Alpina B7 Supercharger backpressure noise?

[MontanaRob]

07 and 08 were sc years. My 07 is making what I would call a chug-chug-chug sound/feel as you let off the gas after you are over 3k rpm or more. It is louder at higher rpm and follows the engine speed as it slows until you cant hear/feel it under about 3k. To me it sounds like backpressure from the sc but I swear I don't recall hearing it before. Most sc and turbo systems have a blow off or bypass valve to allow excess pressure to excape when you let off the throttle, but my understanding is that Alpina's system is different and relies on the Valvetronic system and pre/post sc butterfly valves to modulate that pressure without a bypass valve? See this article here

http://myautoworld.com/BMW/cars/bmwc...07-alpina.html


Also, i usually get a single puff of light smoke come out the right front when I come to a stop. Not out the tailpipe. No visible signs of fluid spray/leak under hood or from underside on exhaust etc. The only other symptom is that when you are BARELY on the throttle (like just touching it), the car stutters and hesitates. This is more frequent when you are on flat ground on cruise control (barely on the throttle to maintain speed) It will do this in any gear at any rpm and it more noticeable at higher rpm. Other than that runs great and hard!

This is an N62 engine platform but with some different Alpina-specific internals and sc components.

Things done/observed lately:

Changed plugs (correct kind) - no effect
Changed ccv diaphrams - no effect except stopped excess oil vapors getting pulled in
Changed rubber seal plates for eccentric shaft sensors - no effect
Did the electronic reset of the adaptive transmission (step on gas for 25 sec while ignition on) - no effect
Intake is clean and clear
Mechanic found bmw vacuum code and cleared it. No other codes or errors. Was not specific enough to locate if at all current. Manual search indicates no broken hoses, pipes, etc in crankcase ventilation or otherwise.
SC runs fine, no noises or leaks

I feel ok driving it but the hesitation issue is a pain and I'm not convinced is normal. Wondering if the backpressure can be damaging or maybe that Valvetronic system is not working the exhaust valves properly.

[geargrinder]

I am with your first take. My First take half way through your post was "failed BPV!" I dont know anything about B7's but apparently thats a clutched supercharger, i am betting the way they deal w off throttle is by clutching the blower. Maybe your clutch is failed? I dont think theres any way they can deal w the BPV type overpressure on lift issue w valvetronic, but it could be the clutch trick, or, some internal bypass mechamism inside the blower. Never seen an ASA face to face so cant say for sure tho. HTH.

[hobbit382]

I am with your first take. My First take half way through your post was "failed BPV!" I dont know anything about B7's but apparently thats a clutched supercharger, i am betting the way they deal w off throttle is by clutching the blower. Maybe your clutch is failed? I dont think theres any way they can deal w the BPV type overpressure on lift issue w valvetronic, but it could be the clutch trick, or, some internal bypass mechamism inside the blower. Never seen an ASA face to face so cant say for sure tho. HTH.

they use a throttle body in front of the blower.

[MontanaRob]

The clutch is centrifugal. Engages at 1100 engine rpm (15x reduction gear fyi)