Dont quote me
But I say 4"....How much do you have?
Daily: 94 POS Accord
Project: 86 2dr Chevy Blazer
Refs: BMXbikes, Wragold, Tommyk, ThumpinTitan, NewAgeKing, BushJ311, l7runner, jco1385, Big Joker, SPY, xxxbasshead
What ever the diameter of the port is the rule of thumb but if you can go with more then do it. Either way you will not be able to tell a difference for the most part.