Is a blow off valve suppose to expel air when the car is just idling? Asking because I just bought a BOV and wondering how tight to adjust the spring.

 

what bov do you have. i have the Greddy type s and it doesn't leak at idl, it is also pretty tight.

 

I have a Go Fast Bits Stealth FX and a non-aftercooled Comptech SC running the stock pulley.

I noticed that the plastic stock Bosch diverter valve used to have a good amount of air coming thought it when the car was idling, wondering if I should set the BOV spring soft enough to allow some air out too.

The SC is always pumping air while the car is running, so where is all this air going when the throttle body is closed?

 

The air is supposed to be released when you are at idle or cruise. The way the bosch valve works is that whenever you have a slight vacuum in the intake plenum (because the throttle is not fully open) this actuates the diaphram and releases the excess boost. When you open the throttle the boost comes on and the diaphram is closed.

 

Monday?

 

Actually the answer is no. The compressor needs more than idle speed to boost. Boost starts to build above about 2000 rpm.

 

Ehh?

When your engine is at partial throttle the engine is trying to suck more air than the throttle (hence name) will let it. As a result the pressure in the intake manifold (between the throttle body and the head) is low (I guess 'high vacuum' is an ass backwards way of saying it). But if you're still revving (by this I mean the engine is still turning) then the SC is still pumping but the air has nowhere to go. The Bosch bypass valve is triggered by the line from the intake manifold (or more correctly by atmospheric pressure from the return line).

Note the difference between the Bosch set up and a blow off valve. In the Comptech install it specifically states that the Bosch valve should point down rather than sideways. This way, boost pressure is pushing on the side of the valve rather than against the diaphram and spring. So it's not a BOV at all, but a vacuum actuated bypass. A BOV uses intake manifold pressure directly against the diaphram vs an adjustable spring + boost feed. The spring is adjustable as is control of the boost feed pressure when a boost controller is used.

 

There is a distinct difference between the Comptech supplied Bosch bypass valve, and a BOV that is used on a Vortech or fitted to a Aftercooled Comptech.

 

Ahh, got ya! I've editted my post but it's still a bit different to yours.

BTW, it's Tuesday here.

 

Now I see where you have gone astray. My understanding is centrifical superchargers have no efficiency at very low speeds for the same reason you can drive the car with the belt off. Roots style blowers are more positive displacement building boost at idle.
At 1000 rpm idle speed the centrifical superchargers are not producing boost in the S2000.