In a message dated 97-09-08 13:47:26 EDT, mgbob@juno.com writes:
> Does anyone know how to calculate dwell settings for contact breaker
> gap, so we can set the gap more accurately than with feeler gauges?
Bob:
Actually, the gap setting is derived from the required dwell angle, rather
than the other way around. For each complete combustion cycle, ie. two
revolutions of the crank, the distributer rotor makes one revolution, or 360
deg. For a four cylnder engine, each cylnder has a maximum of 90 deg
distributer rotation. During this 90 deg, 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, which can be a problem at high RPM.
Adjusting the timing sets the crankshaft angle (based on piston position in
the bore) at which the points open, and the dwell angle is set by adjusting
the point gap. A smaller gap increases the time the points are closed, ie,
increases the dwell angle, and a larger gap the opposite. My Haynes Manual
for an MGB gives dwell values of 60 +/- 3 deg, or 51 +/- 5 deg, depending on
the model.
There is no absolute formula for determining dwell angle, and thus the proper
gap, as there are so many variables to be considered. Anywhere within the
ranges above should be ok, but the best way to determine the values of dwell
for your car is to set the gap one time per spec, measure the dwell, and use
that value from then on.
For a more detailed discussion of the ignition system, and dwell angle, go
to:
http://www.vtr.org/maintain/ignition.html
Dan Masters,
Alcoa, TN
'71 TR6---------3000mile/year driver, fully restored
'71 TR6---------undergoing full restoration and Ford 5.0 V8 insertion - see:
http://www.sky.net/~boballen/mg/Masters/
'74 MGBGT---3000mile/year driver, original condition
'68 MGBGT---organ donor for the '74
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