Vacuum Advance….Vacuum Retard
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From: Brookshire, Texas
Vacuum Advance….Vacuum Retard
Still learning about my 1970 Series II and what may or may not have been dismantled in the engine system over the years. Noticed last week that the hose from the diaphragm on the distributor was missing but also noticed how well the engine starts and idles. From what I have read this hose runs from the distributor diaphragm to the front carburetor and initiates dropping the timing from 10 degrees BTDC to zero degrees at idle. Any damage running the engine without this part of the vacuum set up? I believe it is more of a anti pollution system.
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Joined: Mar 2019
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From: The wilds of Haliburton
Early XK — and high-performance engines generally through the mid-1960s — used purely mechanical (centrifugal) advance.
Vacuum advance — where manifold vacuum pulls a diaphragm that rotates the distributor plate, adding ignition advance — had been used on economy cars for years before emissions became a legislative concern. Its original purpose was fuel economy at part-throttle cruise: under light load, manifold vacuum is high, and advancing the timing further improves combustion efficiency. Pure economy logic.
As US emissions regulations tightened, retarding ignition timing was used to reduce hydrocarbon and NOx emissions at idle and low load. The idea was that vacuum could retard timing at idle, reducing combustion temperatures and emissions. Open the throttle, vacuum drops, retard releases.
So, over the years, people have done whatever they think suits their needs best - often just deleting it. No, it won’t do harm but it won’t likely do anything that you actively want either.
I’ve gone to the 123ignition bluetooth version - with vacuum - which has the ability to draw very precise and stable curves. It also monitors crankshaft acceleration and modifies each piston separately to adapt to differences in compression or mixture or anything else that might alter the need to advance spark to ensure even combustion results.
Vacuum advance — where manifold vacuum pulls a diaphragm that rotates the distributor plate, adding ignition advance — had been used on economy cars for years before emissions became a legislative concern. Its original purpose was fuel economy at part-throttle cruise: under light load, manifold vacuum is high, and advancing the timing further improves combustion efficiency. Pure economy logic.
As US emissions regulations tightened, retarding ignition timing was used to reduce hydrocarbon and NOx emissions at idle and low load. The idea was that vacuum could retard timing at idle, reducing combustion temperatures and emissions. Open the throttle, vacuum drops, retard releases.
So, over the years, people have done whatever they think suits their needs best - often just deleting it. No, it won’t do harm but it won’t likely do anything that you actively want either.
I’ve gone to the 123ignition bluetooth version - with vacuum - which has the ability to draw very precise and stable curves. It also monitors crankshaft acceleration and modifies each piston separately to adapt to differences in compression or mixture or anything else that might alter the need to advance spark to ensure even combustion results.
Last edited by mikemilton; Today at 05:21 AM.
Senior Member
Joined: Dec 2011
Posts: 784
Likes: 461
From: Central California
Early XK — and high-performance engines generally through the mid-1960s — used purely mechanical (centrifugal) advance.
Vacuum advance — where manifold vacuum pulls a diaphragm that rotates the distributor plate, adding ignition advance — had been used on economy cars for years before emissions became a legislative concern. Its original purpose was fuel economy at part-throttle cruise: under light load, manifold vacuum is high, and advancing the timing further improves combustion efficiency. Pure economy logic.
As US emissions regulations tightened, retarding ignition timing was used to reduce hydrocarbon and NOx emissions at idle and low load. The idea was that vacuum could retard timing at idle, reducing combustion temperatures and emissions. Open the throttle, vacuum drops, retard releases.
So, over the years, people have done whatever they think suits their needs best - often just deleting it. No, it won’t do harm but it won’t likely do anything that you actively want either.
I’ve gone to the 123ignition bluetooth version - with vacuum - which has the ability to draw very precise and stable curves. It also monitors crankshaft acceleration and modifies each piston separately to adapt to differences in compression or mixture or anything else that might alter the need to advance spark to ensure even combustion results.
Vacuum advance — where manifold vacuum pulls a diaphragm that rotates the distributor plate, adding ignition advance — had been used on economy cars for years before emissions became a legislative concern. Its original purpose was fuel economy at part-throttle cruise: under light load, manifold vacuum is high, and advancing the timing further improves combustion efficiency. Pure economy logic.
As US emissions regulations tightened, retarding ignition timing was used to reduce hydrocarbon and NOx emissions at idle and low load. The idea was that vacuum could retard timing at idle, reducing combustion temperatures and emissions. Open the throttle, vacuum drops, retard releases.
So, over the years, people have done whatever they think suits their needs best - often just deleting it. No, it won’t do harm but it won’t likely do anything that you actively want either.
I’ve gone to the 123ignition bluetooth version - with vacuum - which has the ability to draw very precise and stable curves. It also monitors crankshaft acceleration and modifies each piston separately to adapt to differences in compression or mixture or anything else that might alter the need to advance spark to ensure even combustion results.
Please post your curves for an XKE with 123 distributor ... This is the tough part of 123 system, getting the correct curves to input into the system>
Rgds
David
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