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beepbeep beepbeep is offline
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Join Date: Jun 2001
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Originally Posted by sand_man View Post
BTW Goran, it's hard to tell from this whole internet thing, but I didn't mean my tone to have a defensive flair. I realize you know these fuel systems VERY addition to turbo charging as a whole, and I respect your knowledge. And I do understand what you are describing. I'm merely pointing out what one of the more experienced shops in my neck of the woods explained when I was describing my plans to change intercoolers and run a different by-pass valve. Your explanation could very well be right one...



Didn't follow the thread for a while. I did a mental misstake when I wrote the explanation. I'll try to explain what happends when fitting open BOV more correctly further on. First, a naming convention check.

BOV- Blow Off Valve (in my textbook). A thing that releases the air trapped between turbo and throttle that was slapped shut after boost run. There are two main designs.
1. Recirculating one: It re-routes air back inte system. Between CIS/AMM/whatever and turbo. Fitted to ALL OEM cars by factory (930 included).
2. Open one: same as above but it doesn't return the air into system but just releases it, making a whistling sound. Used by youngsters in belief it will make their cars go faster. NEVER used on OEM cars, sometimes used on racing cars.

So, what happends when you run you car to full boost and then instantly step off the accellerator?

1. Turbo is spinning upwards 150000 RPM and boosting. It's shaft has certain inertia.

2. You step off the throttle, shutting air access to engine. Turbo still spins at 150000 RPM.

3. Shaft inertia keeps turbo pumping the air against closed throttle. If nothing happends, impeller cavitation (=stall) occures and turbo eventually slows down. Sometimes a lot. It's blade tips are made of aluminium and cavitation might damage them. Thus we have BOV that opens up this tract and lets air escape.

Now to the interestion point. Let's check OEM recirculating BOV:

Air is drawn trough CIS flapper into turbo. The more air is drawn, the more fuel is injected. You step off the throttle, air gets re-routed into turbo intake. BUT, this is after CIS flapper. So net result is that flapper doesn't draw any more air. Existing air is only whirled around downstream from flapper...everything is nice and easy.

Now you want to be a ricer and install fancy green-eloxated open BOV with Japanese letters on it to make funny sounds while shifting. What happends? Well, it will release the air packed by turbo slowing down into athmosphere. Unfortunately, turbo will draw all that "funny sounds"-air trough the flapper. Flapper believes all that air is being drawn into engine instead of being used for noise. So CIS injects corresponding amount of fuel into non-assuming engine which has it's throttle closed and draws almost no air at all. So engine suddenly has almost no air and heaps of fuel injected into it and goes overrich, burping, farting and burning out the things. It might also produce flames....which some people fancy (funny noises + flames = COOL!).

In reality, there are no gains with open BOV except for noise. In worst case, it will use more fuel, burn out things and make turbo slow down more.

It WILL work without going rich on cars using speed/density systems though as they don't have any AMM or flapper between turbo and filter assembly. So it's possible (but quite cheesy, IMHO) to use open BOV on EFI cars fed by MAP-sensor. Not so on CIS.

Check out which BOV Audi used on their IMSA-cars:
Thank you for your time,
Old 01-17-2008, 04:44 AM
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