Inliners International Forums General Discussion Engines Langdons Headers vs Stock Manifold.

Here is my Gonkulator calculation of it.
These are GROSS (60s style) numbers - open exhaust, etc.
Stock 250, Offy-4v, 390 Holley
Torq 221 at 2700
Powr 161 at 4200

Plus Langdon duals
Torq 230 at 2800
Powr 175 at 4500

If your 250 is old and "loose", ie large clearances, you might get a few more HP than the above but the difference should be comparable.

Will you feel it? Yes. That change is about the same percetage-wise as taking a 283-2v and making it a 283-4v. Most of us have done that swap and felt the diff.

Also, not shown in the numbers above is the fact that you can now go from your 2" (if you're lucky) single exhaust to 2" duals with the Langdons. That will add an additional 10hp or so net and to the wheels. Tube headers are better but at this level probably not by much, and are noisier and leak more.