Constant or Ported Vacuum Source for Distributors? ...AGAIN!
SOOooooooo, here we go again.
Same thing was posted two weeks ago.
Same nonsense.
and I have the same responses.
Those looking for plug and chug answers will be disappointed.
Use of "ported" predates emmissions, unless the author thinks cars from the 1940s were 'smogged'. LOL. see
Which approach should be used on a 'performance' setup depends to some degree on the limitations of what is available. If one is stuck with a distributor that has a long advance curve on an engine that wants 12-20* at idle, then using manifold vacuum at idle may be a way attain that.
Given no such restrictions, the distributor's vacuum advance ought to work with the fuel curve in the rest of the operating range. Then decide what is needed at idle.
Ideally, vacuum advance should go away when the fuel curve needs enrichment. The purpose of vacuum advance is to match the slower burning leaner mixes - that much of the article is true. Enrichment may be anywhere from 60% to 90% max power depending on the engine and drivetrain.
In this example from Larew (details here), enrichment is need around 60% load, which might equate to vacuum of 9 to 12 "Hg. So, lets say this is the case, and we find a PV or step up around 10.5 works best. So we would want the vacuum advance to go away as vacuum drops near 10.5"
And sticking with this example, lets say the engine idles best (strongest in Drive) around 12.5 AFR and timing of 16* BTDC. Finally we note that the idle vacuum in D is 10" Hg.
(Graph shows Fuel Air Ratio. Inverse equals AFR. eg 1/.08 =12.5)
Therefore, in this case, using manifold vacuum at idle really wouldn't work at all. If it was used, it might bring a little advance when the transmission was in Neutral, and then go away when in Drive. That just makes a unstable setup.
The right setup here is to set the initial timing at 16* and have the mechanical advance curve shape and top limit set by the springs, or the springs and a limiter.
SOOooooooo, here we go again.
Same thing was posted two weeks ago.
Same nonsense.
and I have the same responses.
Those looking for plug and chug answers will be disappointed.
Use of "ported" predates emmissions, unless the author thinks cars from the 1940s were 'smogged'. LOL. see
It also was used on both CAP and non-CAP Chryslers in the 60s.
Which approach should be used on a 'performance' setup depends to some degree on the limitations of what is available. If one is stuck with a distributor that has a long advance curve on an engine that wants 12-20* at idle, then using manifold vacuum at idle may be a way attain that.
Given no such restrictions, the distributor's vacuum advance ought to work with the fuel curve in the rest of the operating range. Then decide what is needed at idle.
Ideally, vacuum advance should go away when the fuel curve needs enrichment. The purpose of vacuum advance is to match the slower burning leaner mixes - that much of the article is true. Enrichment may be anywhere from 60% to 90% max power depending on the engine and drivetrain.
In this example from Larew (details here), enrichment is need around 60% load, which might equate to vacuum of 9 to 12 "Hg. So, lets say this is the case, and we find a PV or step up around 10.5 works best. So we would want the vacuum advance to go away as vacuum drops near 10.5"
And sticking with this example, lets say the engine idles best (strongest in Drive) around 12.5 AFR and timing of 16* BTDC. Finally we note that the idle vacuum in D is 10" Hg.
(Graph shows Fuel Air Ratio. Inverse equals AFR. eg 1/.08 =12.5)
Therefore, in this case, using manifold vacuum at idle really wouldn't work at all. If it was used, it might bring a little advance when the transmission was in Neutral, and then go away when in Drive. That just makes a unstable setup.
The right setup here is to set the initial timing at 16* and have the mechanical advance curve shape and top limit set by the springs, or the springs and a limiter.
