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Should a BOV be open at idle or not?


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I did some searches and found varying opinions and it seems that it really does not matter one way or the other. At idle, my BOV is about 1/2 way open and I can hear it whistle/chirp as it opens when I let of the gas, even if I am not in boost. It's kind of annoying. I already have about an inch of washers behind the spring and it's still open at idle. It holds boost fine, I just think it opens too easily during moderate driving with little or no boost being generated.

 

When I hold my hand at the BOV exit and rev the engine lightly to about 2k in neutral, I can feel it blowing the turbo-air out after I snap the throttle shut.

 

It's a TurboXS Racing Bypass Valve Type-H34 vented to atmosphere. I want some opinions/ideas/thoughts on BOV operation.

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Mine is just barely open at idle. If I set it tighter it is too stiff to avoid surge on throttle lift, with low or no boost, but close to boost. If it is above 10 psi I can run it stiff but I can't shift at 10 psi on every shift. I have seen other bov's that are slightly open at idle, enough to mess with the MAF.

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Not sure if this will help, but my friend had the same problem with his old-school HKS BOV. It would be halfway open at idle, so it really messed with the stock FI.

 

There are sometimes two vacuum ports on BOV or re-circulation valves. We ended up teeing them and the valve stayed shut at idle.

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Datman, I love that sig pic!

 

I use MSII with no AFM, so air leaking in through the BOV is not an issue except for unfiltered air at idle. Maybe I'll try really clamping down on the spring pressure to keep it nearly shut at idle. At part throttle shifts, personally, I feel it vents too much. Also, it gets into a flutter at part throttle shifting with light or zero boost. I think it's just the BOV bouncing off the column of air in the intake tract. I find it hard to believe that it's compressor surge because all of this is at fairly low loads and engine speeds.

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Datman, I love that sig pic!

 

Thanks :)

 

This is the style of valve I use....i sticks sometimes in the cold mornings, apart from that it works well.

http://cgi.ebay.co.uk/COLLINS-PERFORMANCE-DOUBLE-PISTON-DUMP-VALVE_W0QQitemZ250145039720QQihZ015QQcategoryZ72205QQrdZ1QQssPageNameZWD1VQQcmdZViewItemQQ_trksidZp1638Q2em118Q2el1247

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Datman, I love that sig pic!

 

At part throttle shifts, personally, I feel it vents too much. Also, it gets into a flutter at part throttle shifting with light or zero boost. I think it's just the BOV bouncing off the column of air in the intake tract. I find it hard to believe that it's compressor surge because all of this is at fairly low loads and engine speeds.

 

 

The flutter is surge. At light or szro boost and lower engine speeds is when mine does it if too soft. It doesn't have to be under boost. You should hear the evel a T72 makes when it flutters :shock:.

 

 

Datman, that is a cool pic and nice car. I like the smoke stack in the background.

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Your problem is the Bypass valve is designed to be open at idle. The spring in it is not the same as the blow off valves. You can get a stiffer spring from us for a couple of bucks, or just shim it a lot.

 

 

I had a hunch that bypass valve were sprung differently but you don't really know until you've seen both. I will try a stronger spring and/or more shimming. Thanks.

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Yeah, a bypass valve is actually designed to remove load on the turbo during incipient spin-up, and close as manifold vacuum nears 5" Hg or thereabouts. They will open at any sort of lift throttle to keep the turbo spinning fast, ready for reboost.

 

This is how mine was set up on the 73 ZT, and I really liked it. I had a separate recirc hose that had a small K&N filter on it, before finally getting one of the big 3" Cones and 3" inlet pipe to allow a proper recirc line that blew into the turbo inlet to prespool the impeller wheel and connecting it to that instead.

 

In either case, you need some sort of filtration on the bypass valve if you have it open at idle. If you recirc it, take the time to do it right and aim the blowdown to prespin the compressor wheel properly instead of simply dumping in the inlet piping to the turbo---all that does it create flow disturbances.

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Aim it to re spin the compressor? Not that much air is diverted and it would have to be dumped into a higher pressure turbine side, not the driven side. It's like dumping a big fart. Not much you can do with a small shot of hot air. They also aren't positive discplacement. The bov opening doesn't keep it spinning faster it lets the volume go somewere so the air doesn't stall. I shouldn't say doesn't keep it from spinning fast. If one could measure the rpms (Turbonetics has turbo tachs) it might show up there but that's it. The only way to keep it fully spun is keep load on it, flat shift.

 

As for the filter. The amount of air coming in through a bov at idle or cruise is next to nothing, like having a vacuum line off. The majority is still coming in through the filter. I guess it's one of the hybridZ over kill things.

 

 

 

Yeah' date=' a bypass valve is actually designed to remove load on the turbo during incipient spin-up, and close as manifold vacuum nears 5" Hg or thereabouts. They will open at any sort of lift throttle to keep the turbo spinning fast, ready for reboost.

 

This is how mine was set up on the 73 ZT, and I really liked it. I had a separate recirc hose that had a small K&N filter on it, before finally getting one of the big 3" Cones and 3" inlet pipe to allow a proper recirc line that blew into the turbo inlet to prespool the impeller wheel and connecting it to that instead.

 

In either case, you need some sort of filtration on the bypass valve if you have it open at idle. If you recirc it, take the time to do it right and aim the blowdown to prespin the compressor wheel properly instead of simply dumping in the inlet piping to the turbo---all that does it create flow disturbances.[/quote']

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I originally had it "Y" into the compressor intake boot blowing back at the compressor, but when I went to MSII, I redesigned the entire air intake for a cone filter right into the compressor. I replaced a VERY restrictive air intake system with a direct feed into the turbo. I got rid of the AFM. At that point, I let my BOV (recirc valve) breath free.

 

It was then that I noticed flutter when I backed off the throttle under light loads.

 

Why:

a) My turbo was blowing more air at lower loads due to the better intake

B) I could hear it now because my valve was vented to atmosphere

c) both

 

Whatever the cause, I now noticed that the recirc valve sits open at idle. I know it's supposed to let the compressor vent so that it can keep up speed between load demands but does it really need to vent that "sensitively"?

 

By "sensitively" I mean: I am going up a hill gently at 1/4 throttle and I am still in vacuum, when I slight back the throttle, I get lots of flutter. It seems there is no hysterisis in the valve. It acts "nervous"

 

I tried a restrictor orifice in the vacuum/boost line. I tried damping grease on the slide valve. I tried loosening the spring. I have NOT tried tightening it so much that it is closed at idle.

 

 

Bear in mind that this is all when "grocery getting". When I put the hammer down, she holds boost and blows off during shifts like a good BOV.

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Tell all that to Porsche, and remind Corky Bell he's also in error, Clifton.

 

I imitate only the best.

 

Prespinning of the compressor side is easily done with tangential entry of the BOV through an appropriately designed duct. Porsche has done it for years. And Corky's systems all had a bypass setup to assist those older turbos to lower boost threshold by removing incipient spinup loadings by bypassing around the compressor while in most N/A operation loadings.

 

If you saeriously think well over 3 square inches of air DIRECTLY bypassing the air filter is not something that is requiring of a filter, then have at it. Remember air takes the path of least resistance. It will flow through an open bypass valve before the filter media...it's a bit more than 'an open vacuum line' if the thing is 1/2 open and even a small one at 19 to 25mm discharge opening. IMO, that's a considerable air leak past your primary filtration.

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Not to hijack the thread, but what is the best way to plumb a 1" recirculation line back into the stock AFM/turbo boot? Recirculating my BOV is has been on my to-do list for a while now. I've spent hours at various hardware stores looking for a fitting that would work without creating a restriction in the the pipe. I've also looked through the Jeggs and Summit catalogs. There must be some type of "hose tap" that could be used for this purpose. I may have to replace the stock boot with a hardpipe with the proper fittings but didn't really want to resort to that.

 

I remember watching an episode of Horsepower TV a while back where they were plumbing a vacuum line into an existing hose. They drilled a hole in the rubber boot and installed some type of tap fitting that matched the interior curvator of boot which was "supplied by the manufacturer" of the device they were installing. Any ideas?

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