So, this is a rudimentary math sheet I did for flow rates out of a 1.8L engine.

 

https://onedrive.live.com/redir?resid=9CA0A508CEACADDB!1327&authkey=!ACJ41AFMcHkhIuI&ithint=file%2cpdf

 

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.

 

Twitch

 

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.

So, I have a 1982 Subaru GL that I am currently doing a restore on. One of the things I want to change, and trust me there is a long list, is exhaust. My plan is to keep the cat in place but rebuild cat back removing the muffler. I was curious if this is a decent idea and if so, what diameter pipe would I experience the best gain with?

i dont know the science but i eliminated middle cat.2inch y pipe to a standard exhaust generic. runs somuch better carb e82 from a 81-83 car 3 spd auto