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BOV
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Peter  
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PostPosted: Tue May 14, 2002 11:23 am    Post subject: Reply with quote

I am thinking of adding a blow-off valve ("BOV")to my 931. After checking the factory manuals, the 931 has what is called an anti-backfire valve built into the turbo unit. Is this the same thing?
-Peter A. Holiat
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Smoothie  
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PostPosted: Tue May 14, 2002 11:55 am    Post subject: Reply with quote

I don't think they're the same. My understanding is that a blow off valve goes on a plastic air box to keep backfires from damaging it when they occur. A BOV would be for a car that's prone to backfire. I don't think there'd be any need for one on a 931 - not likely to backfire (at least mine isn't) (probably due to that valve on the turbo) and the air box is made of metal, but I don't consider myself an expert on this, so hang on for some more opinions.
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Peter  
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PostPosted: Tue May 14, 2002 1:35 pm    Post subject: Reply with quote

You are thinking of the pop-off valve that performance products and others sell that attaches to the plastic airbox on 911 turbos. This is different.
A BOV activates when the throttle is closed during boosting. The boosted air hits the closed throttle valve and shoots back toward the turbo. This surging is damaging to the turbo because it continues to spin and push air forward while the air is being pushed back into it. This surging last for only a short time, but occurs repeatedly and leads to turbo breakdown. A BOV vents the boosted air when the throttle is closed, thus eliminating the surge of boosted air from hitting the turbo. A BOV is intended to help maintain turbo life.
A BOV differs from the wastegate, which dictates the level of boost.
Reason why I ask is because the 911 turbo, and almost every other modern turbo for that fact, have an external BOV. Being that the 924 turbos are old, do they have an internal BOV?
As a point of reference, the recent issue of Excellence magazine has an article about a turboed 914 that is using the CIS & turbo from a 924T. This guy also installed a BOV. So it got me thinking...should we be installing BOVs too?
-Peter A. Holiat
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John H  
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PostPosted: Tue May 14, 2002 3:13 pm    Post subject: Reply with quote

The BOV shouldn’t really be installed on the 924T. The CIS system cannot really handle the BOV, while the BOV only opens for a short length of time it causes the plate in the metering head to flutter and this in turn upsets the amount of fuel being delivered to the engine.
I sold an inter cooler to a guy who supplies hot up kits for 930’s and he had tried this with the 930s and CIS and the result was a few dead motors.
BOV are designed to work with Electronic fuel injection systems and the later 931’s with the by pass valve don’t seem to have problems with the back pressure. I’ve raced mine for quite a few years and found no problems with the turbo caused by backing off on the throttle
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Peter  
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PostPosted: Tue May 14, 2002 3:38 pm    Post subject: Reply with quote

John, it's good to hear that your racecar has not had turbo problems. I guess the anti-backfire valve in these turbos has a similar purpose to the BOV and thus no additional hardware is necessary. However, I read (accuracy always questionable) that many CIS cars had BOVs, and that the air was vented to the inlet of the turbo to stabilize the flapper.
I plan on contacting the guy from the Excellence article (he's on Rennlist & Pelican) and asking him about the BOV he attached to his 914 turbo (along with his configuration for additional fuel enrichment). When I find anything out I will post it here.
-Peter A. Holiat
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TroyDest  
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PostPosted: Tue May 14, 2002 5:30 pm    Post subject: Reply with quote

From what I've seen on my '82 931, I've formed an opinion on adding a BOV. When I first attached my boost gauge upstream of the throttle body, I saw the pressure spike shoot up to 25 psi repeatedly depending on how high I had the boost turned up and how I was driving. It only lasted for a split second then the turbo's internal bypass valve took over. So I think a BOV would help the turbo survive but only if it were a very good, quick acting BOV. Otherwise, the original bypass valve does a pretty good job. As far as what a BOV would do to the CIS plate I'm still pondering.... -Troy
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Peter  
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PostPosted: Wed May 15, 2002 2:36 am    Post subject: Reply with quote

Troy,
This site rates two Bosch BOVs, one from a Porsche that has a "quick response time," which by your post seems what the 924T may need.
http://wwwrsphysse.anu.edu.au/~amh110/Technical_pages/blow_off_valve.htm

Here are some more interesting BOV sites:
http://thedodgegarage.com/turbo_bov.html
http://www.alltrac.net/tuning/bovfaq.html

With a nice DIY BOV install:
http://thedodgegarage.com/turbo_bov_install.html

As for a BOV and CIS, I am not sure. I have read people have done it, but I also trust John H when he says it's problematic. I will be looking into it and post any findings.

-Peter A. Holiat
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Smoothie  
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PostPosted: Wed May 15, 2002 4:37 am    Post subject: Reply with quote

Interesting stuff..Smoothies learning somethin'.

Is a BOV activated just by the surge of pressure that it's there to release (my guess) or is vacuum used as with the bypass valve? I see the vacuum source for the bypass valve is downstream of the throttle (at the bottom of the intake runner to cyl #1). I'd guess that the downstream side of the throttle goes from positive pressure to a vacuum about as fast and at about the same moment that the upstream side gets its' extra blast of pressure. Maybe the two are about in balance (or close enough to not be a problem) at normal boost pressures, but when you up the boost high enough they get out of balance with one occuring before the other? I'm saying maybe the vacuum acting on the bypass valve is slower in relation to the speed at which the air pressure upstream of the throttle builds up when you're running at higher boost pressure. -And just maybe things could be put back in balance by using a larger control line to the bypass valve? -Or an additional CBV (compressor bypass valve) to handle the additional volume of air at higher boost.

Also, just to complicate things, Porsche uses several terms when referring to the valve on the turbo. I've heard of it being called "anti-backfire valve", "bypass valve", and "pop-off valve" in their own manuals.
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numbers  
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PostPosted: Wed May 15, 2002 8:48 am    Post subject: Reply with quote

Yes, and the pop off valve performs the same function as an external BOV. It is just that the external BOV vents to atmosphere, while the internal POV vents to the intake. You will see the peak when the throttle closes, the the POV will open and the pressure will decrease. If anything, you might want to explore a quicker acting POV. By the way, the purpose of a BOV/POV is not so much to protect the turbine blades, as it is to keep the turbine from slowing down during quick off and back on throttle operations. This helps cut down on turbo lag.
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Peter  
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PostPosted: Wed May 15, 2002 11:30 am    Post subject: Reply with quote

A quicker acting POV sounds good. Will help cut down on turbo lag. I also think that it should aid in turbo life. But how would one create a faster acting POV? The parts diagram shows a spring in the turbo outlet side, could changing the spring help?
Maybe Smoothie's suggestion of a larger hose or even a smaller hose to create more pressure and possibly quicker action? It maybe that the POV cannot be changed unless the turbo is rebuilt and the POV is set for higher boost. This may explain why an external BOV or CBV was used in the 914 turbo featured in Excellence.
Anyone have that issue?
-Peter A. Holiat
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Smoothie  
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PostPosted: Wed May 15, 2002 12:33 pm    Post subject: Reply with quote

I think you have to consider the volume of air - if you took a 931 intake full of air at .8 bar and released it to normal atmospheric pressure (1 atm.) the volume of air would be less than the same 931 intake full of air at 1.2 bar released to a pressure of 1 atm. So, what happens with all that extra air at higher boost when the throttle snaps shut? You have more of it both up and downstream of the throttle. The extra air downstream of the throttle is probably slowing the response time of the bypass valve because it's taking longer for the air there to go from positive pressure to a vacuum. In addition, there's more air on the other side - upstream of the throttle. Now it just stays there upstream of the throttle and builds up to a pressure spike because the turbo is still spinning and the bypass valves' opening is delayed by the downstream sides' air not going to a vacuum quick enough.

-But how to take care of the delay in vacuum, (if it's possible) a different cam profile that increases vacuum? -Or a vacuum accumulator with an electric valve wired to provide vacuum to the bypass valve when the throttle shuts. -Or maybe a BOV that releases the air into the intake side of the turbo (as Peter already mentioned). This way, the spike is taken care of and the air which has already been metered for fuel by the CIS is returned to the intake so the air-fuel mix isn't changed.


[ This Message was edited by: Smoothie on 2002-05-16 11:16 ]
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John H  
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PostPosted: Wed May 15, 2002 1:03 pm    Post subject: Reply with quote

The Bentley Turbo Mulsanne mid 80's to 90's and I think current models made a great song and dance about the by pass valve which allowed teh boost back into the compressor side when the throttle was closed to keep the turbo spinning and reduce the lag. Unless you'r constantly applying full throttl and then suddenly backing off and then openign up again the BOV is a waste of money.
One of the guys I race against here has a BOV on his 931 and while he thinks it makes a difference his laptimes etc haven't altered. Also when you drive the car it doesn't seem to have any differnet feel in responsiveness.
I quite often play the throttle through corners keeping the tyres on the edge of wheelspin and with the standard Porsche system I have no lag problems.
I personally won't be installing one till soemone can show real evidence that it is beneficial in lap times.
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Paul  
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PostPosted: Wed May 15, 2002 1:18 pm    Post subject: Reply with quote

FYI some useless info perhaps:

1 bar = 14.7 PSI = standard atmospheric pressure at sea level.

When we boost an engine to .8 bar, the absolute pressure is 1.8 bar (1 bar is 0 on most gauges)
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Smoothie  
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PostPosted: Thu May 16, 2002 12:37 pm    Post subject: Reply with quote

Best thing would be to get the existing compressor bypass valve to respond quicker, but that would require getting the intake manifold (downstream of the throttle) to go to vacuum faster and that could be difficult. I think, if you need it, the BOV releasing to the intake of the turbo idea is a good one and could use pressure to activate the bypass valve quicker. Maybe fit the outlet of the BOV to the end plate of the bypass valve that's opposite the end where the vacuum line is attached. It's pretty easy to get at, so it could be easily removed and drilled and threaded for a fitting. (Or get another end plate like the one on the vacuum side that already has a fitting.) Your bypass valve then would be dual powered - vacuum on one side for normal boost, pressure on the other for higher boost situations and you'd be eliminating the lag caused by the delayed vacuum and also putting that extra air right where it should go at the turbo inlet.

[ This Message was edited by: smoothie on 2002-05-16 14:04 ]
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Peter  
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PostPosted: Sat May 18, 2002 11:15 am    Post subject: Reply with quote

I spoke to the guy with the turboed 914. He is now using a turbocharger from a 944T, which he raves about.
Just a side note he stated that the stock CIS system from the 924T was able to provide enough full enrichment w/o using a 5th injector.
-Peter A. Holiat
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