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Hirevtuner's Avatar
 
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To much VAC on BOV

Can anyone tell me why I am getting to much vac on the line to my BOV. The BOV is aftermarket and I am using the line off the throttle body everyone else is using, but when I connect it the BOV starts to open. Also can I run the BOV without the vac line attached.
Old 07-12-2008, 06:46 PM
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Originally Posted by Hirevtuner View Post
Can anyone tell me why I am getting to much vac on the line to my BOV. The BOV is aftermarket and I am using the line off the throttle body everyone else is using, but when I connect it the BOV starts to open. Also can I run the BOV without the vac line attached.

Opening at idle under vacuum is normal operation for a bypass valve. Internal spring pressure closes the valve as you lay int the throttle and vacuum decreases, and then boost holds it closed as the pressure build in the manifold.

"No" to the second question. Must have a post TB reference line for the valve to function properly.
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Old 07-12-2008, 06:57 PM
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Thanks Boxxersix, I had the BOV vent open cause I wanted some noise but from what you said I need to vent it with a line back to the intercooler. I'm sure this is why it it running rough with the vac on the BOV.
Old 07-12-2008, 07:11 PM
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If you car is still CIS, you shouln't have an open BOV. You must recirculate it.
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1981 UK 930. G50/01 shortened, 964 3.8RS Fibreglass Body Kit, 18" Alloys 8.5" F & 10" R, 225's F & 285's R, Special Colour Metallic Blue Paint, FIA Sparco Evo's, A/C and Air Pump removed, Electronic Boost Controller, GHL Headers, Tial46 WG.
Fitting - New service kit.
Needs Fitting - Innovate XD-16 Kit, Kokeln IC. Stephen's K27 HFS, EVO Intake Assy & his Modded USA Fuel Head.

1983 UK 911 3.2 Carrera Sport Coupe. Black, Black Leather with Red Piping, Black Alloy Gear Knob, K&N Air Filter Element, Turbo Tie rods.
Needs Fitting - K&N CO Sensor, Round A/F Dial Gauge, Factory Short Shift Kit.

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Old 07-13-2008, 12:44 AM
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Vented it back to intercooler and all is well, not sure what I was thinking.
Thanks for the info.
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Old 07-13-2008, 09:01 AM
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Vented it back to intercooler and all is well, not sure what I was thinking.
Thanks for the info.

I don't know why this type of piston bypass valves are setup like this on the car(even stock from the factory I see it)......but the bottom of the piston on a valve like this should face into the boost pressure so that when the there's a differential pressure change in the top chamber the valve reacts quicker. Don't get me wrong it does and is working in the orientation you have there in the picture, but the vale is designed to work flipped around.

I would personally switch the valve ports around(bottom port to the IC and side port back into CIS intake duct). The small signal line is fine(hooked up from the TB).

There are only a couple valves on the market designed to have boost pressure holding the valve close, Synapse Syncronic and HKS SSQV. Those are pull type valves and use boost pressure to push the valve closed and seal.
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Old 07-13-2008, 09:39 AM
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Adam, is one of the reasons they did this so that when the car is off boost that air can bypass the turbo and the IC? This would help when pulling away in traffic, parking etc etc as the engine would not be under quite so much vacuum. Of course the air drawn in will not have gone through the turbo and picked up any heat from the turbo too.

I personally would fit it the way you describe for outright performance though
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1981 UK 930. G50/01 shortened, 964 3.8RS Fibreglass Body Kit, 18" Alloys 8.5" F & 10" R, 225's F & 285's R, Special Colour Metallic Blue Paint, FIA Sparco Evo's, A/C and Air Pump removed, Electronic Boost Controller, GHL Headers, Tial46 WG.
Fitting - New service kit.
Needs Fitting - Innovate XD-16 Kit, Kokeln IC. Stephen's K27 HFS, EVO Intake Assy & his Modded USA Fuel Head.

1983 UK 911 3.2 Carrera Sport Coupe. Black, Black Leather with Red Piping, Black Alloy Gear Knob, K&N Air Filter Element, Turbo Tie rods.
Needs Fitting - K&N CO Sensor, Round A/F Dial Gauge, Factory Short Shift Kit.

http://www.danasoft.com/sig/Iamnotanumber.jpg
Old 07-13-2008, 12:43 PM
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Adam, is one of the reasons they did this so that when the car is off boost that air can bypass the turbo and the IC? This would help when pulling away in traffic, parking etc etc as the engine would not be under quite so much vacuum. Of course the air drawn in will not have gone through the turbo and picked up any heat from the turbo too.

I personally would fit it the way you describe for outright performance though

In either orientation the valve still opens at idle to allow an air bypass, it's just that when the bottom port is place to the boost pressure side(intercooler), upon a differential pressure change in the top chamber(throttle lift) not only does the valve have vacuum pulling the piston up to open but also has the boost pressure/flow pushing it open at the same time. In most cases with this type of valve(like the Forge DV pictured just above) this results in a faster reacting valve. No downsides at all to orienting it in this manner


the only two types of valves that MUST be positioned the other way are the pull type valves(HKS SSQV, Syncronic, etc) as they have a valve that looks just like an intake or exhaust valve and uses the boost pressure to seal against leaks. It's why those types of valves can run insane 60+psi boost pressures(diesel stuff) and not leak. They don't need spring pressure to hold the valve closed like all other piston type valves do
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Old 07-13-2008, 05:41 PM
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Thanks Adam, that makes sense.

Do you think I'm right about the valve helping more when fitted one way than the other when off boost but no high vacuum (pulling away from standstill etc etc)?
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1981 UK 930. G50/01 shortened, 964 3.8RS Fibreglass Body Kit, 18" Alloys 8.5" F & 10" R, 225's F & 285's R, Special Colour Metallic Blue Paint, FIA Sparco Evo's, A/C and Air Pump removed, Electronic Boost Controller, GHL Headers, Tial46 WG.
Fitting - New service kit.
Needs Fitting - Innovate XD-16 Kit, Kokeln IC. Stephen's K27 HFS, EVO Intake Assy & his Modded USA Fuel Head.

1983 UK 911 3.2 Carrera Sport Coupe. Black, Black Leather with Red Piping, Black Alloy Gear Knob, K&N Air Filter Element, Turbo Tie rods.
Needs Fitting - K&N CO Sensor, Round A/F Dial Gauge, Factory Short Shift Kit.

http://www.danasoft.com/sig/Iamnotanumber.jpg
Old 07-14-2008, 10:13 AM
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Thanks Adam, that makes sense.

Do you think I'm right about the valve helping more when fitted one way than the other when off boost but no high vacuum (pulling away from standstill etc etc)?

Well pretty much the instant you hit the throttle to accelerate the valve closes instantly, so no air would bypass through the valve and into the IC(subsequently bypassing the turbo as you suggested) The valves are pretty much setup so that they open when the plenum hits anywhere from 7 to 15hg, pending on the spring pressure and any adjusting shims that are above the piston(top chamber)

So if you have a super light spring in the valve then yea it might stay open long enough to allow a little air bypass but typically the spring is a bit stiffer to keep the valve sealed against boost pressure and as a result usually once the vacuum drops below the 10hg mark the valve just closes up. If you're accelerating at that low of a throttle input from a stop, the people behind you might think granny is driving

Yea your thinking is correct, but as to whether it is doing that really just depends on how your particular valve is adjusted. You really want the lightest amount of spring pressure possible that still provides enough sealing pressure at the valve seat. This give the best(quickest) valve response. These types of valves usually have a brass or aluminum piston that seals into an aluminum cut seat at the bottom of the main body, so it does take some pressure to maintain a good seal unlike if it had a rubber or some other "soft" type sealing surface.
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Old 07-14-2008, 10:28 AM
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At least the way Porsche do it no boost will leak/escape and get wasted. Decisions, decisions, decisions...
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1981 UK 930. G50/01 shortened, 964 3.8RS Fibreglass Body Kit, 18" Alloys 8.5" F & 10" R, 225's F & 285's R, Special Colour Metallic Blue Paint, FIA Sparco Evo's, A/C and Air Pump removed, Electronic Boost Controller, GHL Headers, Tial46 WG.
Fitting - New service kit.
Needs Fitting - Innovate XD-16 Kit, Kokeln IC. Stephen's K27 HFS, EVO Intake Assy & his Modded USA Fuel Head.

1983 UK 911 3.2 Carrera Sport Coupe. Black, Black Leather with Red Piping, Black Alloy Gear Knob, K&N Air Filter Element, Turbo Tie rods.
Needs Fitting - K&N CO Sensor, Round A/F Dial Gauge, Factory Short Shift Kit.

http://www.danasoft.com/sig/Iamnotanumber.jpg
Old 07-14-2008, 02:36 PM
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