I am installing a Mallory distributor, HEI 85 series, on a fresh rebuild. It is a '79 L82, 355 with Edelbrock RPM top end package. I can not find anything telling me which to use and/or the benifits of either.

So I ask you, timed advanced or full?

 

go with full manifold vacuum advance

to save myself some typing, I've cut and pasted this directly off the thread on Timing that is a "sticky" at the top of the forum page.

••• note on vacuum source: older cars such as my ’65 used a full manifold vacuum source off the carb to run the vacuum advance. What this means is that at idle the carb supplied full motor vacuum to the vacuum can. Later cars switched to what is called “ported” or “timed” vacuum. The difference is where the vacuum is picked up on the carb. Full manifold vacuum is picked up BELOW the throttle blades on the carb so it always gets the full vacuum the motor produces. The ported vacuum is drawn off the carb higher up ABOVE the throttle blades. This makes a large difference since at idle full manifold vacuum gives full vacuum but a ported vacuum port will give NO vacuum at idle but than will work the same as a full manifold vacuum source after the throttle blades are cracked open when driving.
If your car is a later car and is set up to run off a ported vacuum source you are typically better off switching it to a full manifold vacuum source port on the carb. If you carb does not have a full manifold vacuum port it's possible to pick it up directly off the intake manifold. Many manifolds have a hole already drilled into them for various vacuum purposes that are simply pluged and you can get a proper fitting to feed vacuum from many of the Corvette parts suppliers such as Paragon, etc.
The ONLY reason they switched to a ported vacuum source was because of emissions requirements. One way they lowered hydrocarbon emissions was the A.I.R. system and to make that work properly they needed a very retarded spark at idle. Since a Ported vacuum gives no advance at idle this worked perfect. The problem is that while a ported vacuum helps reduce emissions it’s lousy for performance!
Switching from a ported vacuum to a full manifold vacuum source will typically give you better idle characteristics, better throttle response, cooler operating temps, and better fuel economy.

 

Agree,..manifold timing is best for vacuum advance,..runs cooler, better off idle response, etc. Just make sure you have a vacuum can that's all "in" at a vacuum level lower than your idle vacuum level or you'll get inconsistent idle speed.

Good luck!

 

i do both ported or manifold depending on the engine and the initial advance the engine needs. a engine that needs 18 degrees or more initial because of a high duration cam may not like another 16 to 20 degrees from the vac advance.
manifold vacuum to the vacuum advance was mainly used in the days when initial timing was at or near top dead center (4 after tdc to 8 btdc). a example is 1967 corvette 327/300 hp california emmissions timing spec was 4 degrees after top dead center and ran very bad when the vac advance went bad.

use what works for you but manifold vacuum is not the best for every application, sometimes it is best to try both.

if the vac advance is all in at too low of a vacuum you will get a vacuum advance induced ping/detonation problem that if ignored can do engine damage. i use a advance that only gives 10 to 12 degrees of advance that is not all in before 12 inches of vacuum

henry @ oles

as far as i remember and i am sure someone will correct me if i am wrong, most engines from the factory that used over 12 degrees initial had a ported vacuum source for the vac advance or did not use a vacuum at all

 

most people such as Lars, JohnZ, and other true experts still recommend running full manifold vacuum even on cars that came with ported vacuum originally because of emissions. If your motor is timed so high that a vac can with 16º of advance is too much, than select a different can with less of an advance.

did cars come from the factory with an initial higher than 12º?
My '65 327/365 motor has a relativley high intial at 12º compared to a 327/300 which I believe was 6º (perhaps 4º as you mentioned but I think it was 6º). Perhaps limited cars such as the L88's had a higher initial timing spec but if they didn't run a vacuum can it was because that was really nothing more than a race car that GM never intended to actually have run on the street and race motors don't need vacuum advance - they typically run at close to or at WOT almost all the time where a vacuum advance system would be useless.

Regardless, it wasn't the initial timing on an engine that dictated full or ported vacuum, it was simply emission requirements. Prior to federal emission standards that required a reduction in HC, all cars that came from the factory with vacuum advance units ran full manifold vacuum. It was only changed to ported vacuum because of a need of retarded spark at idle to keep HC levels down.

Bottom line is yes, you are correct, use what is best for you, BUT in the vast majority of cases full manifold vacuum will be better.

 

look up the specs for a 1967 327/300 california corvette (with air pump) and you will see the timing was 4 degrees after top dead center.

the rest of the nation must have better fuel than we have in california because i am seeing a lot of engines that can not handle 16 to 20 degrees of additional timing at idle (from the vac advance) when the engine has 30 or more degrees of timing at idle you can hear a misfire and see high HC (hydrocarbon) readings in the exhaust. when we put race gas into any of these problem cars the missfire/high hc problem goes away

as you begin to see more of the new reformulated gasoline you may begin to see what we see here in the state that leads the way down the road to reformulated gasoline. we often have to limit the vacuum advance to allow a vintage engine to burn these new fuels.

 

well, that's typical for anything involving Ca!
The 6º initial I was referring to was a non-air '67 327/300.
regardless..........

most cars don't need an initial timing as high as 18 even with a radical cam in them.
If you find some need of an initial timing of 18º select a vac can that supples a lower vac advance. I agree with you that engines can only handle so much advance - that's why there are different cans with different levels of advance in them.
Most vac cans are going to supply 15º or 16º of advance and at a more "normal" initial timing setting of 4º-14º this is not a problem.
Again, using my '65 as just an example, initial timing is 12º and the vac advance (on full manifold vacuum) adds another 16º for a total of 28º at idle and total timing of 36º plus vac adv of 16º = 52º. I run all day long on regular 93 pump gas with no issues of detonation at all.

 

pump fuel here in calif is 87, 89 or 91 octane you may be able to find a station that sells 100 octane race unleaded if you are lucky so what applies to 93 octane non reformulated gasoline may not apply to our fuel. don't worry we lead the nation in the pursuit of cleaner air and reformulated gas so our fuel is most likely coming your way.

I have been doing some work with Chevron and thier new PurEscape fuel additive, it seems to make the fuel act more like the fuel of days past.

henry @ oles

 

that's scary on these older motors with higher compression like my '65.
my '78 runs Ok on 87 or 89 but is happier with the 93 but at least it can handle the lower stuff if needed.
I don't even attempt anything below 93 on the '65.

 

now you see why i differ from the old norm on tuning and always use a 5-gas or wide band o2 for jetting. we did a piece on tuning a 67 427/435 that was in corvette fever and went up on the idle jet and main jet to get the mixture correct. the timing curve that worked for this engine was 16 degrees initial plus 20 mechanical at 3600 rpm, the engine could only take 46 total advance with the vacuum advance hooked up so i had to limit the vac can. any time the total got much over 46 total you could hear a miss in the side exhaust (the sound of the miss was bothering the cars owner) and i could read the missfire as high HC (hydrocarbons) in the exhaust with our portable 5-gas analyzer.

my main point is what works for me or you may not work for everyone since gas is not the same in all parts of the country and here there is a diferent bleand for each season of the year. some engine packages work better with ported such as when you are to fighting to solve a detonation problem just off idle.