total timing reading accurate with no load?
Interesting the manifold vacuum advance is working for you. I've thought about trying it on my 360 but the way it is now it's very happy with 20 initial and 39-40 total (9:1 comp, aluminum heads, high altitude), if I added the 14 or so degrees of vacuum advance at idle I'd probably need to back down the initial and then readjust the mechanical advance limit so total timing stays the same. Kinda doesn't sound worth it, anyone think it's worth a shot...?? It runs perfectly the way it is now.
Khalid,
For your situation, and I think Adam83's, the answer is no.
Adam83 posted he was going to take it for a test drive - but hadn't yet. So be interesting to hear his experience after the test drives.
In my opinion if the distributor can do it, its better to use mechanical advance to match the rpm related timing requirements and use vacuum advance for load related timing adjustment. The only situation where load is somewhat high with closed throttle is right at idle when slipping into gear. Hence the widespread use of ported vacuum.
But there are distributors that can't be made to get enough advance at idle, then manifold vacuum can be used.
There are also combinations where more advance is needed or just works out. Over in the compression testing thread yellow_rose writies about running high compression engines pretty cold (160* thermostat etc). I don't have experience working with anything that high of compression but running cold combustion chamber it makes sense that more advance could help insure (a) the burn gets done (b) little heat gets put into the cylinder walls.
Lets go back to the first situation, distributor that can't get enough advance at idle.
For Joe jr hotrodder that's not comfortable taking apart and modifying a Chrysler distributor, manifold vacuum could be an answer.
A typical Chysler replacement distributor was made in the smog control era and all of the early emissions efforts involved putting more heat into the cylinder walls at idle. The goal was reducing CO and left over hydrocarbons. Trade off was more complete burning in exchange for efficiency (power). Slightly higher idle speeds made up for the loss.
A good comparison of timing for Clean Air Package vs. regular timing can be seen specs for 1968 318 2bbl posted on the bottom of page 1.
The initial timing for a manual transmission with CAP was 5* ATC and the distributor has a 10* longer advance curve than a non-CAP distributor.
If Joe jr hottrodder has a distributor with a long advance in it like that, but doesn't want to take it apart and weld the slots, an easy fix (or crutch) might be to use vacuum advance at idle. Depending on the vacuum Jr hotrodder's engine pulls at idle, and the vacuum canister, it may pull timing at idle up to 10 or 15*. Suddenly his engine seems much happier. In fact it probably is. Especially if has a little hotter than stock cam and a carb that doesn't have idle mixture limiters. :)