Thank you Peter. In my mind “ported” meant “enlarged/polished” as in “ported cylinder head” for performance…
Here is means “closeable”… Got it.
Typically, Advance ports are between the throttle plate and the intake valve. Vacuum is greatest when the throttle is closed and approaches atmospheric pressure as the throttle opening increases. In boosted engines it can reach higher than atmospheric pressures. The vacuum retard is a ported vacuum that is place nearly under the closed throttle plate favoring the intake side instead of the engine. On the Stromberg carbs, it is a very small port at the bottom of the front carb throat only, with the distributor connection being a tube extending from the bottom of the carb mounting flange. Vacuum is greatest at idle with the throttle plate cracked slightly open by the idle screw, enhancing the venturi effect causing maximum retard.
The difference between ported and regular vacuum ports is that the vacuum drawn at a normal advance port is proportional to the position of the throttle plate. As the throttle opens, vacuum gradually decreases. Contrary to what most people think, vacuum advance does not advance the spark during acceleration. That is the job the centrifugal weights. It is during cruise when the throttle is only opened slightly that you need additional advance. High RPM + low throttle = max advance. Whereas the ported vacuum for the retard is negated as soon as the throttle plate opens even slightly, at least that is the way it is intended for the Jaguar US emissions controls.
If you are rebuilding the engine to full US emission specs, go with the retard, maybe.
If you are going all out with non emission controls and SU carbs. Set the engine up with and advance distributor. You wallet will thank you with the savings in fuel economy, if done correctly.
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So right, Lawrence - but we must also remember the other element of engine needs; fuel…
Manifold vacuum defines the degree of ‘fill’ of the cylinders. At ambient pressure fill is 100% - in idle, with 18" vacuum, the nominal ‘fill’ is less than 40% - and any fueling system delivers the relevant amount of fuel to suit. This is the primary function of carbs - delivering distributor vacuum is a secondary function.
In idle, and with light throttle - high advance (within reason) have no negative effects. As throttle opens; manifold vacuum drops, cylinder fill increases - and is desirable to reduce advance to raise torque. And at extreme throttle movement/load, very low manifold vacuum and high cylinder fill, ign advance must be reduced to avoid detonation.
With ‘advance’, manifold controlled vacuum - the advance drops to centrifugal settings. With ‘retard’; ported vacuum is generated by the air speed past the vacuum port - and reduces the centrifugal advance, of course. The characteristics, centrifugal and vacuum, of any distributor is fixed - and the initial advance setting aims for best ignition timing at all times. Ie, the advance is specifed related to compression ratio and petrol octane, but since the mechanical distributors are rather crude; with an aim of avoiding detonation in extreme situations. Or, indeed, in other situation where detonation may occur due to mismatch between engine’s and distributor characteristics…
So ignition timing and fueling are two entirely separate functions, but both are there to satisfy the very precise demands of the engine - which are likewise fixed. If the engine doesn’t get what it want; it won’t do its best - though it may not actually noticeably misbehave…
Frank
xj6 85 Sov Europe (UK/NZ)
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A feature of the DVX distributor fitted to late 40s / early 50s cars is , that the vernier is connected to the vacuum unit and swash plate, so as vacuum varies the vernier adjustment control moves. A snap throttle opening results in the vernier moving in a direction indicating ignition advance, perhaps Rob or Phil Dobson will confirm.
Of course when vacuum decreases the vernier returns towards its static position, but due to increased distributor rotational speed the centrifugal weights take up advance duties, so your assertion that the vacuum advance has no effect on acceleration
is incorrect.
Correct that , at speed with a light throttle, the vacuum unit advances the ignition, this sometimes results in a “shunting” effect, usually due to the static ignition being set to advanced.
Your statement " advance ports are between the throttle plate and the intake valve "
implies that the drilling for the advance connection enters the induction system manifold side of the butterfly ! upon reflection, if this is your understanding, surely you will agree that this is “bollocks”.
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Let’s agree that vacuum, when applied, increases ign advance, in relation to centrifugal, with ‘advance’ dist. With a ‘retard’ dist; vacuum, when applied, retards ignition in relation to centrifugal?
Crudely; manifold vacuum is fairly constant with light throttle - and a vacuum advance curve is some 10 degrees above centrifugal. To the extent carbs/throttle bodies are fitted with ‘advance’ spigots this is still true - and ‘European’ uses this feature. In this context; manifold vacuum variation is used to vary ignition timing according to engine load - and lower advance, whatever system is fitted, is required in this situation. That ‘ported’ and ‘manifold’ are sourced differently complicates matters - with somewhat arguable outcomes…
The ‘European’ set-up is traditionally designed for extracting maximum power for any given engine size. The US preference for ‘ported’ vacuum reflects different priorities - there were no disincentive for fitting larger engines to get more power…
The vacuum, and resulting ignition variations with it, is somewhat more complex with ‘ported’ vacuum - but the fueling duties of carbs, or EFI, is the same in both versions…
As an aside, there are two main ways of regulating idle; one using throttle gap - the other is closed throttle and regulating idle with a throttle bypass. Which has some bearing on carb construction and porting…
Frank
xj6 85 Sov Europe (UK/NZ)
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