This came up on another group and I thought I'd ask the collective wisdom here.
An old style mechanical distributor with both centrifugal and vacuum advance.
The relationship between the rotor arm and cap contacts is called phasing.
The rotor arm rotates relative to the main shaft under the influence of the centrifugal mechanism. The plate the points are fixed to rotates under the influence of the vacuum unit.
Do either, both, or neither of these alter the part of the rotor arm that is closest to the contact on the cap at the firing point? Ie, the phasing between them?
Yes - but not by a huge amount, relative to the contact arc of the rotor arm.
Remember that the switching is done by the LT points opening, not by the HT distributor. If you do find the distributor controlling the spark (it happened, back in the '60s, if you started over-revving faster than design) then you need to change the arm.
The centrifugal advance moves the top part of the shaft that the rotor arm is mounted on so yes this does affect the rotor arm phasing relative to the cap. Vacuum advance only moves the points relative to the rotor arm shaft so no this does not affect the phasing relative to the cap. As long as some part of the rotor arm conductive contact is adjacent to the nearest cap contact when the points open obviously it makes no difference but the contact on the rotor arm needs to be wide enough to cope with all possible amounts of centrifugal advance. This does obviously limit the number of cap contacts you can squeeze into a cap and still have the rotor arm adjacent to only one contact at any time and not create cross firing. It all becomes a bit tight with V8 systems that run large amounts of centrifugal advance.
I hesitate to question your undoubted knowledge of the subject, but I had come to the opposite conclusion - and I don't *think* I'm wrong.
As you say, the rotor arm is attached to the part with the cams on, which rotates relative to the shaft under the influence of the centrifugal weights. The spark occurs when the points open - which happens when the
*cam* gets to a certain position relative to the points. Ignoring vacuum advance for a moment, the rotor arm will always be in the same position relative to the cap when the points open - regardless of the amount of centrifugal advance being applied. [The points will, of course, open at a different crankshaft position, but always at the *same* distributor cam and rotor arm position.]
However, vacuum advance rotates the base plate - moving the points such that the cam (and rotor arm) need to be in a different position relative to the cap in order to open the points. So this *does* change the phasing.
As you say, this is not normally a problem because the rotor arm has quite a long angular contact and will always be close enough to the correct cap contact and far enough away from its neighbours to do the business. Obviously, the more cylinders you have, the less leeway there is. I don't know whether anyone ever made a V12 with a single distributor - that would probably have been a bit of a challenge!
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.
I've not seen this on an 8, but it was common on early V12s (1930s). Poor chamber designs had slow combustion and so required considerable centrifugal advance, sufficient to cross fire. The solution was split distributors for each bank, just to allow more angle for the rotor arm.
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.
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.
Vacuum advance moves the plate with the points on around the cam.
Centrifugal advance moves the cam. The rotor being mounted on the cam, will change its relative position along with the cam. This is allowed for by the strip of brass on the rotor being long enough to accommodate the changes, so HT can still jump the gap.
By pure coincidence I went to get a spanner from my old toolbox the other day and came across the packet of Rizla papers that I always used for setting the timing on the old distributors. I think I last used one on my
1979 MK11 Princess. This thread is bringing back wonderful memories of the days when you could do practically any job on your car yourself.
"Dave Plowman (News)" gurgled happily, sounding much like they were saying:
There was a mid-point, though. Electronic ignition & injection with piss- easy diagnostics - wire with a switch in some cases - and no other computerised silliness. That's gotta be about the home-maintenance sweet- spot.
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