Getting The Most From Your Stock Bov

[4DoorWhore]

This Tech Article is more a primer to give more information of the workings of a RB20DET (possibly more RB motors) and the two common mods used to improve upon it's design to sustain higher boost levels without leaking. I've included links to the Tech articles covering these mods in more detail.

Background:

A BOV acts to release boost pressure between the turbo and throttle body when the throttle plate is shut. This is good for two reasons:

1.) With the pressure released the turbo will continue to spin freely = better transitional boost response (quick spool btw gear changes)

2.) The pressurized air would otherwise hit the closed throttle plate and like an ocean wave, reverse course making it’s way back to the compressor causing it to abruptly slow, possibly causing damage to the wheel.

The 1G BOV while infinitely better than the crappy plastic 2G valves, is still an OEM piece that was never intended to boost much above stock levels (13psi) and will leak at higher boost levels.

It is important to first see how a BOV works to figure out how to improve upon it.

The below pic (minus the air fittings on the mounting flange) are of a 1G BOV. The “top hat” section of the BOV contains a diaphragm and spring which is used to raise/lower a valve at the bottom based on air pressure on either side of the diaphragm combined with the spring tension. The Blue lines represent airflow from the UIP and the Red line airflow from the behind the throttle plate.

The BOV in stock form uses both air pressures to operate the BOV. When under boost (throttle plate open), the Red and Blue pressures are equal and the BOV’s valve will remain closed based solely on the top hat's internal spring pressure (well until too much boost force on the valve itself overcomes the springs tension and pushes it open).

When the throttle closes (btw gear shifts when you let off the gas pedal), the air pressure in the UIP remains positive (Blue), but the air pressure behind the throttle plate goes to negative (Red). This is what helps the BOV to “quick release” as you have complementary forces (pull=Red /push=Blue) at work to open the valve.

This is all good at OEM boost levels, but once you turn the boost up, the pressure inside the UIP can force the BOV valve itself to unseat (blow it open), much like what happens to an upgraded 38mm wastegate flapper if the actuator is not strong enough.

When this happens, you are loosing boost pressure, which the turbo will have to work harder (faster RPMs, more heat, less efficient) to maintain. If you are running 18psi at the intake and you are loosing 2psi from the BOV, your turbo is actually running at 20psi (well more due to pressure drop across the IC/Piping but that’s fodder for another tech article )

Dodge Garage Mod?

The Dodge Garage mod seeks to block the Blue line (UIP Pressure) so that only the Red line (Intake pressure/Vacuum) influences the BOV operation. So now instead of equal pressures above/below the BOV diaphragm under boost, only the top section (Red) from the intake is being fed, so you have additional downward pressure on the BOV valve equal to the boost pressure you are running. More pressure = more boost can be run before the spring/valve is overrun.

To do this, this mod involves "plugging" the inlet on the bottom of the BOV flange (JB Weld) that fed the underside of the diaphram with air pressure, and drilling a hole in the original passageway to relieve any pressure from the underside of the BOV diaphram.

I took this a step further and drilled a second hole on the opposite side the original boost feed and again in the flange to give me the the option to have "quick release" if I ever wanted it (which I haven't). Connecting the two new air tubes will give me OEM BOV operation.

Crushing:

"Crushing" the BOV just means sticking the piece in a vice and pressing the top had down causing the internal spring to shorten and increase pressure on the BOV valve.

Warning, crushing too much will prevent the BOV from opening enough/at all and will cause boost surge. I crush mine so the boost nipple is flush with the middle tier of the top hat (as far as it will go without bending the nipple) and holds well to 25psi

Combining both these mods will elevate the boost threshold even further

any additional info is welcomed as i still consider myself new to RB motors

[mad082]

the problem with the mod you have writen about is that it makes the car run leaner, which isn't safe. instead of just recirculating the air it is getting more air into the system that isn't being measured by the afm. so if you have an aftermarket ecu and the car is tuned to be right on the money DO NOT DO THIS MOD!

a better way to do it is to use a big drill bit to counter sink a hole then screw a screw into the hole. this seels off the recirculation hole, and doesn't introduce any new air into the system.

[mad082]

see here for pics of the screw setup. first pic shows the counter sunk hole. seconds shows the screw in place.

[RANDY]

So the above mod is to reduce the chances of boost leak at higher levels of boost, right? Squashing the hat down to put more pressure on the spring, and relocating the recirculation hole.

And mad082's link shows how to block just the recirculation hole to retain a bit of pressure and theoretically make the turbo not work as hard.

So can I ask: What is the recirculation hole there for in the first place? What benefit (relilability?) did Nissan/Mitsubishi have in mind when they designed it?

[4DoorWhore]

honestly i didnt mess with drilling and capping... in the past i have only used the crush-mod. So read and heed what mad082 has to say thanks for the feedback on this.

ill look into the other mod more in depth..... i have access to alot of the jap shops out here so ill check around.

[mad082]

i have a theory on the recirculation hole. they aren't just on skylines. i did a cooler install on a my99 wrx and it had the same bov, and i have seen them on other cars too.

my theory is that at full noise the compressor has to work harder (spin faster) to keep up the boost at the set level. this may sound like a bad thing but it isn't that bad. WHY? because since the compressor has to work harder, the exhaust wheel has to spin faster. this allows more air to pass out through the exhaust wheel, and less out of the wastegate.

another positive of the hole is that it is recirculating cold air. the air coming out of the hole has just been cooled by the intercooler. and then as it escapes out the hole it cooles again as it expands (the act of compressing air heats it up, and decompressing it cools it - thats how a fridge works) this cold air is going back into the intake. this cools the air between the AFM and the turbo.

and yet another positive is that when the hole is blocked the vacuum created in the return hose can cause the bov to be sucked open slightly (that is the theory behind the mod in the top post of this thread). this is 1 of the design flaws of the stock bov. since it is a flat plate, the moment it moves it leaks boost. most aftermarket bovs have a plug that has to move a few mm before it will release air, so the vacuum has less effect on them.

the only other downside to the stock bov is the fact that when the hole is open it takes longer for boost to build. when it is blocked boost will build quicker, but i found that more boost bleeds off due to the hole being blocked, but that may just due to my bov being weak. i did scuash it a bit and it held better, but i find the release of boost is quicker with the bov unplugged.

[fr0st]

Does anyone have a picture of a properly crushed BOV they could post up?

[mad082]

will take 1 tonight. i just took it off, got a big socket (about a 21mm) sat it on the top and gave it a few medium strenght taps. now i don't mean medium hits, as if you were trying to break something off, but taps, like tapping a nail in. you use gentle taps to get the nail to go in a bit so it holds itself up, then slightly harder taps once it is about 1/4-1/2 way in. you only need a few taps to do it. you will see the top start to squash in.

Please note: i take no responsability for BOV's that are damaged by this process.

[RANDY]

That first pic at the top of the page shows a pushed in hat? It looks like the top is intact, but where it joins the rest of the BOV seems to be in more?

[4DoorWhore]

interesting thanks Mad, i have been doing some testing also and pretty much confirming most of your notions.

to properly crush a bov use a bench-vise. it works to crush it perfectly semetrical, crush it until the outlet pipe is flush with the top of the hat, in better words, when the pipe touchs the hat your good.

[Guest101]

I am also new to RB motors, so you may have to excuse me.

I would like to know if there is any decent proof that this standard BOV does leak at higher boost levels..?

You compared it to a wastegate, which is completely different, if the wastegate can't flow enough, then boost rises.. The opposite doesn't apply for a BOV.

And as you stated, it works based on the pressure difference from either side of the throttle plate... Therefore, no matter how much boost you throw at it, it will still remain neutral with basically identical pressures both pushing the BOV open, and holding it shut. (So, because of the internal spring it will stay shut).

Unless this device is built to act as a POV ((Pop off valve) don't know if they're still called that).

The POV was featured on most of the earlier turbo cars, 70's & 80's.. It's purpose was to vent excess boost pressure if the boost got to high. So if you have a failure with the wastegate system of the turbo, the motor wont then be forced everything the turbo can throw at it. sorry if that didn't make sense.

Or perhaps because of higher boost, and inlet modifications, the internal spring can't hold the small difference in pressure because of a big front mount cooler etc.

In that case, why not run the vaccum hoses from somewhere else..? (hook up 1 either side of the throttle body, or at least both hoses from the same side of the cooler (after it))..

Or if it is meant to act as a POV (or whatever), why can't that bit be removed/disabled from the BOV...?

Now I'm confusing myself even more....

Hope all that made sense to somebody, coz I aint proof readin...

[mad082]

the reason why the BOV will creep open is due to the force of the air passing by it, not pressure, and the vaccum of the plumback pipe sucking it open. and it is getting direct pressure by the air passing by it, where as the pressure holding it shut is coming through a small tube.

also once there is a slight leak, the air in the pressurised area will try to escape out of the place where the leak is to a lower pressure area.

[4DoorWhore]

quick question bit off topic... but why on some occasions do i see people use the TBO rubber caps to cap off the BOV and the BOV outlet piping.

Edited September 24, 2006 by 4DoorWhore

[mad082]

because they are idiots. plain and simple.

they do it for 2 reasons other than that though.

1. to make it flutter

2. to stop it leaking boost, and make it spool up faster.

the down side to blocking off the bov completely is that you have much more lag on gearchanges (unless you don't lift off the throttle). if you have an auto then it is ok to do it, but the immediate response if you lift off then step on the gas again will be slower.

[justin911]

you probs wana becareful if you do this... or maybe not squash it to much...

hehe i learnt the hard way I did it to my bov the other night.. i squashed it so the outlet pipe went as far down as it could go without bending.

Anyways now after i change gears and put my foot down, boost builds up realy realy fast but it takes the car like a second or 2 to actualy start moving...

goto find another bov now

to properly crush a bov use a bench-vise. it works to crush it perfectly semetrical, crush it until the outlet pipe is flush with the top of the hat, in better words, when the pipe touchs the hat your good.

[mad082]

this is due to it being squashed to far and it can't release enough boost to stop it going back into the compressor and slowing it down.

[4DoorWhore]

worst DIY ever

dude the crush works... i just wouldnt drill anything...

just gotta be conservative when you crush it... and use a bench-vise.

[mad082]

i think the bit about crushing it till the outlet pipe is flush with the hat should be removed. i seriously think that is too far as well. sure it would stop the bov from leaking, but it will also limit the amount of air that can escape, so people running higher boost are going to get more lag on gear changes.

[4DoorWhore]

ill adjust it i crushed mine to just above the hat and its still running like a champ definatly better than uncrushed, even with the upgraded boost level.

[mad082]

how is it on gear chages?