A big misconception I see a lot is that people think BOOST equals horsepower. It doesn’t. Airflow does.
(for the nerds)
If you look up Boyle’s law, it says that pressure multiplied by volume always equals a constant. If the volume increases, you’ll need LESS pressure(boost) to meet the constant. If you think of horsepower as the constant and exhaust size as the volume, this means you’ll need LESS boost to make the same amount of horsepower with a bigger/better flowing exhaust.
Because we are dealing with a combustion engine and not just gas in a lab with a constant temperature, you also have to take into account Charles' law which, in simple terms, states that when you increase the temperature of a gas, you'll also need to increase the volume(of the exhaust) to maintain the same pressure. If you increase the temperature of the gas within a given space but not the space itself, you increase (back)pressure. More boost equals more heat. More heat equals more pressure.
When you combine these two together, it explains that when the air is compressed, the pressure and temperature increase as the volume of space containing the air molecules decreases. By pushing air into a smaller space, more force is exerted on it making it hotter and more pressurized. The molecules within the air will try to spread apart. It is this constant attempt to move away from one another that creates additional pressure. Now for the simpler explanation..
If you look at boost as a physical measurement of restriction and NOT as a means to measure horsepower, you’ll see that simply adding more boost doesn’t always mean a faster car…it can potentially mean a less reliable one. For example, 15lb of boost on a car with a full 3” downpipe back exhaust will make more power than 15lb of boost on the same turbos through a stock exhaust(or a smaller exhaust). This is due to the fact that the bigger exhaust system allows more AIRFLOW through the engine while simultaneously reducing back pressure and heat at any given boost number . The more back pressure a system has, the harder the turbos have to work to force the air(which is hotter and now has more volume–taking up more room but not increasing airflow) through the system and even more heat is created in the process. As you increase boost on both systems, you’ll notice a few things. Each lb of boost past our example will make less horsepower per lb on the stock exhaust system than with the full exhaust. As you continue to increase boost(heat/pressure) the increase in horsepower per lb of boost will start to exponentially drop onon both systems. The full exhaust system will maintain a bigger horsepower per lb margin and take longer to completely run out of steam as you are traveling further out of the efficient range of the turbocharger. Eventually, you won’t be making any more power, just adding more heat and stress until something in the engine or turbo gives out. This will be due to not only the small exhaust incapable of airflow, but the tiny turbos themselves ALSO incapable of increasing airflow past their limits.