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post #1 of 25 Old December 10th, 2018, 08:20 PM Thread Starter
 
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Someone explain why the BOV leaks boost

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.
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post #2 of 25 Old July 18th, 2019, 10:24 AM
 
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Re: Someone explain why the BOV leaks boost

Quote:
Originally Posted by Steamfitters38 View Post
Can anyone explain to me why the stock, AA, Evo etc. BOVs leak boost under light/part throttle?

I feel like Iíve read fifteen threads about it but none have really explained why this phenomena is confined to the XPT. Cars and trucks donít do it so why do our RZRs?

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

If Iím making 2lbs of boost under light acceleration, why is the BOV bleeding? Shouldnít 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?

Iím sorry this has been beat to death, but I just donít understand. I canít find these answers.
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-tur...one-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.
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post #3 of 25 Old July 19th, 2019, 10:03 AM
 
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Re: Someone explain why the BOV leaks boost

Quote:
Originally Posted by RZR_Joe View Post
Quote:
Originally Posted by Steamfitters38 View Post
Can anyone explain to me why the stock, AA, Evo etc. BOVs leak boost under light/part throttle?

I feel like Iíve read fifteen threads about it but none have really explained why this phenomena is confined to the XPT. Cars and trucks donít do it so why do our RZRs?

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

If Iím making 2lbs of boost under light acceleration, why is the BOV bleeding? Shouldnít 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?

Iím sorry this has been beat to death, but I just donít understand. I canít find these answers.
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-tur...one-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.
I have my BOV set-up as Polaris did, should I reverse the orientation? I have it connected as recirc currently.

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post #4 of 25 Old July 19th, 2019, 10:31 AM
 
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Re: Someone explain why the BOV leaks boost

Quote:
Originally Posted by Bullitt427 View Post
Quote:
Originally Posted by RZR_Joe View Post
Quote:
Originally Posted by Steamfitters38 View Post
Can anyone explain to me why the stock, AA, Evo etc. BOVs leak boost under light/part throttle?

I feel like Iíve read fifteen threads about it but none have really explained why this phenomena is confined to the XPT. Cars and trucks donít do it so why do our RZRs?

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

If Iím making 2lbs of boost under light acceleration, why is the BOV bleeding? Shouldnít 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?

Iím sorry this has been beat to death, but I just donít understand. I canít find these answers.
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-tur...one-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.
I have my BOV set-up as Polaris did, should I reverse the orientation? I have it connected as recirc currently.
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?
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post #5 of 25 Old July 19th, 2019, 11:42 AM
 
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Re: Someone explain why the BOV leaks boost

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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post #6 of 25 Old July 19th, 2019, 02:03 PM
 
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Re: Someone explain why the BOV leaks boost

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Originally Posted by Max H View Post
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
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:



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 ~13.5Ēhg 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.
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post #7 of 25 Old July 19th, 2019, 02:13 PM
 
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Quote:
Originally Posted by RZR_Joe View Post
Quote:
Originally Posted by Max H View Post
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
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:



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.
That pic is opposite of what Polaris has

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post #8 of 25 Old July 19th, 2019, 02:27 PM
 
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Re: Someone explain why the BOV leaks boost

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 !!
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post #9 of 25 Old July 19th, 2019, 04:23 PM
 
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Re: Someone explain why the BOV leaks boost

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Originally Posted by Bullitt427 View Post

That pic is opposite of what Polaris has
The Polaris OEM diverter is unique, this is an aftermarket. I don't think the OEM unit uses a spring but rather just a diaphragm which is why it would be backwards from the factory. I'll locate an old one and break it open later


Update.
Ah hell I can't find it right now. If you are still using the OEM diverter I would leave it plumbed as factory.

Last edited by RZR_Joe; July 19th, 2019 at 04:35 PM.
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post #10 of 25 Old July 19th, 2019, 04:35 PM
 
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Quote:
Originally Posted by RZR_Joe View Post
Quote:
Originally Posted by Bullitt427 View Post

That pic is opposite of what Polaris has
The Polaris OEM diverter is unique, this is an aftermarket. I don't think the OEM unit uses a spring but rather just a diaphram which is why it would be backwards from the factory. I'll locate an old one and break it open later
As I said earlier... aftermarket BOV is installed exactly like the factory valve as per instructions, but it’s backwards from the posted pics you have provided.

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post #11 of 25 Old July 19th, 2019, 04:36 PM
 
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Re: Someone explain why the BOV leaks boost

Quote:
Originally Posted by Bullitt427 View Post
Quote:
Originally Posted by RZR_Joe View Post
Quote:
Originally Posted by Bullitt427 View Post

That pic is opposite of what Polaris has
The Polaris OEM diverter is unique, this is an aftermarket. I don't think the OEM unit uses a spring but rather just a diaphram which is why it would be backwards from the factory. I'll locate an old one and break it open later
As I said earlier... aftermarket BOV is installed exactly like the factory valve as per instructions, but itís backwards from the posted pics you have provided.
Then it's installed wrong.
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post #12 of 25 Old July 19th, 2019, 04:38 PM
 
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Quote:
Originally Posted by RZR_Joe View Post
Quote:
Originally Posted by Bullitt427 View Post
Quote:
Originally Posted by RZR_Joe View Post
Quote:
Originally Posted by Bullitt427 View Post

That pic is opposite of what Polaris has
The Polaris OEM diverter is unique, this is an aftermarket. I don't think the OEM unit uses a spring but rather just a diaphram which is why it would be backwards from the factory. I'll locate an old one and break it open later
As I said earlier... aftermarket BOV is installed exactly like the factory valve as per instructions, but it’s backwards from the posted pics you have provided.
Then it's installed wrong.
Aftermarket Assassins is wrong?
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post #13 of 25 Old July 19th, 2019, 04:41 PM
 
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Quote:
Originally Posted by RZR_Joe View Post
Quote:
Originally Posted by Bullitt427 View Post
Quote:
Originally Posted by RZR_Joe View Post
Quote:
Originally Posted by Bullitt427 View Post

That pic is opposite of what Polaris has
The Polaris OEM diverter is unique, this is an aftermarket. I don't think the OEM unit uses a spring but rather just a diaphram which is why it would be backwards from the factory. I'll locate an old one and break it open later
As I said earlier... aftermarket BOV is installed exactly like the factory valve as per instructions, but it’s backwards from the posted pics you have provided.
Then it's installed wrong.
Super ATV is wrong? As per their install video
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post #14 of 25 Old July 19th, 2019, 04:45 PM
 
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Re: Someone explain why the BOV leaks boost

That is just a generic BOV, same as SATV Rev 1 BOV and 50 others. They just put their label on it and charge you 4x times the price. As yes, if it's installed where the manifold pressure works on the top of the valve then it's backwards. That was the entire point of the article I wrote.
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post #15 of 25 Old July 19th, 2019, 05:09 PM
 
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That is just a generic BOV, same as SATV Rev 1 BOV and 50 others. They just put their label on it and charge you 4x times the price. As yes, if it's installed where the manifold pressure works on the top of the valve then it's backwards. That was the entire point of the article I wrote.
Hmmmm... even Evo is wrong too huh?
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