Supercharger Boost Return - Options for a conversion

Hi all,

Whilst I have mostly been playing with a turbocharged engine, I've recently picked up a supercharged car - a VW Corrado G60 - which I am converting to Haltech (using a Nexus S2). It's a scroll-type positive displacement charger.

I plan to also fit DBW - and have a Bosch TB for this.

I'm now wondering how best to handle the boost return...

The OEM throttle body is cable operated with an integral boost return butterfly connected to the throttle linkage in what looks like 1:1 ratio with fully closed throttle = fully open boost return and vice versa. So it's progressive, half throttle has boost return half open.

On my car the standard return pipework has been deleted, it previously fed back into charger intake and now just vents to atmosphere. It doesn't run MAF.

Given my new TB won't incorporate this mechanical return - what might now be the best way to handle this?

Some boost return valves (R53 mini for example) use a butterfly valve but vacuum actuated - so only opening during negative manifold pressure and not as per the vw version.

It would be nice to electronically control this - maybe with a small DBW throttle?? or could I use a ISC valve (although I can't imagine it would offer sufficient flow) or am I overcomplicating things??

Is the throttle before or after the charger?

You can use a wastegate, BOV or adapt a vacuum diaphragm like many chargers use either way. Depending on setup they will be more or less progressive with throttle differential pressure.

If using a wastegate or BOV you can vary the sensitivity via valve orientation to pressure source and spring pressure/pre-load. Use of a BOV or wastegate allows potentially more flexible packaging and their behaviour is self regulating.

I think DBW is generally a waste of money in itself, mounting arrangements, packaging and tuning time as well as reliability and available electrical system drive current.

Thanks Michael. I’ll go down that route then.

I’ve been driving the car a bit now, with the stock setup and a boost gauge so I’m learning a bit about manifold pressure behaviour in certain scenarios. Again I reiterate it’s my first experience with anything supercharged so this may be a dumb question..

If using a BOV, what happens with the excess boost pressure during part throttle application at high rpm? Does it apply ‘back pressure’ to the charger and accelerate wear?

In the standard config, because the return is bound by throttle position and not manifold pressure, I can apply small amounts of throttle, see the manifold pressure return to positive while it’s dumping the ~1 bar pressure through the return, since it’s almost fully open still.

With a BOV controlled by manifold vacuum, once I get to this ‘slightly positive’ (or at least atmospheric) pressure, it would close and I would be seeing the full 1 bar in the system.

Might this cause drivability issues? Or accelerated charger wear?

Or this what you mean by using spring pressure on a BOV? Such that it needs more than atmospheric pressure to close?

The specific arrangement depends on if the throttle position relative to the blower and where you have your reference line for your guage, can you put up a diagram or link to an engine system diagram?

Or even a word sequence

Throttle > [boost ref?]>blower>[boostbref?] Manifold?

The gauge is retro fitted and I’ve taken a reference from the manifold (after throttle valve) so am measuring manifold pressure.

I can create a diagram when at my PC tomorrow but in words it’s:

Air filter -> Supercharger -> Intercooler -> Throttle body -> Manifold (Reference line for gauge fitted here)

I’ve attached an image I had already but it’s not the clearest and probably doesn’t help. Supercharger is bottom left (air filter out of shot to left of it). Intercooler is out of shot on bottom right (you can see the feed and return pipes) and throttle body is top centre.

That helps.

Next question is what is the maximum boost the blower can generate as that can also impact orientation and positioning of a wastegate or BOV but isn't as critical.

Base on what you have said if you reference the signal port of a BOV post throttle and the piston face to the supercharger outlet it should work, can do similar with an external wastegate, top of diaphragm post throttle, bottom of diaphragm pre-throttle, valve face to boost with softest spring available.

That should behave in a similar way to factory and mostly (or fully at cruise and idle) unload the blower at partial throttle. If you could build slight positive pressure on a partial throttle with the original setup it still would have been making some boost against the timed bypass, so under that transitional boost condition it always would have been partially loaded.

Depending on how well matched the mechanical arrangement was previously you may well find you can make it smoother and more progressive with a pneumatic control of BOV or wastegate.

The only condition where it may load a but harder is initial cranking but that may actually be advantageous, provided you add a but if cranking fuel.