MyE28.com

so i've been around long enough and i KNOW i should know this... but i can't find my Maximum Boost book in the midst of all my packed stuff from my apartment...

what is the difference between a blow-off valve and a wastegate???

if i am building a custom turbo kit for my 325i will i be able to be use a HKS super-sequential blow-off valve for it??? because my friend has an extra one... i know... dumb question. or at least for me. haha

man... you really can't be building a turbo kit without knowing that -- it's a very basic concept in turbocharging.

But to answer:

The wastegate controls the amount of boost produced by the turbocharger. It is a part of the exhaust system, either a part (bolt-on) of the exhaust manifold, or built into the turbocharger (internal wastegate). When the turbo creates the desired boost - typically based on the spring within the wastegate, the wastegate is signaled via a boost/vacuum line and opens up. This allows gasses to flow around the turbo to control the speed of the turbo, thus, not overboosting.

The Blow Off Valve - or Bypass Valve - there is a difference - is a part of the intake system. It is connected to a vacuum line located behind the throttle body. Once throttle is released, the vacuum on a car will jump and will open a BOV/BPV. This design is so that the excess air has somewhere to go once the throttle body is closed - otherwise your intake pipes will build up pressure and look for the path of least resistance... back through the turbo. Without the BPV/BOV, closing the throttle could cause your turbo to slow down from full speed to little-to-no speed... which damages the turbo.

A BOV typically blows pressure into the atmosphere - your Fast and the Furious "Pshhhh" sound.

A BPV typically blows pressure back infront of your turbocharger -- which helps spooling when shifting.

A car - like ours, or e30s - with an AFM cannot use a BOV. While other people have run them before - blowing pre-metered air out of the system will cause some bad running issues. The system will need to be designed with a BPV that recirculates the excess pressure back into the system, infront of the turbo, but behind the AFM.

Hope that helps...

haha... yeah thanks for the help... i just read this post and i think i had a little too much wine for the 4th... lol... because i knew all that... except for the part with the AFM and BOV not working together... so thanks very much... that will be very useful information...

i'm going to try refraining from drinking and posting now...

it's dangerous.



Bryan

now... say i were planning on building this kit from the get-go with a MS2 engine management with an Innovative LC1.... wouldn't that eliminate the problems caused by using a BOV??? or are there other downfalls to using it? and how much does a BPV actually help the turbo spool during shifts???

I believe if you get rid of the flap door AFM and replace it with a MS setup then ... yeah... I suppose you can run a blow off valve.

Do you really NEED to make that PSHHHH sound? Get a recircing HKS SSQ - I'm pretty sure they have them - It should make some noise and recircs the boost.

I mean, I got a buddy with a VW 1.8t with an aftermarket BPV, it sounds like a slightly muffled BOV, sounds good.

Heck, at 10psi you can hear mine from the cabin kinda.

ill vouch, 8psi I can hear my BPV just fine from inside the cabin or with the head out the window, but who cares what sound it makes when the car you beat is soooooo far behind you .......

i was running 13 psi with the hx diverter valve and that thing was loud as hell. sounded similar to a BOV but was more or a sharp pop and not a whoosh.

also, say one designs a turbo system still utilizing the AFM, if it is moved up closer to the throttle body you could use a BOV before the afm and still have the air metered correctly right? (BOV------>AFM------>throttle body). i just ask because i believe technique tuning does something like this on their m3 kits and i think they call it a blow through system.

gol10dr1 wrote:i was running 13 psi with the hx diverter valve and that thing was loud as hell. sounded similar to a BOV but was more or a sharp pop and not a whoosh.

also, say one designs a turbo system still utilizing the AFM, if it is moved up closer to the throttle body you could use a BOV before the afm and still have the air metered correctly right? (BOV------>AFM------>throttle body). i just ask because i believe technique tuning does something like this on their m3 kits and i think they call it a blow through system.

The AFM in this situation will get slammed shut every time you get off throttle. This could damage the meter.

Sweeneys right,
The AFM was never intended to be a blow thru meter.

RussC

ahhhh.... very good input guys... thanks... it helps a lot... i'm on the verge of doing 2 turbo projects at the same time... i'm pretty excited.. because i'm getting closer and closer to having my own shop...