To vac advance or not to vac advance

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fnaramore

fnaramore

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So got the msd ignition in, car is running great, question is, is there a point to having the vacuum advance/is it bad to have it. Msd says it advances the timing up to 10* at 15 lbs. of vacuum. Currently, the timing is 12-13* initial, 22* mechanical and 34-35* total. Any opinions? As I understand it, the only point of vac adv is economy. 1973 dart, 318
 
hook the vacuum advance up...it provides additional advance during light and part throttle driving ...helping fuel mileage...
 
I did away with the vacuum advance on any engines that produced less then 12/13 in of vacuum out of gear. I may or may not have left some economy on the table, but once the engines got larger "performance" cams, a performance dizzy was usually included in the build and very few in my day even offered a vac advance option. :D
 
Well right now it is a XE256H cam, weiand stealth manifold, edelbrock 600 cfm carb, msd pro billet ready to run distributor, blaster 2 coil and 2.25 in pipes off the manifolds. In april or may Im gonna put in a XE262H cam and eddy performer rpm heads.
 
Is there any danger that with the initial and mechanical advance I have, that the vacuum advance could raise it too high? does the vacuum advance hurt performance at all?
 
I've always thought that the biggest reason that vacuum advance was engineered by developers was to allow an engine to be started easier with the timing in a retarded position. - And that as soon as an engine started it would produce enough vacuum to advance the timing to a 'run' position. Although not essential on all setups, surely some must still benefit from it. I'm not running any vacuum advance setups on any of my vehicles now because I've switched them all over to electronically controlled timing. Even our Roadrunner with a new FAST EZ efi 2.0 with an MSD Digital 6 has the timing controlled by the ECU.
 
Yea, one more here, run the vacuum advance.
 
I think with your engine, cam, and carb size, I'd run the vac adv for now. Just remember, from a economy standpoint, your right foot will be a better friend then any can hanging on the dizzy. :D
 
Here is a curve ball.... Most of the factory electronic distibutors,have a adjustable vacuum advance. It ,takes some time and allen key.....
?
 
OldmanRick said:
I think with your engine, cam, and carb size, I'd run the vac adv for now. Just remember, from a economy standpoint, your right foot will be a better friend then any can hanging on the dizzy. :D
Click to expand...

Very true... if only I could trust my foot :burnout:
 
The more spark advance the better, for mileage, performance, and low emissions, until you reach the knock limit. That limit depends on gas quality, temperature, throttle position, altitude, and engine design (c.r., cam).

I like 1967 'cuda's comment (post #9). That makes a lot of sense, and I never thought of it before. You don't want it sparking before TDC since the engine will want to kick back and fight the starter. Better to retard the spark while cranking. I think some engines are designed like that (tractors, etc). Probably the Model T, so the owner didn't break his arm as often.
 
Mild engine w vacuum advance hooked to manifold or "timmed port" will help part throttle torque when adjusted in harmony w your mechanical timing curve.
 
I'm sure there "is a point" where a wild cam, poor mileage, wild--*** car won't benefit much from vacuum advance. I don't know "exactly" what that point is.

I'd say run it.

I freely admit that "there were times" back when I was younger and stupid'er that I didn't, LOL
 
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