Last night driving around I noticed the intake noise sounded louder than usual. Sounded pretty cool actually, but I knew what the cause was. Sure enough, when I got home and popped open the hood, my CAI hose had come off the blower again (why Vortech didn't see fit to put a bead on the intake lip is a mystery to me).

I still had the engine running, and I was curious about how much suction the blower makes so I put my hand down near the intake to feel the rush of air entering the blower (no, I didn't chop my fingers off - I read that thread so I was careful).

To my very great surprise, I felt air coming OUT of the intake, not entering in. I blipped the throttle a few times and every time, when the throttle was open air would enter, but when it closed the air flow would reverse. It would also make the low moaning sound people have mentioned in the past and tried to identify. (I get this sound every time I shift in the 2500-3000 rpm range.) Well, it's definitely compressor surge.

There is only one possible explanation - the BOV is not opening enough when the throttle is closed. To test this theory, I took the top of my Blitz DD BOV and took out the spring and the plunger. The result was no back flow and no compressor surge. I put the plunger back in without the spring and also got no back flow. I went for a drive, and I am happy to report the moaning sound when shifting is gone. This is a great relief, because it was really starting to bug me. Unfortunately, I'm also losing about 1-2 psi of boost pressure at redline since without the spring the plunger isn't fully closing when boosting.

Anyway, my conclusion is that the spring in the Blitz DD bov is way too stiff. I'm considering cutting it one coil at a time to gradually reduce its stiffness until I get no surging and still get full boost pressure. I don't know if this happy medium can be reached, though. I think now I understand why Mardi uses the Vortech mondo bov.

Anybody else gone through this?

 

interesting. I thought when the throttle is closed, there should be some air coming out of the blower because your blower is still spinning and packing air to the t.b. but only in a smaller amount of air.

do you "feel" compressor surge during boosting before you take out the spring in the BOV? have you tried to take your car to peak boost and release the pedal. then quickly tap it again and see if there's any lag? You said that you shifted around 2500-3000 rpm and during that rpm, the boost is still quite low...around 1-2psi maybe? (that's what I see on my 2.87.) If so, I think for that small amount of boost, your BOV is not able to detect it and release the pressure. But it should be fine.

in my case....the greddy type S is known to leak. it leaks a little at really low rpm when the throttle is closed. I rev up the car to 1500rpm and quickly let go of the throttle and there's some air coming out of the greddy. Maybe it is just the design. Does your Blitz do this at low rpm?

 

Actually, the blower pumps out quite a bit of air at idle. It's surprising how much. Much more air than the bov is able to release, hence the backflow. The bov opens partially at idle (by design) because it is actuated by the manifold vacuum, which sucks the plunger up against the spring force. But the spring is so strong that the plunger doesn't open up enough to fully release all the air being pumped by the blower.

The compressor surges during shifting under normal partial throttle operation, not during boosting. When boosting the boost pressure is high enough to force the bov open and release air sufficiently. This results in the familiar bov pssssht sound rather than the low moaning sound of compressor surge during a partial throttle shift.


Yes. The only time air does not come out of the bov is when boosting, when the manifold pressure along with the spring force keeps the plunger fully closed. The problem is that the bov does not let enough air out at idle and when backing off the throttle during shifting during low-moderate acceleration.

 

Is there an easy solution to this?

 

I went to Pepboys at lunch and poked around in their spring drawer until I found one about the right size but considerably softer. I'll try it out and report back later.

 

Okay, I'm back. The spring I found at Pepboys was soft enough to avoid compressor surge, and I gained back 1 psi to get to 8 psi at redline. But I was getting 9 psi before, so I'm not through yet. I'm gonna run down to Home Depot and see if they have a better selection of springs.

 

you the man!

I wonder if I can fix my greddy by getting a bit stiffer spring.

 

Well that's good to know. I got that sound when I first installed the SC, but after playing with the BOV (Greddy) I was able to make it go away. Its good to know what to listen for now!