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Author Topic: Blow Off Valve  (Read 2007 times)

Kiwimagic

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    • 1990 SA SOHC, 1991 SA Verde DOHC, 1994 XR2 Turbo
Blow Off Valve
« on: July 02, 2020, 07:34:58 PM »

Hi All
I have just purchased a VJ11 Turbo which I will fit to the VJ14 to make the hybrid for the Capri.
My questions are;
What is the best Blow off Valve to use with this setup.
What should I be looking for. There are so many variants out there.
Adjustable or fixed boost.
What should the max boost be.

cheers
Kiwimagic
 
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Rocketman

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Re: Blow Off Valve
« Reply #1 on: July 02, 2020, 09:18:43 PM »

Your post is a bit confusing. I'm reading blowoff valve but the context you are using in implies a wastegate?

Blowoff valve vents air to prevent overpressure in the intake piping when shifting or letting off the throttle.

The wastegate controls overall boost pressure by bypassing exhaust gasses around the turbocharger's turbine.

If I'm reading wrong forgive me.

Regarding blowoff valves:
I'm not sure what is out there on the modern market, but the classic BOV style that seemed to work best were the HKS SSQV and the Greddy type RS.
A lot of folks nowadays will argue that BOV are terrible for performance ( I beg to differ) and that a recirculating type Bypass Valve (BPV) is the superior option.
The stock type on the Capri is a Bypass Valve. I have seen the Forge brand recommended for an aftermarket bypass valve, the stock one is garbage.
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Kiwimagic

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    • 1990 SA SOHC, 1991 SA Verde DOHC, 1994 XR2 Turbo
Re: Blow Off Valve
« Reply #2 on: July 03, 2020, 05:11:39 AM »

hi
Yes I just read it myself. It is a bit confusing.
The way I understand it is;
The BOV releases pressure build up in the inlet manifold when you have taken your foot off the accelerator, to change gear or slow down. As the pressurized air now has no where to go it will try to go back through the compressor turbine and thus slowing the turbine.It then is released by the BOV into the atmosphere or recirculated back into the air intake. The BOV is actuated by pressure in the inlet manifold.
The waste gate is to divert the exhaust gas to prevent the turbine over spinning and producing too much boost. This is controlled by a boost controller which is connected to the waste gate actuator. This boost controller is triggered by the pressure in the inlet manifold.
Does the BOV work in conjunction with the waste gate. Too much pressure in the intake manifold opens the waste gate and opens the BOV.
 Is this where the term Turbo Lag comes from. The time taken for the air pressure "boost" to build up.After the waste gate and the BOV have both closed.

This is all new to me so please correct me if I'm wrong.
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Rocketman

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Re: Blow Off Valve
« Reply #3 on: July 04, 2020, 01:11:33 AM »

Kiwimagic,

Not quite, they are two completely independent systems.

The wastegate controls boost pressure. The boost source is the inlet pipe across the valvecover, so it is seeing boost pre-inlet manifold. You can see the small line with the nipple just over top of the turbocharger, with a line that goes down to the turbocharger's wastegate actuator. When boost pressure reaches the maximum pre-set in the actuator (by spring pressure) the diaphragm pushes a rod, which opens a port on the turbine side of the wastegate to allow exhaust to bypass the turbine. This creates a self-regulating feedback loop to control boost pressure. Installing a Boost Controller in the line to the actuator fools the actuator, allowing you to accurately raise the overall boost pressure in the system.

The blowoff valve does nothing to control the boost, it just there to prevent overpressure in the inlet pipes or stalling the compressor in the turbocharger. It vents or bypasses the already moving air so that it doesn't build up in the inlet pipes when the throttle suddenly closes. It vents air pre-inlet manifold, using a vacuum line from the inlet manifold. The throttle closing generates vacuum that goes to the BOV. The pressure differential in the diaphragm of the BOV between the inlet pipe & inlet manifold vacuum are what cause it to actuate.

The BOV does nothing to control the boost, it is there to save the intake piping & turbo compressor

Turbocharger lag comes from the inertia of the turbocharger itself, it takes time for the turbine to get the compressor up to speed. It does not take much time for the air to build up, it takes time for the machinery that moves the air to come up to speed
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greywolf27030

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Re: Blow Off Valve
« Reply #4 on: July 04, 2020, 10:40:14 AM »

Does the BOV become more important the higher the boost pressures?

Jack Byrd
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Jack Byrd

Rocketman

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Re: Blow Off Valve
« Reply #5 on: July 04, 2020, 09:48:11 PM »

Sort of, but not really? The higher the boost pressure the more it has to flow, and the louder it will be. But our stock or even upgraded hybrid turbos are so tiny that it's basically irrelevant. If you have a big inline six or a twin turbo big block v8 with some massive turbos, yes you have to get a matching BOV that can flow appropriately

The stock bypass valve is tiny, and has a convoluted flow path inside, so while you can get away with it on a stock turbo at higher boost, it's probably a good idea to upgrade it. There are some stock/OEM bypass valves from other cars that have a better design that will drop right in its place. I can't remember which ones off the top of my head. This will also get rid of that annoying "Mooo" noise you hear with a cut airbox or cone filter while running the stock bypass valve. It's the rubber diaphragm oscillating inside the BPV.

You will have to tune the BOV a bit. On most BOV's there's an adjustment screw to adjust the spring pressure which changes it's sensitivity. You want it to stay closed during boost and open sharply on shifts. If you have it poorly adjusted you may notice stalling during shifts. It's not a hard thing to dial in, a couple passes on a quiet street and you'll have it set.
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greywolf27030

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Re: Blow Off Valve
« Reply #6 on: July 05, 2020, 08:26:50 AM »

thanks
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Jack Byrd