What is a Dump Valve? and What is a Wastegate?

[Glenn Butcher]

I recently put this post together for another mailing list, and thought it would be of use to people here.

What is a Dump Valve/BOV?

A dump valve (aka BOV - Blow Off Valve) is a valve that relieves the pressure on the turbo\'s intake side.  The standard BOV vents back into the intake (Pretty sure it is called the "Charge relief valve" by Mazda).  When you take your foot off the gas peddle, the throttle bodies close, the turbo\'s compressor wheel is still spinning down - if there is no BOV then the turbo is actually forced to stall (it has been known for Turbo shafts to snap because of this continual force on and off). I believe that BOV\'s are not important if you are running low boost, but there is a BOV on the standard RX, so it must be important on a rotor.  If that excess pressure is relieved from the system, then the turbo takes less time to spool up again.

It is only relevant to boost control if the valve is designed to leak under increased boost pressure - then it becomes part of the boost regulation system.

What is a Wastegate?

A Wastegate relieves the excess pressure on the turbine (exhaust side) of the turbo.  i.e. if a wastegate is set to 10psi, then the wastegate will open when the pressure reaches 10psi, venting the exhaust gasses back into the exhaust (or into the atmosphere - external wastegate).  If the wastegate does not open in time, or does not open correctly - this is when Boost Spikes, Boost Creep occur.

i.e. Open the wastegate and the turbo will slow, reducing the amount of boost being developed. Close the wastegate, and more exhaust gas is forced through the turbine, increasing turbo speed and thus the boost level being developed.

This is a problem with the standard wastegate on virtually all standard Mazda\'s, when you put a larger exhaust on the car - the engine will rev quicker and of course the turbo will spin quicker as there is less back pressure in the exhaust.  The standard wastegate usually cannot cope with this - hence causing Spikes, and creep --> lean mixtures --> detonation --> etc.

[catboy]

i think boost creep can occur if the wastegate size is not large enough to flow the gasses away quickly enough too...

basically the wastegate cannot get rid of enough gas to maintain the pre-set pressure, hence the boost slowly increases even after it has opened...

[hIGGI]

u could also include what is

Pop of valve / relief valve

- valve which does not have any vac hose going into, with preset (adjustable) level of boost
- when u are going to go over that boost, it will bleed it

Its used like safety device when wastegate is not working etc.....

Simply said, it controls your boost directly on intercooler piping.....and protect you from overboosting

(boost controllers controls how much is wastegate opening, but would not give you control over top level of boost)

[Glenn Butcher]

Cheers Higgi,

I believe that some of the good quality BOV\'s actually are adjustable like a POV also.

Check out //www.turbosmart.com.au.

[hIGGI]

never saw BOV with boost control function but i might be wrong....

and i don`t see it on turbosmarts\' site....just variety of BOV\'s vented/closed loop, dual.....

[Glenn Butcher]

I am sure I read it somewhere, on AutoSpeed.  I found this link http://www.autospeed.com/A_0878/cms/article.html which gives you are run-down on the various BOV\'s. (May have been one of the site links off the article).

Another one here: http://www.autospeed.com/A_0145/cms/article.html

But I can\'t find the specific one.  I\'ll keep an eye out.  

I could be wrong as I may have miss-read it as it was a while ago.

[mmilner]

Thanks for the info all. Never been too sure about turbo related things so all this has been very interesting.

Martin.