Originally Posted by NYC6
You could actually lose a good bit of power increasing the diameter. Dont forget about scavenging, especially for N/A motors you need to pull all the air out quickly to fill the cylinders fully with the fresh incoming charge. F/I cars like the Evo/Sti easily fill the cylinders through turbo pressure.
the difference between being .25" over is still better for power than being .25" under.
the biggest issue is back pressure, you ideally want none, no leftover gases in the cylinder chamber.
regardless of the turbo being able to push in tons of air, it doesn't PUSH out burnt exhaust gases (being the exhaust valve is open when the intake is closed, mostly)