I'm interested in the DIY BOV (venting the stock BOV into the air) and I just wanted to know two things..

1)Will it cause you to momentarily run rich when venting(is it venting before or after the AFM)?

2)Does it improve turbo life, like the aftermarkets claim, by stopping compressor surges?

I did some searching but there seemed to be some differences in opinions.


Thanks a lot, Colby

 

1) Yes. After.

2) Well, kind of. The cars have a stock BOV anyway. All you're doing is putting a check valve after the BOV and making it blow the excess air to atmo instead of back into the intake. So yeah, it'll improve turbo life vs. not having a BOV, but you've already got one, so there's no difference.

Brandon

 

Only getting a bigger BOV will stop compressor surge. I started getting surge in my 1990 at about 12 - 13psi with the stock BOV, the opening just couldn't flow enough air. The Blitz BOV fixed it, but it was still to small when I went to the larger hybrid at 15psi or so.

 

oh, so you only have to worry about compressor surges at high boost level, e.g. 13? That's good.

I forgot to mention, is this DIY BOV okay to do with the stock airbox?

 

Uh, I suppose, but why? Usually one open-air vents their BOV because they don't want to deal with the hassle of plumbing it back in to the intake. Just do a TID mod and you won't be so ricey

Brandon

 

tid mod instead? I'm not really looking for more power(or boost) right now.. is there something else it would do?

I was gonna get an aftermarket BOV for my birthday in a month, but looking at this DIY one got me interested..

I want to do either one for these reasons:

1) extend turbo life

2) I just got a full RB exhaust and while it's great, I'm disappointed that I'm not seeing any flames. I guess it's because I'm not running rich enough.. someone told me a BOV would fix this..

3) and in a distant last, the sound I guess.. I'm sure it's cool but it's not too important to me. Plus I didn't think this DIY one would be that loud.

and I want to do it with my stock airbox, because, right now it's keeping my boost down and down the road when I get my ecu tuned, I'm gonna add an intake I think, so this'll make that easier I guess..


Trust me, I want to stay away from rice..

 

*shrug* A BOV would certainly help flames, but won't really help turbo life. IMO there's no reason to go with a "real" aftermarket BOV until you NEED to plumb one into your intake because of your FMIC.

I'd just hold out until you get the tuned ECU & intake...

Brandon

 

Keep in mind the stock BOV leaks if you open-vent it.&nbsp The easy way to fix this "problem" is to bump up your idle above 1kRPM.&nbsp If you don't, the engine will die trying to idle lower.


This is the part I'm kinda puzzled about.&nbsp Lots of people claim it does, but the facts seem to point otherwise.&nbsp As soon as you lift off the throttle, the ECU cuts all fuel at the fuel injectors.&nbsp So even though you're venting the boost away from the engine, I don't see how it gets temporarily rich...&nbsp hmmm...


The stock BOV already does it.&nbsp Venting it back in or venting it to the atmosphere doesn't really change anything.



-Ted

 

This is what I was thinking, but whenever I bring it up, no one responds.

I think its lots of 2nd- and 3rd- hand info spreading around.

 

The 2nd-gen runs a Bosch L-Jetronic FI system. The extra air that you're venting has already been metered, and fuel injected accordingly. Thus, a rich condition.

And AFAIK, I think you're getting the 1st-gen anti-afterburn system (cuts fuel to the rear rotor on throttle drop) mixed up with the second-gens. It's been demonstrated on a few AF Meters (both non- and semi-wideband) that the car goes rich under these conditions.

But then, what do I know?

Brandon

 

Why does mine drop off the lean scale on off throttle then?

 

That's right, it is metered, but what happens when you lift off the throttle?&nbsp 1) it slams the throttle plates shut, and 2) it kills the fuel injectors, so you get ZERO fuel.&nbsp How can it be suddenly rich if the fuel injectors are cut off?&nbsp So the facts seen to NOT point to a rich condition...


-Ted

 

I will check with the wideband if I can persuade my friend to use it on my car

I don't trust the narrow band one, but I do know when I let off to shift, it goes from 2-3 in the green all the way over to lean right as the BOV sounds off, and then back up to green when I get back in the gas.

 

all very interesting.. but will the stock BOV(open vented) and an aftermarket one cause the same rich condition or do they work differently somehow?

also..

quote from RETed:
-----------------------
Keep in mind the stock BOV leaks if you open-vent it. The easy way to fix this "problem" is to bump up your idle above 1kRPM. If you don't, the engine will die trying to idle lower.
-----------------------

How exactly do I adjust the idle?

 

Okay, but when the throttle plates close, the turbo is still spinning, pulling air through the AFM (and out the BOV). The ECU is going to try to "compensate" for that air, even though the throttle plates are closed.

I think that the big point of contention here is whether the injectors completely shut off, or if not, to what degree they lessen their pulsewidths. In that case, Ted, I'm going to have to "bow to your superior wisdom". Though I still think you're getting first- and second-gens mixed up :

Oh, and Felix - when the throttle plates are closed, the engine doesn't flow enough air to get an accurate reading from the O2 sensor - that's why it reads "lean". Check its reading at constant no-load high-RPM.

Brandon <--

 

I'll admit to spreading this "fact", but I must admit, the more I think about it, the more I'm not sure. Yes, the injectors are completely shut off whenever the TPS reads zero throttle opening (and revs are over 1500rpm). So there should be no fuel entering the engine. One possibility I thought of was that the throttles closing is not an instantaneous action. The manifold pressure should drop below atmospheric (triggering the BOV) before the throttles actually close and cut fuel. So maybe there's enough metered air just sneaking in before fuel shut-off to cause some fuel to be injected without its air, which causes a momentary rich condition. This is just me thinking it through in my head, I dunno if it's right! This definitely needs some research. Fact is though, some people do report a stumbling after gearchanges after fitting an open-vented BOV to an AFM-equipped car. I might try the open-venting the stock BOV (with a check valve) just to see what happens, but I'll probably put it back to stock once I know.

 

Anyone got a good "high resolution" datalogger to monitor boost, fuel injector delivery, and EGT channels?



-Ted

 

When I let off the gas, my A/F guage does full lean....Basically off the scale...Now I know that an A/F guage is NOT a precision instrument, but this leads me to believe that the injectors close up almost completely when you suddenly let off the throttle. It wouldn't make sense for Mazda to dump so much fuel into the cats otherwise, regardless of what kind of BOV you have.

 

I had an open vented blitz on my 1990 and had no problems EXCEPT when the car was falling back down to idle it would stall occasionally. I have noticed since coming back from the DSM side of the house there hasn't been anyone who really dug deep into the FC ECU code, not that I have seen anyway. On the DSM people have known EXACTLY what the car was going to do by changing one input (I.E. Lower the Karmen HZ value by X and you get into a more agressive timing table over or under a certain Hz #). On the DSM if you have an open vented BOV (be it GReddy, Tial ect) you will get stumbling and stalling, but the injection system there is completely different. On the TII to me it didn't seem to matter much, with an aftermarket BOV that remained closed atleast, the stock one would leak like a sieve.

 

ok, Ted and BlackFC, you say that the stock BOV will leak when open-vented.. is this when it is *just* open-vented? Which is why you suggest adjusting the idle?

Is it correct that when you perform the DIY one, you don't have to worry about leaks or idle adjustement because of the check valve?


and just to reiterate, will everything still work alright if I perform the DIY BOV with the stock airbox?
Also will this prepare me, or eliminate a step, when and if I decide to do the TID mod and cone intake in the future?


Thanks for all the help!

 

This is a cheap and easily reversible mod, so just do it and see what happens. If you don’t like it or it causes problems, just undo it. Just make sure you block of the BOV’s connection to the airbox when you pull it off.

 

It leaks all the time.&nbsp It's not going to bother you except at idle; an intake vacuum leak at idle is going to drop the revs low enough so the engine will die.


I don't bother with that crap.&nbsp Why would I want to band-aid this BOV just cause I want to vent the stock BOV?&nbsp You're adding another restriction to a valve which needs to vent boost rather quickly.&nbsp Either get a real, aftermarket BOV or don't mess with the stock one.


The BOV does nothing to the stock air box.&nbsp It does normally vent back into the stock TID, so you need to plug that hole up, else the car will NOT idle at all.


It depends on the design of the TID...&nbsp As with the stock air box, the stock BOV has nothing to do with the air filter.



-Ted

 

Some types of cars will have this problem, others won’t. It’s all to do with how the manufacturer sets up the stock BOV’s spring and what vacuum level opens it. It’s nothing to do with the EFI. The only significant difference is the type of airflow meter (Karman Vortex type); everything else is quite conventional.

 

The only time I would think that no fuel is entering the engine is when the car is off. I mean the engine needs some fuel to idle, correct?

 

Reread my post. I said “the injectors are completely shut off whenever the TPS reads zero throttle opening (and revs are over 1500rpm)”. That last bit’s important. When you lift off the gas, because no fuel is required the injectors are switched off to conserve fuel (a carb just keeps pumping it in). When the revs drop below 1500rpm, the injectors are switched back on so that the engine can idle. All EFI systems do this.