Posted from JROD on ls1tech.com as to why 1 3/4 headers are, generally, better than the 1 7/8 headers on stock 346 engines. This is the info I"ve known about for a while. If you can show me info as to why the 1 7/8" headers are just as good, I’m always willing to learn.
“…I would agree with you that a good 1 7/8” header should make power over a poorly designed 1 3/4" header any day of the week. I happen to prefer a long primary tube for a street/strip 346 CID engine, especially in a combination that wants to place the emphasis on the street side of that equation (better low speed and midrange TQ and power, throttle response, fuel economy, etc.)
On a 383 the larger (but much shorter) tube is worth about 10 more peak HP and 12 or so past peak…it will also widen the peak numbers a bit (a good thing), but expect to lose about 20 ft/lbs at 4K or so. You can tune all you want to try to fix this hole, but you can’t fix it. Thats simply where the long smaller primary just overpowers the shorter larger primary…
As I said, what we have typically seen is that a larger header wont hurt top end power figures, but it will hurt the area under the curve and lower your “average” HP and TQ figures, and as stated slightly penalize you in the fuel economy department as well.
On a 346 this is 20 to as much as 30 lb ft of Tq. in my experience."