I am trying to decide on which way to go with the exhaust on my car (stroked 400 in a Dart). Everything else being equal, what I would like to know to aid in my decision is how much difference in horsepower gain could be expected from Schumacher Tri-Y headers with 1 5/8" tubes over stock HP manifolds and then from TTI full length headers with 1 3/4" tubes over the Schumachers. I'm not concerned with ease of installation or anything else, just what difference in HP to expect going from manifolds to the Tri-Y's to the TTI's.

It depends on the horsepower per cylinder you have, but if you go to Mopar Muscles' website I wrote a tech article comparing manifolds to a few different header designs on a mild 360. It would at least give you rough idea.
Keith

I can't tell you HP figures , but long tubes will be the best , Schumachers will be second , HP manifolds worst ...

It will be a much larger difference on a 500 hp engine compared to a 300 hp engine.Saying you have a stroked 400 I would be looking 1 3/4 longtubes at a minimum for maximum hp and depending on other variable such as gear,converter,compression,cam,etc. I may suggest 1 7/8 tubes.

That being said for what the tri y headers cost I would go straight to the long tubes myself.If the budget is tight go with the hedman shorty headers and a full mandrel bent exhaust.

What I know about BB in A bodies, especially stroker BB, use the largest set of headers you can get, same on the exhaust pipes and mufflers I ran a stock stroke 440 in a 1969 Dart GTS years ago that ran 12;23 at 109.5 MPH in the 1/4 with a set of Hooker 1 7/8 fenderwell headers with 3.5 inch collectors feeding into a set of 2.5 inch pipes into a 2.5 inlet and outlet "Turbo" mufflers and then into a set of 2.5 inch outlet pipes that exited in front of the rear tires.(total length was less than 5 ft. ) I removed the exhaust system at the colletors and the car ran 11.39 at 117.8 MPH on the next run BB Mopars like to breath, bigger exhaust systems are way better than small ones My next BB A body car had 2 inch Hooker fenderwells with 3.5 inch collectors a three inch exhaus system from the 3.5 inch collectors to the 3 inch in and out 14x7 oval Magnaflow muffs, that sytem was .042 quicker(6.97 without the sytem and 7.01 with it on) ET and .3 MPH faster in the 1/8 mile by removing the exhaust system, not really worth while for that little gain That car would run low tens at 127 MPH through the exhaust sytem in the 1/4, I never remove the exhaust at a 1/4 mile track based on what I had learned on the 1/8 mile tests IHTHs