I read in CAR CRAFT and a F-Body mod book that if your car is not producing up to 450hp it won't beneifit from 1-3/4 headers. From the 270 to 450 range better gains are made with 1-5/8 headers. They say that too large of a header will slow exaust flow. Is this true??

 

There are two things to consider when looking at headers, length and ID. The ID is important when considering the shear volume of gas it is expected to see but the length has a distinct affect on scavenging. Most "shorty" headers are simply more effecient at merging the exhaust gasses but do not scavenge all that well. For a bolt on car, full length 1 5/8" primary headers would probably be best but nobody makes them that I know of. Probably because the people who install full length headers also install at least heads/cam, which will work best with 1 3/4" primaries. Some other things to consider if you really want to get "into it" are the collectors, stepped primaries, tri-Y's, etc.

When it's all said and done, it's been proven that the 1 3/4" full length headers outpeform the shorties all over the place. If someone made full length headers with 1 5/8" primaries and you only planned on bolt-ons it might be more appropriate, but that isn't a very large market so I wouldn't hold my breath.

 

Hooker makes 1-5/8 long tubes http://www.rit.edu/~jar5968/headers/headers.html