I'm lead to believe the purpose of a turbo is to basically "pack in" as much air as possible into the cylinder, therefore yielding more powerful combustion, but wouldn't doing so upset the fuel-air ratio inside the cylinder, meaning I would have to add more fuel to maintain the proper ratio, thereby decreasing fuel economy?

And on a similar note, having an exhaust that's too big for your engine decreases hp, but why? I mean all the exhaust does is allow exhaust gases to escape the cylinder more quickly, so why is having "back pressure" so important? What is back pressure doing that's aiding the engine?

I'm told that once you add a means of forced induction into an engine, (like a turbo), the bigger the exhaust the better, why is back pressure suddenly a non-issue in a forced induction system?

Thanks much,

Azure