The slant or the 318
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.
Well. I neglected to do the necessary math relative to the valve AREA (as opposed to just the diameter)of the valves in question, and that failure to do so, resulted in a very incomplete and basically misleading contention.
Here are the real numbers; valve AREA as opposed to diameters, in these cases:
Stock slant six valves:
Intake....2.06 sq. in.
Exhaust..1.45 sq. in.
Oversize /6 valves:
Intake....2.38 sq. in.
Exhaust..1.77 sq. in.
Z-28 302 Chevy valves (stock)
Intake....3.20 sq. in.
Exhaust..2.01 sq. in.
On a percentage basis, the Chevy valves are 34% larger in area than the oversize (1.75") slant six intake valves, and 14% larger than the oversize (1.5") /6 exhausts.
Personally, ~I~ would rather PUSH a Mopar than drive a Chevrolet, but that's not pertinent to this discussion.
The fact is, the Chevy head will flow 300 cfm on the intake side with some porting; a great port job on the /6 head doesn't seem to be able to flow much more than 200. Please correct me if I'm wrong (I would LIKE to be!)
Given that set of figures, you can see that getting 2, or even 1.5 horsepower per cubic inch out of a normally aspirated 225 on gasoline is a monumental task, and expensive.
Using Guzzi Mark's car as an example, his car weighs, maybe 2,500 pounds in A/Gas trim and goes 115 mph. That takes 313 horsepower, according to the Wallace online computer (a bll-park figure, admittdly, but probably close.) If he has a 235 cubic inch engine, that's 1.33 horsepower per cubic inch.
An admirable figure, given the difficulty of putting air through the slant six head, with its smallish ports and valves. I am always amazed when I see the vidoes of that car... it moves!
On the turbocharged side, we have this video of Ryan Peterson's '66 Valiant
going 127 mph at 2700 pounds...
[ame="http://www.youtube.com/watch?v=7QzUfV8iTpQ"]Turbo Slant Six 10.74 @ 127 mph 7-19-10 - YouTube[/ame]
That's 455 horsepower, according to the same Wallace calculator, and I may be light on the weight at 2700 dunno if that included Ryan's weight.
That figures out to 1.94 horsepower per cubic inch. And that's with a "power-robbing" 727 T-Flite. His engine is 234 cubes.
So, yes, turbos work and yes, they're expensive to build to this level, but if all you want is a peppy daily driver to give the street-driven 340s fits, stock internals and a 10-pound boost limit will probably work for low 14's/high 13's.
Just sayin', before you mill that /6 head and spend money on deep gears and a nice, equal-length header, you might consider a low-buck turbo setup. Easy to upgrade, later...