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Old 08-25-2003, 12:24 PM
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Originally posted by dmorris1200
The reason you disconnect your vacuum to set timing is because you're setting BASE timing. Base timing means timing with no mechanical or vacuum advance. Also even under WOT as your engine rpms increase so does your manifold vacuum. That is why your accelerator pump only needs to give you a short momentary boost of gas, then as your engine builds up rpms fuel is drawn through the jets because of the engines intakevacuum. If you have a vacuum advance can set to provide advance at a low vacuum signal you will see this advance even under heavy acceleration. Maybe you should read this link HERE were it tells you to add your vacuum advance to your mechanical advance to see your total ignition advance. At 40mph when you step on the gas to pass someone your engine is seeing your distributors mechanical+vacuum advance. Too much of this combined advance will cause that annoying little 'ping,ping,ping'.

OK I read the article. Below is a portion of it that I have copied and pasted. The section that is bold does not seem to be what you claim the article stated. Did I miss something here?

What is Initial Timing and Total Timing?
Initial timing is that setting you make while your engine is idling with a timing light. This is typically between 4 ATDC (After Top Dead Center) to 16 BTDC (Before Top Dead Center). For performance applications "in most cases" you want as much initial timing as the starter can handle (the more timing the more cylinder pressure that the starter must overcome to crank the engine). Too much initial timing, besides the starter load, can also run the HC (Hydrocarbon) emission levels beyond legal or breathable levels <hint to the smog machine as a tuning tool above>.
Total timing is the calculation of the combined initial and mechanical advance timing settings (max). This "can" also include your vacuum advance though, I like to describe it as "total timing plus vacuum advance". Since vacuum advance is a nearly inconsistent value I like to use it after I have set up the correct mechanical values in the distributor. At times I must use the vacuum advance to generate the timing values I require, but I try to only use it as an "economy" tool to give a street driven vehicle more efficiency and throttle response by this addition.

There are two types of vacuum advance functions available:
One uses a PORTED vacuum source that as RPM increases, so does
the vacuum to the port .... this is a BAD connection in Racing, especially
since we typically retard our timing at the upper RPM levels. But it can be
used as a tuning tool so long as you do not exceed the MAX total timing
figure you need. In this case you will be using the vacuum advance as part
of your total timing.
The second uses FULL MANIFOLD vacuum, which is how I prefer to
use vacuum advance (street cars only). Some may disagree, but it works for
me! To do full manifold mandates an adjustable vacuum canister on the
distributor. The way this system works is when you smash the throttle and
vacuum levels go to atmospheric the vacuum advance is stopped. No ping, no
excess unneeded timing that is only good for street car efficiency anyway.

The above is from the article you linked to.

Originally posted by bad502
so your saying that I should leave the vac adv. hooked up
when I set the initial timing? Should I leave it at 15 deg.
or should I change it.
Absolutely NO as to hooking up the vacuum advance when you are setting the timing! Hook it back up AFTER you have set the timing. For now leave it at the 15 degrees and try it. If you do not get any pinging you can increase it 2 degrees at a time. Once you experience ping then drop it back down 2 degrees. The best performance will be at the highest degree of advance WITHOUT any ping.

Last edited by Frisco; 08-25-2003 at 12:24 PM.
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