Originally posted by "Someone"
Originally posted by v8killer
***At idle most engines make between 17-22 inHg of vacuum. So if you adjust the BOV towards hard so that it is closed at idle that means that BOV will need to be set to open at a vacuum level greater than 22 inHg. After a shift your vacuum will go to around 25-27 inHg which is just barely over the threshold of 22 inHg you have set the BOV at. This hard setting means that the BOV will open a lot later than normal which allows more time for compressor surge to occur.
You are much better off adjusting the BOV soft to open at only 10 inHg of vacuum and reroute it. So when you shift gears and create 25-27 inHg of vacuum you are way above the threshold to open the valve. Thus the valve opens early and quickly negating the chance of experiencing compressor surge.
Jay
you are missing one very big point in your post here...A BOV operates on the premise of pressure differential. when the pressure differential between the intake plumbing and the manifold is greater than the set point the valve will open.
granted set up like it is my BOV may not open until 22 inHg of pure vacuum to prevent leaks at idle. I also agree that when i shift there will be greater than 25 inHg in the manifold but you forget that there is also 14 p.s.i. (28.504 inHg of pressure) pressing on the head of the valve forcing it to open.
So for arguments sake lets say that the valve head has an area that is .75 that of the vacuum diaphram (area of 1). you get an effective pressure differential of
(25 * 1) + (28.5 * .75) = 46.375
this is more than enough to open my valve set @ 22 inHg
but for a more practical sake lets see how shell do just crusing around town at low boost of say 3 p.s.i. (6.108 inHg) on the highway at part throttle.
(25 * 1) + (6.108 * .75) = 29.581
Again plenty to open the valve. So i really dont see how setting the valve to stay closed at idle can hurt that much?
Its like this for me...I like it venting to the atmosphere. If the t-25 is to fragile to handle a stiffly set BOV then its a piece of crap as far as a PERFORMANCE turbo goes. Im not worried about it...
EDIT: to further this discussion (because i know someone other than Jay will ask..) the reason that the BOV will not open under Wide Open Throttle due to the 14 p.s.i. is that the diaphram is pushing the valve back closed with approxamately the same pressure that the intake piping is putting on the valve head. remember the top line into a BOV should supply boost as well as vaccum to the diaphram.
(-28.5 * 1) + (28.5 *.75) = -7.125
the minus represents force that is acting to keep the vlave closed on top of the pressure that is being applied by the spring that we are adjusting.
END EDIT
Scott