How's boost controlled on turbo/super chargers? How's do people increase and decrease it?

The boost levels are controlled by the wastegate. Generally these are spring controlled, so the spring opens the wastegate at a certain pressure to control the boost.

Max boost on a supercharger is controlled by the speed in which it spins.

So, fitting a smaller pulley to the supercharger increases the RPM the compressor is spinning at and changes the max boost. To alter the way in which boost comes on is a result of the compresser wheel design. Superchargers have complex flow diagrams much like turbos do.

Also important to note is Blow off valves work differently in positive displacement supercharger compared to turbocharger setups.
Basically BOV's should be open at idle/low load to releve pressure from positive superchargers, as well as gearchanges.

Where in turbo's and centrifugal S/C's you want them to be shut at idle/low load but still open during gearchanges.

Boost is controlled by a wastegate on turbo engines as they can spin at any rpm [up to 150.000 rpm]
S/C is generally controlled by the size pullies on either crank or s/c itself..They all have flow numbers and maps..S/C being driven off crank has more control as engine rpm and s/c rpm are always the same ratio to each other..Without belt slippage...
Here's a link to most things about turbo's...

Bloody good read!!!

http://www.turbomustangs.com/turbotech/main.htm

..

This is how external wastegate controlls boost... Its very similar to an internal wastegate, though the internal vents directly from the exhaust housing.

The external wastegate is a valve, thats basically it. When you a driving OFF boost the valve is shut tight and held closed by its spring. Now this spring is at a tension that will not move until a certain amount of air pressure is applyed to it - this being the desired max boost level, say 10psi or 0.7bar.
A thin hose coming off the manifold runs to the wastegate and when the preasure inside reaches 10psi it pushes the valve open, due to the spring not being able to stay closed at 10psi.
Once this valve opens, air from the manifold can escape through the valve ( wastegate ) and out back into the exhaust AFTER the turbo- this reduces the air pressure/speed inside the exhaust manifold and as a result the turbos exhaust housing recives alot less air and wont spin the exhaust wheel any faster... thus boost preasure stays at 10psi

To raise the boost pressure you have some options... You can retain the 10psi spring in the wastegate and use a boostcontroller. The boostcontroller have to be set up so it recives air from the turbo's compressor housing ( straight from the turbos outlet ) and pushes this air back into the top of the wastegate forcing the spring/valve closed until a predetermind level, say 15psi

Another usfull feature of the boost controller is that it will hold the wastgate shut until it reaches this new boost level, unlike the wastegate which can start to slightly open at lower boost making for a slower boost.

Does that all make sense?

Very well explained CHEF! I knew what a wastegate was and roughly how it worked but that really got a clear picture in my head. I think alot of people would feel the same way.

No worries

On a side note about the XR6 turbos internal wastegate, they suffer from massive boost creep on anything over mid 200wkws... APS and other tuner shops grind out the wastegate to let it flow more air and settle down the boost creep. Alot of poeple have it welded shut and fit an external wasgate. This was the main reason I chose an external gate over a turbo with an internal gate... they just dont flow enough air, thus Garrett are/have stopped making internal gated GT turbos

not all turbos have wastegates though, as used more in newer common rail disels of today.

You'll find with higher boost levels the wastegate will cope ..Latter model Garretts of allmost the same turbo have bigger dump pipe flange...Wastegates work harder at low boost levels.....