV for around $45. It comes with a 1” inlet/outlet and can be made to recirculate or vent to atmosphere. If you are going to vent it to atmosphere I recommend installing a filter on the outlet side.

Here is a picture of the RKX BOV and associated pieces. Throw away the instructions that come with as they could not be more wrong. Another fact: Spring choice is chosen to keep the BOV closed at idle and is NOT chosen based on boost level! You want to run as light spring pressure as possible while keeping the BOV closed at idle. This will allow for quick pressure relief when the throttle is lifted.



I tried to go about finding the right spring tension systematically with a pressure rig I created and a vacuum pump. The pressure rig is nothing more than a pressure regulator, gauge, and some fitting with get connected to an air compressor. I bench tested the RKX at 20psi and leakage was very minimal. I checked the vacuum at idle to determine what would be required to keep the valve closed at idle. At idle the XPT develops about 13.5”hg or -6.6psi.The RKX comes with 3 different spring weights and two spacing washers. The washer are to increase the pressure in between spring rates. I started with the lightest spring and worked up to the heaviest spring (yellow) with both washers. This was still not enough tension to keep the valve closed with ~7psi of vacuum applied. I started adding additional washer until I found adding .400” of spacers worked. With the spring tension correct I plumbed it up in the RZR. I plugged the recirculating line and used a 1” house to plumb the inlet side (bottom) of the BOV to the intake. I left the outlet side vented to atmosphere. I fired up the engine and verified the BOV was closed at idle. I gave it a quick rev and let off the throttle, SWOOOOOSH. It works as intended. Now the OCD in me could not stand having wahers stacked inside the BOV. I'm sure they will work fine but that's not me. I had some 1.5” round delrin stock laying around. A few minutes on the lathe and I hacked out a couple of spacers. (.400, .375. 335) For adjust as needed.




Note: When changing springs very careful placement of the o-ring is crictal to a good seal. The ONLY way to verify it's sealed is with a vacuum test every time you open the top.

The stock BOV works fine for stock boost levels and can be made to vent to atmosphere for the cool swooosh factor. I recently turned up the boost some and wanted something a bit stronger. Anyway I hope this clears up some confusion about BOVs and provides some useful info on how to setup a BOV inexpensively.

RKX BOV = ~$45
1” Filter = ~$8
1” Plug = ~$8
Total = $61

I'm sure I left a few things so please ask if you have any questions.
 

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Thank you for the write-up and info provided. Quick question... I'm considering doing this upgraded, but after watching several videos, it looks like there isn't a right or wrong way to install? For simplicity I'll call the vacuum line connection the "top". On my stock RZR the BVO is installed with the top facing up. But I've seen others where the top is rotated 90 degrees, so the top is now the "side" (in-line with the turbo hose). Does it matter? Thanks from a newbie...
 

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Discussion Starter · #3 · (Edited)
Thank you for the write-up and info provided. Quick question... I'm considering doing this upgraded, but after watching several videos, it looks like there isn't a right or wrong way to install? For simplicity I'll call the vacuum line connection the "top". On my stock RZR the BVO is installed with the top facing up. But I've seen others where the top is rotated 90 degrees, so the top is now the "side" (in-line with the turbo hose). Does it matter? Thanks from a newbie...
The stock Diverter valve is plumbed backwards from a standard valveg. A standard BOV has the inlet (boost side) on the bottom and work the piston against the spring pressure. There is definitely a right and wrong way and if installed incorrectly it will not function correctly. The BOV needs be installed so the boost pressure is on the bottom of the piston. If installed backwards the boost pressure will not work against the spring and the valve will not see the "differential pressure" required for proper operation. Also they are designed with a air flow path where the piston and the casing forms a "seat" (seal). If plumbed backwards against the seat it will leak. Does this make sense?

 

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Thank you for the write-up and info provided. Quick question... I'm considering doing this upgraded, but after watching several videos, it looks like there isn't a right or wrong way to install? For simplicity I'll call the vacuum line connection the "top". On my stock RZR the BVO is installed with the top facing up. But I've seen others where the top is rotated 90 degrees, so the top is now the "side" (in-line with the turbo hose). Does it matter? Thanks from a newbie...
The stock Diverter valve is plumbed backwards from a standard valve but work using a diagram rather than a spring. A standard BOV has the inlet (boost side) on the bottom and work the piston against the spring pressure. There is definitely a right and wrong way and if installed incorrectly it will not function correctly. The BOV needs be installed so the boost pressure is on the bottom of the piston. If installed backwards the boost pressure will not work against the spring and the valve will not see the "differential pressure" required for proper operation. Also they are designed with a air flow path where the piston and the casing forms a "seat" (seal). If plumbed backwards against the seat it will leak. Does this make sense?

Thank you for the reply. Yes it makes sense... Using your original picture and how you have the parts oriented, I labeled them they way it should be installed, based on your description... is this correct?

Auto part Screw Wheel
 

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Great how to write up Joe.
This should be a sticky.
 

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Great Info and write up Joe! Thanks!

I think you're stalking me from the other side of the country tho.... I was just looking into BOV's the other day lol....
 

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Now the OCD in me could not stand having wahers stacked inside the BOV. I'm sure they will work fine but that's not me. I had some 1.5” round delrin stock laying around. A few minutes on the lathe and I hacked out a couple of spacers. (.400, .375. 335) For adjust as needed.
Did you have to turn any off the 1.5" OD of that stock Joe? or was the OD good and you just turned the face down to thickness?
 

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Discussion Starter · #11 ·
Now the OCD in me could not stand having wahers stacked inside the BOV. I'm sure they will work fine but that's not me. I had some 1.5” round delrin stock laying around. A few minutes on the lathe and I hacked out a couple of spacers. (.400, .375. 335) For adjust as needed.
Did you have to turn any off the 1.5" OD of that stock Joe? or was the OD good and you just turned the face down to thickness?
I did, I think it was 1.45 or so. I made a couple extras if you want one.
 

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Now the OCD in me could not stand having wahers stacked inside the BOV. I'm sure they will work fine but that's not me. I had some 1.5” round delrin stock laying around. A few minutes on the lathe and I hacked out a couple of spacers. (.400, .375. 335) For adjust as needed.
Did you have to turn any off the 1.5" OD of that stock Joe? or was the OD good and you just turned the face down to thickness?
I did, I think it was 1.45 or so. I made a couple extras if you want one.
I appreciate it, I have some laying around as well actually, just curious to the info so I can have everything ready when i get the BOV.
 

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Just came across this: With the diagram as indicated that BOV is installed backwards from what Polaris, SuperATV (Rev1) and Evo suggest for their BOV's

Soooo what is correct?
 

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OH ..here we go...I think there maybe some unfinished business here...good question :popcorn:
 
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