I was just thinking that same thing but about the Audi version of 1.8t... now I have to talk to my A4 gurus and see if my memory is correct.

Sounds like what you might need is one of these in your charge air pipe:
http://www.greddy.com/img/PHP/products/jpg/560.jpg
Second item from the bottom and it it called "Relief Valve"

It is designed for overboost protection and is typically set one or two psi above the max pressure you want to run. They are sometimes used as protection from sneaky and dangerous boost creep. But in your scenario, it could be used in place of the BOV to maintain charge air pressure at your preset level, but not exceed it. -No electronics or gizmos to break with one of these. And it would also protect against the pressure spike when the throttle slammed shut.

Most guys who run them just use them for cheap insurance against blown engines.

It says its adjustable from 0.8 to 2.0 kg/cm^2,
which is about 11 to 28 psi

That wouldn't address compressor surge when he does come off the throttle for more than a split second.

is if possible for external wastegate? internal I can see how it is easier. with your technical abilities it is possible.

I understand the basic premise- keep air flowing, while maintaining MAP at a level just above the BC level.

In practice, with the throttle closed, the power available to drive the turbine is zero, and I can't believe that the wheel / shaft assembly, as light as it is, carries sufficient momentum to continue pumping a significant mass of air at PR>2.

In practice, I expect that it might relieve that initial surge as the throttle plate snaps shut, but that the turbo will decelerate much more rapidly than it would with a vacuum-assisted bypass valve in the picture.

Going back to the original post, isnt that why most BOVs have a spring tension adjustment? So that a properly adjusted BOV wouldnt open until the compressor is about to go into surge?

There is one really good way to fix this problem, but its a bit complicated.

Throttle body at compressor inlet (and on the manifold as well)

If you have it the manifold I would still assume you would need to run a bov to release the air after the tb at the manifold shuts if you mean intake manifold. Why not ditch the TB on the manifold itself, why would you keep it?

Are you familiar with the chronic idle problems that owners of intercooled hotside superchargers seem to suffer from?

A solenoid inline between a regular spring BOV and the vac source, similar to a EBC setup, seems like the simple way to electronically controll it, but it's starting to sound like a pointless mod.

Joe is probably right. There may not be a way to keep pressure. Either realease it or deal with surge.

Whats the idle problems they suffer from? No I'm not aware of them.

Constant hunting idle, often dies when returning to idle, needs a high idle to be stable, good luck if you want a decent idle with the A/C on

Couldn't this be corrected by installing some sort of additional idle valve on the manifold itself? ( not that it matters )

Well it looks like I'm about a week of so away from my trial code. It will at least be interesting to see if I can get the code to work the way I want. Still a little new to the C# environment.

It seems to me that the turbine does keep making pressure during the time when the throttle is closed, for a moment anyway. Thats where our surge comes from, excessive pressure ratio without flow. All I'm looking to do is wait until I'm close to the surge line before I release pressure.

A pre-turbine throttle may fix this senerio perfectly, i don't know. I do know that that's a lot of hardware changes. This will just be a processor a few sensors and a valve on the vacuum line like ARTech mentioned. We'll already be in a bit of a control system development stage as we'll be looking for a way to control a VNT turbo as well.

--Alex

No. Or at least, "It's not that simple."

The problem isn't so much the location of the idle valve as the throttled volume. That is to say, the total volume of the space between the throttle plate and the intake valves. You see, when the idle control valve makes a small adjustment, it has to flow enough air to raise the pressure in the whole post-throttle space before the engine really responds. As this volume increases, the responsiveness slows down more and more. Remember that with a positive-displacement supercharger, the throttle is relocated to the inlet of the S/C.

With the stock intake manifold and the stock throttle location, the throttled volume is the space inside the plenum and the intake runners.

With a coldside, it's the volume of the supercharger itself, plus the manifold and runners, which isn't much of an increase.

With a hotside, it's the stock throttled volume, plus the volume of the S/C, plus the volume of the crossover tube.

With an intercooled hotside, it's all of the above, plus the volume of the I/C. This, incidentally, is the primary reason why the S/C guys love water-air intercoolers. It's not that they aren't worse than an air-air intercooler from a thermodynamic standpoint, but they have a much smaller internal volume so they don't make idle suck as much.


If you're in a situation where you are stuck with a very large throttled volume, one solution is to leave the stock throttle plate in its usual location, and add a second (presumably larger) throttle upstream of the S/C.


The wheel/shaft has a certain amount of momentum, so it'll keep spinning as long as it can. If it is acting against a restriction, then it will make pressure, but at the same time it will decelerate rapidly. If it's simply blowing out an open hole, then of course the PR will sit at 1, however the shaft will keep spinning for longer.

An ideal BOV is tuned principally to keep the turbine spun-up during the shift by eliminating any restriction downstream of the compressor. In doing so, it also prevents the compressor from operating the the surge area.

Compressors will surge even at low outlet boost when the flow is very low.
So a simple "regulate the pipe pressure at a tad above the boost target" won't work. It will surge. In fact my GT2560 surges during the BOV delay, if I lift while making say 7 psi boost at 2900 RPM, as I lift smoothly and the needle passes 2 psi.

Besides, regulating boost in the charge pipes at a tad above target boost is not simple.

Which is why VAG uses an electronic bypass controlled by the ecu...

Which is what my plan is as well. It will be a little more complex than a pressure switch on the BOV....
--Alex

What I'm saying is that the algorithm isn't simply "open the BOV when intake pipe pressure exceeds x boost".

There needs to be a lookup table to open the BOV x seconds, depending on RPM and rate of rise of intake tract pressure.