Should I run it? I have a modified 350 around 400 horse. Total timing is 36 all in by 2500. Initial is around 12. I can set my msd e-curve to give me 15 degrees of advance at either 7 or 10 inches of vac. Which would be better if I choose to go with vac advance. Thanks, Justin

 

http://65corvette.nonethewiser.net/t...um_advance.pdf

 

Thanks, that was a good article.

 

I believe in it strong enough that I just spent $400 on a new distributor with vacuum advance. My old distributor was a mallory unilite in perfect shape but with no vacuum advance.
Watch out that you don't end up with too much vacuum advance. I ran into this yesterday.
I set my total timing at 36 but with vacuum I was at 60, way too much.
I am custom building a limiter so i can fine tune the total advance.
I want 50 -52 degrees total of all 3

 

Well, I have 11 initial 25 electronic then 15 vac. Should equal 51 toal with vac.

 

Norval,
I just went through this and found an Echlin VC1862 AR31 (NAPA)can has 8 distributer degrees,(16 crank) and pulls fully with only 8 inches a vacuum. This with a 36 degree mechanical gives 52 degrees. So far no pinging,but with the hot weather in the afternoon,I think it's pretty close. In the morning with cool air this curve rocks. Throttle response is explosive off idle,and at cruise speed it just sails along effortlessly.

I tried a homemade limiter myself at first(limited about 4 degrees) and it felt like it was holding back some while cruising at 60-65 mph.

 

yes you should run vacuum advance on a street car. Hook it up to full manifold vacuum source.

as for what level to set your adjustable unit at, first check to see what vacuum level your motor is producing at idle and set the vacuum advance to come in at approx 2" hg below that.

 

This has to be one of the most misunderstood and hotly debated issues in performance engine tuning. Quite frankly, I am absolutely amazed at the number of supposed "corvette experts" that just do not understand the issue of vacuum advance and why it is used. I took my own 69 big block into a guy (who will go unnamed) here in South Florida with a carbuertor problem I was having. This guy has been working on vettes exclusively for 30 years! He swore up and down it was the vacuum advance and not the carb, and that I needed to disconnect it and make things "simple." He then went on to tell me boats don't have it so you don't need it. I have a performance boat too, and I told him the reason for that is because boats are constantly under load and never operate in a light throttle condition. He went on and on and I got so pissed I nearly left. I finally threw down the gauntlet and said if you can make it run better without the vacuum advance lets see it. I told him the car would run hot with it disconnected and the mileage would take a nose dive. He said bull****. He readjusted the mechanical advance to 42 degrees total with no vacuum advance. The car idled like crap and sure enough temp gauge went up 30 degrees and mileage dropped 3 miles per gallon. Throttle response noticeably degraded.

The bottom line is vacuum advance virtually always improves throttle response, gas mileage and makes the engine run cooler (due to less transfer of combustion heat to the head as a result of the increased advance) on a street car, period. This assumes of course that you follow the guidelines others have given here which is right on (approximately 36 degrees mechanical and the rest vacuum to a total of no more than 52 degrees). Good luck

 

I can't agree with you enough. When I got my first Vette a few years ago I knew the distributor needed to be recurved and I took it to 3 or 4 mechanics in a row. Without fail their "solution" to fixing my timing problem was to put a ball bearing into the vacuum advance hose disabling the vacuum advance system and therefore lowering my timing.
The car ran like crap each time when they returned the car to me.
My neighbor found what they did and removed it and the next mechanic would do the same thing.

That'shen I realized that if I wanted to have my older vette with points ignition system, vacuum advance, and carb run correctly that I needed to start to learn to do things myself.

The vacuum advance system is widely misunderstood, even by so-called "professional" mechanics.
Yes, you can run the car without it but on a street driven car it's important and greatly helps the car run better in terms of idle characterics, throttle response, fuel ecomony, and lower operating temps.

Boats and race cars don't need it as they are typically run WOT most of the time but street cars run at varying loads constantly and it's the vacuum system that adjusts timing based on load.

 

Well then, looks like I will be running vacuum advance then. I can't really get 52 withoug having my total timing be 37. It would be 51 with vac. This doesn't seem to be an issue from what I've read.

 

My car has the vacuum advance unpluged, timing is 22 at idle, 36 at 2500 and runs 180 degrees, As a matter of fact this little ole 180 horse engine shot to 130 MPH Monday like nothing, must be running OK with my set up?
Truth is if you run it then set the rest of the car up with it that way but don't go changing idle and timing everyday or it all will be messed up.
If your car has full time vacuum and the timing is set with it, then if you unplug it the car won't idle, if you don't use vacuum then the car will idel high if you plug in up to vacuum.
Pick your poison and drink it!!!

 

I was playing with the vac adv a little bit ago and found it didn't help me. I guess I have everything set up already for non vac adv.

 

This is when you would want to use a ported vacuum advance (does not activate until off idle due to the vaccum port being above the throttle blades rather than below) instead of direct vacuum of the manifold. If you use it this way it will still perform better. But with your initial timing set at 22 and a short curve in the distributor this is the only way for it to work together properly with your set up now. Most carbs have a "ported" fitting on them to connect it this way.

 

amen, brother.

 

That was a typo I meant 11 initial, 36 total.

 

One degree one way or the other isn't going to hurt you.
If what you say is true, then your timing is right and time to move on to something else.

 

I'm using the E-curve distributor. initial-21, mech-15, ported vac-15. Runs+idles great. Demon 825 carb/XR288HR cam

 

I just ran across this post on another forum and had to copy and paste it here. I couldn't have stated it any better if a gun were to my head.
All you Warren Johnson wanabe's read and learn

Not having full vacuum advance at idle (timing retarded from where it should be at idle) increases exhaust gas temperature, which heats the coolant more, driving up temperature at idle; this was the factory design "ported spark" condition in the 60's with air-injection systems to ensure a good "afterburn" in the exhaust manifolds when the injected air hit the exhaust stream at idle, which was where emissions testing was done in those days.

Most of those "ported-spark" cars had factory base timing at 0 degrees (some were actually 2-4 degrees AFTER TDC), and had 30-34 degrees of centrifugal advance in the distributor to try and make up for it. Cars with full manifold vacuum on the distributor at idle had factory base timing set at 8-12 degrees, and their distributors had 20-26 degrees of centrifugal advance. Same WOT total timing for both, but the "ported spark" engines with air injection systems and retarded idle timing ran like crap at idle and part-throttle, and ran hot at idle.

Properly-matched vacuum advance can calibration (so it's fully-deployed at idle) with manifold vacuum makes for stable idle and a cooler engine at idle, better part-throttle driveability and throttle response, and improved fuel economy.

If you had a timing light rigged so you could see it cruising down the highway at 50-60mph with vacuum advance, you'd see 50-52 degrees advance at highway cruise; the lean cruise mixture takes longer to burn than a rich mixture, so the vacuum advance "lights the fire" sooner so maximum cylinder pressure is still reached just past TDC for peak fuel efficiency. When you accelerate, the mixture is instantly richer and takes less time to burn, so the vacuum advance drops off, retarding the timing slightly to accommodate the faster burn time, again maintaining peak fuel efficiency.

Without vacuum advance (just centrifugal), timing is only affected by rpm, not engine load (manifold vacuum), so optimum torque isn't reached and timing isn't optimized for engine load and mixture burn rate. Vacuum advance is useless on a race engine that is run at WOT all the time, but it's essential for a good-running street engine that has to run at idle and part-throttle too.

People that run fancy chrome-and-polished whizbang centrifugal-only distributors on a street engine have been suckered by the Summit/Jeg's race-car marketing hype, and are missing out on the joys of a really good-running street combination. Absolutely. Positively.

 

Gotta agree with ya on the last paragraph there.

 

You're right, but the point being those who think the vac timing can can't be valuble tuning aid for a street driven hot rod ain't usin all his tools