Ok recircs what are they and what do they do? do bovs ( dump valves ) do a better job? and what goes wrong when recircs go bad?

 

The basic premise of the recirc or bov is to release built up pressure when the throttle butterflies slam shut because if you do not do that, the returning pressure wave can damage the turbo`s.

Recircs ( see pic below ) operate by been piped either side of the turbo or in simple terms form a regulated short cut from the turbo output back to the turbo input.

 

 

Here I have removed the lid of the recirc to reveal the diaphragm that rises and falls by the vacuum generated within the intake( plenum ) the diaphragm then acts upon the spring loaded plunger which itself is connected to the valve disc, this is positioned so that the in and out ports can be opened ( when throttle closed ) or closed fully ( when on the gas).

 

 

The top and spacer ring removed in the next photo reveals the diaphragm rubber seal, this isolates ( seals ) the top section from the bottom and means the vacuum will draw the diaphragm up and pull the plunger rod etc.

 

 

In the next photo the plunger is held up to show the lift angle and the plunger rod below.

 

 

By cutting off the lower section elbow it is possible to see the actual valve which is connected to the plunger rod and moved by the actuator diaphragm.

 

 

In the next photo the rod is been held in the full lift ( high vacuum, over run etc. ) the valve disc can be seen open.

 

 

Finally cutting the side out reveals all of the above in one go and allows a clearer understanding of what is happening.

 

 

 

The recircs as mentioned have a slam spring which has a pre-load effect upon the valve, however the vacuum produced at tick over lifts the valve and for this reason this kind of unit cannot be used to dump to atmosphere as it would be the cause of a big air leak and not forgetting the zed is equipped with an air flow meter this would cause fuelling errors too, this is where piston type bovs ( dump valves ) are required as they do not lift at soft idle conditions.

 

Bovs have a place on big bhp engines but a recirc in good condition will keep all pressure and air read by the air flow meter INSIDE the engine pipe work, bovs let it out so by their design cause momentary fuelling errors.

 

Recircs do have their problems however, in the main due to wear and tear, the slam spring becomes weakened and results in very early lift of the valve. Now this in its first stages will not be that noticeable but as it worsens will mean the valve is leaking ( recircing ) turbo boost not to atmosphere but back to the front of the turbo, now of course this will mean no signs of this will be particular noticeable, BUT slowly you will start to get increased turbo lag as the turbo rise pressure takes longer to attain, eventually and seen many times even making standard boost can be an issue, most owners think their zed is in safety boost and although this can feel similar it is actually much worse and harder to find and prove if you don't know how and because they recirc back into the pipework a boost leak check will not even show a problem.

 

Professional inspection and testing is the safest way as the easiest method to check if they have failed involves very controlled road testing with the turbo wastegate actuators disconnected to measure unregulated boost rise against revs and road speed, a very keen eye is required to not major over boost if the recircs are ok.

 

Hope that helps with a little insight into the original question of what they are and what they do.

 

Jeff TT

Excellent read again Jeff! Can you do recirc testing? I've got two sets and i'd really like to know if they are doing the job, will then use two best ones.

Fantastic write up jeff.

 

Love the stickers ;)

a fountain of knowledge!:dance:

 

dan.:D

 

 

Bovs have a place on big bhp engines but a recirc in good condition will keep all pressure and air read by the air flow meter INSIDE the engine pipe work, bovs let it out so by their design cause momentary fuelling errors.

 

 

 

Do these fuelling errors cause damage jeff and are they noticable?

 

ps good write up

Great stuff Jeff! :bow:

 

Is there any way of reconditioning or are new ones the best/only option?

Do these fuelling errors cause damage jeff and are they noticable?

 

ps good write up

 

The ecu will pulse the injectors for the air it measured coming in, if you "dump" some of this to atmosphere, there is reduced air intake and therefore your AFR becomes richer or goes down in value.

 

Running rich is not as catastrophic as running lean but will cause things such as bore wash where the oil is cleaned from the cylinder walls, that accelerates wear on the components. Also lamda sensors really don't like it and a serious rich mixture causes the cats alot of problems aswell.

 

Ps it's good for the forum for Jeff to take the time to write these up! Maybe a series starting lol?

Edited June 16, 200916 yr by stenorth

The ecu will pulse the injectors for the air it measured coming in, if you "dump" some of this to atmosphere, there is reduced air intake and therefore your AFR becomes richer or goes down in value.

 

Running rich is not as catastrophic as running lean but will cause things such as bore wash where the oil is cleaned from the cylinder walls, that accelerates wear on the components. Also lamda sensors really don't like it and a serious rich mixture causes the cats alot of problems aswell.

 

Ps it's good for the forum for Jeff to take the time to write these up! Maybe a series starting lol?

 

so is the fuelling error from bov's enough to cause bore wash then?

great stuff Jeff keep them coming! :bow:

so is the fuelling error from bov's enough to cause bore wash then?

 

Lots of variables in that one.

 

Didn't mean to put the jumpers up you, sorry lol, I wouldn't worry too much, the short answer is no. It's a gradual over time thing like consistant hard acceleration/heavy loading. Just bear in mind the more that you hear that stupid sound, your running rich on that cycle. :p

 

Just thought i'd raise the issue because running rich is sometimes seen as safe and it can be as bad as running lean, albeit not quite as instant!

Edited June 16, 200916 yr by stenorth

Lots of variables in that one.

 

Didn't mean to put the jumpers up you, sorry lol, I wouldn't worry too much, the short answer is no. It's a gradual over time thing like consistant hard acceleration/heavy loading. Just bear in mind the more that you hear that stupid sound, your running rich on that cycle. :p

 

Just thought i'd raise the issue because running rich is sometimes seen as safe and it can be as bad as running lean, albeit not quite as instant!

 

I only bought them to get rid of goose honk and ain't had them fitted yet mines an auto so prob won't be so bad.

I'd rather have goose-honk than 'scania-airbrake' disease....lol

Si, some of the stuff you type sounds like goose honk

:rofl: true, but then some of the stuff you type is toilet. :wack:

Said si

I'd rather have goose-honk than 'scania-airbrake' disease....lol

 

I have two tone goose honk each recirc must be a little worn as they "honk" in sequence and slightly different pitch and it is a little :o to say the least. I think I can tolerate 'scania-airbrakes'