Overboost caused by plumb-back BOV wrong way round! Discuss...

[Stefan]

I have always had an overboost problem since my 1G conversion - boost hits the internal gate's 12psi, stays there a little while, then quickly ramps right up.

I had put this down to bad gate (KKR430) or dump design.

However, the plumb-back BOV (stock late-model EVO one) has always been really loud on shifts when on boost. I got thinking recently that this loud CH! CH! CH! might have been compressor surge....which is what a BOV is supposed to prevent.

Looked at some photos on net, looked at my car... the compressor and intake sides were back to front, like this (obviously with the fitting connected to plenum):


Swapped it, and now it makes a gentle quiet long shhhhhhhhhhhhhh noise on shifts, and doesn't accelerate anywhere near as hard (no boost gauge currently in car).




My question is - what exactly was happening before? And is the BOV merely leaking to control boost now?

[oldcorollas]

the BOV has no purpose in controlling boost.

it simply opens when vacuum is applied to the signal line....

it SHOULD stay completely closed under any other conditions.

perhaps it is now farked and is leaking badly, so you are losing boost?

edit: the way it is mean tto work, the boost pressure pushes against piston, and that is held shut by spring and ref line boost.

what you had, was manifold air trying to enter thru the side of the BOV, and the spring and ref line had nothing pushing against them, so the BOV stayed closed.
the Ch CH CH might be because the side loading from the manifold air mmight have caused piston to bind?

[1jz68u]

is that pic the old one or new one?

if its the old one it looks like you dont have the vacuum hose attached to the top so the CH CH CH as you describe it would have just been from the wastegate and the BOV would be closed all the time therefore not working at all.

if this is the new pic still there is no hose attached to the vacuum nipple (or whatever they are called) so i have no idea why it would be working unless it is now just constantly leaking boost due to being the wrong way around which would explain why the car doesnt pull as hard, but also means your turbo will be working its tits off.

hope this makes some sense. i also could be totally wrong.

[PlacentaJuan]

you have no clamps on the joiners maybe thats why its not working.

and bovs go ZUUUTUTUTUT not CH-CH-CH get it right next time.

but yea dunno how you can hook up a BOV to affect boost since it is all after the compressor unless you had the output going into the wastegate or something.

[YLD-16L]

First time I've ever heard of anyone fitting a BOV the wrong way round +rep

Edit: Just read the bit where you said you do actually run a reference line from manifold to BOV.

boost hits the internal gate's 12psi, stays there a little while, then quickly ramps right up.

no boost gauge currently in car

It had a boost gauge?????? You say it hit 12psi then rapidly ramped up, ramped up to what? Does it still ramp up or does it hold a preset boost now?

Oh and given you have the BOV mounted on the turbo side of the IC, referenced from the inlet manifold you can end up with the BOV cracking open on boost IF there is substantial pressure drop across your intercooler. Measure both sides of the core under boost and check the difference.

[Stefan]

To clarify:

• This is the photo of how the BOV was. The only pic I had of the way the bov was installed was this mid-build pic, hence missing clamps, reference lines etc - it was not running like this.
  
• When I was installing it, I asked a mate which way round - he said 'definitely this way' so that's what I did :roll
  
• When the car was on the dyno after the conversion, it was overboosting - ramping up to say 20psi. Since putting the BOV around the correct way, it doesn't accelerate nearly as logramithically, and I strongly suspect that it is no longer overboosting. I will confirm this by installing a boost gauge again (there isn't currently one in the car).

[YLD-16L]

Measure the pressure drop across you IC too (or move the BOV to between the throttle and IC) because if there is an issue with pressure dop across the core then the only BOV that's not going to creep open on boost will be a Bok Choy shitter with some god aweful hard spring in it that'll struggle to even open on throttle lift.

[oldcorollas]

When the car was on the dyno after the conversion, it was overboosting - ramping up to say 20psi. Since putting the BOV around the correct way, it doesn't accelerate nearly as logramithically, and I strongly suspect that it is no longer overboosting. I will confirm this by installing a boost gauge again (there isn't currently one in the car).

I'd think of it a different way.
after the conversion it was boosting, and your wastegate was not controlling it.
since putting the BOV around the correct way, it is now leaking and not making boost.
I suspect it is leaking shitloads of boooost..

turbo cars with high boost usually accelerate logarithmically thats what their dyno curves look like.

[Duk]

Your valve appears to be a diaphagm type (like the Bosch BOVs) and it is installed back to front. The air flow from the intercooler pipe should go to the underside of the valve so as not to side load the diaphragm (or piston, it's a piston type).

[YLD-16L]

^^^^ tomorrow the WA residents should start replying

[IN 05 NT]

how long is your vacuum line from the blow of valve? if your running back over to the manifold....thats nearly 1mtr long, will cause a delay...causing the presure surge back thru the turbo,

you need to keep the vacuum line a short as possible to help open the valve.....

my vacuum line is about 35cm, and i still suffer surge....(also a old BOV).....

[Stefan]

Your valve appears to be a diaphagm type (like the Bosch BOVs) and it is installed back to front. The air flow from the intercooler pipe should go to the underside of the valve so as not to side load the diaphragm (or piston, it's a piston type).

Yes... that's a photo of how it was and it has already been changed as you described. This thread is about the effects of changing it to how it should be...

[Stefan]

how long is your vacuum line from the blow of valve? if your running back over to the manifold....thats nearly 1mtr long, will cause a delay...causing the presure surge back thru the turbo,

you need to keep the vacuum line a short as possible to help open the valve.....

my vacuum line is about 35cm, and i still suffer surge....(also a old BOV).....

Good point - it is a rather long line.

Interestingly though, I get no surge now (at least, no surge noise whatsoever).

[Stefan]

Measure the pressure drop across you IC too (or move the BOV to between the throttle and IC) because if there is an issue with pressure dop across the core then the only BOV that's not going to creep open on boost will be a Bok Choy shitter with some god aweful hard spring in it that'll struggle to even open on throttle lift.

Even the designs that work the other way, and seal the valve harder the more boost there is?

So, to summarise, the ideal position of the BOV is between the cooler and throttle. This eliminates any problems with cooler pressure drop, and allows a short line to reference the intake plenum pressure. Unfortunately, this also means running a big thick hose across to the turbo intake if recirculating - now I see why non-recirculating BOVs are easier to install. My setup uses a MAP sensor so there are no AFM issues to worry about, but I really dislike the ricey noise...

Any recommendations on a reasonably-priced BOV that will hold up to 1.5 bar and work well?

[Stefan]

turbo cars with high boost usually accelerate logarithmically thats what their dyno curves look like.

Well, they certainly do when the boost isn't gated!