Blow Off, Bypass (Diverter-Recirc) Valves - the little stuff that counts - The Saab Link Forums

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Performance Modifications for the NG900 / Old 9-3 This forum contains PERFORMANCE related Q&A's for the NG900 and 9-3. This may also include suspension.

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Old 09-29-2014, 06:13 PM   #1
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Blow Off, Bypass (Diverter-Recirc) Valves - the little stuff that counts

Hey guys.. while I'm searching here to learn more BOV's.. thought I'd mention some of the things learned as I climb up the HP ladder. Had a few silly problems, but will try to avoid mentioning any vendor names although it might be obvious if you know the style they are known for.

So although there are many types of BOV or BPV, Dual piston, sequential, super sequential, with and without diaphragms.. some are both a BOV and Diverter, the end-result of the BOV/Diverter is to blow-off or recirculate boost pressure off in throttle off states/between shifts on the inlet side to the throttle body (in most cases). I'll keep my discussion to BOV's... so jump in.

1. Why are BOV/BPV important on Turbo cars? -

Basically to keep the back pressure or compressor surge off of your turbo, and therefore extend the life of it. When they sense vacuum (off throttle) from the throttle body base, they open or divert the boosted air.

Improve performance response, by either getting rid of hotter inlet charges by blowing off the charge or re-routing to pre-charge the intake for the next boost cycle, or both. Those of us that can use BOV's like the sound, but do you know when to upgrade them?

2. Believe it or not.. when you increase your boost levels 5psi, to 10 psi, to 20psi, or maybe even 30psi and up, you need to also consider upgrading the BOV/BPV ability to hold that boost level. Some are adjustable and some are fixed.

The BOV operates a lot like your wastegate holding boost up to a certain level only they open between shifts or off the throttle to blow off boost pressure. If your BOV can't hold the boost pressure of your inlet flow.. you may hear a strange fluttering... or the whistle sometimes called boost leak. If you're shifting up hard under full throttle and manage to find that rpm sweet spot for max boost.. then suddenly you lose boost pressure.. you might have a BOV issue (in my best Jeff Foxworthy voice imitation)..

3. Single Piston versus Double Piston .. Double piston is supposed to offer a slight performance advantage over single piston.. keeping some of the charge in the inlet for the T5 during idle periods, during off throttle, but blows off as well. The theory is that you have some of the charge available to be added to the new charge, improving lagtime for big house turbos..

MAF (Mass Air Flow) systems require BPV or re-circulated because the air passing thru is accounted for or measured and matched for the correct fuel needed. Otherwise if you run a BOV on the T7 car, it will sense a boost leak and you'll run into fuel rich/poor performance issues. In most cases the T7 and above require BPV/Diverter valves.

4. Long story short.. update your BOV/BPV according to the boost levels you plan to run.. are you doing 15psi? 20psi? 25psi or more? No matter what the style, make sure it matches the boost pressure for the Engine Management system.

It's the little things that count.

edited.. but there's the theory that a lighter spring actually benefits the BPV, because it is a closed system. However, not the same for the Blow Off Valve (BOV)., because you need to hold the boost with higher spring rate.
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Old 09-30-2014, 03:42 AM   #2
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and here I thought that would cause at least a little debate.. What's the world of Saablink coming to..?

I've read where a weaker spring might be of benefit to a MAF or T7 car.. but can't see it on the T5.
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Old 09-30-2014, 03:51 AM   #3
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the springs that a lot of the BPV's (forge type for example) come with have nothing to do with the amount of boost you are running. Bogan had a thread from a year or so ago where he was running in circles trying to "tune" his BOV and all the while he was running way too stiff a spring. Here: https://www.saablink.net/forum/perfor...-9-3/65634.htm

The springs are actually for tuning to the amount of vacuum, so that the valve opens correctly, and NOT to the amount of boost and holding the valve shut. Between the boost in the inlet, and the spring (even if it's a very weak one) the valve will be shut.

I run a forge on my 85 c900, and a turbosmart kompact on my 9-5. I should actually swap them because there is a lot less space in the c900 and the turbosmart valve is a good bit smaller than the forge. In the 9-5 there is plenty of room around the BPV area.
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Old 09-30-2014, 06:30 AM   #4
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Thanks for the link.. and it actually points out what I found to be true as well.. you need the higher spring rate (red).. to support the higher psi levels.. vacuum/boost.. using the blow off valve.

The BPV.. recirc type. benefits from the lower spring rates..
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Old 10-02-2014, 09:40 AM   #5
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Yeah....i suppose if you run them backwards (can be done, in which case you rely partly on the spring to force the frustum of sealing mechanism into its seat) or you can run it properly, wherein the charge pressure is acting normal-to (perpendicular) to the travel of the piston/frustum. When run properly, the pressure of the throttle body acts on the back of the frustum (sealing piston) to counter the breeching pressure of the charge-air.
Since the system is theoretically neutral on boost, i.e, wide throttle angle, then there is literally no need for a spring. However, a stiffer spring will yield snappier performance on partial throttle WHERE THERE IS A PRESSURE DIFFERENTIAL, but this is at the expense of responsiveness and more often than not, you get pressure surge when you let off (fluttering sound of death).

Bogan, bless his tenacity, was running after dead-ends. If you check that thread, I posted numerous times about also experiencing the same problems. Since Bogan and I have the same setup, the two of us, and several forum members, exhaustively discussed whether it was from the BPV/BOV, the springs, the design, or turbo surging (which is different from the flutter you experience when you snap off the throttle and i caused by too much boost on too small of a hotside wheel).

After hundreds of dollars and many hours logging, I have come to conclusion that this turbo flutter we were hearing is actually the death knell of a weak DIC. When I replaced my aging DIC with a new unit, I could boost to 35psi with nary a complaint from my system. Beforehand, I used to get this "chchchchchchchchchchchchhhhh" as soon as the pressure went above ~20psi.

If you don't believe me, make sure your BOV/BPV is oriented correctly and then remove the spring. I promise you it will work just fine.
YMMV. Good luck,
-Cm
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Old 10-04-2014, 01:14 AM   #6
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Great read. You know that's weird you say that.. because the DIC was the first thing I suspected. But with the car idling nice and smooth. Didn't think that was the issue. I did a test run, and she was boosting pretty good up to 20psi.. the same thing you describe.. Almost like she bucked with a strange back fire and now she won't boost past 3 to 4 psi.. Runs at idle pretty good and picks up pretty good, but won't boost

I looked at your sig, and we are running the same gear.. p0172 (rich), and p0300 (backfire).. saw some white sparks in the corner of my eye just before the engine died.. but she started right up and ran like nothing happened.. just couldn't boost anymore.

bpc cleaned.. then replaced.. silicon lines checked for leaks.. smoke tested.. now looking for a pigtail for my new map sensor.. any of this sound familiar to your story..?

Funny thing.. I actually ordered the DIC .. but returned it.., because I swaped to an old one thinking what are the chances.. and she fired up fine so didn't think anything of it because I also checked the spark plugs for discoloration... but they looked good too.

Well.. after I replace this MAP sensor.. I'll be looking into another cassette..
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Old 10-04-2014, 01:29 AM   #7
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It would be nice if we had a forum that has cars with similar setups.. like "Stage 1&2 Setup", Stage 2&3 Setup" and the "OH S*IT Setup".. Seems like I've read a lot of similar posts for cars with a lot of performance add-ons... because after trying to bullet proof everything comes back to something simple at times.
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Old 10-15-2014, 08:07 AM   #8
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Does it cut out once you pass a certain pressure threshold, or does it just not get above a certain PSI? These cars are very simple, actually. If you are in limp mode because of a software or hardware problem, then you certainly would not be able to boost past 3-5psi. How the hell did you see white sparks? Was the car on a dyno? Sparks inside the cabin? :O
These cars will idle and derp around town fine on a totally munched DIC. When you call upon the thing to accelerate, however, it will initially feel way down on power, and then it will at some point after that cease to ignite the charge under boost, and then, miles after that, it will cease to work completely. Generally, these things indicate failure by reporting more and more knocks. The failure mode of these goddamn DICs is not binary, it is progressive, IMHO. I F***ING HATE the DIC system.
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Old 10-15-2014, 08:09 AM   #9
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Can you data log? If you can live-log the ECU with a laptop that will be helpful.
Did you tune this yourself or is it mailbox tuned?
-Cm
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Old 10-17-2014, 03:41 AM   #10
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Quote:
Originally Posted by Cm452 View Post
Can you data log? If you can live-log the ECU with a laptop that will be helpful.
Did you tune this yourself or is it mailbox tuned?
-Cm
It's a mailbox tune.. Was on my way to the local tuner just before all of this to have them get the most out of it. Have to resolve the issues first.

embarrassed to say, but found the wastegate adjuster sleeve (too long) binding and causing waste gate to leak (hence the rich CEL). Gave me a false sense that it was preloaded properly. replaced it with a new one.. what hell??? No boost.. (NB)

I have two springs in my Forge BOV, but reverted back to the yellow (original) to see if any difference.. same thing. Running another boost leak test on the compressor side to see if it will hold, and maybe close off the delivery pipe near the throttle body to test in the other direction as well. I will need to do the same thing on the exhaust side of the turbo, but more so for back-pressure, which I suspect is where my problem is.

New DIC just arrived.. on standby until I learn more.
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Old 10-17-2014, 03:56 PM   #11
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Clogged Cat converter..

Sequence of events..

1. Wastegate actuator arm adjuster was binding
2. Car ran rich
3. Cat blew out
4. Cat remains solidified just aft of resonator before muffler
5. = No boost

Too much back pressure.. i.e. blockage in the exhaust system = no boost

Tested for boost leak; tested back-pressure.. (7psi).. dang

She's definitely boosting now.... ahhhhh finally
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Old 10-20-2014, 07:17 PM   #12
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Wow, nice catch! Still doesn't solve your rich condition. A jammed wastegate won't affect afr, just boost.
something is not quite right, as I've run my car at moronically rich afr, and the cat is okay.
-Cm
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Old 10-22-2014, 05:41 AM   #13
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Please, I don't know what the sence of DIC is. The abbrevation for Diverter reCirculation?

Thanks, regards, Kay
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Old 10-24-2014, 06:24 AM   #14
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Talking

I was running very rich.. seems like pressing the pedal more didn't help with power while she was trying to find a way to breath out.. was getting p0172 a lot (no longer).. but back to the p0453 for emissions.

running rich caused back-fire in the exhaust; which burnt up the cat; to cause the clog in my exhaust; causing the hi back-pressure; resulting in lack of boost.. whew.. funny how one thing leads to the other.. but its not over yet.

She boost fine now.. but need to solve for the stalling. Even with a new CPS and DIC (Direct Ignition Cassette), she stalls. All my vacuum lines are fine.

Seems like when it rains it pours.. so to speak. SID popping coolant low when she has plenty and no indication of oil/water mixing (whew), but now a P0453 for emissions... WTF So I'm noticing air coming from the fuel tank when I pop off the filler cap to refuel. Runs great when up to speed but won't hold idle, and stalls when coming to a stop. She never did that before.

More research indicates the potential of leaking emissions system, or an issue with charcoal canister (filled with fuel) may be causing the stalling. I need to run more tests to check it, and with 225k on the odometer.. maybe its just time to replace it too. Will be a brand new car after all of this.. lol..
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Old 10-31-2014, 06:36 AM   #15
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Again, super simple.Just disconnect and plug the vacuum lines for the EVAp system. It will run fine, just might throw a code, though, you seem to be well accustomed to the CEL,so i doubt this will bother you. Anyway, the trick with these cars is to start simple and just work your way back. They really are just lowered tractors. Unlike a MAF car,T5 will idle under pretty much any circumstance, save for too little fuel or too much fuel. Your FPR might be tired as well. I am not too experienced with FPR failures modes. I just replace mine every 60-100kmiles. Your idle problem may also very well be the IAC valve. These go periodically.Remove the valve entirely. Start the car. It will idle very high (like 2500rpm) but if it holds an idle then that is your problem. They also usually need a good cleaning, so blow the IAC with carburetor cleaner as well.
Again, don't sweat the small stuff. If you have compression, use of all your gears, and decent ignition cassette, it'll run forever.
-Cm
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Old 10-31-2014, 11:25 AM   #16
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So I replaced the charcoal canister in the wheel well cleaned the IAC using electrical cleaner (all I had and evaporates), then a light oiling before I replaced it. She turns over more before starting for some reason.. normally cranks about 3 seconds or so before firing up.. now about 5-8 seconds longer. It actually maintained idle better as it didn't quit.

But I then put in some BG44K and things got interesting.. probably over-kill. Took her for a 20 mile ride and CEL started flashing.. didn't bother checking anything after I rushed her home.

I'll try your test tonight to see if a difference, and haven't heard about replacing the FPR that often....
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