I heard Dump/Blow off Values are BAD!!!!

I was chatting to a mate of mine this weekend and he said he was on the skyline forum talking about new parts he wants for his motor one of which was a dump value.

He got a load of posts back saying they are really bad for the turbo. Now i am not a technical person and he did explain it to me but not sure i got it.

So my question is are they bad and if so why?

http://www.max-boost.co.uk/max-boost/intake/DV.htm

The above link (if I have done it right) makes interesting reading, as does the whole site.

I was chatting to a mate of mine this weekend and he said he was on the skyline forum talking about new parts he wants for his motor one of which was a dump value.

 

He got a load of posts back saying they are really bad for the turbo. Now i am not a technical person and he did explain it to me but not sure i got it.

So my question is are they bad and if so why?

he sounds like a bit of a pleb.....UK spec cars dont like em as it reeks havoc with Air Flow sensor and can cause rough idle and often the engine stalling, they release compression surge into the atmosphere so there is no way of it backing up into the turbo housing and possibly shattering it

stock supras are fitted with a recirculating type valve and these are ok, so I cant see the problem with venting to the atmosphere

http://www.max-boost.co.uk/max-boost/intake/DV.htm

 

The above link (if I have done it right) makes interesting reading, as does the whole site.

 

good site that:innocent:

good site that:innocent:

I heard that the chap responsible for it smells of wee

...stinks actually...

I was just being nice, he's a creep (if you really want to know):

steer clear

Skyline RB26 plus vent to atmo BOV personal observations below:

The following is based on my Skyline experience, with air usage

measured by air flow meters before the turbos. It is not applicable to

cars that solely use a MAP sensor and throttle angle for air measurement .

To recap I put in a bigger intercooler and the standard Skyline duct

that runs across the lower front of the car taking dumped air from the

2 blow off valves to the intake between the air flow meters and the

twin turbo inlets was masking air exiting the lower 4 rows of the new

IC.

Having spent a good deal of money on this item it pained me to see some

of effectively blanked off. So I decided to blank the inlet off

altogether where the cross pipe entered the turbo inlet ducting, remove

the cross pipe and vent to atmosphere. The dump valves were being held

open at idle by the idle vacuum level, so after ages of pondering i

made a set up of solenoids to control when vacuum was applied to the

dump valves. This set up worked perfectly. *HOWEVER* a much more

serious problem occurred, which any similarly inducted turbo car will

potentially suffer if the boost air is dumped to atmosphere rather than

as standard into the air intakes after the AFM (s). On the overrun,

after a period of boost running, the standard set up will recirculate

the excess boost back into the turbos, through the IC, and back through

the dump valves to the turbo inlet again until the turbos inertia has

slowed them to the point of little or no boost. The AFM's do not see

any more air entering the engine, as it is being recirculated.

However, when we dump to atmosphere, that air is lost from the system

and the turbos draw in fresh air via the AFM (s). This causes the

engine to go mad rich, as the throttles are closed and no fuel is

really needed. Hence the black smoke seen on the overrun after my mods.

Worse still was a noticeable but very short period of detonation when

coming hard back on the throttle. This puzzled me totally, then it

dawned. As the fuel system started working normally again the Lambda

sensor "caught up" with what was happening, saw a very overly rich

mixture, and shut down the injector pulse width, creating a very lean

mixture, causing a brief, but very dangerous period of detonation!

I spent the afternoon and evening making new bracketry for the IC, new

hoses and adaptors and shifting the IC forward that critical 40 mm or

so, enabling me to fit the original moulded duct from the BOV's behind

the IC without blocking it. Quite a lot of work and fabrication...

However, the mod I was planning could well have caused damage, and is

certainly something to be very aware of if one has a similar AFM pre

turbo set up changed to dump boost to atmosphere. The type of BOV

matters not, it's the fact that the air no longer re circulates but is

lost from the system, confusing the AFM (s) into thinking the engine is

wanting more air/fuel mixture. An oscilloscope on the O2 sensors

confirmed what was happening. We live and learn, often the hard way,

but in this case not as hard a lesson as it might have been. I hope

this helps, I searched the web long and hard for references to BOV

problems, and failed to find any details of why dumping to atmosphere on

none MAP sensor systems, that aren't mapped for this, is potentially

dangerous.

How many turbocharged cars in the world have BOV's?

How many turbocharged cars in the world have had trouble due to a BOV being fitted?

i currently have a 200sx S14, bailey dump valve fitted and it does over fuel upon lifting off the throttle means a loud pop and occasionally followed by a foot long flame thats all!

No turbo engine I have ever built has had one, I never use them at all.

How many turbocharged cars in the world have BOV's?

45

How many turbocharged cars in the world have had trouble due to a BOV being fitted?

34

I don't think anyone is saying the generic 'BOV' is the issue here, it's the type of BOV that causes the problems on certain cars (UK Supras, certain MR2s etc), usually when a standard recirc is replaced by a vent to atmos type.

http://www.max-boost.co.uk/max-boost/intake/DV.htm

 

The above link (if I have done it right) makes interesting reading, as does the whole site.

 

argh! the c20let, bad expensive memories!

45

 

 

 

34

 

I don't think anyone is saying the generic 'BOV' is the issue here, it's the type of BOV that causes the problems on certain cars (UK Supras, certain MR2s etc), usually when a standard recirc is replaced by a vent to atmos type.

Are your sources accurate? I think not.

No turbo engine I have ever built has had one, I never use them at all.

Not even a recirculating one? Why not? In my book they're a longevity increasing component. I suppose the engines you're building are generally going to be rebuilt every few races, or maybe at best a season, so you wouldn't really have to worry about it?

I wouldn't have thought many race engines would have an AFM? We all know how much of a restriction they are, and the lost air doesn't bother a MAP based system.

IMO and that of some of the UK's top turbo experts, the BOV was "created" by Bosch for the 944 Turbo and the Cossie Sierra solely to reduce noise from the compressor on gearchanges. None of them have seen ANY evidence to suggest they decrease spool time or aid longevity. Neither have I, it's one more thing to go wrong, added plumbing and weight, and something else to package. I have never had issues even on fairly wildly boosted road car engines doing pretty high annual mileages (Volvo 240 2.1 litre B21A engine, running 1.6 bar and 330 BHP, doing 15,000 a year over 2.5 years, on plain bearing Holset turbo, which is old tech, with no water cooling to cassette, and a pretty cavalier attitude to cool down, warm up, and general well being of the engine. Experts are the top guys at Turbo Technics and Turbo Dynamics, and Ken Brittain of Brodie Brittain Racing, all of whom have pretty impeccable long standing credentials in this arena.

And me of course, but I am just an over opinionated geezer from the sticks

Very interesting reading Chris

And me of course, but I am just an over opinionated geezer from the sticks

If that's the case Chris, why would you suppose Toyota fitted one as stock? Same thing to reduce compressor noise?

For sure, yes, most originak owners wouldn't like the squeak on lift off at high boost. IMO.

dont car's like the new Lotus Exise (how ever you spell it) and Noble have very loud waste gate/bov noise

I don't think anyone is saying the generic 'BOV' is the issue here, it's the type of BOV that causes the problems on certain cars (UK Supras, certain MR2s etc), usually when a standard recirc is replaced by a vent to atmos type.

The ONLY problem I've heard about BOVs on UK Supras are on the autos and regard lumpy idle and stalling , I've used the HKS SSV for the past 3 years without issue.

Interesting, so removing the bov wouldn't have a detrimental affect on the system?

..the BOV was "created" by Bosch for the 944 Turbo and the Cossie Sierra solely to reduce noise from the compressor on gearchanges.

That is my recollection as well.

It was meant to make the engines more 'civilised', not faster

None of them have seen ANY evidence to suggest they decrease spool time or aid longevity...

I have seen some anectotal evidence of this on bikes.

Up to 1 bar it made no difference, but over 1 bar it made a difference in spoolup when shutting the throttle and opening it wide again. I don't think it was placebo.

For sure, yes, most originak owners wouldn't like the squeak on lift off at high boost. IMO.

What do you think would have been causing the squeak?

My thinking is that effectively your reducing thrust loading by using a BOV of some kind. It may be have the effect of nine tenths of feck all, but the physics doesn't lie. There will be a direct reduction in axial loading on the compressor by fitting a system to relieve pressure once the TB is closed. There's an indirect effect in that your not accelerating and decelerating the turbine as drastically, which will have an effect of life. Again probably a gnat's cock in the grand scale of things, but I guesss that's where the level at which modern engine design is. IMHO.