Is there a strong argument to run vacuum advance?

[HeavyFuel]

Heard and read both sides of the fence...just wanted to get the latest opinions.


Background: I just can't seem to get my timing dialed in to my liking and it's not the dizzy (new Firecore 'plug and play'  )  My 3 speed torqueflight with 3.23 gears just seems to limp around, and I can't seem to get rid of the mild occasional ping.

So tonight I unplugged the vacuum advance, and went for a drive around town.  The street manners were improved, and the off-idle hesitation was reduced.   Nice cool temps outside, about 65*.

The timing is sitting at about 16* initial, and about 35* with mechanical....all in around 2800 rpm.  (gotta verify all that....my timing light went to hell recently)


Soooo... whats the downside?



Running a little richer, maybe hotter when puttering around town?  Perhaps a little worse gas mileage?  Fouling plugs?


I may just leave it like this for a while and see what happens.   

[A383Wing]

gas mileage will probably be a little less....but if that's not a big concern, and if it runs better without the vacuum advance connected, then cap it off and run it this way.

Just out of curiosity, did you have the advance connected to ported or constant vacuum nipple on the carb?

[firefighter3931]

Most modified builds will run better with the vacuum advance disconnected. If you have an aftermarket cam and the manifold vacuum is less than 15in at idle I would recommend leaving the vac advance capped and run without it. 


Ron

[myk]

What carb are you running?

It's my understanding that vacuum advance is just a gas mileage thing.  I ain't worried about that too much...

[HeavyFuel]

The advance was hooked to the ported nipple....the one right up front on my stock 4429s AVS Carter carb.


The motor isn't really modified, internally anyway.  Stock 440 HP, CH4B intake, stock repo 6-pack purple shaft cam.  It makes good vacuum above idle.....not so great at idle.  I thought that was normal?   Ported vacuum is supposed to by nada at idle...right?

[ODZKing]

Haven't tried anything with my 73 but my 67 I did. Wanted the stock look so I cut the va hose and simply put a same size nail in there to block any air passage.
However I didn't think it made any difference performance wise, but as stated my 383 is stock.  It's a cruiser!   
What did change is the mileage on the road and even around town. 
We used to do a lot of traveling with the car and with 2.94s in the rear, it is no race car. However it was getting great gas mileage for a 383. 17/18 
Gas mileage really went to hell, like down to 12 and single digits around town, so I put mine back the way it was.

[John_Kunkel]

It's my understanding that vacuum advance is just a gas mileage thing. 

Not so, vacuum advance is about efficiency, better mileage is a side-effect of efficiency.

The engine's ignition timing requirements vary with load, and if you have the centrifugal advance timing optimized for full load at a given rpm then it will be far too retarded for part load operation at that rpm. IOW, vacuum adjusts the timing for load while centrifugal advance alone only adjusts timing according to rpm.

[Ghoste]

So if you do any highway driving or steady load cruising at part throttle, you should have it?

[A383Wing]

this might help if you care to read up on it

http://www.setyourtiming.com/Timing_Settings.html#VaccumAdvance

[Mebsuta]

Diaphragm went bad in mine, so I bought another distributor just for the vacuum advance canister.  When that one turned out to be bad too, I just capped it off and forgot about it.  The only time I ever noticed it when it was working was if I dialed in too much vacuum advance, and then it would surge at part throttle.  This is for regular street-driven 383 HP.

[myk]

What do you mean by too much advance?  Did you have it set "advanced" or retarded? 

[Kern Dog]

What carb are you running?

It's my understanding that vacuum advance is just a gas mileage thing.  I ain't worried about that too much...

Yes....and a lower emissions thing as well as a cleaner spark plug thing. Not much to be gained in terms of power though.

[c00nhunterjoe]

I have found that it varies from build to build. Your geographic location plays into it as well. My advice- experiment. Some builds like it, some dont. Try it. Adjust it, try ported and manifold. See what works and what doesnt on your particular build.

[flyinlow]

I have found with mild build 440 street cars that once you have the centrifugal set up right for best WOT performance there is too much total timing at part throttle with the stock vacuum advance. I limit mine. I cut a washer to slip over the advance arm and glue to the distributor to restrict the vacuum advance arm movement to about 8*. About half of the original.

I hate to leave part throttle torque and gas millage on the table. Not a strong argument but my 

[Paul G]

Keep in mind the stock base timing setting. What was it, in the single digits BTDC maybe 6° to 8° right? Now you have the base at 16°. You are all in at 35° about 2500 RPM. The factory used much less base timing, then used vacuum advance to improve part throttle performance and economy. By moving the base timing way up the chart as you have done, and most of us have done when modifying the engine for improved power, the need for vacuum advance has been negated. All it will do at this point is cause pre-ignition.

 

[HeavyFuel]

Keep in mind the stock base timing setting. What was it, in the single digits BTDC maybe 6° to 8° right? Now you have the base at 16°. You are all in at 35° about 2500 RPM. The factory used much less base timing, then used vacuum advance to improve part throttle performance and economy. By moving the base timing way up the chart as you have done, and most of us have done when modifying the engine for improved power, the need for vacuum advance has been negated. All it will do at this point is cause pre-ignition.

 

[ottawamerc]

Keep in mind the stock base timing setting. What was it, in the single digits BTDC maybe 6° to 8° right? Now you have the base at 16°. You are all in at 35° about 2500 RPM. The factory used much less base timing, then used vacuum advance to improve part throttle performance and economy. By moving the base timing way up the chart as you have done, and most of us have done when modifying the engine for improved power, the need for vacuum advance has been negated. All it will do at this point is cause pre-ignition.

 

Now there's an explanation that I understand thanks.

Scott