Rundown on the situation:

Car pulls around 20in at idle. However when I'm preparing to launch the car at around 3-4k rpm, the motor is pulling around 25in (not creating boost). While I'm not too concerned about the car not creating boost, I am concerned about what sort of ramification this much vacuum created may have on a BOV and the car when I try to launch at that rpm.

The spring that was recommended to me was for an SVO running around 20. The "tech" explained this was the spring they always used for the SVO, and a stiffer spring (used on those pulling more vacuum) was "too much" for the SVO.

So...knowing that the spring was optimal for a car pulling 20 at idle, am I going to run into any problems when I try to launch the car? My concern is the vacuum pulling open the BOV and venting air when I am trying to launch the car. This is a VAM/MAF car and it will vent to atmosphere when the BOV is installed.

 

no load on the motor = no boost.... just cause your revving to 4k means nothing without load....you can sit at 4k in 3rd gear at a steady speed and be in vacuum.


and your going to have a fun time trying to keep the car running venting to atmosphere with a MAF...unless you put the BOV before the MAF. the maf will count all that air and add fuel as it thinks its needed...then you blow it out, but it doesnt know that, so you run rich and stall...more commonly happens on high RPM shifts.

 

There are plenty of SVO's and other MAF/VAM cars running around without stalling issues with a vent to atmosphere bypass/BOV. Something I am not too concerned about.

I realize the reason I am not building boost, that is also not the issue. My concern is because it is pulling 5in more over idle, will that cause the BOV to open on me? Or perhaps since there is no pressure difference (i.e. no forced air trying to force the BOV to open) it will not open?

Be nice...I'm a pseudo-newb to turbos.

 

basically, your throttle body isnt open as much with no load on the car than it would be with you driving around at 3k, and im guessing you tapped your boost gauge to the intake manifold. so the cyls are moving quicker...needing to suck in more air so thats whats creating more vacuum. its quite normal. in my laser after i high RPM run and then left in that gear i would see as much as 25-30 inches of vac.


try to visualize your throttle body. it may only need 5% throttle to spin 3k in neutral. now giving it 5% throttle in 3rd gear at 3kRPM the car wont stay at 3k, itll come down in the rpms. youd have to open the tb more..which creates less vacuum as there is more area for air to flow. hope this made some sense.

 

Makes sense to me. I believe the boost gauge is tapped into the intake manifold. Instead of running my own line, I bypassed my stock boost gauge and ran the line to my VDO. Pretty sure the stock line goes to the manifold or a vacuum feed source on the firewall (haven't totally investigated the source just yet). Perhaps this is a reason why my needle "vibrates" when the car is warmed up at idle?

So from what I am gathering, as far as the BOV goes I should be fine launching the car as normal. Correct?

 

yea, i dont see why not. the way i understand BOVs to work is that if they use a vacuum source to stay open, when atmospheric pressure is achieved they are closed and hold X amount of psi. i know there are a couple different types of BOV's though. also, ive never dealt with an svo, so there may be some things i dont know about, but usually boosted applications are relatively alike.

 

sneakin explained it pretty well, but i dont really see what your worrried about. ever car is going to how your describing. unless you have a 2step your car is always going to see less vacuum at idle than at a steady higher rpm(in neutral with practically no load) mainly because if your throttle is only open 5% and your car is spinning at 4k it needs more air than at idle but that 5% throttle your giving it isnt giving the motor enough air to accelerate and the motor is pulling in so much more air because it revving higher that it actually starts creating a vacuum inside the manifold as it trys to "pull" in more air to sustain the rpm.

this strong vacuum also usually pulls the bov open because thats how a blow off valve works, it gets pulled open by the vacuum of the intake manifold under strong vacuum and/or deceleration. as soon as u stomp the pedal to the floor and vacuum slowly starts to turn into boost the blow of valve shuts because its now being pushed shut by the boost being fed onto the top of the bov spring from the intake manifold. and as soon as u let off the gas theres still pressure in the pipes and theres now vaccum in the manifold, both of these force the bov to open instantly.

kinda of confusing and its kind of hard to turn knowledge into words sometimes but i hope that helps

 

Thanks for the info. I've just never had a car that required a boost gauge before, so I'm a bit ignorant on the whole turbo thing and I was just curious.

Thus ends my newb post this month.