The posts from Dave Baker and Roger Mills mirror exactly the discussion on
the other group. ;-)

--
*Money isn't everything, but it sure keeps the kids in touch *

Dave Plowman London SW
To e-mail, change noise into sound.

"Dave Plowman (News)" wrote in message
...
The posts from Dave Baker and Roger Mills mirror exactly the discussion on
the other group. ;-)

If the other group has numpties like me on it then not surprising.
--
Dave Baker

In article ,
Dave Baker wrote:

"Dave Plowman (News)" wrote in message
...
The posts from Dave Baker and Roger Mills mirror exactly the discussion on
the other group. ;-)

If the other group has numpties like me on it then not surprising.

My position is I'm happy that moving the plate the points are on via the
vacuum unit does effect the phasing - but I'm still not sure about the
centrifugal advance. Thing is some dizzies without a vacuum advance still
have a rotor arm with a hockey stick shaped contact.

The discussion came about when details were sought about locking a
distributor so it is only a trigger and HT switch when using a mapped
electronic ignition. Do you set the phasing on full retard, half way, or
full advance?

--
*You can't teach an old mouse new clicks *

Dave Plowman London SW
To e-mail, change noise into sound.

"Dave Plowman (News)" wrote in message
...
In article ,
Dave Baker wrote:

"Dave Plowman (News)" wrote in message
...
The posts from Dave Baker and Roger Mills mirror exactly the discussion
on
the other group. ;-)

If the other group has numpties like me on it then not surprising.

My position is I'm happy that moving the plate the points are on via the
vacuum unit does effect the phasing - but I'm still not sure about the
centrifugal advance. Thing is some dizzies without a vacuum advance still
have a rotor arm with a hockey stick shaped contact.

The discussion came about when details were sought about locking a
distributor so it is only a trigger and HT switch when using a mapped
electronic ignition. Do you set the phasing on full retard, half way, or
full advance?

There's no real difference in where you want the rotor arm with mapped
ignition. It needs to be far enough alongside each cap contact at minimal
ignition advance so that it can advance by another 30 degrees or whatever
and still be adjacent. Given that's 15 degrees of rotor arm movement you can
work it all out by measuring the length of the rotor arm contact. In terms
of direction of rotation you therefore want the rotor arm to be well
alongside the cap contact i.e. towards the back end of it when the crank is
at TDC or just before that.

Easiest way is to check that with the crank anywhere between 5 degrees BTDC
and 40 degrees BTDC the rotor arm is still adjacent to its cap contact. Even
at cranking speed you're never going to spark after TDC, probably 5 to 10
degrees BTDC whether with mapped ignition or points and max advance is
unlikely to be more than 40 degrees of combined vacuum and centrifugal
advance.

What you can do with mapped which you can't with dizzy ignition is run lots
of low rpm timing. Most engines actually want about 20 degrees of advance
even at tickover but if you put that into a dizzy system you can't then add
enough centrifugal advance for higher rpm and there'll be too much advance
for starting purposes. The engine will try to kick back when it's cranking
over. With mapped you can crank at say 8 degrees advance and then go
straight to 20 at idle speed. In fact you can set optimum idle advance just
by getting the highest idle speed as you alter the advance with the throttle
shut then adjust the throttle stop to pull the idle rpm back.
--
Dave Baker

In an earlier contribution to this discussion, Dave Plowman (News)
wrote:
In article ,
Dave Baker wrote:

"Dave Plowman (News)" wrote in message
...
The posts from Dave Baker and Roger Mills mirror exactly the
discussion on the other group. ;-)

If the other group has numpties like me on it then not surprising.

My position is I'm happy that moving the plate the points are on via
the vacuum unit does effect the phasing - but I'm still not sure
about the centrifugal advance. Thing is some dizzies without a vacuum
advance still have a rotor arm with a hockey stick shaped contact.

The discussion came about when details were sought about locking a
distributor so it is only a trigger and HT switch when using a mapped
electronic ignition. Do you set the phasing on full retard, half way,
or full advance?


Ah well, that's an entirely different question! If the spark timing is not
controlled by the cam opening a pair of points, some part or other of the
hockey-stick-shaped contact needs to be close to the contact in the cap over
the whole angular range in which the spark may occur. I've no idea where
you'd have to 'fix' it to achieve that - if indeed it's possible at all. In
a conventional distributor, the contact length only needs to cover the range
of vacuum advance angles - but in this new situation you need to cater for
the *total* advance range.
--
Cheers,
Roger
_______
Please reply to Newsgroup. Whilst email address is valid, it is seldom
checked.

In article ,
Roger Mills wrote:
The discussion came about when details were sought about locking a
distributor so it is only a trigger and HT switch when using a mapped
electronic ignition. Do you set the phasing on full retard, half way,
or full advance?


Ah well, that's an entirely different question! If the spark timing is
not controlled by the cam opening a pair of points,

Effectively it would be - but a sensor replacing the points, still
triggered from the 'cam' underneath the rotor arm.

some part or other of the hockey-stick-shaped contact needs to be close
to the contact in the cap over the whole angular range in which the
spark may occur. I've no idea where you'd have to 'fix' it to achieve
that - if indeed it's possible at all. In a conventional distributor,
the contact length only needs to cover the range of vacuum advance
angles - but in this new situation you need to cater for the *total*
advance range.

Think it may be easier to swap to EDIS. ;-)

--
*If you can read this, thank a teecher

Dave Plowman London SW
To e-mail, change noise into sound.

"Dave Baker" wrote in message
...

"Dave Plowman (News)" wrote in message
...
In article ,
Dave Baker wrote:

"Dave Plowman (News)" wrote in message
...
The posts from Dave Baker and Roger Mills mirror exactly the
discussion on
the other group. ;-)

If the other group has numpties like me on it then not surprising.

My position is I'm happy that moving the plate the points are on via the
vacuum unit does effect the phasing - but I'm still not sure about the
centrifugal advance. Thing is some dizzies without a vacuum advance still
have a rotor arm with a hockey stick shaped contact.

The discussion came about when details were sought about locking a
distributor so it is only a trigger and HT switch when using a mapped
electronic ignition. Do you set the phasing on full retard, half way, or
full advance?

There's no real difference in where you want the rotor arm with mapped
ignition. It needs to be far enough alongside each cap contact at minimal
ignition advance so that it can advance by another 30 degrees or whatever
and still be adjacent. Given that's 15 degrees of rotor arm movement you
can work it all out by measuring the length of the rotor arm contact. In
terms of direction of rotation you therefore want the rotor arm to be well
alongside the cap contact i.e. towards the back end of it when the crank
is at TDC or just before that.

Easiest way is to check that with the crank anywhere between 5 degrees
BTDC and 40 degrees BTDC the rotor arm is still adjacent to its cap
contact. Even at cranking speed you're never going to spark after TDC,
probably 5 to 10 degrees BTDC whether with mapped ignition or points and
max advance is unlikely to be more than 40 degrees of combined vacuum and
centrifugal advance.

What you can do with mapped which you can't with dizzy ignition is run
lots of low rpm timing. Most engines actually want about 20 degrees of
advance even at tickover but if you put that into a dizzy system you can't
then add enough centrifugal advance for higher rpm and there'll be too
much advance for starting purposes. The engine will try to kick back when
it's cranking over. With mapped you can crank at say 8 degrees advance and
then go straight to 20 at idle speed. In fact you can set optimum idle
advance just by getting the highest idle speed as you alter the advance
with the throttle shut then adjust the throttle stop to pull the idle rpm
back.
--
Dave Baker

PS - I should just add that because on a normal dizzy ignition only the
vacuum advance affects the phasing whereas with mapped both vac and
centrifugal elements do it's possible you can run into a situation where you
don't have enough phasing with mapped. It's never going to affect a 4 pot
but maybe a V8 or V12 if the rotor arm is too wide. Can't say I've ever come
across it though but then these days most mapped systems use coil packs not
dizzies. It must be 15 years ago when I was first getting into mapped ecus
that there was any great demand for dizzy triggered mapped timing. Nowadays
you fit a toothed wheel on the crank (or use the existing flywheel) and a
coil pack (or two) off a Fiesta or similar. Crank triggering is more precise
although let's face it dizzies have worked well enough for 100 years. I
doubt if anything less than a top end race engine would be able to
differentiate between the odd degree or two of timing error from dizzy
ignition.
--
Dave Baker