I've seen alot of people not running the vacuum advance on their distributors. Any opinions on this, why or why not? Also when changing main jet sizes in a holley, what increments should you increase or decrease? How sensitive are they?
Engine Tuninng
- Thread starter moparguy
- Start date
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jamesmcclain340
Well-Known Member
why not? big cam= low or erratic vacuum signal at lower rpm's
When I ran the Mopar Performance Dist. I didn't notice any difference in milage or performance on my combo. Now I have the MSD dist. and there is no vacuum advance. I was told to up jet sizes two numbers at a time.
Fred B
Fred B
I've heard you can do away with the vacuum advance by welding up the slots in the the mechanical to limit the full advance to your preference, then bumping the initial up to compensate for no vacuum. Any takes on this. How much do the slots need to be closed or is it just trial and error? In a stock distributor. Are there any benefits to doing this?
C130 Chief
Mechanical Genius
Vacuum advance is strictly there for part throttle economy. It has no effect at full throttle so if changing you mecanical advance made a difference, it was wrong to begin with. It also has no effect at idle, so if it is affecting your idle it is either hooked up incorrectly or your carb has not been set up properly (additional airflow at idle) for the large cam. If power is your main goal, it will make little dfference to you. As for jetting, 2 #'s at a time till you quit going faster then back one.
lead69
hopeless car junkie
On the vaccum adavnce you need to limit the amount of advance by limiting the slot and its an age old debate but I run it to a port with full vaccum(intake manifold is best or carb base) so its advanced at idle and steady rpm (high vaccum).This helps smooth out the cam and give you better throttle response and will help gas mileage slightly.Ive run vaccum advance on my cars and havent-just depends on how much I feel like tuning it.I set up the initial with no regard to the vaccum and run around 16 crank degrees with 18-20 degrees in the distributor-then I limit the vaccum advance to 5-7 degrees.It sounds like alot of timing with it all combined but it works well in conjunction with pump gas street engines,I learned this recipe from several prfessional tuners.As far as the holley two sizes at a time is correct and you shouldnt have to jet more than four sizes up or down-if you do then thats an indication of either too small or too large a carberator.
lead69
hopeless car junkie
I forgot to say one of the biggest advantages to running the above set up is a reduction in engine temps.I had a bigblock and running the advance to a port with full vaccum was worth a true 10-15 degree drop in temp.
C130 Chief
Mechanical Genius
Ported and manifold vacuum are not the same. What you describe (having vacuum at idle) is connecting to manifold, not ported vacuum.
lead69
hopeless car junkie
Just so theres no confusion I edited my posts to clarify what I was saying,hopefully its clearer now,thanks.
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