So, this is a rudimentary math sheet I did for flow rates out of a 1.8L engine.
This doesn't account for expansion or restriction. It is merely a maximum flow for an NA engine.
Turbo charging changes all of these calculations drastically with compression rates and thermal expansion.
But this allows you guys a formula to start with if you want to figure out the best exhaust size for your car.
PS: The stock exhaust on an EA car is about as awful as you can get.
The flow rate of a stock EA exhaust is still creating back pressure in top gear.