Low vacuum - should I ditch my vacuum advance distributor?
So I’ve got a recently built 340 with a Comp cam in it with 274/286 of duration and 488/391 lift. It only makes around 8-10hg of vac at idle, and I’m pretty sure there are no leaks and compression is good. I’ve got it set at around 13-15 initial and 28 total right now, which is as far as my current mechanical will allow right now. That level of vac is about as much as I’ll get.
That cam in a 340 probably wants 16 to 18* initial, and it that should not advance until 750 or even as much as 850 rpm. Its critical to measure the rpm of the timing.
https://www.forabodiesonly.com/mopar/ams/distributor-starting-point-for-a-curve.182/
The vac advance is useless at idle as it won’t do anything below like 15hg (I’ve checked this).
Factory distributors generally were designed so the vacuum advance is used only to increase timing when the mixture is low density under light to moderate load.
Whether the mechanical advance in your distributor will allow you to do that will ahve to be determined by measuring the timing vs rpm and examining the mechanism.
Measure the timing vs rpm from as slow as the engine will go to as high of rpm as you feel safe.
If the advance is too long or too quick and you can't or don't want to make adjustments to it, then sometimes using manifold vacuum at idle can be used to advantage.
You'll have to adjust the vacuum advance as Halifaxhops shows in the How To.
There should be no vacuum advance under heavy load (close to wide open throttle). As
Chrysler tech explained, its purpose is to make oup for the slower combustion of low density, lower loads in the combustion chamber. Heavy load is when mixtures are richer, but exactly what the manifold vacuum will be varies from engine to engine. No vacuum advance under 8.5"Hg was typical for hi-performance Chryslers.
see
https://www.forabodiesonly.com/mopa...ting-from-park-to-gear.473761/post-1973170999
https://www.forabodiesonly.com/mopar/threads/cruise-seems-lean.493425/post-1973492093
good luck. I'm not on much, but you can use the search here for examples of hi performance timing.
