vacuum advance

I run it, I see no reason not to. Mine doesn't ping at cruise.

If you can, use it.
It has benefits, like mentioned before.

However, if you have a large cam, the vacuum will fluctuate too much, causing the timing to go all over the place.
Not good for fuel economy or the engine.
In this case you'd run locked timing.

I've also heard they can be slow to respond to vacuum, like when you go from cruise to WOT...giving you way too much vacuum at WOT, if you have an aggressive base tune due to a lopey cam. Plus, if you adjust the mechanical advance so that it's all in at a lower RPM, then you need less vacuum advance

with the performance oriented distributors that don't have vacuum advance, should tell you that it's not critical to have it.

run it if you want, leave it unhooked if you don't.

for what it's worth, my Dakota with EFI never gives more than 38-39 degrees of total timing. apparently the magnum head design doesn't require the advance that older heads do.

thanks for starting the post. is vac adv hooked up to manifold or ported vac?

curious to see where this thread goes...



i never ran it for the longest time. always ran my small blocks with the total in by about 2200 rpm and no vac advance.


recently i figured lets see if i can get some better mileage from the car and not lose any performance. i was going to set up a mp distributor i had here laying around be me being an idiot and in a rush broke a few things pulling it apart.

over the years i have heard good and bad about FBO. i figured i may as well give them a try so i can make my own informed opinion. well he set me up a distributor that has more mechanical initial timing, the same total and actually has the mechanical total come in about 500 rpm later. he also said to hook the vac. advance up to constant vac. i thought that was weird but i did like he said.

so now when i set the timing i set the initial at 14* which made the total 34* and with the vac. advance hooked up i have 31* at idle. i have to say so far i only have about 150 miles on it since i put it in but the car cruises smoother and seems to run about 5* cooler. when cruising at say 35-45 mph i do notice i need a lot less throttle then before to pick up speed. really hard to explain. you would have to had to drive the car before and after to understand i think. car starts great when cold and hot. infact i think it actually runs better cold with he extra timing. carlisle will probably be the first long trip i take with it so we will see how the mileage is then.

I've messed with it for years in all configurations (ported vac, crusing vac), locked out timing, etc.....

Right now I run it and I've found a sweet spot that I like.

414 cube smallblock 34 degrees full out mechanical advance.
Intitial timing at 20 degrees that starts to mechanically advance up to the 34 degrees around 1500rpm, full out around 2500rpm.
Vacuum can pulls 15 degrees more for a total of 49 degrees after 2500rpm under light/no load.
Vacuum is plugged into the timed spark port on the carb,...that way it is not messing with idle.

I've found the key to good idle is having no timing fluctuation! So make sure the mechanical advance doesn't start until a few hundred rpm after idle speed, and make sure the vacuum advance doesn't kick in until crusing.

Works great.

15.5 MPG with a low 11 second 3400lbs 550hp car on pump gas!

4.30 gears, 30" tall tires, no OD....3400rpm going 65mph down the highway. This was with the automatic trans....it will probably do better MPG with the stick that's in it now.

Also I run TWO 650 demon carbs on a cross ram.

Not to hijack the post but, I'm getting a slight surging when driving under a light load at part throttle. A friend told me to disconnect the vacuum advance and that should solve the problem. Have not tried it yet, but does that sound right?
The car is a bone stock 68 Charger R/T. I checked the timing and found that I have 14 degs at 650 rpm. A total of 34 degs mechanical advance. Not sure at what RPM and with the vacuum reconnected I get a total of 53 degs advance. The car runs good other wise and does not ping.

block the metering rods all the way up on the power step & take it back out for a spin/holler back

10.5/1 470 w [Email]265@.050[/Email] I run 20* initial w 35* total. No vaccuum advance. I am also using an FBO setup distributor and would reccommend him to anybody. He removed the vac mechanism on my dist and it came setup for a great price. Dropped in and works great.

I dont use it on my street/strip cars. I run a very fast mech advance all in by 1800 and I really dont need it as they run fine like that on pump gas. Ron

I just went back and made a corection on my post. I see where I wrote that my total mechanical advance was 43 degs. I ment to write 34 degs.
Thanks Frederick for pointing that 43 degs is way too much. You are right about that and I'm sure the motor would be pinging if I was that advance.