Peter
RE:"Is this a good thing to do? I would like to plug off the vacuum
retard
on my ignition as well but am not sure of all the consequences. Wouldn't
the distributor have to be recalibrated to provide the required level of
retard (mechanically) at different rpms which the vacuum unit provides
based on manifold pressure?"
The vacuum retard does not come into play unless the throttle
butterfly is almost completely closed (idle setting), regardless of engine
RPM. To achieve this, early cars (TR250 and '69 TR6 for sure) had a valve
in the vacuum retard line located under the forward carb. If the throttle
linkage wasn't in the idle position, the valve would block the vacuum
signal to the retard diaphragm on the distributor. Later cars avoided
having this valve by using a "ported" vacuum pickup that only senses vacuum
with the butterfly valve closed. (Note: if you hook your retard line to the
wrong vacuum pickup, you'll have retarded ignition at all RPM unless the
throttle is wide open, and the car will lose a lot of power.)
When calibrating a distributor, you are adjusting both the "total
advance" (static advance plus centrifugal advance plus vacuum advance if
installed) as well as the RPM and manifold depression they kick in at. When
"dynamic" timing is set to 4 deg ATDC, "static" is still at 10 or 12 BTDC,
depending on the year of the car. Therefore, the "total advance" available
to the engine at high RPM, as well as the advance at any other given RPM
(provided the throttle is not closed), is unchanged by plugging the retard
line and setting dynamic timing at 10 or 12 BTDC. Therefore the distributor
does not need to be recalibrated to achieve exactly the same performance as
it had before. Only idle timing will be changed.
I'm trying to be brief, and I'm no great writer. If you want me to
amplify this point, please e-mail me privately.
Best Regards,
Tom Marincic
CD3574L
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