Just wondering how much difference the size madeperformance wise.

 

Size for what????

 

Hmmn...we're on an exhaust board, common header sizes are 1-1/2, 1-5/8ths, and 1-3/4's inches in diameter...my guess is the original poster is talking about header primary size.

Gennerally the smaller primaries promote lowend torque at a sacrafice of high end HP vs. a loss od lowend tq. and gain of highend HP with larger primary headers.

Really depends on your gain and losses (not really losses, but lost potential) you may incur would have to take into consideration the engine your building/have.

 

what is a good size for a 300 hp 305

 

For a 300hp 305, I'd go with a 1 3/4" primary tube header, but that's if you really are making the number. Most 305s do well with 1 5/8" primaries. 350s need the 1 3/4". Only a 455 or a racecar would require the 1 7/8".