Hi
Just a query to something thats been bugging me for a while -

Under normal conditions have you noticed that the boost solenoid starts switching under mild accelleration when still under vacuum.

Anyone have any idea what its doing? i would have thought it would only operate to control boost to the ECU set point.

I have a theory (which is probably total rubbish) that it may be reducing the vacuum in manifold and turbo to reduce oil being sucked in (through turbo bearings?).

I seen a few posts on people seeing a greater vacuum after fitting dawes which sorta ties in.

Just wondering whether instead of disconnecting the solenoid completely, fitting a one way valve in the line may be benificial to allow flow from into the manifold under vacuum.

Any ideas?

Rich

Flow into the manifold will determine engine RPM (throttle).

The usage of the Boost control solenoid at low RPM / light throttle is to help give a fast turbo spool-up.

Although you may see a strong vacuum in the manifold when not accelerating, there _may_ (not always) still be a strong positive pressure on the other side of the throttle (in the intercooler etc).

When you suddenly 'boot it' again, there is a good reserve of boost ready to surge into the manifold.

If the solenoid was off, then the max pressure behind the throttle would be around 8 psi (inner-loop boost control determined by the actuator).

From what I have noticed from MY98 ECU, as soon as you touch the throttle, the boost solenoid duty is set to around 90% (spool-up phase 1). It stays at that setting until RPM is around 2500. The duty ratio then drops suddenly to 75% (spool-up phase 2). Then at some combination of RPM approaching 3000, or the boost is getting near target, it lowers the duty ratio to the static value.

It stays at static value until RPM is about 4200, and then tapers down to reduce boost at high RPM.

I will soon publish some pretty graphs to show this stuff actually happening (when I get round to copying them off the laptop).

I also have a cool graph of what a fuel-cut due to high boost looks like (showing a delayed lambda hole).

James, you sound like you know what your talking about! I am interested in the role of this solenoid (I have a MY01) What does it actualy do ? does it allow air to bypass the throttle butterfly or bleed to atmosphere, either way the pipes are very small and I cant imagine the air flow would be particulary great or is it oiperating some other valve ?

Cheers

It is a recirculating (into the intake after the MAF sensor) bleed which (along with the restrictor orifice) only lets the wastegate actuator see a variable fraction of the boost thus altering equilibrium boost from the wastegate spring value of about 8 PSI.

If you want that in English (I tried to make it concise!) then see http://www.autospeed.com/A_0028/article.html and try a google search on wastegate solenoid bleed. Couldn't get the Scoobmania site to work which also has some nice diagrams.

many thanks John. Let's see if I understand this, the solenoid bleeds off some of the pressure from the wastgate actuator, in order to 'fool' the actuator into thinking there is less boost being produced.

I presume this is required, because the wastegate actuator starts to open the wastegate valve (in a semi-linear fashion) as soon as pressure starts acting upon it and would therefore reduce affective boost pressure at low RPM ?

As the RPM and boost pressure rises, it then allows the wastegate actuator to see more of the 'true' pressure and thus allows the wastegate actuator to take more control ?

Please correct me if I am wrong, cos to me the turbo is the heart of these cars and I find it quite fascinating stuff...!

NB , why dont we fit the Dawes between the turbo output and the solenoid and leave it to do it's stuff as per the Autospeed article?

I think you are about right. Connecting the Dawes with the solenoid still connected will cause a war between the two of them trying to control boost.

Thanks for the info - put my mind at rest.

(so much for my rubbish theory ;) )

Rich