Originally Posted by phunk

These pistons would cost you a little power in a NA car.

The factory compression ratio is 11:1. These pistons are 10:1. Reducing the engines natural compression ratio will reduce overall power output at any given airflow.

When people decide to upgrade their internals for more horsepower with a boosted engine, if that engine was considered a "high compression" engine in its original form, then it is common to slightly lower the compression ratio. So this is a typical procedure when doing an engine build for a NA car converted to FI.

High compression ratios are ideal for overall power and efficiency. However, once you force more air in with boost, the high compression ratio really starts to compound the pre-ignition cylinder pressures. Too much heat and pressure can cause the air-fuel mixture to self ignite early, before the spark plug sets it off. This is very bad for the engine.

So it is a balancing act. Like every choice we have to make in a performance car, there are compromises. For the sake of maximized efficiency, you want the compression ratio as high as possible. Luckily, the VQ37VHR makes lots of power with very little boost pressure. Most people will never even attempt to run as much boost as, for example, a *stock* Evo X runs. Those who go so far, would have one of the most powerful VQ's around. So, generally we do not have to lower our compression ratio nearly as much as other guys. This is great because it lets our engine be more powerful and responsive without using the boost as a crutch. Cars like EVOs and STIs drive rather poorly in comparison due to having extremely low natural power output.

Sorry if I went overboard and touched on things you already know. Just covering all bases!