What would I gain from a 4bbl on a 273?

Hey Flatlander;

It will be a nice upgrade, mostly in the HP department. I can not say for sure what the increase will be, but I do think all the exhaust work and a 4bbl will be nice. I tend to do the same to all my mondaine cars to spice them up. It is very satisfying.
(sp, all over)
On the HP to torque thing you ask about, torque, based on the engine configuration alone and nothing else is more readily avaliable with a longer stroke engine. You can see this in a 383 MoPar vs. a stoked SBC from a 350 to a 383. The Chevy engine with a longer stroke makes more torque in the low RPM band than the short stroke MoPar 383. Then the HP that comes in with the higher RPM's tends to be favored with the MoPar.

Your 273 is a short stroke engine. A few things that can help in the torque producing area would be dual exhaust with or without headers. Headers a plus. Change of cam. I would call up Comp or Crane for help and ask about customizing a cam to enhance torque in a small engine.
Now that your going to a 4bbl. , a small one, like a 500 cfm unit is more than enuff for moderate engines and small ones as well. Going to a larger one will weaken the snappy response and hinder good air and fuel flow for good torque making mixtures.

Some head work can enhance low lift cylinder filling. (This an a cam designed for this thought train) By addressing low lift cams and enhancing the cylinder head to take advantage of this all, you can help fill up the cylinder with more air and fuel.
Now that theres more in the cylinder, the result is a more powerful charge and bang.