Thread: V8 Vacuum Advance / Ignition Timing settings

Hi

I have been studying the FORUMS on here for hours and can't find an exact answer to my question....

I have recently installed a 390cfm 4 Barrel Holley, Edelbrock Manifold, new HEI ignition, leads, internal coil, New iridium Spark Plugs, and Pacemakers and Dual Exhaust to my Stage One v8.

I took it to the local Tuning shop to get sorted out... not exactly sure what they have done but seems to go ok, but just checking a Few things wioth my limited, knowledge and having trouble getting my mind around it...

They moved the Vacum Advance for the distributor, from the Carby port above butterflys, to the port below, manifold vacum, and the advance timing seems awfully high, reading way off the marks on the Pulley.

But i must say I checked it with the vacum line put back on the port above butterflys.




What is the best settings, and how should I check what is best for my particular setup, Modifications.

Should the Vacum line be placed to manifold vacum, or above on Carby port?

I have read tonight I should check timing at idle, with vacum line disconnected/plugged leading from distributor?

What are the gains from advancing the timing?


I found this on here but Im even more confused now...




This section puts into words something I have learned in the past:

Now, to the widely-misunderstood manifold-vs.-ported vacuum aberration. After 30-40 years of controlling vacuum advance with full manifold vacuum, along came emissions requirements, years before catalytic converter technology had been developed, and all manner of crude band-aid systems were developed to try and reduce hydrocarbons and oxides of nitrogen in the exhaust stream. One of these band-aids was "ported spark", which moved the vacuum pickup orifice in the carburetor venturi from below the throttle plate (where it was exposed to full manifold vacuum at idle) to above the throttle plate, where it saw no manifold vacuum at all at idle. This meant the vacuum advance was inoperative at idle (retarding spark timing from its optimum value), and these applications also had VERY low initial static timing (usually 4 degrees or less, and some actually were set at 2 degrees AFTER TDC). This was done in order to increase exhaust gas temperature (due to "lighting the fire late") to improve the effectiveness of the "afterburning" of hydrocarbons by the air injected into the exhaust manifolds by the A.I.R. system; as a result, these engines ran like crap, and an enormous amount of wasted heat energy was transferred through the exhaust port walls into the coolant, causing them to run hot at idle - cylinder pressure fell off, engine temperatures went up, combustion efficiency went down the drain, and fuel economy went down with it.

When I got my RRC with the Chev the local V8 gurus said to get the vacuum from the manifold for ign advance for lpg, rather than the port on the carb above the butterflies, which I have done ever since to get more advance at idle. Noticeable improvement in mileage.

Didn't know that it increases advance at cruise until I read the above article. Explains why it ran warm on a 40* C day on a long trip (1/2 on the gauge) and I found the vac advance had a leaky diaphragm. Replaced it and all has been good since.


Thanks for your help/input.

Chad

Hi Chad,

That quote is a bit out of context. I found I had an inoperative vac advance that was causing the ignition to be retarded and the motor to run hotter than normal.

Running the vac from the manifold gives more advance lower in the rev range and is a common crude way of getting a more suitable advance curve for LPG.

My POS runs much better on LPG as a result. Can't really tell with petrol because the gauge needle drops too fast either way.

cheers, DL