1.6 HP per cubic inch of displacement that the 86 CID Cruze engine has for a stock production engine with a 100K mile warranty isn't very bad. Some very exotic engines hit 5 HP per cubic inch of displacement, but are only good for a couple of hours of racing and have to be completely overhauled. And certainly not using convenience store pump gas.
A turbo can be looked at in two different ways, for the Cruze to recovered a small portion of that wasted exhaust heat where about 85% of the energy is not being used to operate the engine, but causing global warming. The Cruze is classified in the mpg race.
Another is increased performance, but a turbo does have limitations, more like a bell shaped curve. The more boost it produces, the more restriction it adds to the exhaust so performance will suffer over that peak. It has to be optimized for both fuel economy and performance.
For the big stuff, a supercharger is far better driven by the engine crankshaft, but for an 8,000 HP engine, that supercharger would consume about a thousand of that HP just to run it. Just about everything is a tradeoff in engineering, and for a "cheap" vehicle like the Cruze, manufacturing cost enters the equation.
HP = Torque X RPM/5252, torque is mostly gained by increasing displacement, some motorcycle racing engines run at 17,000 rpm, another way to gain more power, but have to be geared down substantially. Piston and bearing friction also increase exponentially, another tradeoff, forget about durability and engine life.
Also compressing air produces a lot of heat so intercoolers are required, the restrictions of the plumbing become a very strong consideration.
Unleaded gas is a major problem, but beats using leaded gas that explains why we have so many nuts today. Its combustion chamber is 2,600*F that can fry exhaust valves in a hurry and put large holes in your pistons. To counter this, either an EGR valve is used, problems with buildup, or the exhaust valves are closed early in the case of the Cruze. But this can effectively decrease your displacement by as much as 30% making 1.6 HP per cube even a lot more attractive.
Water injection would be far more superior, but not for the average vehicle driver that don't even know how to fill their own gas tank let alone anything about what is under the hood. Only thing the EPA can talk about is excess NOx emissions, nothing about frying up your engine.
A ton of books have been written on this subject since 1885, should do some reading first before asking questions, and never will get a straight answer.
