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Old 08-24-2006, 11:08 AM   #1
scooby24
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Default Ultimate BOV discussion

Yup, there's a million of them...but what I'm about to post may prove the stock BPV does not need the "crush" mod or the ".16" mod and can hold boost to infinity.

Posted on a local forum by a well respected and knowledgeable employee of a major supercharger manufacturer

Quote:
Originally Posted by Local Boards
More Supercharger and Turbo 101:

There is NO such thing as a "valve that holds more pressure"
They only FLOW More air.....
Or make a cool, different sound......


Forget what you hear on the internet,
Forget what you read in the ads in Magazines,
Forget the hype, and girls in tiny clothes at the NOPI events,

Just go with the basic facts of how the thing works.


When we design a new Blow off/diverter/byspass valve. We only design it for a couple things.....

- Max FLOW (CFM)
- Longevity of internal parts
- Mounting methods
- Astetics, and Branding Placement.


So do you see what i didn't list..... PRESSURE. Pressure isn't something of a concern, because the valve it's self isn't what holds back that pressure. Mother nature does that for us, along with common physics.





So the way a BOV/Bypass Valve/Diverter Valve works is........

By having a Vacuum / Boost line on the TOP side of a Rubber Daiphram.
This Diaphram is ALWAYS bigger then the hole the air is coming through.

The reason for that is, that is the way it gets a Mechanical Advantage over the air that is trying to force it open......


Here comes the math section

Here is the math for the amount of pressure that holds it SHUT
So think about it this way..... if you have 30psi of boost.
And the diaphram with a 2" diameter around on top. (this is just an example)

That means you have 3.1416" of surface area on the Diaphram.
Take that surface area, times the amount of boost pressure.

And you have 94.24 lbs of pressure forcing that diaphram shut.


Now here is the math at how much pressure is trying to force it open

So lets say on the inside of the valve, you have a 1.25" Diameter Hole.
This is the hole that leads to your tubing that is under boost pressure.
When this hole opens, boost will be let out.

Ok, so the surface area of this hole is 1.22" square inches.

Now take that times that same 30psi of boost pressure. And you get.

36.6 lbs of force trying to blow that hole open......


So take the two pressures, and subtract the holding pressure vs. the pressure forcing it open.


94.4 lbs ( - ) 36.6 lbs = 57.8 lbs of force that HOLDS that valve SHUT.



And this just with the mechanical advantage.... NOT counting the spring inside the unit. So now you can see that the Spring does NOTHING in terms of holding a valve shut.....


The more boost you make...... the harder it wants to stay Shut!



The only reason to get a different valve, is to get one that flows more air..... (when you let off the throttle) Or one that sounds better, or looks prettier.......


You want to get a valve that is the correct size...... if it's to big... It will be sluggish.....and you will have "tip in throttle issues"
If it's to small, when you let off you will still get compressor surge, because it can't dump enough air fast enough.



Moral is... if you had a boost leak at your valve, and now it's gone....
It's only 1 of two things.....

1) Your boost refrence line (vacuum/boost) was not on all the way.....
2) One of your clamps was loose on your tubing.....


Your valve works the same way both ways, and it holds the same pressure no matter what. Plus like I said..... it would hold 150psi if it needed to. Because of the way they work..... they will always stay shut.......... Unless there is a hole in that diaphram, and it can't hold it's mechanical adantage.




I hope this helps..... I am no teacher......


But I want to make sure that the stuff said in "AUTO TECH"
Is actuall Tech, and not just made up internet bs, or marketing bs.



Thanks
XX
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Old 08-24-2006, 11:17 AM   #2
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so why does the stock BPV "leak?"
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Old 08-24-2006, 11:21 AM   #3
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Quote:
Originally Posted by Capt Crunch
so why does the stock BPV "leak?"
Theory/speculation/lies?

Has anyone physically metered air through the recirculating line to see if the stocker is not holding boost?

Only thing I could possibly think of, if his post is truth, is they aren't taking into account the vaccuum on the recirculation line that may be helping to open the valve.
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Old 08-24-2006, 11:27 AM   #4
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Explain to me why my fuel trims at idle changed completely after crushing my stock bpv. They went from negative to positive.

And with the spring set too lightly on my current forge bov, it runs rich at both idle and wot, and idle vacuum is too low.

There is plenty of BS out there on the forums. I think that plenty of people have proven that a BOV can leak. I know I've already proven it to myself.
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Old 08-24-2006, 11:31 AM   #5
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Quote:
Originally Posted by jays05
Explain to me why my fuel trims at idle changed completely after crushing my stock bpv. They went from negative to positive.

And with the spring set too lightly on my current forge bov, it runs rich at both idle and wot, and idle vacuum is too low.

There is plenty of BS out there on the forums. I think that plenty of people have proven that a BOV can leak. I know I've already proven it to myself.
Leaking at idle is different than leaking under boost. At idle there is a vacuum on both lines and if the math is correct..that means there is more vacuum to open the valve than close it...thus that's when spring pressure is important.

So the post refers to leaking under boost only.

How are you seeing it running richer at WOT? Wideband? What are the injector duty cycles showing?
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Old 08-24-2006, 11:34 AM   #6
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I saw it go rich much past 24psi on the stock BPV. I've never seen IDC's go past 70% with my PE850's. Crushing the bov put AFR's right back where they should have been. This is with an LC-1 wideband o2 logging via streettuner, and an XD-1 afr gauge.
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Old 08-24-2006, 11:37 AM   #7
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Quote:
Originally Posted by jays05
I saw it go rich much past 24psi on the stock BPV. I've never seen IDC's go past 70% with my PE850's. Crushing the bov put AFR's right back where they should have been. This is with an LC-1 wideband o2 logging via streettuner, and an XD-1 afr gauge.
So IDC stayed constant and AFR decreased with no other changes than modding the BOV?

That would definately be a valid arguement.
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Old 08-24-2006, 11:46 AM   #8
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Just out of curiosity, what is his position at the major supercharger manufacturer?
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Old 08-24-2006, 11:49 AM   #9
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Quote:
Originally Posted by jays05
Explain to me why my fuel trims at idle changed completely after crushing my stock bpv. They went from negative to positive.
The stock BPV *definitely* is not 100% closed at idle... but it's OK, because the complete system is closed.
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Old 08-24-2006, 11:54 AM   #10
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I wrote this thread:
http://forums.nasioc.com/forums/show...hed+modded+bov

and jays05's post is #15....I thought i remembered him posting.

the fact still remains that bov's DO leak. Aren't there multiple ways to build a bov? such as a diaphram or a piston design. yes some bov's are designed for the higher the boost the higher the clamping force shut. However, are they all like that?
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Old 08-24-2006, 11:55 AM   #11
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Quote:
Originally Posted by scooby24
Yup, there's a million of them...but what I'm about to post may prove the stock BPV does not need the "crush" mod or the ".16" mod and can hold boost to infinity.
You're going to try to prove that people (incuding me) that have experienced a leaky BPV were just making it up?

Counter proof: http://all.dejonpowerhouse.com/edit/BOV1.htm
(tests performed with a 1G DSM BPV, which is essentially the same as ours... possibly sprung differently)


Side note: I'm selling a Subaru BPV equipped with the Dejon leak-stop kit. PM if interested.
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Old 08-24-2006, 11:58 AM   #12
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There stock BPVs are different between USDM WRX and JDM STi. The JDM one uses pressure to hold the valve close, and actually has a softer spring than the USDM WRX one. My USDM WRX leaked at 22 psi. My JDM STi did not at 24 psi.
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Old 08-24-2006, 12:05 PM   #13
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Quote:
Originally Posted by LastResort
Just out of curiosity, what is his position at the major supercharger manufacturer?
not sure the title but it's behind a desk talking to people on the phone.
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Old 08-24-2006, 01:24 PM   #14
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Is it just me or does his math seem off?
A 3" diaphragm, which I assume would be a circle, would have an area of 7.06. Or is my math off? Area of a circle is radius squared times pi, is it not? So... 1.5 x 1.5 x 3.14159 = 7.069.
And the 1.25 circle would be about what he said.
This strengthens his argument.

Of course all that math is irrelevant if it doesn't take into account all of the variables. In this case mainly pressure drop and latency.
Forgive my grammar from here on out as I plan on using sentence fragments

*WOT, the chargepipes are pressurized beneath the BOV/DV

*x number of milliseconds later the intake is pressurized

*during this gap the top of the valve will be under vac and the bottom is pressurized

*if the spring tension is not great enough the valve can open, even if only a few mm

*at this point the surface area that the charge air is acting against will be THE SAME as the top of the piston/diaphragm (or at least in every valve that I have seen)

*now you have a leak at WOT

*And I didn't even take into account the drop in pressure that the top of the valve will see

The pressures that you see, the spring tensions, etc. none of these variables are likely to be constant or linear. The valve may well reach equilibrium with the sealing surface open a few millimeters, allowing a small amount of boost to escape/recirc.


And then we have the variable of DESIGN. What if the surface area of the intake side of the valve and the charge-air side are nearly the same? Will the pressure drop alone be enough to overcome spring tension? What if the valve has a side entry for the charge-air? The surface area on the underside of the piston could be greater than the surface area available for the vac/boost reference to act upon, and spring tension will be the only thing keeping it closed.

A negative cannot be proved, you can only prove that the opposite is possible.

Last edited by patty AT forge; 08-24-2006 at 01:32 PM.
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Old 08-24-2006, 02:51 PM   #15
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i love greg and kcsr
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Old 08-24-2006, 02:56 PM   #16
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Quote:
Originally Posted by wrxwhat
i love greg and kcsr
Hi Trevor! :wave:
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