I'm probably gonna take a lot of flack for this, but I'm gonna say it anyway, because after reading all the analytical performace information I could get ahold of on this site and others, I really believe it to be true.
That slant six can be made into a real powerhouse IF you don't try to do it using the tried-and-true, conventional methods of hot rodding.
These engines have several factors that make them very poor candidates for horsepower upgrades by increasing the breathing using say, a 4bbl manifold, headers, a longer duration, high-lift cam and higher compression.
Those are all methods that work well on MOST normally-aspirated engines.
They don't work very well on slant sixes BECAUSE, the basic engine design of the slant six was arrived at through the dictates of upper management at Chrysler REQUIRING a short engine (fan-to firewall) in order to produce an engine that would fit a "short" engine compartment (fan-to-firewall) for the new Valiant and Dart. They also wanted to have a low hood line, but that's another story.
The engineers were required to design an inline six that was smaller in a front-to-rear dimension than was really practical for an inline, but they did it anyway, the only way they could: They designed it with small-diameter pistons (and bores) and the 170 was born.
The 170 had a lot going for it; it had a good rod-length-to stroke ratio, a short stroke (low piston speed) great rpm potential and a cylinder head that fit the displacement very well; it breathed so well that even at 7,000 rpm, it made good power. The Hyper Pak was born and proved to be just about unbeatable in racing for engines of that class (displacement.)
The following year, Ma Mopar needed a station wagon/grocery-getter engine for the B Bodies. What to do?? The 170 was just too small.
Some genuis got the idea to stroke the 170 a full INCH, which did several things:
Made gobs more torque than the 170.
Changed a great breathing 170 into an asthmatic, strangulated also-ran, because in their haste to get that engine (170) to market, the guys who designed it didn't leave much room for improvements in breathing for the new 225, which was THIRTY-FIVE PERCENT bigger than a 170.
They never changed the valve or port size of that original 170 head.
So, you can port that head and add bigger (1.75" X 1.5") valves to it, but it's never going to breathe like what is needed, to make good horsepower/cu. in. numbers.
A 300 HP 225 slant six is a real accomplishment, and that's only 1.3 HP/CI.
You can build a 3,000-pound A body street car that will run low 14's, but it will have to be a radical-cammed engine with lots of compression and a good exhaust system. Deep gears will be needed to make it really perform...
If low 14's is fast enough for you, that's probably what you should do, but swapping a 360 Magnum in there with a mild cam and headers will give you better performance, cheaper.
The alternative, if you want to run quicker than that, and keep your slant 6, is to put a turbo or supercharger on it. The slant six basic engine has a very strong infrastructure and can stand quite a bit of boost before bad things start to happen... like over 20 pounds...
A 300 HP boosted slant six would probably last forever (nominally) and is pretty easy to put together.
Food for thought...
