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Someone explain why the BOV leaks boost

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29K views 36 replies 12 participants last post by  RZR_Joe  
#1 ·
Can anyone explain to me why the stock, AA, Evo etc. BOVs leak boost under light/part throttle?

I feel like I’ve read fifteen threads about it but none have really explained why this phenomena is confined to the XPT. Cars and trucks don’t do it so why do our RZRs?

I just bought the AA BOV and it leaks till like half throttle, and as soon as I’m cruising its leaking again.

If I’m making 2lbs of boost under light acceleration, why is the BOV bleeding? Shouldn’t it be closed??

And if I swap for an adjustable one and fix it so it only opens for excess boost pressure, will I see a performance gain?

I’m sorry this has been beat to death, but I just don’t understand. I can’t find these answers.
 
#2 ·
Sorry I did not see this way back when you posted it. I assume you read my write on this issue? https://www.rzrforums.net/rzr-xp-turbo/534688-inexpensive-bov-done-right.html


The common RZR aftermarket BOVs use additive spring pressure plus boost pressure to close the BOV. Could not tell why Polaris set them up this way ( I have my suspicions). I prefer it to ONLY open under vacuum post TB & over pressure pre TB.

The whole I bring this up it that I was helping another member with some boost issues and I saw this thread unanswered.
 
#3 ·
RZR_Joe said:
Steamfitters38 said:
Can anyone explain to me why the stock, AA, Evo etc. BOVs leak boost under light/part throttle?

I feel like I’ve read fifteen threads about it but none have really explained why this phenomena is confined to the XPT. Cars and trucks don’t do it so why do our RZRs?

I just bought the AA BOV and it leaks till like half throttle, and as soon as I’m cruising its leaking again.

If I’m making 2lbs of boost under light acceleration, why is the BOV bleeding? Shouldn’t it be closed??

And if I swap for an adjustable one and fix it so it only opens for excess boost pressure, will I see a performance gain?

I’m sorry this has been beat to death, but I just don’t understand. I can’t find these answers.
Click to expand...
Sorry I did not see this way back when you posted it. I assume you read my write on this issue? https://www.rzrforums.net/rzr-xp-turbo/534688-inexpensive-bov-done-right.html


The common RZR aftermarket BOVs use additive spring pressure plus boost pressure to close the BOV. Could not tell why Polaris set them up this way ( I have my suspicions). I prefer it to ONLY open under vacuum post TB & over pressure pre TB.

The whole I bring this up it that I was helping another member with some boost issues and I saw this thread unanswered.
Click to expand...
I have my BOV set-up as Polaris did, should I reverse the orientation? I have it connected as recirc currently.
 
#4 ·
Bullitt427 said:
RZR_Joe said:
Steamfitters38 said:
Can anyone explain to me why the stock, AA, Evo etc. BOVs leak boost under light/part throttle?

I feel like I’ve read fifteen threads about it but none have really explained why this phenomena is confined to the XPT. Cars and trucks don’t do it so why do our RZRs?

I just bought the AA BOV and it leaks till like half throttle, and as soon as I’m cruising its leaking again.

If I’m making 2lbs of boost under light acceleration, why is the BOV bleeding? Shouldn’t it be closed??

And if I swap for an adjustable one and fix it so it only opens for excess boost pressure, will I see a performance gain?

I’m sorry this has been beat to death, but I just don’t understand. I can’t find these answers.
Click to expand...
Sorry I did not see this way back when you posted it. I assume you read my write on this issue? https://www.rzrforums.net/rzr-xp-turbo/534688-inexpensive-bov-done-right.html


The common RZR aftermarket BOVs use additive spring pressure plus boost pressure to close the BOV. Could not tell why Polaris set them up this way ( I have my suspicions). I prefer it to ONLY open under vacuum post TB & over pressure pre TB.

The whole I bring this up it that I was helping another member with some boost issues and I saw this thread unanswered.
Click to expand...
I have my BOV set-up as Polaris did, should I reverse the orientation? I have it connected as recirc currently.
Click to expand...
No recirculation is fine. The problem is with them being setup to be open at idle and requiring positive manifold pressure to close them. Simply reversing the orientation does not solve that problem. It should be orientated so manifold pressure is working against the bottom of the piston & spring. This issue is that most are not setup with enough spring pressure to overcome the vacuum at idle / part throttle.

"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."


Does that help?
 
#5 ·
Joe ....So if I am under boost like 2 or 3 psi like the OP described ... it is still OK to hear it leaking? I thought from reading your write up that you would only hear this when transitioning from vac to boost ? but not when under boost even at low psi?

If it is leaking when under very low boost wouldn't this mean I am loosing boost ?

Sorry I have ask you this before ...I must be a little Tarded
 
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#6 ·
That is the way Polaris setup the stock BOV and a lot of the aftermarket. It is open at idle / part throttle and requires positive manifold pressure to close

Like this picture I stole off the web:

Image


This is done to prevent compressor surge at the detriment of throttle response. If the system is designed and tuned correctly it is not necessary to bleed off boost at idle / part throttle.

I will try to explain better with boost / vacuum numbers.

Given that at idle the XPT makes ~13.5”hg or -6.6psi of vacuum you need at least ~8psi spring pressure to keep the valve closed at idle. At this point the vacuum is working against the spring thus subtracting from it's rating.

As the throttle opens the vacuum decreases with rpm to eventually zero. At this point there is zero differential across the piston and the spring is at it's given rating.

As the rpms build further and the same positive manifold pressure is applied to each side of the valve (intake port of the BOV & vacuum reference) do the differential across the valve remains zero. This spring still maintains it's rating and keeps the valve closed.

When the throttle is closed vacuum is created post TB. Via vacuum reference this value is applied to the top of the valve (piston) and it subtracts from the spring pressure. The intake side (bottom of the piston) still sees positive manifold pressure. So now you have a differential across the valve greater than the spring pressure and the valve opens and dissipates the manifold pressure until the differential pressure is less than the spring pressure at which time the valve closed again.

Example with general numers: You're on the throttle and making 15psi boost. 15psi is seen at the bottom and top of the piston so the spring pressure remains ~8psi keeping the valve closed. Then you let off the throttle, ~7psi of vacuum develops post TB, this is applied to the top of the piston while the bottom still has 15psi. So the vacuum of 7psi is subtracted from the spring rate of 8psi making it 1psi. Now a there's a differential of 14psi across the piston. The force on the bottom of the piston (15psi) oversomes the spring rate (8-7=1psi) and the valve open until the differential equalizes.

Sorry I got carried away with a long explanation.
 
#8 ·
Thanks.. Joe I think I understand now ...pictures are worth a thousand words ...the middle picture really answers my question it show that even at 5 psi of boost the BOV will still dump some pressure.
Hopefully I understand it correctly ...Thank you !!
 
#16 · (Edited)
I guess you just like to argue which I'm not going to do. The EVO is another generic BOV you can buy for about $35 or you can pay them $189, your choice. I explained the technical side in detail in the original write up again in this one. The atuomotive world does it differently than the aftermarket RZR world. I just offered my opinions and thoughts, you do as you like I could care less. Have a great one, I'm done.
 
#17 ·
The XPT uses a unique system as the charge to throttle body circuit (Charge tube) is very small. This means you have more pressure in your charge tube compared to other setups that have long charge tubes and air/air intercoolers to fill. It's also a CVT transmission which loads the motor differently so you see more vacuum under cruising. If we install the valve with the piston side into the charge tube we need a much stiffer spring to overcome the boost pressure to keep the valve closed. When we do this we notice much higher charge temps as the valve is closed under lighter throttle positions when it should remain open slightly. Other true BOV's with no diverter port out of them are setup this way and often times show hotter charge temps. It's all about pressure difference. When your post TB readings are vacuum the valve should be open.

RZR_JOE has some valid points and truly either way you can make the valve work. I just found it to work better the factory orientation on the XPT through data logging and running a long 1" tube near my ear so I could hear when different valves & setups were leaking.

-Mike
 
#19 · (Edited)
Mike I appreciate you taking the time to chime and I know how valuable time is when yo run your own business. But since you did I have a couple of observation / questions that maybe you can clarify. Please don't misunderstand, I'm not knocking how you are doing things. My experience is with much larger applications and while the concepts are the same the practical application maybe be different. I am genuinely interested in hearing your opinions and learning something for someone who has spent countless hours tuning these engines.

Agreed the XPT has a very short intake volume (it's a tiny engine) but you said it has more pressure as opposed to larger systems. Pressure is pressure and has nothing to do volume so it sees no more pressure than any other system. It will see pressure differences quicker because of the small volume. A non-bypassing BOV has to be tuned and setup correctly for the application.
Given it is, I have trouble understanding a couple of things.

You said that with the BOV clsoed at part throttle the IAT is much higher.
Pressure is what creates heat. So under part throttle (vacuum) how is additional heat generated by not recirculating? Are you saying that at part throttle the turbo generates positive pressure pre-TB? If so what kind of higher IAT and pressure are we talking? I can see this on engines with a large turbo but on a 925cc engine with a turbo that is barely adequate to supply the required airflow?

The whole topic about BOVs bypassing at idle / part the has been a subject of debate forever and will continue long into the future.

Pressure ports and the direction of flow on the BOV is a different subject all together. This style BOV uses a piston and not a true valve & seat as the above picture so the dynamic if a bit different. When pressure and flow is applied to the bottom of the piston and exits the side it creates a linear movement that works with the direction of the BOV cylinder. However, when pressure / flow is applied to the side port it side loads the piston. This is known to cause sticking issues with the piston. The same style BOV can be setup to bypass using the bottom port that works with the direction of piston travel. However, it does require a different spring setup. Would this not be better or is flow so low for this application that it does not matter? 95% of the automotive world uses this method (positive pressure on the bottom) so why would the XPT be different? Low flow volume make it irrelevant?


For the masses who do not want to tune / setup their BOV for their machine I can understand setting it up to recirc at idle / part throttle. However, I see better performance with increased throttle response setting it up to be closed at idle part / throttle, at least that's what my butt dyno tells me LOL I don't have any desire or time to log dyno runs for these tiny engine so that's why I'm asking the questions. Thanks and look forward to your answers.
 
#25 ·
RZR_Joe said:
Mike I appreciate you taking the time to chime and I know how valuable time is when yo run your own business. But since you did I have a couple of observation / questions that maybe you can clarify. Please don't misunderstand, I'm not knocking how you are doing things. My experience is with much larger applications and while the concepts are the same the practical application maybe be different. I am genuinely interested in hearing your opinions and learning something for someone who has spent countless hours tuning these engines.

Agreed the XPT has a very short intake volume (it's a tiny engine) but you said it has more pressure as opposed to larger systems. Pressure is pressure and has nothing to do volume so it sees no more pressure than any other system. It will see pressure differences quicker because of the small volume. A non-bypassing BOV has to be tuned and setup correctly for the application.
Given it is, I have trouble understanding a couple of things.

You said that with the BOV clsoed at part throttle the IAT is much higher.
Pressure is what creates heat. So under part throttle (vacuum) how is additional heat generated by not recirculating? Are you saying that at part throttle the turbo generates positive pressure pre-TB? If so what kind of higher IAT and pressure are we talking? I can see this on engines with a large turbo but on a 925cc engine with a turbo that is barely adequate to supply the required airflow?

The whole topic about BOVs bypassing at idle / part the has been a subject of debate forever and will continue long into the future.

Pressure ports and the direction of flow on the BOV is a different subject all together. This style BOV uses a piston and not a true valve & seat as the above picture so the dynamic if a bit different. When pressure and flow is applied to the bottom of the piston and exits the side it creates a linear movement that works with the direction of the BOV cylinder. However, when pressure / flow is applied to the side port it side loads the piston. This is known to cause sticking issues with the piston. The same style BOV can be setup to bypass using the bottom port that works with the direction of piston travel. However, it does require a different spring setup. Would this not be better or is flow so low for this application that it does not matter? 95% of the automotive world uses this method (positive pressure on the bottom) so why would the XPT be different? Low flow volume make it irrelevant?

For the masses who do not want to tune / setup their BOV for their machine I can understand setting it up to recirc at idle / part throttle. However, I see better performance with increased throttle response setting it up to be closed at idle part / throttle, at least that's what my butt dyno tells me LOL I don't have any desire or time to log dyno runs for these tiny engine so that's why I'm asking the questions. Thanks and look forward to your answers.
Click to expand...
Sorry for the delay, it is hard to find the time to answer as in depth as I want. Keep in mind I tested the BOV's years ago now so the data is not fresh in my mind. I agree, volume has everything to do with it.... especially pre throttle body intake volume. If you have a small volume charge circuit it takes less RPM of your turbo to create the same amount of pressure. Most common turbo systems you have longer charge tubes and an intercooler to fill before positive pressure(boost) is seen. Take an X3 for example... very similar turbo size and much more volume to fill before the air hits the throttle body. We can use a much weaker spring in our BOV for that model. Unfortunately I don't have any data on the actual air pressure in the charge tube vs the vacuum in the intake plenum at cruising with no BOV or a very stiff spring to tell you if there is actual pressure or not. Either way the air created from the turbo is not needed. If it was, the torque request would go up and the throttle would then open.

I have had many customers with other BOV's with stiff springs that threw over charge temp codes on 2016's (the 17-Up now ignores that code for the most part) and when switching to our valve or a softer spring, the issue went away. We have logged this many times with different setups. I have not looked into it to see if this is an issue because it does not recirc when cruising (causing pressure on the back side of the throttle blade = heat) or if it's simply that the stiff spring does not open the valve enough or quick enough when off the throttle... possibly a combo. I do know that the softer spring solves the issue on multiple occasions. Keep in mind the spring pressure on our valve is greater than the stock one still, but not overkill.

The orientation of the valve really does not seem matter as I did some logs with this the other week. I had not tested like that in quite some time and maybe not even with our current style BOV. With the piston(bottom) towards the charge tube you have a much more metallic sound with the BOV but the function seems to remains the same. I switched back to the stock orientation as the sound is better IMO. We have been running this way for years without any issues and many BOV's sold.
 
#20 ·
This thread really has me intrigued. I’m new to RZRs but not necessarily new to turbos, been a farm guy all my life an Turbo diesels ain’t nothing new but these lil engines do operate a little bit differently. Also been really thinking a BOV for my 19 xp4t after I tune it an do exhaust. So guys please keep up this good info
 
#23 ·
If it's not broke don't fix it. On a stock machine there is zero benefit to replacing the stock diverter valve. The stock valve is manufactured by Bosch and works well at the stock boost levels. Anyone wanting to raise the boost or vent to atmosphere then I would suggest changing it. Personally I like the RKX for the quality and cost. You can read the write up I did and how to set it up here: https://www.rzrforums.net/rzr-xp-turbo/534688-inexpensive-bov-done-right.html
 
#24 ·
I'm in Joe's camp here and appreciate all his work and data on the BOVs that he has provided over the last year. It never made sense to me to run it as Polaris does from the factory. I run mine as described in his pics. The first time I took my stock one-off, I thought it was installed wrong from the factory. I was in disbelief that's how they came. That was before all these posts to educate myself to find out the why.

Beyond that though, I've had many turbo applications yes on bigger engines and they too were always this way. While I don't claim to be a turbo expert, just evaluating a BOV and knowing how they open and close, it seems that would be the only way to run them (obviously it isn't). On my Polaris, I run a stiff spring cranked down to minimize leak. It has been this way for some time now with no detected issues. I run it with AA oversized turbo and stage 3 tune for that turbo.
 
#26 ·
So here was my experience this weekend, I recently installed RZR Joes BOV due to my last one leaking boost ( it was one I had picked up from KMS about 2 years ago with the o-ring seal at the bottom of the piston) I installed a .400 spacer under the spring and re-orientated the BOV with the piston side installed in the charge tube, I tested it in the garage and was pleased that it no longer leaked boost at all yet dumped pressure whenever I let off the throttle. I was looking forward to trying it at our dunes here in Utah so last Saturday I was leading the pack and we were maybe 5 minutes into a hard charging ride up some of our long steep stuff and my car throws a code and cuts the power back to the point I had to turn out of a climb and the whole group comes to a halt. I was like WTH, never seen a code 2629 before. I looked it up and it’s for for high compressor charge temps, so I decided to uninstall my stage 2 AA tune and ran the stock tune for the rest of the day( definitely not as fun) and did not have anymore issues. Just for the record I have run Mikes tune for about 2-1/2 years on my 2016 so the only variable here was the BOV and BOV orientation.
I did not remember reading this particular thread in the past but Mike’s observations in the previous thread are spot on, for whatever reason under heavy sustained loads the intake charge temps really spike under this BOV configuration. I am going back to The Polaris orientation and hope this is the end of that issue.
 
#27 ·
Interesting, I don't suppose you had the data log running? Here's the thing, under throttle and load all BOVs are closed and no recirculation occurs. This is regardless of orientation or BOV. Only under part throttle does the OEM or AA BOVs recirculate. If you change the orientation you will also need to change the spring rate. PM me your #, I'd like to hear more and see if we can figure this out.
 
#28 ·
Did you do any data logs to see what the actual charge temps were? I know Mike mentioned the 2016's had an issue with that, I have and 18 (that ignores the code according to Mike above), but haven't had any issues. Highest pre TB charge temp I've recorded is about 215°F while running the RKX in the same orientation as Joe and yours was setup. When i had the stock valve and charge tube ( I have an agency charge tube now, changed the same time as the BOV), the temps were about the same, maybe 5° cooler at most.

If you want to run the RKX in the stock orientation & function, just change the spring/spacer combo to open at around 12 in/hg, it will leak a little boost at idle/low throttle like the factory does there and function in the OEM manner.
 
#31 ·
So just a few weeks ago I was trying to do some more tweaking of my tune on my turbo rs1. Since day one of me putting this setup together I've had issues with fueling in vac and light load. I would get lean tip in spikes or even slight lean condition light load from 1-3psi of boost. I was also getting random compressor surge on let off above 12psi of boost or so. I was running a high dollar turbosmart "plumb back" compact bov. What I never realized is this valve works just as the factory Polaris valve. It is open at idle as well as light load. After multiple spring changes and shims I gave up. I bought a turbosmart duel Port bov that works like a normal bov. Now all of my weird erratic fueling issues are all gone. Just thought it was weird I never came across this posting until now.
 
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