Good Initial Timing and Too much Total Timing Problem. Seeking guidance to minimize total timing. 1969 Swinger 340 4 Gear
Emissions & Timing.
Chrysler's answer to reduce excessive HC and CO was to make idle fuel mixtures leaner and at the same time retard the timing at idle. Reducing the initial timing put more heat into the cylinder wall (burn completes while the piston is further down the hole) at idle. To make up for the loss in power, idle speed was bumped up - especially needed with automatic transmissions.
The 340 was released in '68; all came with CAP (Clean Air Package) AFAIK.
So lets look at timing for a 273 4bbl to see how these idle settings in the CAP package were compensated for in the distributor advance.
By 1800 rpm the timing is essentially the same in both.
Your 340 probably has a distributor advance intended for initial timing of TDC or 5* but we don't know (unless you have the original distributor).
So you need to measure and plot the timing from as low of rpm as it will run, to at least 2000 rpm, preferably to 3000.
Then, if and only if it has a curve shaped like above, you set a baseline that will be ballpark:
1. Set the timing so from 1800 to 3000 rpm so its close to factory.
2. If, and only if, it has a long primary advance like the CAP 273, you can try connecting the vacuum advance to a manifold vacuum source.
3. Modify the advance governer by welding the inside of the slots and use the ported vac source for vacuum advance.
Here's the '69 340 timing for a 4-speed graphed.
My opinion, and its only my opinion, is the upper rpm timing is conservative. Even for street use (long trip with engine fully heat soaked) the engine might like a bit more advance assuming high octane fuel is available. Mopar may have done this on purpose in 68-69, knowing that mechanics might reset the inital a little higher if the specific engine and local conditions allowed.
Emissions & Timing.
Chrysler's answer to reduce excessive HC and CO was to make idle fuel mixtures leaner and at the same time retard the timing at idle. Reducing the initial timing put more heat into the cylinder wall (burn completes while the piston is further down the hole) at idle. To make up for the loss in power, idle speed was bumped up - especially needed with automatic transmissions.
The 340 was released in '68; all came with CAP (Clean Air Package) AFAIK.
So lets look at timing for a 273 4bbl to see how these idle settings in the CAP package were compensated for in the distributor advance.
By 1800 rpm the timing is essentially the same in both.
Your 340 probably has a distributor advance intended for initial timing of TDC or 5* but we don't know (unless you have the original distributor).
So you need to measure and plot the timing from as low of rpm as it will run, to at least 2000 rpm, preferably to 3000.
Then, if and only if it has a curve shaped like above, you set a baseline that will be ballpark:
1. Set the timing so from 1800 to 3000 rpm so its close to factory.
2. If, and only if, it has a long primary advance like the CAP 273, you can try connecting the vacuum advance to a manifold vacuum source.
3. Modify the advance governer by welding the inside of the slots and use the ported vac source for vacuum advance.
Here's the '69 340 timing for a 4-speed graphed.
My opinion, and its only my opinion, is the upper rpm timing is conservative. Even for street use (long trip with engine fully heat soaked) the engine might like a bit more advance assuming high octane fuel is available. Mopar may have done this on purpose in 68-69, knowing that mechanics might reset the inital a little higher if the specific engine and local conditions allowed.