General spark advance thoughts

[Anonymous]

After being asked several times elsewhere, I thought I'd post the science here. For some of you, this is simple, but for newcomers, it's news.

Inside most of the I6 distributors there are 2 advance limiter slots. One is usually larger than the other. The total mechanical advance is TWICE the value that is stamped on the distributor plate. The moving plate that advances has a post that is inserted into one of these slots. Whichever slot contains the post is the one your distributor is using.

Using the larger numbered slot requires less static timing and vice-versa. If your engine has strong compression, using the LARGER numbered slot will allow wider advance angle and less static. This will help starting, especially in cold weather or when the engine has hot-soaked. Using the SMALLER numbered slot will let you run more static advance, which can help generate a "peppy" feeling at lower engine speeds.

Generally, if your carb is aged and the throttle plate is worn, you can get a better idling characteristic by using the LARGER slot and setting less static timing. This is because it will require a larger throttle opening to generate the idle, which allows more adjustment range with the carb.

If most of your engine is in good shape, try using lighter-weight springs in the distributor. Whatever advance curve you're using, it will advance faster and will improve gas mileage, if it does not cause knock.

To change your distributor from one slot to the other (distributor out of the car), remove the plate that supports the points (or pickup, if Duraspark) and then the screw in the center of the shaft. Pull the advance mechanism out of the distributor. Most advance units have a small spring that also needs to be removed to release the advance cam (on points distributors, especially). Then pull off the advance cam, turn it 180 degrees and reinstall in the reverse of the above.

These are general rules, but actually apply to all engines, not just these sixes.

 

[addo]

I wish ours (Bosch) were that easy.

You have to remove the transistor cover (2 screws), transistor unit, vac advance (2 screws), two circlips that hold the iron rotor to the shaft plus the rotor itself, the two allen head screws that hold the breaker plate, lift out the breaker plate and undo a fine circlip to separate the parts. The springs and weights remain in the distributor.

THEN try lighter or heavier springs, or welding/reaming the advance slot. And aligning the iron rotor on reassembly is a bugger.

Cheers, Adam.

 

[Anonymous]

Mark,

The vacuum advance does actually advance the timing more right??

It might sound like a stupid question, but there has been several people that think it actually retards the timing judging from their posts and some of the questions that have been PM'd to me....

I assumed it was called a vacuum advance because that is what it does...

I've had my Duraspark apart a couple of different times. There is a small clip that sets in the shaft that you have to pull out to switch settings....Little bugger has flown across the work bench more than once when I was trying to reinstall it!!!

Right now I am running a pair of really light springs that allows all the mech adv to come in right around 2500-2600 rpms and I set the mark on the smaller number when I was running my dual carb setup. I want to switch it to the larger number with my current 2 Bbl carb.

Also, If your vacuum advance cannister is not adjustable (mine isnt) I found that most of the aftermarket ones are a direct bolt in. I have a Crane adjustable unit waiting for me to install it when I get the chance.

Nice topic

Later,

Doug

 

[LEROY POLL]

HELLO MARKP

TO EVERYONE:

YOU WILL GET A FALLING OR LOSS OF ADVANCE WHEN YOU USE THE MANIFOLD PORT. IF YOU HAVE A LOSS OF ADVANCE, IT IS CALL RETARDING.

THIS IS TRUE ON THE LOADAMATIC. IT IS ALSO TRUE ON THE DUAL ADVANCE AS WELL. BUT THE RETARDING IS AT IDLE ONLY.

THE LOADAMATIC WILL RUN FULL ADVANCE WITH THROTTLE OPEN JUST A BIT AND AT NO-LOAD. IT RUNS OFF THE VACUUM OF THE MANIFOLD AND GET FULL ADVANCE THE HIGHER THE MANIFOLD VACUUM GETS. AS THE MOTOR TAKES ON LOAD THE MANIFOLD VACUUM WILL DROP. THIS WILL CAUSE A DROP IN THE AMOUNT ADVANCE. FORD SAYS THAT IT FALLS BACK TO WHERE THE MANIFOLD VACUUM LOAD PUTS IT. THEY SAID THE ADVANCE WILL FELLOW THE LOAD. THEY ALSO NAMED IT LOAD-A-MATIC FOR THIS REASON.

I WAS IN SCHOOL WHEN THIS DESIGN CAME OUT AND NO ONE COULD FELLOW THE THINKING. ALL MY FRIENDS SAID "I DON'T KNOW HOW IT WORKS,...BUT IT WORKS." EVEN MY TEACHER SAIDS THAT THE THINKING WAS WRONG. HE ALSO DIDN'T LIKE FORDS.

IN ELECTRONICS WE LOOK AT THINGS FROM TWO WAYS. YOU EITHER HAVE RISING VOLTAGE OR FALLING AMPERAGE. IT ALL DEPENDS ON WHERE YOU ARE COMING FROM, OR WHAT YOU NEED.

SO..... IF WE HAVE FULL (HIGHEST) ADVANCE....WE NEED NO MORE. SO THEN IF WE NEED NO MORE ADVANCE WE WILL BE NEEDING LESS ADVANCE. IF WE DID NOT LOWER THE ADVAMCE WE WOULD GET PINGING. WE RETARD THE ADV. TO GET RID OF THE PINGING. THE LOADAMATIC IS MADE TO MATCH THE LOAD SO WE WILL NOT GET PINGING. THE MANIFOLD VACUUM IS TRACKING THE LOAD OF THE ENGINE.

IT START AT FULL ADVANCE AND FALL BACK TO A LOWER ADVANCE, NOT START AT A LOW ADVANCE AND RISE TO A HIGHER ADVANCE.

THE OLD TEST OF TAKING A HILL IN HIGH GEAR AT LOW SPEEDS TO SEE IF THE TIMING IS SET RIGHT IS NOT A GOOD TEST FOR THE LOADAMATIC. FOR AT THE POINT OF WIDE OPEN THROTTLE THE LOADAMATIC WILL GO TO IT'S LOWEST ADVANCE.

EVEN THE VENTURI PORT WILL HAVE A DROP IN ITS VALUE GIVING LESS ADVANCE. WITH THE VENTURI PORT YOU GET A GREATER ADVANCE AT W.O.T. AT 65 MPH THAN AT A STEADY 65 MPH. THIS IS CALL A JUMP IN ADV. THIS IS DO TO THE MORE RAPIDLY MOVING AIR AT W.O.T. THIS GIVES A GREATER (OR LARGER)VACUUM TO THE PORT.

IF YOU WILL WATCH THE TIMING ON AN AUTOMATIC TRANS. CAR YOU CAN SEE THIS FALLING BACK AND MATCHING THE LOAD. AT IDLE THE TIMING MARK IS ALMOST OUT OF SIGHT AT NO-LOAD. FULLY ADVANCED...! PUT IT IN DRIVE AND THE TIMING WILL DROP,.... PUSH ON THE THROTTLE (WITH BRAKE SET) AND IT WILL DROP AGAIN,..... BUT RISE AS THE LOAD OF THE TRANS. IS MATCHED.

THIS IS A GOOD DESIGN FOR POWER PUMPS THAT STAND ALONE AND PUMP DIFFERENT LOADS. WATER PUMPS IN NEBRASKA USE FALCON MOTORS FOR THIS LOAD TRACKING FEATURE. (THAT'S WHAT I WAS TOLD.) AND ALSO BECAUSE THEY WILL KEPT RUNNING AND NOT BREAK.

REMEMBER THIS WHOLE POST WAS ON MANIFOLD VACUUM ADVANCE. MARKS POST IS GOOD ON THE VENTURI ADVANCE.

THANKS EVERYONE FOR YOUR TIME. I TRULY HOPE THIS WILL GIVE SOME UNDERSTANDING AND LESS CONFUSSION ON OUR TWO OR THREE DIFFERENT KINDS OF DIST. USED ON THESE MOTORS.

THANKS MARKP FOR TAKING ON THREAD.

LOTS OF LUCK 2 EVERYONE....LOL2E

LIVE IN GRACE

LEROY POLL.

 

[Anonymous]

LEROY-

I can't say that ALL vac advance systems are this way, but they are on a PORTED vacuum source for a reason- so they don't see vacuum at idle, only during off-idle and transition throttle settings. It's called an advance for a reason- it controls amount of advance, not retard.

A typical sequence would be:

1) engine at idle, ported OR timed vacuum port sees no signal (the orfice for the port is typically above the throttle blade at rest), therefore NO vac advance. Engine is running on static timing with maybe a trickle of mechanical adv (doubtfull).

2) Transmission is put into gear , or clutch is let out, and a load is applied- at this time, the vac advance port should be exposed to the vacuum signal on the backside of the throttle blade, and the vacuum advance should start to 'see' a little of the signal (TOTALLY dependent on throttle setting- in gear at idle, an auto won't see it).

3) The throttle is pressed, the blade opens, and manifold vacuum (although dropping slightly) is applied to the timed port (the blade is now 'ahead' of the port). This vacuum now pulls back on the timing plate, which advances the timing- to add power for the additional load. In cases of WOT, the systems plays catch up to the load as engine speed increases. This is where the proper tuning comes in- you want your adv curve matched to your engine's ability to accelerate / hold a load.

4) as engine speed rises, and vehicle reaches cruise speed, the load decreases, and a lighter throttle setting is used. NOW- this is where tuning and vehicle differences come into play: some will still have a throttle setting PAST the timed port, which means it cruises using mechanical (RPM dependent) and vac advance. Some designs don't use a vac adv, especially low-vac signal engines.

It's all a dynamic environment- some engines (and atmospheric conditions) like different highs and lows; the key is to get those boundaries right. Getting an adjustable vac adv can be a great tuning tool.

You're completely right about WOT / highest load / lowest advance- the port, like all the others in the carburetor (except the high-speed signal in the venturi/booster) sees negligible vacuum, which reduces the amount of advance. They balance an engine's timing needs using a combination of mechanical (higher engine speeds, low vac or not) and vacuum (engine not up to, or needs to accelerate to full load) advances.

JEFF