View Full Version : How does a dump valve work?
Sounds like a stupid question.... but
How exactly does a recirculating dump valve work?
As far as I know, it dumps when it gets vacum on the control pipe?
How can I set it to dump when the pressure gets to a certain point? can someone please enlighten me as to there operation so I might set mine up!
Paid2getdirty
05-11-2005, 22:11
Er. everyone else tell me if I'm wrong but I was under the impression that a dump valve's purpose is to prevent stalling of the turbo compressor wheel when you go from an open throttle on boost to a closed throttle. The blower is filling the pipework to the inlet manifold with air at up to 25 psi, depending on application, and when you snap the throttle shut the air pressure has nowhere to go. This air pressure backs up down towards the compressor and tries to stall it, in some high pressure applications it can damage the wheel so I've been told. The dump valve is a spring loaded diaphragm and when excessive boost pressure acts on the diaphragm it opens against the spring pressure and the air escapes past the diaphragm. A recirculating dump valve pipes this excess air back into the inlet side of the turbo so you don't hear it working as much as one that exhausts to atmosphere. I figure you adjust the spring pressure so it blows off at just above max boost pressure.
Let me know lads if I'm talkin bollox, could do with knowing how to setup one myself cos sooty' has an adjustable valve on it instead of the standard one. Do you get an accurate boost gauge and make sure its blowing off no earlier than after max boost is seen??
Ok just found this :
http://www.jekylhyderacing.com/bov.htm
it answers most of my questions.... but now i need to know.... what dump valve to get? any recomendations for a cheap recirculating dump valve? And how do I set it up to dump at a certain pressure, do i just cut the spring?
Sorry bout lots of questions!! --
Simon
some of them have a screw/nut adjusting mech, just un-nip the nut, turn the screw..it adjusts the cut off point etc.
Paul Woods
06-11-2005, 17:49
oooh where'd ya get the cool gold spinny azev!
yep the adjusting nut provides more/less tension on the bov spring.
oooh where'd ya get the cool gold spinny azev!
yep the adjusting nut provides more/less tension on the bov spring.
aha.. the one embossed with the site so no git runs off with it :D good advertisin..... i made it using adobe image ready.. and erm.. a solid modelling prog..solidworks. my azevs be bak from welders.. 4 months no less.. jesus wept. i'll be machining them soon and tinging them this here colour=P~
what dump valve to get? jeez..really wouldn know.. im sure the imoc posse would have a better idea there. re-circ is always the best option unless you have something to control the overfuelling at lift off.
Too late, i've purchased a brand new audi 1.8T bosch recirc dump valve. It doesn't look pretty but it only cost me ?5!!! :). I might change to something a bit snazzier when it all works!
Paul Woods
07-11-2005, 06:37
simon did you just buy that off ebay by chance? its just i just bought a brand new audi recirc for a fiver off a guy on ebay for my upcoming TT project.....sounds very similar...
http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=8010104077&rd=1&sspagename=STRK%3AMEWN%3AIT&rd=1
i thought,simple but effective... is it the same one?
yea exactly the same infact!!!
I also paid ?5.50, how spooky is that!!
http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=8010470335&rd=1&sspagename=STRK%3AMEWN%3AIT&rd=1
I figured for the money it'll do the job, they are supposed to be good up to 15psi :D
Paul Woods
07-11-2005, 17:20
now that is some weird shit going on right there....identical DV's for identical money... spooky!
they are adjustable somehow ?
Driftin_AW
07-11-2005, 21:55
I just want to point out something about dump valves/blowoff valves.
They are designed to prevent compressor stall by diverting pressurised air out of the pressure side of the inlet tract. They are not designed to be used to protect against overboosting etc. The adjustment is so you can tighten it up if the valve is leaking under boost or loosen it if it's not opening when it should. The better designs, such as HKS's SSQV (super sequential valve) dont have the pressurised air in the inlet tract acting on the face of the piston, but have it acting on the side. The opening and closing is then controlled entirely by vacuum/pressure in the intake manifold, so when it's under pressure it forces the piston down and shuts the valve, when it's under vacuum it sucks the piston up and opens the valve.
You used to be able to get something called a "pop off valve". Some people use the term interchangably with blowoff valve or bypass valve, but they are different. A pop off valve does exactly what it sounds like you're after, it's designed to open at a preset (usually adjustable) pressure value to vent excess boost in the event that you get a boost spike or boost creep. It's like a overboost protection device, which is not what a blowoff/bypass valve is supposed to do.
Paul Woods
08-11-2005, 06:35
yes thats a good point from malcolm simon,i will be using mine as a true BOV to protect the turbos,with your SC there is no need for it as the SC isnt affected by the throttle closing being mechanically driven....and the BOV wont work as an overboost limiting device unless you get one that adjusts right down to 5psi and hang it off the manifold somewhere.These audi ones are non adjustable,its just a recirc BOV with a spring that lets go at a max of 15psi....so the only way that will regulate pressure on your set up is if you want 15psi to be your boost limit! not a good idea.
Adam worked out some pretty impressive formulae on flow rate/sc output etc its worth searching for...that will tell you what sort of boost pressures you can expect running the m45 on your engine.
Cheers paul, I've already calculated boost pressures and pully sizes.. It should be limited around 5psi (i think just under with the current pully sizes)
I wanted the dump valve as i've read stories of pipes blowing off on sudden throttle closure, and the resistance can aparently heat up the supercharger rotors a little.
The thought was that I could also use the bov to prevent excess boosting, in case something goes "pete tong" in my calculations, however looking back if the pully size is calculated right, then there shouldn't be any problems as the boost should only go down if there is a problem!!
So in short I've come to the conclusion that i can't/don't really need to limit boost with the dump valve, so its simply going to be used as a "pipe blow off" safety feature (I think!!) :roll:
Paul Woods
08-11-2005, 17:18
well....i put a bit of thought into this for you today,just to find a way that you can use the BOV as more of a boost control valve than a lift off device.
What i came up with was if you used a hobbs pressure switch (these are adjustable to any psi) and set that to say 6psi and had that wired so it was breaking a circuit to a toyota VSV (found on any old t-vis system) and this VSV was controlling the vac feed to the bov then in effect what you would have is a boost controlling device.
Basically if you get your calcs wrong,even by a fraction it may well over boost and you certainly dont want that! 15psi is too high a safety limit.....so using my idea the bov would just sit doing nothing until the psi hits 6,the hobbs would open the vsv which would let vacuum open the bov to let the extra pressure out,as soon as it dropped to 5psi (instantaneous) it would shut off again....now you're thinking "where do you get the vacuum from on a manifold thats full of boost!" well if you took a feed from just after the AFM,or even before the AFM itself im pretty sure theres enough vac there to open the bov.
Just a little idea from the depths of me brain! people please feel free to pull this idea apart as ive only just come up with it and it may well be flawed.
Intresting idea... I read a 306 website about using a VSV or "solenoid" as they called it to operate the dump valve when the throttle closed. The switch was on the throttle body. see>> http://johns306.com/content/view/17/29/
The VSV was connected to the brake servo vaccum lines to overcome the problem of pressure!
In theory this would work.
How bout setting up the VSV to a toggle switch so you could decide when you wanted boost or not (mad max stylee) pretty much like an air bypass valve... would that work??
Boosting to 15psi is not an issue with the supercharger setup at present \\:D/
Paul Woods
08-11-2005, 18:30
yes a switch would work but feeding from the brake servo line is never going to work under throttle open conditions,the manifold will have pressure in it not vac (thank god we never need to brake at the same time as accelerating!) ...but there will be vac at your AFM you could use...
I just want to point out something about dump valves/blowoff valves.
They are designed to prevent compressor stall by diverting pressurised air out of the pressure side of the inlet tract. They are not designed to be used to protect against overboosting etc. The adjustment is so you can tighten it up if the valve is leaking under boost or loosen it if it's not opening when it should. The better designs, such as HKS's SSQV (super sequential valve) dont have the pressurised air in the inlet tract acting on the face of the piston, but have it acting on the side. The opening and closing is then controlled entirely by vacuum/pressure in the intake manifold, so when it's under pressure it forces the piston down and shuts the valve, when it's under vacuum it sucks the piston up and opens the valve.
You used to be able to get something called a "pop off valve". Some people use the term interchangably with blowoff valve or bypass valve, but they are different. A pop off valve does exactly what it sounds like you're after, it's designed to open at a preset (usually adjustable) pressure value to vent excess boost in the event that you get a boost spike or boost creep. It's like a overboost protection device, which is not what a blowoff/bypass valve is supposed to do.
ahh... i understand the bov is to help with back pressure stalling, what i didnt know was that the adjustement on it was for not your boost cut off point.. well informed malc, thanks very much. so what device in the engine is the boost limiting/trimming device? is it adjustable?... if not..its going out the window for a dash mount digital adjustable gauge/readout.. your going to tell me the bosst is proportional to the fuelling and controlled by the ecu now?.. how does it work..
Driftin_AW
08-11-2005, 23:40
paul - that's a brilliant idea. the only problem is that some BOV's need pressure on both sides to stop them opening, and if you block the line that normally goes to the intake manifold then a non adjustable valve may leak. On a plumb back valve you could probably switch the inlet and outlet so the boost pressure acts on the side of the piston (like I described on the HKS type valves), and it should then work brilliantly...
adam, the wastegate is the only mechanical device on the engine that can directly effect boost. conveniently it is usually very effective ;) the only thing the ECU can do to control boost is either shut the Turbo-VSV (if you still have it connected), or go into safety mode (fuel cut). Obviously this applies only to turbocharged engines. Most factory SC set ups use an air bypass valve to limit overboosting
Interestingly some simple tests have shown that on gen 2 3sgte's, if you unplug the MAP sensor (electrically) you will still see fuel cut at around 12.5psi. It seems the ecu uses some rather intelligent combination of AFM, TPS and RPM signals to decide how much boost pressure is present, without the use of a pressure sensing device.
Paul Woods
09-11-2005, 06:29
ah right malcolm...didnt think about the bov needing pressure both sides to keep it shut....yes a vsv that switched between manifold pressure acting on the bov and vac acting on it should work.
Powered by vBulletin® Version 4.2.5 Copyright © 2024 vBulletin Solutions Inc. All rights reserved.