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The advantages and disadvantages of the various sorts of ignition control can
be summarised as follows:
Fixed timing
Fixed timing ensures that the magneto always delivers its sparks at the
optimal position of the armature to the magnet in the instrument's housing - the
point of maximum flux change. This ensures maximum spark intensity at all times.
It's a big 'Plus'.
An ATD in combination with a fixed-timing magneto therefore has the advantage
over a manually-controlled cam that retarding the timing is done otherwise than
by altering the internal timing of the magneto.
However, fixed timing with an ATD has the disadvantage that no allowance can
be made for load, as the only factor governing the amount of advance applied is
engine speed. A manual lever can always be retarded as necessary, even if the
sparks weaken slightly in the process.
Special considerations apply in the case of V engines. Although a magneto can
perform satisfactorily in a somewhat retarded mode internally (vis à vis the
optimal fully advanced setting), in V engines one spark will inescapably be
'retarded' in relation to the other because magnetos are not designed to work
assymetrically. Therefore, ATDs were adopted early by V engine manufacturers as
they avoided the need to weaken additionally the spark supplied to the 'Number
Two' cylinder. In a 60° V twin with a half-engine speed magneto, one spark is
inevitably supplied 15° 'late' at the magneto - adding to it by using a manual
cam can push the magneto to its limit of operation or beyond.
Manual Systems
It follows then that manual advance and retard systems suffer from the
disadvantage that they demand interference with the optimal internal timing of
the magneto to obtain 'retard' for starting or other purposes (such as
hill-climbing under heavy throttle at modest rpm), while they have the advantage
that the operator can at least make small timing adjustments to cater for road
conditions, throttle position and load.
Many 'sporting' versions of British motorcycles retained manual operation
long after their more mildly-tuned siblings went to ATDs. Probably to give the
rider some control as described, possibly also because the sporting rider liked
to have an array of knobs and levers to play with, rather as car owners favoured
additional gauges and gizmos so they could have more to worry about while out
and about.
Further issues relate to wear and sloppiness in the cam mechanisms of
manually-controlled magnetos. With fixed timing, the cam can be positively
located in position by the maker and will stay there. A manual magneto requires
contra-rotation of a steel cam in (usually) an alloy housing. Despite efforts to
lubricate with oiled felt, wear inevitably occurs over time, and some degree of
chatter is induced. This chatter will not always be evident when conducting
static timing tests, but dynamic tests reveal a sometimes considerable amount of
variation in the firing intervals on multi-cylinder engines. Slop in the fit of
housing to cam is to be avoided if at all possible, and the locating peg which
limits movement of the cam needs to be set correctly to ensure the internal
timing is preserved.
We devote a good proportion of our energy to trying to eliminate such
variations on multis, by careful measurement of firing angles, contact breaker
performance and the condition of bearings and housings. It is not rare to find
as much as 10° of error between cylinders - at the crankshaft - on twin cylinder
magnetos, which is a recipe for rough running at best, and mechanical woe at
worst on a high compression engine used hard. It is, however, in the nature of
decades-old machinery for there to be some small degree of error - the trick is
to reduce it to the bare minimum.
General Pros and Cons
As far as we are aware, no magneto was ever fitted with a secondary system of
advance and retard using inlet manifold depression to override or offset the
advance offered by an ATD (or indeed to vary a manual setting). The lack of such
a parallel means of control may have extended the life of manual control on
motorcycles way beyond its prevalence in the automobile industry. Assuming
variations in pressure in a typical inlet manifold are up to the task, it would
theoretically (at the price of interfering with the internal timing on an
otherwise 'fixed' magneto) be possible to do so, using a manual cam on a
fixed-timing magneto, or a dual means of control on a manual magneto. We'd be
fascinated to hear from anyone who has managed to do this - and what effect it
had on the road manners of the machine in question.
ATDs generally advance the engine to its maximum by 2000-2500 rpm; most
operators of manual systems run their engines in the fully advanced position
most of the time, except for starting and tick-over or occasionally when loads
are heavy. This contrasts with the more sensitive advance curves to be found
with many modern electronic ignition systems, and has to be included as a
disadvantage.
However, these and any other negatives are outweighed, we believe, by the
beauty of having an independent ignition generator which is not dependent on the
sorts of charging systems to be found on the majority of elderly vehicles which
have little power to spare for coil ignition on top of lighting requirements and
battery needs.
On balance, an ATD in good shape is probably the easiest and most foolproof
way of managing the timing, but it's horses for courses, and personal preference
and originality will usually be the deciding factors in the decision whether to
go 'auto' or 'manual'. That and the question of space under the timing cover in
many motorcycle applications.
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Brightspark Magnetos