I'll take a crack at a simple explaination...

Power = (Volume of Fuel-Air Mixture Burned) X (Combustion Efficiency)

A higher compression ratio improves combustion efficiency, but only by a relatively small percentage (say 10-20% increase in power).

A higher boost pressure increases the volume of fuel-air that enters the engine... in effect, increasing the engines virtual displacement. If you run a boost of 14.5 psi, this doubles the fuel/air volume processed by the engine at a given speed. Note that atmospheric pressure is 14.5 psi so in this example inlet pressure = 14.5 psi boost + 14.5 psi atmospheric = double or 100% increase in power. Similarly, running 7.25 psi boost would increase power by 50%.

Complicating things a bit is the fact that adding boost also raises the effective compression ratio at the same proportion. So if you consider that most engines even on high octane fuel can only handle about 15:1 CR, then at a boost of 14.5 psi, the mechanical compression ratio of the engine could be no more than 7.5:1 and similarly at 7.25 psi boost the mechnical CR should be no more than 10:1.

So, as you can see the objective with boost is to increase the virtual displacement of the engine without exceeding the limits of compression ratio which would cause detonation.

For the purists out there, please excuse the gross approximations and simplifications in the above explaination. I am only attempting to show the greater influence of boost on power as compaired to compression ratio.

__________________
-Wayne

1994 E320 Coupe (120k)
1990 300SL (BBS,RENNtech adds)
1988 260E Sedan (180k)