tl;dr: They do. It's just harder to tell how much.
The longer answer is that they do and that effective compression is failing you as an approximation for actual effects.
Think about detonation (AKA premature ignition of the fuel-air mixture). Normally we consider two causes: compression (the change in the space enclosed by the cylinder as the piston moves up and down) and temperature (e.g., measured temperature of the intake air).
In reality, there is only temperature.
Let's back all the way to the ideal gas law:
pV = nRT
where V is volume and T is temperature (in degrees Kelvin, remember!) and the rest are interesting constants that aren't germane to this discussion. Compression causes that V value to decrease and, therefore, the T value to increase, maintaining equality on both sides.
Think of it this way: you have a fixed temperature budget for a certain octane gas. If T gets higher than T_ignition, bang. So, as you point out, you can add an intercooler to the system, reducing the input air temperature.
Likewise, you can change the amount that V changes. This increases the amount of temperature increase that your engine can tolerate before detonating.
Now, adding a turbo on the intake air compresses the normal atmospheric pressure to something significantly higher, resulting in a change in those other constants that I previously brushed off (check turbo volumetric efficiency for more information) and increases the temperature.
That eats into my temperature budget. If I used lower octane gas, that would lower the threshold for detonation and, at boost, I could be looking at engine damage.
So, after all that, what do you do?
- Research research research: don't build in a vacuum. Copy other people's layouts or improve upon them.
- Measure your air intake temperature, before and after the turbo.
- Find the best gas that you can.
- Tune the engine computer to keep your engine from blowing up.
On tuning: one thing the ECU can do is add extra fuel to the mixture, thereby cooling the mixture down. Admittedly, using fuel as a coolant is not conducive to absolute efficiency but shouldn't be an issue when driving around out of the boost. As always, less right foot = less gas spent.
All of the above is discussed in Corky Bell's Turbocharging book Maximum Boost - a very entertaining read for geeky people like myself.
Following up some time later: I just noticed the specific question about 9.1 static compression ratio running 10 psi of boost. As an example, my WRX runs 8:1 at about 13.5 psi so, on the face of it, 9:1 with 10 psi seems achievable.
Let's look at one of the more arguably sensible equations for effective compression ratio (which, as we noted, is still an approximation of fairly complex thermodynamics):
ECR = sqrt((boost+14.7)/14.7) * CR
ECR is the "effective compression ratio" and
CR is the "static compression ratio" (what you started with before adding boost).
boost is measured in psi (pounds per square inch). Remember, the goal of this equation is to tell us whether our proposed setup is feasible at all and will it be able to run on gas that I can purchase on the street vs. the racetrack.
So, using my car as an example:
ECR = sqrt((13.5 + 14.7) / 14.7) * 8 = sqrt(1.92) * 8 = 11.08
Using this equation, the implication is that my effective compression ratio is about 11:1 at peak boost. That's within the bounds of what you could expect to run a normally aspirated motor on with pump gas (93 octane). And, proof by existence, my car does run on 93 octane just fine.
So, let's look at the setup in question:
ECR = sqrt((10 + 14.7) / 14.7) * 9.1 = sqrt(1.68) * 9.1 = 11.79
As cited in the reference, 12:1 is really about as far as you can go with a street car so this setup would still be within those limits.
For completeness, we should note that there is also another ECR equation that wanders around the internet that omits the square root. There are two problems with that function:
First, that would result in an ECR for my car of 15:1. That's a bit ridiculous: I wouldn't even want to start a motor like that with street gas.
ECR is an approximation anyway: the real answer to the question of "how much boost can I run?" is derived from critical factors such as intake air temperature and compressor efficiency. If you're using an approximation, don't use one that one that immediately gives you useless answers (see point 1).