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Bov / dump valve position and diameter

 
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Dutch924-racer  



Joined: 23 Jul 2007
Posts: 1045
Location: The Netherlands

PostPosted: Wed Jan 22, 2020 7:45 am    Post subject: Bov / dump valve position and diameter Reply with quote

A question for 931 with a non original turbo.

You read different stories about bov/ dump valves. Some say it need to be as close to the throttle body as possible, others say it needs to be just after the turbo (just like the original design).

The diameter of a 25mm (inlet size) bov enough?
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937 trackday car
Series 2 engine
951S turbo and intercooler
937 DITC
Fast road camshaft
Forge recirculation valve + boost controller
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Fasteddie313  



Joined: 29 Sep 2013
Posts: 1843
Location: MI

PostPosted: Wed Jan 22, 2020 8:55 am    Post subject: Reply with quote

I have a 25mm BOV dumping to atmo just before my TB..

It is enough to dump off atleast up to 15-16 lbs of boost in my experience to prevent rev-hang problems..

Recirculating it causes lean spikes when it blows off back under the AFM, slapping the plate closed, but lean doesn't really matter under no-load conditions..
I also run complete fuel cut on decel, iirc anything under 33 KPA at <1.5% throttle or something goes complete fuel cut at no load..

I have sense removed the recirculation component and have been tuning for it only for the fact that I like the blow off sound...

I have found that in order to run with the BOV dumping to atmosphere I have to set the BOV spring VERY tight, to the point of even using shims to increase spring tension, to keep the darn thing closed under light load and light decel..


With a normal MAP tuned engine it doesn't matter if air gets sucked in the BOV under light load conditions, or slightly blows out under decel or whatever, but I experience that with CIS the BOV eating any air or expelling any air under average conditions messes the tune all up badly, mostly causing very lean conditions on slight decel due to the BOV opening and letting more air in unmetered..

So basically in my experience to dump to atmo the BOV needs to be very heavily sprung to keep a good predictable tune, which is also going to slightly hurt spooling performance between shifts compared to a recirculating setup I believe.. But sounds real good..

I have made a bunch of logs of different BOV settings to study in order to understand these effects and have settled on running a very tight open-to-atmo BOV..

This cheap bov (actually pretty nice IMO)
https://www.ebay.com/itm/Wilsondyno-25mm-Turbo-Dump-Blow-Off-Valve-Kit-Aluminum-Universal-for-Golf-Polo/111792777503

k26/6 951 compressor on a 4.10 931 turbo up to around 1.1 Bar..
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Cédric  



Joined: 27 Aug 2004
Posts: 1924
Location: Sweden

PostPosted: Wed Jan 22, 2020 9:24 am    Post subject: Reply with quote

I have the recirc valve close to the turbo, since its the turbo im trying to save here i want to load it of as soon as possible. I use a 25mm forge copy, as far as performance go it works very well. Boost drops instantly and comes back instantly if i let of the throttle and then goes back on full again.

The only issue i have atm is that i get a pop/bang/small fireball in the tailpipe when come of the throttle at full boost. I havent had the time to find the cause of that, maybe next year
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Carrera RSR  



Joined: 08 Jan 2010
Posts: 1885
Location: Somerset, UK

PostPosted: Wed Jan 22, 2020 8:09 pm    Post subject: Reply with quote

I din't want to run a BOV initially, rev hang forced me to. I'm now using GoFastBits/GFB https://gfb.com.au/products/blow-off-and-diverter-valves/respons-tms/ dual port BOV. I looked at many. Chose this as it looked 'nicer' than others. I also wanted the flexibility to recirculation and atmosphere venting and play with variations. Recirc did not work back to A/F meter. Revs hang too high between shifts and when you let off the gas the car surges forward briefly. Gutted as I was looking for a certain noise I got from the stock turbo. The other option with the GFB Response was the adjustability between recirc and atmosphere. So now its venting 100% to atmosphere, its positioned next to the TB and the spring is adjusted to very light so it dumps on relatively light boost. When the spring was stiffened, I still suffer with rev hang. Every car and BOV will perform differently due to differing set ups. I wasn't bothered at protecting the turbo, in fact I wanted more compressor surge noises....

Since taking the pic below I have moved the hose from the lower port to recirc back to A/F meter to venting into the lower engine bay to avoid oil misting in the bay. Also put a blanking plug on the port you can still see open. What I hate about my set up is when boost dumps to atmosphere it sounds like a trucks air brakes venting. Would prefer compressor surge sound than this. My previous stock turbo sounded great.

IIRC Boost tube to BOV =32/33mm. BOV to A?F meter was 32mm reducing to 25mm. Now venting. Using the TB nipple for control line feed to BOV. Its now as short as possible to give instant signal to BOV to vent.

The sound - https://www.instagram.com/p/BxJ9wnfFVx0/?utm_source=ig_web_copy_link

Untitled by stevenmcooper, on Flickr
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1980 931 - forged pistons, Piper cam, K27/26 3257 6.10 hybrid turbo, 951 FMIC, custom intake, Mittelmotor dizzy & cam pulley, H&S exhaust, GAZ Gold, Fuch'ed, Quaife
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Fasteddie313  



Joined: 29 Sep 2013
Posts: 1843
Location: MI

PostPosted: Fri Jan 24, 2020 2:13 pm    Post subject: Reply with quote

Carrera RSR wrote:
BOV


You ever rev you engine fromunder the hoodand watch it?
Does it open at all from just freereving at decel?

Are you still running the decel valve/vacuum limiter valve?
I am not..

Maybe my high intake vacuum due to no limiter is what gave me problems with mine opening too easily until I really sprung it up...

BTW your car sounds good to me..
Maybe you have less turbo sound on blowoff because it isn't at as high of an RPM as your old turbo..
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Carrera RSR  



Joined: 08 Jan 2010
Posts: 1885
Location: Somerset, UK

PostPosted: Fri Jan 24, 2020 6:13 pm    Post subject: Reply with quote

Fasteddie313 wrote:
Carrera RSR wrote:
BOV


You ever rev you engine fromunder the hoodand watch it?
Does it open at all from just freereving at decel?

Are you still running the decel valve/vacuum limiter valve?
I am not..

Maybe my high intake vacuum due to no limiter is what gave me problems with mine opening too easily until I really sprung it up...

BTW your car sounds good to me..
Maybe you have less turbo sound on blowoff because it isn't at as high of an RPM as your old turbo..


Vacuum valve is long gone....

Blow off sound isn’t ‘period’. If you listen to late 70’s/early 80’s turbo race cars they mostly never ran BOV. They had closed systems. I tried to emulate this. The rev hang may be tolerated on track, if they suffered this. For ‘civilised’ road use its not.
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1980 931 - forged pistons, Piper cam, K27/26 3257 6.10 hybrid turbo, 951 FMIC, custom intake, Mittelmotor dizzy & cam pulley, H&S exhaust, GAZ Gold, Fuch'ed, Quaife
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Carrera RSR  



Joined: 08 Jan 2010
Posts: 1885
Location: Somerset, UK

PostPosted: Sat Jan 25, 2020 9:23 am    Post subject: Reply with quote

As for old school turbo sounds......this ticks all the aural boxes

https://youtu.be/zIHceBG0cHw
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1980 931 - forged pistons, Piper cam, K27/26 3257 6.10 hybrid turbo, 951 FMIC, custom intake, Mittelmotor dizzy & cam pulley, H&S exhaust, GAZ Gold, Fuch'ed, Quaife
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Fasteddie313  



Joined: 29 Sep 2013
Posts: 1843
Location: MI

PostPosted: Sat Jan 25, 2020 10:27 am    Post subject: Reply with quote

Right on..

I wonder what could be done to combat rev-hang and still get your comp surge sound..

Maybe a very tight closing TB and an idle valve that is shut until RPM drops under 1500 RPM or so?


The setup I think would be the best for maintaining spool inbetween shifts, and needing no BOV, would be to have the TB before the turbocharger, so when the throttle closes the turbo can spin very freely in the intake vacuum..

Such a system though would require almost 100% hard piping with no elbows or spans of hosing that will collapse under vacuum..



World record power/displacement there so such a setup must work and be superior in some way..
https://youtu.be/Ty_r7JO05Qo
https://youtu.be/9t4yfHfTfDs

The hero of turbocharging..
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Dutch924-racer  



Joined: 23 Jul 2007
Posts: 1045
Location: The Netherlands

PostPosted: Sun Jan 26, 2020 7:28 am    Post subject: Reply with quote

for now I have put it just after the turbo, but the TB direct after the turbo also is a good idea

I have several strength springs with it so will test it later


_________________
937 trackday car
Series 2 engine
951S turbo and intercooler
937 DITC
Fast road camshaft
Forge recirculation valve + boost controller


Last edited by Dutch924-racer on Sun Jan 26, 2020 7:31 am; edited 3 times in total
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