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-   -   vacuum advance for fuel mileage? (http://www.hotrodders.com/forum/vacuum-advance-fuel-mileage-223616.html)

LEROYDOZOIS 09-01-2012 10:07 PM

vacuum advance for fuel mileage?
 
my neighbor decided to come over and tell me if i unhook my vac adv that ill get better fuel mileage and wont notice a change in performance. he then also said that he would rebuild the 900 cfm holly that he had on his 496 and that i should buy if from him..............

although i dont believe him in any way i have noticed in a few posts that people dont hook up their vac advance for 1 reason or another so now im curious, is there any advantage to having/not having a vac adv ???

vinniekq2 09-01-2012 10:12 PM

Vacuum advance is strickly for fuel economy. If your engine can use it then it should be hooked up.A holley anything is not likely the best carb for mileage,they are better known for power

LEROYDOZOIS 09-01-2012 10:19 PM

i figured
 
whenever i would be changing a carb on the suburban or something like that nature he would run over and tell me how hes a carb master and that his boat doesnt use any gas and that he could build me the perfect carb

i dont trust him with my mechanical pencil let alone a carb lol =]

cobalt327 09-02-2012 04:03 AM

Quote:

Originally Posted by LEROYDOZOIS (Post 1587701)
whenever i would be changing a carb on the suburban or something like that nature he would run over and tell me how hes a carb master and that his boat doesnt use any gas and that he could build me the perfect carb

i dont trust him with my mechanical pencil let alone a carb lol =]

A vacuum advance is one of the most important things to use if economy is a priority!!! I've linked to a page w/info on timing, etc. earlier (on another thread), but here is info on the vacuum advance that may help. You should take a look at the entire article, for that matter.

Be sure the advance is actually working. Often the original vacuum advance units are no good by now, being as they've not been installed on an HEI distributor for many years.

Greg T 09-02-2012 06:23 AM

Also, anyone who so loosely makes a statement such as "disconnecting your vac advance gives you better mileage", should never be trusted to rebuild anything on a motor vehicle.

LEROYDOZOIS 09-02-2012 08:04 AM

yeah
 
Quote:

Originally Posted by Greg T (Post 1587752)
Also, anyone who so loosely makes a statement such as "disconnecting your vac advance gives you better mileage", should never be trusted to rebuild anything on a motor vehicle.

i agree and truly hes a joke he, comes over when im rejetting out trucks or boat and starts ranting about how q-jets/eddys are POS carbs and how i need to buy his boat carb. following by ( my boat gets really good fuel mileage ) following the ( the condensation off my blower motor i built makes my oil green, so my blower must be kickass ) i feel bad because hes lonley and sad in his life but theres times when i have to remember my 16' screw driver is for my mixture screws and not for screwballs ;)



and 327. thanks for the link ill read it when i have down time at work. timing on the truck should be correct but ive kinda wanted t ocheck the ( total timing ) i think its called, i saw a thing on here before and used it when i was doing the timing on the truck, but i like to double check =]

DoubleVision 09-02-2012 08:25 AM

So now this goes back to your other thread where I asked where your timing was set. Was vacuum advance in use? Was it connected to a manifold source? You never did tell me what the timing was set at, other than you saying somebody else said it was okay. The base timing should be 12 degrees before top dead center with vacuum advance disconnected and plugged. After it's set lock the distributor. Connect the vacuum advance to a manifold or full time vacuum source (one that pulls vacuum full time including idle)
Now recheck the timing, it should be 18 to 24 degrees before top dead center. When you connect the vac advance right away it's going to idle up.
This enables you to further lower the idle speed, placing the butterflies back on the idle slots on the carb.
As the other guys have said vacuum advance is very important from a mileage stand point. It's also very important in relation to other things such as idle quality, idle speed, and yes without it in use the engine will run hotter and in some cases run hot. Many have asked what use vacuum advance at idle? There are several reasons, one of those is idle and off idle mixtures, such as at cruise speeds the mixture is lean. Lean mixtures burn slower than do richer ones. Due to lean mixtures burning slower more timing advance is needed. Another reason is try and set the base timing at 24 degrees without the aide of vacuum advance, kick back, starter dragging with broke starters will result as there is no vacuum when trying to start the engine so the engine only "sees" 12 degrees before top dead center until it starts.
with the base timing at 12 degrees or lower without vacuum advance another side symptom is poor throttle response. You can have a nice small block with excellant components to make lots of low end torque and without the correct spark advance it will be a powerless slug that drinks gas. I've been down this road several times and that's how I got to where I am now. I was told and I learned. It taught me my brother is right, there is no substitute for experience.

1brewmaster1 09-02-2012 01:03 PM

DoubleVision is right on target... There are MANY postings on the internet that argue the issue of ported vacuum (no vacuum at idle) and manifold vacuum (constant vacuum). The engine will run better, cooler, more effecient, faster and just about everything positive that is not mentioned when running off of manifold vacuum to the vacuum advance. Either take the vacuum off of the base of the carburetor or off of the intake manifold and your vacuum will do the rest. What ever your timing spec is, it will increase 12 - 14 degrees at idle when the manifold vacuum is applied. After modifying the vacuum source, readjust your idle and idle mixture screws and reset your idle again. Ported vacuum was introduced prior to the cat when polution control started controlling emissions. Instead of BTC (before top dead center), we went to ATC (after top dead center) and ported vacuum was required. Without all of the emissions on street built engines, ported vacuum is no longer needed.

LEROYDOZOIS 09-02-2012 02:42 PM

Quote:

Originally Posted by DoubleVision (Post 1587778)
So now this goes back to your other thread where I asked where your timing was set. Was vacuum advance in use? Was it connected to a manifold source? You never did tell me what the timing was set at, other than you saying somebody else said it was okay. The base timing should be 12 degrees before top dead center with vacuum advance disconnected and plugged. After it's set lock the distributor. Connect the vacuum advance to a manifold or full time vacuum source (one that pulls vacuum full time including idle)
Now recheck the timing, it should be 18 to 24 degrees before top dead center. When you connect the vac advance right away it's going to idle up.
This enables you to further lower the idle speed, placing the butterflies back on the idle slots on the carb.
As the other guys have said vacuum advance is very important from a mileage stand point. It's also very important in relation to other things such as idle quality, idle speed, and yes without it in use the engine will run hotter and in some cases run hot. Many have asked what use vacuum advance at idle? There are several reasons, one of those is idle and off idle mixtures, such as at cruise speeds the mixture is lean. Lean mixtures burn slower than do richer ones. Due to lean mixtures burning slower more timing advance is needed. Another reason is try and set the base timing at 24 degrees without the aide of vacuum advance, kick back, starter dragging with broke starters will result as there is no vacuum when trying to start the engine so the engine only "sees" 12 degrees before top dead center until it starts.
with the base timing at 12 degrees or lower without vacuum advance another side symptom is poor throttle response. You can have a nice small block with excellant components to make lots of low end torque and without the correct spark advance it will be a powerless slug that drinks gas. I've been down this road several times and that's how I got to where I am now. I was told and I learned. It taught me my brother is right, there is no substitute for experience.

thankyou for the detailed response =] when i get the suburban back ill make sure i go through the steps you wrote above. and i might just hook the vac adv to the port on the manifold so theres no issues as to what its hooked up to on the carb. hopefully ill get the quadrajet working so i can use it instead of the eddy. but ill do the tuning with the eddy being i know it works flawlessly atm.


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