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Discussion Starter #1
Running an RK Stage 1 on my e39 M5 - the previous owner had a Tial QR recirculating BoV to cut down on blow off sound, however I just replaced the bottom portion with the Q as I prefer the sound.

The problem now is that sometimes in first or second the BoV sounds like its rapidly opening and closing, and the blow off continues to blow off after letting off the throttle. (two separate issues)

Is this normal? It's probably placebo but I feel like the car does not pull as hard as it did, maybe I'm just getting used to it.

Any thoughts? I checked all BOV seals and O-rings.
 

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2002 M5, Imola / Imola
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Not a SC owner, but understanding the concept - I think that sounds normal. While you are off throttle, but still in gear the motor is still turning - meaning the SC is still producing boost. You don't want that going through the intake while off throttle so the BOV is going to dump it.

When it is rapidly opening and closing, it is likely normal just now you can hear it. It is controlling the amount of boost pressure based on your throttle input and RPM.
 

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Is it opening and closing only under part throttle or also full?

It is intended to dump boost off throttle. A superchargers boost is rpm dependent, so if you're at 5k rpm and lift, the supercharger is still creating boost that would normally back up against the closed throttle bodies and put pressure on the throttles, hoses and impeller/turbine, that's why the blowoff valve is there.

Obvious points first- is the b/o valve spring included with the Q valve? If there is a new spring is the it correct one? Is there a vacuum leak in the system?

Simply because I'm curious (and I have never owned an RK kit) did the previous owner vent the QR to atmosphere? Am I correct in thinking the kit comes originally with the Q valve and all the PO did was replace the Q with the QR or did he modify the kit to actually recirculate? I ask as I haven't heard any valve venting to atmosphere being quieter than another; only vs ones that recirculate, would love to find one that is.
 

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The BOV should remain closed if you are on throttle in 1st and 2nd. If its opening in gear while on throttle, I would look at the vacuum line feeding the BOV. On throttle = closed BOV. Off throttle = open.
 

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The BOV should remain closed if you are on throttle in 1st and 2nd. If its opening in gear while on throttle, I would look at the vacuum line feeding the BOV. On throttle = closed BOV. Off throttle = open.
It should remain closed to a POINT (your max boost level desired by spring choice) - ie. it will remain closed until your max boost is achieved and then bleed off air to maintain your max boost, no?

Disclaimer: not a SC/Turbo owner :)
 

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The spring rate in a blowoff valve should be matched to the manifold vacuum at idle so it opens. The valve shouldn't open under boost on throttle (as the PSI will be balanced on both sides of the valve). The spring does not counter the boost pressure, it counters the pressure differential between the manifold (by that I mean after the throttle, s62 has a plenum with individual throttle bodies) and the charge pipe. Obviously this can get way more complex for high boost turbo systems with multi-stage blowoff valves, but this is a 6psi supercharger with a single stage valve.

Boost will be determined by the pulley diameter and corresponding supercharger RPM.

My thought about the spring was a red herring, I see the lowest they offer is -2psi for superchargers and it goes up from there, so if it were too high it wouldn't open properly (assuming the -2ps spring is what's included in the original kit).

I am not the be all end all guru of forced induction though, so please anyone correct me if I have something wrong, I love these forums because I often learn something.
 

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It should remain closed to a POINT (your max boost level desired by spring choice) - ie. it will remain closed until your max boost is achieved and then bleed off air to maintain your max boost, no?

Disclaimer: not a SC/Turbo owner :)
Thats usually the job of a boost controller, but im not very familiar with the Tial unit. But in effect you are right. The Boost Controller controls when the BOV opens once max pressure is met.

In this case, it could be the boost controller opening the BOV in throttle possibly?
 

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I think some of you are confusing the BOV with a wastegate in a turbo (sometimes SC) application. As Matt said above, a BOV is only used to manage the pressure difference in the manifold.

BOV should not be open at idle or that'd be a serious vacuum leak, same with on throttle, unless the spring is too weak to hold it (edit: actually, I think boost is what holds it closed) closed on boost, which is also a problem.

Typically supercharger boost pressures are limited by the pulley size, and that's why pulley upgrades are listed with a PSI (generally the max PSI you'll see at redline).

Anecdotally, people do add wastegates to SC applications to keep boost pressure consistent throughout the rev range, but that's not typical.
 

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BOV should not be open at idle or that'd be a serious vacuum leak,
Quite right- I didn’t mean it should be open at idle, just that the amount of vacuum generated at idle should be (with correct spring) the amount needed to open the valve if the throttle closes and there is pressure in the charge pipe (which would drop at idle, valve closes).

But I believe it’s not the spring holding it closed on boost though- that same line that pulls vacuum from the valve with a closed throttle also sends positive pressure when the throttle is open. Once the spring sees greater than -2psi of pressure (in this particular case) it will close as it goes from -2 to 0. Once closed, the pressure will remain equal on either side of it, holding the valve shut. The spring does move it to the closed position, but given that it has positive pressure behind it it does not need to be strong to do this.
 
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