[beepbeep]

Wayne, you are a bit wrong actually...

First of all, there are two "valves" on most turbocharged engines.

One is called "wastegate" and is used to re-route exhaust gases around turbo to prevent turbo boosting too much or overreving...this as often spring based vacuum thingie. On newer cars, there is a solenoid valve in-between that is pulse-width-modulated by ECU to achieve right boost. Wastegates can be integrated with turbo (smaller Garrett-turbos, modern cars) or separate (older Porsches). They can also vent directly into athmosphere creating a helluva of noise @ full boost. (like mine :-)

Second one is caled BOV which is short for "Blow Off Valve" and is used to prevent excessive pressure build-up when turbo is spinning wildly and throttle is suddently closed (as in upshifts). Ir releases air between turbo-compressor (the "cold" half) and throttle-body. If BOV was missing, turbo would face sudden pressure-rise in pipes going to the throttle-body (and intercooler if available) and would likely start to surge, with potential compressor-blade failure as result. There are two types of BOV's, recirculating and open-air. Recirculating ones short-circuit turbos compressor-intake and outlet, recirculating the air...thus the name. Open-air variant just vents compressed air into the athmosphere...creating "pshyyy"-sound of questionable coolnes. Open-air dumps are not to be used on engines with hot-wire AMM as it usually confuses ECU who thinks dumped air is still in the loop and richens the mixture considerably.

There are some cars without wastegates:
- Old ones which use so called pop-up vavle which just vents air to athmosphere over certain pressure (BMW 2002 Turbo)
- New ones with VNT-turbos (SAAB 9-5 TiD etc.)

There are also rally cars which don't have BOV's but use funky anti-lag ECU's to prevent turbo-lag...reducing turbo lifelength considerably...

About bigger turbos:

Yes, bigger turbo *can* produce more power @ same boost, due to fact that it will be nearer it efficiency peak for given volume of air compared to small one. With other words, small Garret T3 blowing 1 bar into 3-liter engine @ 6000 RPM will be out of limits, surging and creating a lot of backpressure, where KKK K27 will be just on-spot, creating moderate backpressure and heating air nearer it's optimal PV=nRT limit...thus producing more power at same boost. But all that is another complicated story...