To Vacuum Advance or Not to Vacuum Advance
Never found a benefit...
Not going to disagree.
I never disagree when somone says that's what they found. Heck I wasn't there!
As long as they don't say that's what the rest of us will find.
To be exact we'll have to pick a year, transmission, carb as to what the factory did. Dodge tended spec more initial than Plymouth, at least in the 60s. Don't know why. Same distributor.
So looking at strictly stock here:
For '72, initial timing TDC, (-2.5 to 2.5)
'69 Dodge (CAP only) TDC
'69 Plymouth (CAP), 5 ATC
'69 Plymouth, 5 BTC
Where I'm going with this is open the throttle and timing advances quickly with rpm.
Non-CAP '68 Plymouth 318 Timing.
The stock 318 was relatively efficient even at low rpm, and so it didn't need a lot of lead time for the burns if you floored it from idle.
But most of the time we are looking at distributors for emissions equiped vehicles.
Take a '68 CAP equiped 318 with manual transmission as an extreme example. A super quick and relatively long primary advance gets the timing where it needs to be by the rpms where the engine is fully loaded.
On a more gentle throttle application, vacuum advance for a '68 318 starts as soon as the port is exposed.
So to use the example, if vacuum is at 10"Hg, could pull in as much as 11*.
If the engine is at 1000 rpm, timing should be around10*, for a total of 21* BTC.
As soon as we change compression, combustion chamber, cam, smog controls, or anything else that effects mixture, pressure, load, then timing conditions change.
Here's a comparison with factory 4 bbl 273 timing.
We can see the hi-po 273 neads more time at lower rpm. But around 1500 rpm, combustion efficiencies improve with rpm.
Never found a benefit...
Not going to disagree.
I never disagree when somone says that's what they found. Heck I wasn't there!
As long as they don't say that's what the rest of us will find.
Pretty much; if gentle on the throttle.
To be exact we'll have to pick a year, transmission, carb as to what the factory did. Dodge tended spec more initial than Plymouth, at least in the 60s. Don't know why. Same distributor.
So looking at strictly stock here:
For '72, initial timing TDC, (-2.5 to 2.5)
'69 Dodge (CAP only) TDC
'69 Plymouth (CAP), 5 ATC
'69 Plymouth, 5 BTC
Where I'm going with this is open the throttle and timing advances quickly with rpm.
Non-CAP '68 Plymouth 318 Timing.
The stock 318 was relatively efficient even at low rpm, and so it didn't need a lot of lead time for the burns if you floored it from idle.
But most of the time we are looking at distributors for emissions equiped vehicles.
Take a '68 CAP equiped 318 with manual transmission as an extreme example. A super quick and relatively long primary advance gets the timing where it needs to be by the rpms where the engine is fully loaded.
On a more gentle throttle application, vacuum advance for a '68 318 starts as soon as the port is exposed.
So to use the example, if vacuum is at 10"Hg, could pull in as much as 11*.
If the engine is at 1000 rpm, timing should be around10*, for a total of 21* BTC.
As soon as we change compression, combustion chamber, cam, smog controls, or anything else that effects mixture, pressure, load, then timing conditions change.
Here's a comparison with factory 4 bbl 273 timing.
We can see the hi-po 273 neads more time at lower rpm. But around 1500 rpm, combustion efficiencies improve with rpm.
