mark chandler
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| posted on 13/11/11 at 11:26 PM |
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Assume all vacumn pipes are disconnected and its running 10psi of boost, set the spring a little above this so its not open when running on boost,
however if you ge an overboost situation then is will bleed off pressure.
Regards Mark
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Craigorypeck
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| posted on 14/11/11 at 12:04 AM |
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But the vacuum line is an integral part of the valve operation??? The vac line turns to boost when on throttle and this is what really keeps the valve
shut and the spring just adds a bit of extra pressure to tip the balance..?
From the forge website
The quoted spring pressure (Psi) relates to the un-assisted clamping force of the spring and should NOT be taken as an equivalent to boost pressure
when determining suitability. Once a valve is fitted to a car it is subjected to various pressures that act upon the diaphragm or piston to aid its
respective ability to seal or hold boost. An example of this, would be when using a FMCL007P valve on a 1.8T engine as found in the TT 225hp. The
boost pressure on a "chipped" car can peak at 26psi. The recommended spring would be the Green or Yellow
The green spring is a 6psi item..
Also found this online...
First thing is first: The purpose of the spring in a BOV IS NOT to hold your BOV closed under boost pressure! Selecting a spring based on PSI is
incredibly confusing/meaningless and it makes us wonder if the people who pioneered aftermarket BOV design in the first place even understood how they
work.
All BOVs have a reference line coming into the top of the BOV from your post-throttle body intake manifold. Under high boost, the force holding the
BOV closed is BOOST! The pressure coming through the reference line is equal to the pressure under the BOV piston. Therefore a VERY mild spring will
hold it shut just fine under these conditions.
Rather, the purpose of the spring in a BOV is to hold the BOV closed when your throttle plate is closed, IE during idle and deceleration conditions.
Under these conditions, the pressure in your intake manifold is much lower than the pressure in your intake piping (therefore under the BOV piston) so
the BOV has a natural tendency to spring open. Being open isn't even a problem if you are recirculating your BOV into your intake. However if
you are NOT recirculating, it can either allow unfiltered air into your intake or push metered air out of your intake which can be a problem.
Therefore, your goal when selecting a BOV spring and adjusting the BOV should NOT be to select a spring based on your boost level; that makes no
sense. Your goal is to get the BOV to stay CLOSED during high intake manifold vacuum/closed throttle plate conditions, using the very smallest amount
of spring energy possible. Using less spring energy will allow the BOV to snap open as rapidly as possible when pressure release is necessary.
I'm no expert I tell theee but that makes sense to me..
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Stott
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| posted on 14/11/11 at 09:42 AM |
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quote:
Originally posted by Craigorypeck
I'm no expert I tell theee but that makes sense to me..
That's right, the info you put above is correct, it won't bleed off pressure in an overboost situation regardless of the spring pressure,
that's not what it's there for.
The only way it would bleed boost is if your wastegate jammed shut, and the vac line to the top became blocked/disconnected, and in this scenario the
bov will let all the boost out as the pressure will easily overcome the spring so for the purpose of this example it's an open port, it
wouldn't matter what spring rating your bov had it would leak boost.
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