BOV with carburetor

[zak_62]

I was reading the Turbo Esprit Wiki article about installation of a Blow Off Valve.
http://esprit.driestone.com/index.php/Esprit/InstallingABOV

Under "The Problem" section, the author stated that "On an EFI car there is a single butterfly and running a vacuum line from behind it is easy. On a carbureted engine we're not so lucky (not only do we have four butterflies, the pressure after the butterfly is also an air/fuel mixture.) "

I am not sure if my observation is correct, but at the side of the front carburetor, there is a vacuum connection that is utilized for vacuum advancing the distributor. Is this connection vents behind the butterfly of the first cylinder? Why we can't (or don't want to) use a fuel air mixture to control a BOV? Finally, would it be better if we can vent the blowed off pressure back to the air box via the Crank Case Breather Connection already existed?

[superdave]

Is yours a carb turbo? If so I had a BOV fitted to my carb turbo for over a year until I've just sold it at the weekend. Very easy to fit, works well and no problems whatsoever. My firend Alan Croft also has one fitted to his carb turbo as well.


Cheers,



Dave Walters

[DrieStone]

I wrote that, and there's a lot of theory there. My comment about not wanting to run the air fuel to the BOV was based on the assumption that a fuel mixture would be detrimental to the BOV, certainly this is the case on a cheaper BOV (with plastic components.) I wouldn't run an A/F mixture to a BOV, but you're right, you could do this if you weren't worried about the fuel weakening components. (additionally I realized my comment about four butterflies isn't all that valid I realize since who cares if the pressure is equalized as long as it's close.)

As for the breather, I would be worried about forcing compressed air into the crankcase. Again, the idea of venting the excess back into the system is theory based on how many modern turbo charged cars work. I need to do more work to change my theories into facts. Again, if it were me, I would vent to atmosphere and just accept the minimal loss.

As for others with a BOV, I have still yet to see a proper installation of a BOV. It sounds like everyone who has installed a BOV is using it like a dump valve. It doesn't mean that it hasn't been done, just that I haven't seen it.

>> Edited by DrieStone on Monday 16th August 15:23

[sanj]

DrieStone said:

As for others with a BOV, I have still yet to see a proper installation of a BOV. It sounds like everyone who has installed a BOV is using it like a dump valve. It doesn't mean that it hasn't been done, just that I haven't seen it.

Would you care to elaborate on that statement?

Cheers,
Sanj

[zak_62]

superdave said:

Is yours a carb turbo? If so I had a BOV fitted to my carb turbo for over a year until I've just sold it at the weekend. Very easy to fit, works well and no problems whatsoever. My firend Alan Croft also has one fitted to his carb turbo as well. Cheers, Dave Walters

How did you fit yours Dave?

[zak_62]

DrieStone said:

As for the breather, I would be worried about forcing compressed air into the crankcase. Again, the idea of venting the excess back into the system is theory based on how many modern turbo charged cars work. I need to do more work to change my theories into facts. Again, if it were me, I would vent to atmosphere and just accept the minimal loss.

Yeah, and people sometimes do enjoy the added sound effect~

And thanks for the comprehensive information on the site.

[DrieStone]

sanj said:

Would you care to elaborate on that statement?

Sure, those that I know of use only spring pressure to open the valve, and not spring + pressure differential. In my investigations, non of the owners that I knew of that had a BOV had explained how they created the pressure differential.

I'm not saying that it hasn't been done. I just haven't seen/heard of it being done in a proper way.

[th4neuk]

Hi,

I have some pics of my BOV installation on webshots.

http://community.webshots.com/album/83835046qsoOUP

Fitting was fairly easy.

Cheers

Alan Croft
89 Turbo

[DrieStone]

Where does the vacuum line on the back side of the BOV go? I can't tell from the pictures.

[th4neuk]

I linked it into one of the vacuum pipes near the carbs. I cut into it and used a t-piece from my local aquarium store. cost was pennies. The BOV on mine is a bailey twin piston unit and the only problem I had was getting the silicone t hose made up. I'll check on the colour of the vacuum pipe and let you know exactly which one.

Cheers

Alan

[zak_62]

th4neuk said:

I linked it into one of the vacuum pipes near the carbs. I cut into it and used a t-piece from my local aquarium store. cost was pennies. The BOV on mine is a bailey twin piston unit and the only problem I had was getting the silicone t hose made up. I'll check on the colour of the vacuum pipe and let you know exactly which one. Cheers Alan

So you linked it to the outlet on the side of the front carburetor that we normally used for vacuum advance? How's the pressure holding up when driving?

[sanj]

DrieStone said:

sanj said: Would you care to elaborate on that statement? Sure, those that I know of use only spring pressure to open the valve, and not spring + pressure differential. In my investigations, non of the owners that I knew of that had a BOV had explained how they created the pressure differential. I'm not saying that it hasn't been done. I just haven't seen/heard of it being done in a proper way.

The BOV kits that I have sold in the past, as well as those still sold by Marcus, are all of the differential type. There is a port on one side of the piston which receives vacuum from the intake manifold on the far side of the throttle body when the throttle is closed.

Cheers,
Sanj

Cheers,
Sanj

[DrieStone]

sanj said:

The BOV kits that I have sold in the past, as well as those still sold by Marcus, are all of the differential type. There is a port on one side of the piston which receives vacuum from the intake manifold on the far side of the throttle body when the throttle is closed.

So the question is... does the fuel mixture affect the BOV in any way? Previously I had heard that the mixture can shorten the life of the BOV and lead to failure. This is why I didn't think I wanted to use this method. I'm sure it depends on the BOV you are using. It certainly is easier to plumb the BOV in this way. What brand BOV is used in Marcus' kit?

[sanj]

DrieStone said:

So the question is... does the fuel mixture affect the BOV in any way?

The BOV is positioned before the fuel is mixed in with the airstream, at the hose between the chargecooler and the plenum crossover. There really shouldn't be much in the way of fuel in the vicinity of the BOV, with the possible exception of a tiny bit at the vacuum port under boost. The valve is aluminium, with some sort of composite piston, and a steel spring. The piston o-ring is probably fairly immune to fuel as well.

Cheers,
Sanj

[th4neuk]

Hi,

Yes the line does go into the black pipes which then link into the carbs. It also then goes into a balck and white line heading towards the airfilter (i think this is for the cutoff valve??). I haven't really noticed any difference performance wise other than the turbo spools a bi quicker and you no longer have the horse whinny sound as the wastegate shuts.

Cheers

Alan