I've read Lars' article on timing, and several other posts on this site and others, but I am still partially confused about adding vacuum advance. I will be using a full manifold port, with no pcv valve, and a 1116163 (ms 163 16) vacuum advance canister on a stock 1970 396/350 motor and points distributor. My question is, when I set the initial timing without the vacuum advance do I need to back off that timing when I add the vacuum back in? My reason for asking is that If my initial/base timing is 12* and my vacuum advance gives 16* fully deployed, then I would have to retard the timing to 4* BTDC to still have the 12* of initial, which seems way too low; on the other hand if I add the 16* of vac. adv. onto the 12* initial, then I would have 28* at idle, which seems too high.
A question on vacuum advance
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X2 ...... your motor will be happy ........is it still a a true 10.25-1 cr L34.....you "may " have to limit that 16* to about 10-12 if it pings at cruise on lite accelration with initial at 12 but maybe not just try it .......no downside to hooking up the vacuum advance ....... is your mech advance curve still stock so not al lin til 5k if it modified to come in quicker again it may ping at cruise witha 16* degree can but you wont know till you try it
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You should ABSOLUTELY NOT subtract the vac acv from the static.
Expect somewhere around 50° of adv at cruise.
You may find that full manifold vac at idle gives too much adv, if you don't use ported vac; but then again you may not. That's why they call it "tuning": you tune the parts YOU have, to work together. Not, look up some "number" in some "book" and "set it to the spec" and somehow it's now magically perfect. Doesn't work like that. Experiment and find what YOU like best for YOUR motor in YOUR car the way YOU use it with the fuel YOU can buy in the conditions YOU operate it.
Expect somewhere around 50° of adv at cruise.
You may find that full manifold vac at idle gives too much adv, if you don't use ported vac; but then again you may not. That's why they call it "tuning": you tune the parts YOU have, to work together. Not, look up some "number" in some "book" and "set it to the spec" and somehow it's now magically perfect. Doesn't work like that. Experiment and find what YOU like best for YOUR motor in YOUR car the way YOU use it with the fuel YOU can buy in the conditions YOU operate it.
Thanks for the replies. My engine is an original L34 that is basically stock so I may need to limit the vacuum advance considering that it's stamped with a 16 which translates to 32 crankshaft degrees. As far as the distributor is concerned, I'm assuming that the weights and springs are the original. I'll see how it plays out when I go to tune it.
No PCV? looking for an engine full of water and sludge? whassupwiddat?
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that can should give you 16* of timing not double it. think that might be the issue.
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No
It's 16°
That, in and of itself, should be fine.
More important would be, the vac at which it starts to drop out, and the vac at which it's fully out; you may well find that with 2013 fuel, a timing curve set up for 1970 fuel will ping excessively. Usually those older vac adv units have too weak of a spring for modern fuel, and therefore hold the full high-vac adv down too low and don't drop fully out until the car is literally floored.
What your timing ACTUALLY IS, is the 12° of "static" or "initial", plus the amount of "centrifugal" that the springs weights & cams give it, which is usually something int he range of 24 - 28° for factory distributors, resulting in a "total" timing of around 36 - 38° at high speeds and high loads; then, the 16° of vac adv ON TOP OF THAT at high speeds and low loads (cruise), for better gas mileage, cooler running, and various other benefits. The actual end result may well be more than 50° BTDC under some conditions.
Use a PCV valve. There are NO BENEFITS WHATSOEVER to removing it, and every reason TO use it. That would be a bonehead move for anything but a PURE STRIP car.
Thank you for this clarification, 16* seems much more reasonable for a vacuum advance. My mistake about the PCV valve; it is connected and fully functional. I meant to say that I removed the TCS module and I am running a single hose from the carb to the advance can.
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