In a message dated 97-05-30 19:20:47 EDT, randyr@starwave.com (Randy Rees)
writes:
> I never could figure out how you could set dwell and gap with only one
> set screw.
Randy:
As a matter of fact, dwell and gap are two ways of saying the same thing.
Actually, the gap setting is derived from the required dwell angle. Dwell is
determined as follows:
For each complete combustion cycle, ie. two revolutions of the crank, the
distributer rotor makes one revolution, or 360 degrees. For a four cylnder
engine, each cylinder has a maximum of 90 deg distributer rotation (360/4).
During this 90 degrees, the points must remain closed long enough to allow
the magnetic field to build up in the primary side of the coil, and then open
to allow the field to collapse, generating a high voltage discharge from the
secondary side. If the points don't stay closed long enough to build the
magnetic field, a weak spark will result. Too long, and the discharge time is
reduced. This is a particular problem at high RPM. For a 6 cylinder engine,
the maximum rotation for each cylinder is 60 degrees, and for an 8 cylinder,
45 degrees. Typically, the ratio of closed to open is on the order of 3 to 1,
ie closed 60 degrees, open 30, for a 4 cylinder.
Adjusting the timing sets the crankshaft angle (based on piston position in
the bore) at which the points open, and adjusting the point gap determines
how long they stay open, thus setting the dwell angle. A smaller gap
increases the time the points are closed, and a larger gap the opposite.
Hope this helps!
Dan Masters,
Alcoa, TN
'71 TR6---------3000mile/year driver, fully restored
'71 TR6---------undergoing full restoration and Ford 5.0 V8 insertion - see:
www.sky.net/~boballen/mg/Masters/
'74 MGBGT---3000mile/year driver, original condition
'68 MGBGT---organ donor for the '74
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