Hi Scott, there must be a misconception here somewhere, I have an
article about 30 years old showing the build-up of a 392 Hemi with '57
iron heads and the photo clearly shows the spark plug is offset a little
from the top center of the chamber, about the same amount that the Ardun
is. My understanding is all the Chrysler Hemis were just like this one,
and today when they have aluminum aftermarket heads with two plugs, they
just have one on each side in about the same position.
I believe the plug is almost always offset with a two-valve hemi because
if it were central between the valves it would either (a.) make the
valves smaller or (b.) create a tendency to crack from the plug hole to
the valve seat with an iron head or to the valve seat insert in an
aluminum head. There may be some exceptions to this rule (Rich Fox, what
about the Nissan SOHC two valve Hemi?) if they use 12mm or 10mm plugs.
With today's four-valve heads the practice is to put the plug dead
center between all the valves.
The original Ardun has a "cartridge fire" setup where an 18mm flat-seat
plug has its electrodes up in a 5/16" or so hole about 1/2" from the
combustion chamber. This principle appeared on the old Miller and Offy
engines from the '20s on, and works just fine, needless to say. Most
people today use a Champion steel adaptor to go from 18mm to 14mm plugs
with cartridge fire, but a few use 18 mm plugs, which are still
available. Sometimes people alter the original Ardun setup to put a
14mm plug down on the combustion chamber surface per today's practice.
The "Bean Bandits" are said to have been the first to do this. The new
repro Arduns now being manufactured by Don Ferguson Jr. have a 12mm plug
right down on the combustion chamber surface. The Chrysler Hemi has
always used a 14mm plug down on the combustion chamber.
As to the flame travel in all these heads, they all work. The cartridge
fire setup seems to retard the flame propagation about 10 or 15 degrees
compared with having the plug electrodes down on the chamber proper, but
all you do is just advance your timing accordingly, most people use
about 45 to 50 that way. The power output seems to be about the same
either way the Ardun is set up, ancient or modern, or so I am told by
several of my veteran Ardun friends, if not all.
The bore and stroke of a 24 stud Ford/Merc in '47 when the Ardun
appeared were 3-3/16 x 3-3/4 (239 cubes). The '49 to '53 Merc used a 4"
interchangeable crank, the Ford kept on with the 3-3/4. In the '50s,
people bored their Flatheads to as much as 3-7/16" (madness but it
worked), often 3-3/8" (too big for me, but they did it), and commonly
3-5/16. The Merc 4" crank could be offset reground to get 4-1/8" using
21A con-rods and the full-floating bearings meant for them, since the
later crank had 2.139" rod journals and the early had 1.999". Other
forms of cranks, welded or billet, were and are produced, up to 4-1/2"
stroke which is the limit that can be squeezed in and still get
clearance. When you go all the way with this you get to the 325 inch
SCTA rule.
Cheers Bill
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