1965 273 Heads

PolarBear said
I had some new duals bent, and attached to the existing exhaust manifolds (which I'd also like to replace with some TTI's)......... that's another story, though.

Tell me what I'm missing - the spec manual says that the 273 Commando (235 HP) had something like 12:1 compression... and that the 273 2bbl only had something like 9:1. That difference doesn't come from the heads? ...... or do I just need to change the valves? .... or what? How would the pistons make that difference?

The spec manual also says that the Commando had a different cam. This, I'm sure, would give the old block a little kick.

The addition of a slightly bigger cam would help,
but if you really want more power go with a larger engine. ;)

The 273 High compression pistons {10:5} are hard to find,
and not cheap.

Another problem, with the High compression 273 is fuel quality/octane
of the fuels you can buy at the pump.

Are you planning on street driving the car? What rear gear is in the car?
If its the stock 2:93 you don't want too large of a cam,
in a small CID engine.{I have learned that the hard way.}

273;
8:8- 9:1 to 1 2bbl compression,
10:3- 10:5 to 1 4bbl compression, bigger cam, Same exact heads.

1966 D Dart race only 273 was even higher compression,
but was a Race only setup, with about 50 cars built.

factory 4 bbl,273, with the 10:5 compression made 235 hp.

in 1967 the "New" 2 bbl 318 made 230 hp, and More torque.

"No replacement for more displacement"

Your 273 heads are limited in valve size, because the 273 bore is so small.

If you want to use larger valves, which are not needed, even with a 4bbl,
the block needs to be notched, which is not cheap,
or the larger valves will hit, and ruin the valves, and the heads.

For all these reasons, I built a 318 when my 273 was very tired.

Think I have no clue, what I am talking about, read Larry Shepards books
on small block Mopars, and buy some of the older Hustle Stuff books, etc.

A 1965 Service manual has a lot of good info too. :)