The slant or the 318
I like that Offy manifold! With a stock bottom end it would work well. Your super six is manifold only?
Have you ever considered turbocharging? The slant six motor has two things relevant to its construction that make turbocharging it a good thing, overall:
#1: It was originally designed to be an aluminum engine, so the areas of the block requiring strength equal to a cast iron motor, were beefed up (main bearing webs and cylinder head mating surfaces, for instance.) When Ma Mopar decided the aluminum was not going to work, due to a number of things (casting porosity, etc.) she said, let's make it out of cast iron, then... and didn't change much in the way of "infrastructure." The result, was an extremely robust, stout, ALMOST unbreakable piece with a forged steel crank with main bearings the same size as the mains in a 426 Hemi. Slant sixes go with boost like ham goes with eggs!
#2: The slant six was originally a 170 cubic-inch engine with small bore center spacing, and small bores. The cylinder head that was dictated by this small-displacement/small bore package had smallish valves and ports that worked well with the 170. The Ramchargers had one such 170 running 7,000+ rpm an an early Willys coupe at the Indy Nationals in '61, and it EASILY won its class against the best G.M.C. six-cylinders and flathead Ford V8's in competition, cars that had years of developmental time to their credit. The 170 was a high-rpm, absolute KILLER motor for that type of racing (pounds-per-cubic inch.)
But, Ma wanted to use that motor in some full size B-Body Dodges and Plymouths; what to do??? The answer was, to stroke the 170 crank
a full inch, for a displacement of 225 cubes. The small ports and valves were not a hinderance for this application, because they enhanced torque at low engine speeds, which was exactly what these cars needed to keep up with traffic. Making a lot of horsepower was never a factor; people who wanted to go fast just bought a Hemi...
So Ma never changed a thing to enhance the breathing potential for the 225 (and, later, the 198-inch) versions of the /6, a fact that makes getting one of the larger-displacement (225) motors to have enough air flow at higher rpms, a real problem. The 225 cylinders are 39 cubic inches each. That's the same size as the cylinders on a 302 cubic inch Z-28 1969 Chevy engine.
The valve sizes tell the story:
Chevy STOCK valves are 2.02" intakes and 1.6" exhausts.
The slant 6 STOCK valves are 1.62" intakes, and 1.36" exhausts
Remember; that's for the SAME SIZE cylinders...
Slant six cylinder head port sizes reflect this same disparity in flow potential.
Most "performance-minded"-folks have their slant 6 head ported and install 1.75" (nominal) intake valves, and 1.5" exhaust valves, which helps a lot.
But it's still far short (16-percent on the intake and 18 percent on the exhaust) of a STOCK Chevy V8 head from their 43-year-old, hi-po V8.
So, it turns out that getting a 225 slant 6 head to flow enough air to generate enough cylinder pressure to make a reasonably-impresive amount of horsepower, normally-aspirated, is going to be a tough nut to crack.
You can be very happy with a low 14-second 3,000-pound Duster (for example) that has a 4-bbl carb, a slightly lumpy cam, 12:1 compression ratio and a nice-sounding set of headers and makes, probably, 215 horsepower (according to Wallace online racing computers.)
Moparkid305 is a good case in point. He contends that his car is streetable, and if you like a 1,000rpm idle, and deep gears in your daily driver, that may be for you. You can have a LOT of fun with a low-14 second car....
There is an alternative, however, that may be just as effective and, overall, a cheaper way to get this performance.
Tom Wolfe had a '70, /6 Dart to which he did the following: Added a Super Six 2-bbl intake manifold, went to the junkyard and bought a used, '87 Buick Grand National turbocharger, mounted it on his stock exhaust maifold, and used stock-ratio rear end gearing and a stock converter and tranny. Ran 12.95 @ 104 mph. Here's the video:
[ame="http://www.youtube.com/watch?v=pPe_vHwZsF4"]Slant Six Turbo 1970 Dodge Dart 1/4 Mile pass - YouTube[/ame]
In all fairness, he did have the boost cranked up to 22 pounds for that run, but the head was stock; the short block was stock.... stock cam and he hadn't spent a lot of money in the engine (I don't believe the head had ever been off.)
Tom has since, raised the bar for his car with some good (expensive) forged pistons and aftermarket rods, and has a higher stall converter and enjoys a ported head with bigger valves, now.
The Dart has rewarded him with an 11-second-flat timeslip, at a little over 120 mph in the quarter. (That was run into a 15-mph headwind, BTW.) That takes around 500 horsepower.
It would take a pretty healthy (not cheap) small block V8 to compete with those times, I think.
Tom is a FABO "regular" and is always willing to help out with his experience and information RE: Turbos.
This is just ONE alternative to "hopping up" a slant 6 in the "conventional way."
Nitrous oxide would also be a way to some impressive times that would not break the bank.
Knowledge is power; examine all of the avenues, and then make your decision.... only YOU know what would be the best route for you.
Hope this helps!:cheers: