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Discussion Starter #1
I got this off a turbo tunning site and it made me think about air issues.

"louder BOV will conflict with vehicle performance. In order to make a BOV loud, you have to vent to the atmosphere (as opposed to back into the intake, like our cars do from the factory), releasing the excess air instead of routing it back into your turbo system. The problem with this, is that your ECU has already accounted for this "excess air" and adjusts fuel curves accordingly. If it's not in the system anymore, the ECU is compensating incorrectly. This means that the car will not run the way it's supposed to. It may idle poorly, or it may not be noticeable - but it will have a negative effect, even if you can't feel it (the logger will prove it). In any event, it is not the best thing to do if you are serious about making maximum performance. And if you are more interested in making your car sound "furious" and not making it "fast", expect a good deal of heckling, as this is a performance community."

So, any thoughts?

 

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Yeah thats correct. Basically the only time you can use a BOV without the idling or rich running problems is if the car is fueled based on speed density vs air flow. Basically all you can do to fix this is get a piggyback fuel controller or get a stand-alone ECU. When it comes down to it, there really is no need for an atmosphere vented BOV, unless you are running in excess of 500hp. A good diverter valve will be more than adequate. I know the stock bosch units are responsive but the diaphragm is prone to ripping which leads to boost leak and poor performance. Id look for a good quality of aftermarket diverter valve, you will get a little BOV sound but there wont be a decrease in performance. As long as the valve can hold boost without leaking,not leak under vacuum, and flow enough air to prevent compressor surge you will be in good shape.
 

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important to note that this is only true for cars with mass air meters. cars like the 94-98 GM-900 have MAP sensors. thus, the air being dumped never made it's way into the intake manifold where the reading is being taken from and the fueling will never be added for it.
 

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Agreed, Delorean... But this has already been covered in the HUGE thread about bov vs Recirc-valve...
Daniel.
 

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Discussion Starter #5
Saab-Daniel said:
...this has already been covered in the HUGE thread about bov vs Recirc-valve...
Daniel.
Forgive me but I've read the awesome soapbox post in the thread you speak of and read peoples responses, however I feel it necessary to make this it's own thread because I personally (and regrettably) only read posts about how awesome it is and issues on BOV mounting before I mounted my own. I think that I may not have been the only one who did as I did (only seeing the benifits). Wanting people to discuss the actual drawbacks and possible ill effect BOVs might have on their cars is main the reason that I posted this. I feel this issue deserves it's own thread considering that the other one is 95% glorification of BOVs.... and who knows maybe someone will come up with an idea of effectively mounting the air meter after the BOV or some other means of resolving the issue. It really can't be that bad to post this thread if you ask me but I do apologize for annoying you in any way with my approach to the use of this forum. {btw - I really am not trying to be sarcastic or disrespectful to Daniel even though it may come across as such. I'm thankful to share this place with every sc member.}
oh... and the mocking of the furious sound over the fast gave me a smirk even though I now have a BOV.
 

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I understand that a BOV on a car with an AMM (even a dual piston BOV) will affect the air flow reading, causing the car to run rich momentarily when it vents. In addition it can be quite tricky to get the BOV set up right to avoid idling or stalling issues, and it often requires a bit of moving thigs around. But one thing is for sure - having done rolling roads before and after BOV installation, there was precisely no difference in BHP, torque, A/F ratio or anything else on my car. In fact there was an extra 0.1 bhp after BOV installation ;) . The gear changes on boost are definitely quicker and smoother - previously revs would drop and the turbo would have to spool up again when you entered the new gear, but now it is a seamless continuation of boost. So they may not be the most effective for maximum performance, but to imply that they are detrimental to performance seems a bit strong as the difference would appear to be miniscule. I like the sound of my Forge, which is why I put it on - it's worth a few more pence in petrol as the mixture goes a bit rich, and I'd rather be swinging too rich than too lean.
 

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Evan: I must have mistaken your intend for this thread, and as such the thread has to be here...I thought your intend was to recieve info on something I already thought as covered...
So keep on sharing, or I will be really mad ;)
Daniel.
 

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I fitted the Forge FMDV004 BOV to my GM900 but it was sooo loud I took it off again. When I started using my T16, I found that the standard BPV was split and leaking, so rather than fork out for a new one, I just slapped on the Forge BOV, and like Jezzadee say's, I too have noticed that the Forge unit is able to hold pressure through the gears better, and I have a lot less lag after a gear change. The way mine is mounted, it's actually not that noticable in volume, and again like Jezzadee says, I'd rather be running too rich momentarily than too lean. I'd also like to point out, that I've never had the car stall at idle, and it ticks over smoothly without problem.
 

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One problem I found with BOVs (and one of the reasons I switched back to using a recirc unit) is that they dump boost, and mess up the AMM readings, when I'm feathering the throttle during cornering. This can lead to jerkiness and loss of driveability.
 

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thats why you need a "T8 or T8S" no AMM to mess up when you fit a Atmos DumpValve :cheesy: :cheesy:

Paul

_____________________________________
Full Throttle until you see God, then Brake...
 

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it's as simple as this. if your fuel metering is BEFORE the BOV, then it's going to mess up your fueling. if it's after (such as a system that uses a MAP sensor) it will be just fine and won't affect fueling at all.

I am almost amazed that a K-jet car will even start with a BOV fitted! :eek:
 

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One of the things I was hoping to get across in my post is that a lot of the time you will see better results with a quality recirculating valve that is up to the demands of the engine it is bolted to. By better results I mean that shifting characteristics and boost recovery will be as good and the other aspects of the system will be better- stabler idle and part throttle, smoother integration into the entier sytem. On my rally car with boost set to 15 psi I used a standard Bosch valve from a turbo audi rated at 18 psi. It never leaked. For a quality unit Forge make a very good bypass valve.

Those of you that post improvements over the stock Bosch unit frequently say things such as 'I replaced my old stock (read worn out or already failed) with a BOV and noticed improvements.' I contest that you would have noticed those same improvements fitting a new standard Bosch valve or upgrading to the Bosch plastic 'viggen' bypass valve rated at 20psi. Are that many of you pushing more than 20psi? If so the Forge unit, TAS make one and a few other companies as well.


Please do not see this as an attack on those of you who run a BOV and are perfectly happy with it. This is an attempt to get real information out there for people to make educated decisions about tuning their cars. As I stated in my 'soapbox post' there are certain times a BOV is more apropriate and certain times a bypass valve is more apropriate.


James
Team Saabworks
http://www.saabworks.com
 

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rally900 said:
Please do not see this as an attack on those of you who run a BOV and are perfectly happy with it. This is an attempt to get real information out there for people to make educated decisions about tuning their cars. As I stated in my 'soapbox post' there are certain times a BOV is more apropriate and certain times a bypass valve is more apropriate.
You've certainly not come across that way - I think you've given a very clear and balanced view.

When I fitted my dump valve, I fitted a BOV since I didn't know a re-circ could be fitted; no-one seemed to have done it at that point in time on a T8. Well, no-one came forward ;) If I was to fit a re-circ valve (the standard Bosch valve at VW is much cheaper than SAAB, BTW), would a silicon T section with a 25mm take-off clamped over a cut in the plastic pipe be sufficient? What about just a hole and a grommet for the re-circ end to be pushed into? How does the T16 valve connect the re-circ valve to the pre-turbo pipe? Inquiring minds need to know :eek:
 
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