1981 straight six manual 4x4 project

[Ford F834]

Here is what I posted in another area on here a while back concerning spark advance:

On distributor curves, the mechanical advance is based on the best power under load without detonation, this basic idea goes way back as far as the moveable advance system whether it was a level on the steering column or a centrifugal advance mechanism. The Windsor V8s can take 36-38° total mechanical advance at around 5000 rpm, mechanical advance is 2 stage, a quick initial, up to around 3000 rpm for a street engine advance of 28-32°, the remainder coming in slowly to max rpm.

The vacuum advance came later as an economy measure and it makes up the difference between under load and light load conditions. There are two main styles, a vacuum brake system used by Ford on the flathead V8s and 6s with the front of the cam distributor and used by Mallory on their street distributors for years. The second is a vacuum can that has a diaphragm attached to a rod which moves the breaker plate (except on the old Chevy in-line 6, on those the whole distributor moves) this system has either an adjustment screw in side the can, or on older Fords a removable cap under which was a spring, shims and a hollow stop tube inside the spring. These are pretty straight forward, at X inches of vacuum you get Y degrees of advance.

What happens with the EGR is not that you can get more advance, it dilutes the mixture enough that the vacuum advance does not cause detonation. A specific example, the mid 80s Oldsmobile built 307 ci V8, if the EGR wasn't opening, you would get a light throttle spark knock at roughly 35-40 mph in 4th gear lockup, a bit more throttle and it would vanish when the computer added fuel by decreasing the down time on the metering rods on the E4ME primaries.

Now, to further confuse everyone, for a long time, Ford used Holley distributors, with no mechanical advance, only vacuum, called a Loadamatic distributor These had a 2 stage set of springs on the breaker plate, one very light one for the venturii signal, the other a heavier one for the throttle port signal. Depending on the year and application many had a spark control valve on the side of the distributor. This blocked the throttle port so that the very weak venturii signal didn't leak, when the throttle signal was there the valve moved in under the higher vacuum to allow it to the distributor diaphragm to receive the full vacuum. Some carburetors simply used a small check ball in the throttle passage. Because this system ran at maximum advance at cruise conditions, Ford found that it was necessary on some applications to use a second diaphragm on the back of the main one connected directly to manifold vacuum to quickly retard the main system under sudden WOT conditions. A quick way to recognize these systems, the diaphragm "can" is very flat on these as there are no springs inside it.

The next piece came with the emission systems, 1966 in California, and 1968 nationwide. One of the problems with many engines was idle emissions, unburned hydrocarbons and carbon monoxide (CO) also oxides of nitrogen. It was found that if the carburetor idle circuits were at the transfer slot that the idle could be set lean enough in the jetting that the engine could run cleaner. In order to do this, the initial timing needed to be reduced. The other solution was air injection. Each of the big three used a different approach, Ford used two systems, IMCO for IMproved COmbustion and Thermactor or air injection. Both systems used a lazier distributor curve and in some applications a lower initial (static) timing. It also was the death knell for the Loadamatic distributor as it's advance would vary with altitude and to some degree barometric pressure differences. The leaner low speed mixtures also would cause the Loadamatic equipped engines to misfire at low speed cruise due to the advance being fully in.

I hope this isn't too confusing, but hopefully it will help understand how the spark advance systems relate to other systems. All have to work together in harmony for best power and economy.

As for a choice of manifold vs. ported vacuum, the port in most carburetors and non computer controlled fuel injection systems is very close to the bottom or back side of the throttle plate (only one is used on multiple barrels) most Ford and Chrysler engines and some GM used a ported spark. One of the problems with direct manifold vacuum is the timing can retard if the vacuum goes below a certain level. This causes the engine rpm to drop, further reducing the vacuum. The manufacturer can use a light spring in the vacuum can so it has to fall way off in order to be a problem. It is more of a problem with automatic transmissions due to the engine loading in gear. This can also be a problem if the centrifugal advance comes in at too low an rpm. I used to see the older small block Chevrolets with a Powerglide come into my shop and the complaint was "I just got a curve kit and installed it, now I can't keep it running in gear and it races in neutral." Stall speed on the old Powerglide was real low and even a mild engine would drop 3-400 rpm from neutral to drive. As far as total vacuum advance, I never saw any difference between manifold and ported vacuum, the difference was what it did at idle. Whatever system, you still need to disconnect the vacuum when setting the timing, just be glad you aren't trying to set timing on a pre computer Oldsmobile V8, it was set at 1100 rpm, no vacuum because they wanted to be sure the initial centrifugal advance was in. Royal PITA, move the distributor, reset the rpm, check the timing, it would take 10-15 mins to set one correctly, but boy did it make a difference!

Wow.... THANK YOU ALL for the detailed explanations and advice! The history lesson on ignition advance might need to be a web page or resource link? So much for my delusion about owning an old, dirt-simple straight six 😉.

Please do not take offense if I do not immediately put this information into practice. I promise it will not go to waste. My short term goal was to have a decently reliable 4x4 while I am building the diesel. I'm pretty much there. The long term goal is then to revisit the '81 and perfect it. My take-away from this is that I probably need an aftermarket distributor/ignition upgrade that is more tuneable, and perhaps a better or different carburetor. For now I have to deal with what I have (which certainly isn't bad).

Since I have a timing light, a vacuum gauge and a tach/dwell meter I can at least measure what my current centrifuge and vacuum advance are doing and see if there is some opportunity there. Is there a good way to read centrifuge advance or advance with the vacuum attached since the timing cover only goes to 14*? The damper has a second notch in it... is that used? Sorry if these are questions I should be googling, but so far your answers are 10,000x better than anything I've found online. 👍

[Ford F834]

The problem here is that the Carter doesn't have true ported vacuum, and still gives ~5" at idle and only something like a max of ~12". So, in order to get a vacuum advance that is responsive to the load we changed David's to manifold.

Gary, granted I am running a JUNKyard carburetor which may be defective, but I was not able to measure any vacuum on the carburetor ports at idle.

 

[Gary Lewis]

Wow.... THANK YOU ALL for the detailed explanations and advice! The history lesson on ignition advance might need to be a web page or resource link? So much for my delusion about owning an old, dirt-simple straight six 😉.

Please do not take offense if I do not immediately put this information into practice. I promise it will not go to waste. My short term goal was to have a decently reliable 4x4 while I am building the diesel. I'm pretty much there. The long term goal is then to revisit the '81 and perfect it. My take-away from this is that I probably need an aftermarket distributor/ignition upgrade that is more tuneable, and perhaps a better or different carburetor. For now I have to deal with what I have (which certainly isn't bad).

Since I have a timing light, a vacuum gauge and a tach/dwell meter I can at least measure what my current centrifuge and vacuum advance are doing and see if there is some opportunity there. Is there a good way to read centrifuge advance or advance with the vacuum attached since the timing cover only goes to 14*? The damper has a second notch in it... is that used? Sorry if these are questions I should be googling, but so far your answers are 10,000x better than anything I've found online. 👍

Actually, they are good questions. On the timing marks, they sell tapes that stick on and add more degrees to the marks on the damper. But I've gone the cheap way and divided the circumference by 180 to find the distance between every 2 degree marks. Then lay them out on the damper with a light-colored Sharpie.

But you check the centrifugal/mechanical advance w/o the vacuum connected or you won't know which is doing what. So disconnect and plug the vacuum line and then slowly rev the engine up, noting the total advance at each, say, 500 RPM. Or 200 RPM if you want. But before you build a chart remember to subtract the initial advance you have dialed in.

[Gary Lewis]

The problem here is that the Carter doesn't have true ported vacuum, and still gives ~5" at idle and only something like a max of ~12". So, in order to get a vacuum advance that is responsive to the load we changed David's to manifold.

Gary, granted I am running a JUNKyard carburetor which may be defective, but I was not able to measure any vacuum on the carburetor ports at idle.

Ahh! So yours is different than David's. Connect your gauge and find out what range of vacuum you get.

[PetesPonies]

Ahh! So yours is different than David's. Connect your gauge and find out what range of vacuum you get.

I mentioned earlier about the timing marks and the confusion in regards to the 300 engine. It's easy to think you have the engine timing at one setting but be way off. I suspected something like this when you said it wasn't running right on manifold vacuum.

As for what was posted above . . this is not true >>>>>> "What happens with the EGR is not that you can get more advance, it dilutes the mixture enough that the vacuum advance does not cause detonation."

Far from it. A "diluted" mixture as it was termed, would cause a scenario that would be more likely to have detonation. Detonation is when the fuel mixture lights off itself, with no ignition spark.Typically it is because of excess heat. That heat can be generated in a few different ways, but a contributing factor is a lean mixture, which causes heat. By using the term diluted, I have to assume you mean leaner. If I am wrong, correct me. A fat, or richer mixture, is less prone to detonation than a lean mixture. What EGR, in fact, does is cool the charge. EGR removes some of the oxygen in the mixture, and that creates a cooler burn. Removing the oxygen is the same as adding fuel, it makes for a richer, fatter mixture. Richer mixtures are less prone to detonation as I said before.

[85lebaront2]

Bill - How much centrifugal/mechanical can a 300 six take? And when should it be all in, and how fast should it come in?

Ditto the vacuum? The problem here is that the Carter doesn't have true ported vacuum, and still gives ~5" at idle and only something like a max of ~12". So, in order to get a vacuum advance that is responsive to the load we changed David's to manifold.

Last, what process should one use in order to get the initial, centrifugal, and vacuum set up properly?

Due to the relatively long stroke (3.98") and low redline probably 32-34° max, listen for sounds of pinging under load and back off if needed.

[Gary Lewis]

I mentioned earlier about the timing marks and the confusion in regards to the 300 engine. It's easy to think you have the engine timing at one setting but be way off. I suspected something like this when you said it wasn't running right on manifold vacuum.

As for what was posted above . . this is not true >>>>>> "What happens with the EGR is not that you can get more advance, it dilutes the mixture enough that the vacuum advance does not cause detonation."

Far from it. A "diluted" mixture as it was termed, would cause a scenario that would be more likely to have detonation. Detonation is when the fuel mixture lights off itself, with no ignition spark.Typically it is because of excess heat. That heat can be generated in a few different ways, but a contributing factor is a lean mixture, which causes heat. By using the term diluted, I have to assume you mean leaner. If I am wrong, correct me. A fat, or richer mixture, is less prone to detonation than a lean mixture. What EGR, in fact, does is cool the charge. EGR removes some of the oxygen in the mixture, and that creates a cooler burn. Removing the oxygen is the same as adding fuel, it makes for a richer, fatter mixture. Richer mixtures are less prone to detonation as I said before.

Here is some info I found with a bit of reading. (I don't have access to my Internal Combustion Engines textbook where I am.) Most of the sources were consistent, but this one is the most succinct with respect to detonation and pre-ignition. It comes from an article in Contact Magazine, which is a forum for experimental aircraft and powerplants:

Detonation: Detonation is the spontaneous combustion of the end-gas (remaining fuel/air mixture) in the chamber. It always occurs after normal combustion is initiated by the spark plug. The initial combustion at the spark plug is followed by a normal combustion burn. For some reason, likely heat and pressure, the end gas in the chamber spontaneously combusts. The key point here is that detonation occurs after you have initiated the normal combustion with the spark plug.

Pre-ignition: Pre-ignition is defined as the ignition of the mixture prior to the spark plug firing. Anytime something causes the mixture in the chamber to ignite prior to the spark plug event it is classified as pre-ignition. The two are completely different and abnormal phenomenon.

There is another discussion in the article about optimum ignition timing, and helps explain why we need ignition advance:

There is another factor that engineers look for to quantify combustion. It is called "location of peak pressure (LPP)." It is measured by an in-cylinder pressure transducer. Ideally, the LPP should occur at 14 degrees after top dead center. Depending on the chamber design and the burn rate, if one would initiate the spark at its optimum timing (20 degrees BTDC, for example) the burn would progress through the chamber and reach LPP, or peak pressure at 14 degrees after top dead center. LPP is a mechanical factor just as an engine is a mechanical device. The piston can only go up and down so fast. If you peak the pressure too soon or too late in the cycle, you won't have optimum work. Therefore, LPP is always 14 degrees ATDC for any engine.

So, if we need LPP at 14 degrees ATDC then we need to light off the controlled-burn well ahead of that. But, "well ahead" is a function of a number of things, including the load on the engine and the RPM of the engine. The load on the engine is determined by the vacuum, hence the use of vacuum advance. And the RPM factor is applied by use of the mechanical advance.

As for EGR and "dilution", I think the intention of the comment was that exhaust gas "dilutes" the air/fuel mix. Not that it "leans" the mix by reducing the ratio of air to fuel, but that there is less air & fuel in the incoming charge - but the air/fuel ratio didn't change. However, I think the issue is really what EGR does to ignition timing. Here's a quote from Wikipedia: "Although EGR does measurably slow combustion, this can largely be compensated for by advancing spark timing."

In other words, distributors on engines with EGR will have more advance built in than a comparable engine w/o EGR. So, if we block off the EGR then we are going to have too much advance - like Big Blue when I add EFI but delete the EGR.

[85lebaront2]

The problem here is that the Carter doesn't have true ported vacuum, and still gives ~5" at idle and only something like a max of ~12". So, in order to get a vacuum advance that is responsive to the load we changed David's to manifold.

Gary, granted I am running a JUNKyard carburetor which may be defective, but I was not able to measure any vacuum on the carburetor ports at idle.

Ok, first item, on the carburetor, look at the flange at the base of the float bowl section, there may be a Carter number stamped into the side just above the throttle body. This may help in identifying at least the year. One fast identification is the choke design, if it has only the internal piston it is definitely an older model. One other, the older carbs had a taller throttle body. The pictures I saw, look like it is probably mid 70s as it has the tab on the front of the throttle shaft to operated the EGR kill valve.

On timing, distributors, etc. the 300 is known to "slip" the balancer and will result in a real problem setting the initial or static timing. The other issue which Pete referenced, almost every 300 built still has the old cast in timing scale on the timing cover. Now is where it gets interesting, some engines have a scale like the V8s do on the balancer, others have a single TDC mark and a bolted on metal scale. The thing to remember is the engine turns clockwise viewed from the front. On the balancer scale version, the advance side is clockwise from 0° TDC (Ford scales usually go from 10° ATDC to 30° BTDC), on the metal scale, advance is counterclockwise from 0° TDC.

If you have access to a timing light that can be used to read spark advance, you can verify how much total mechanical advance you have by running the engine up till the advance stops, then use the light to get it back to TDC on the scale. Ford distributors have an advance cam with a notch that sets the total mechanical advance, example would be a stamp of 24L, this will give a total mechanical advance of 24° which is added to the initial of say 10° for a total of 34° timing. Unfortunately on a Dura Spark distributor, to get to the cam where you can see it, the reluctor and pickup plate have to come out.

Gary is correct in his post on detonation or "pinging" it is caused by a second flame front starting on the side opposite the spark plug (one of the reasons many newer engines have the plug centered), pre-ignition and run-on or after-run are a separate cause, generally from carbon deposits getting red hot and igniting the mixture before the plug fires or continuing to ignite the mixture after the ignition is off.

EGR, EGR was designed to reduce the combustion temperature be diluting the incoming fuel/air mixture with burned gases and as a result, can reduce light throttle pinging or spark knock since the mixture burns slower (it also killed gas mileage),

[PetesPonies]

Here is some info I found with a bit of reading. (I don't have access to my Internal Combustion Engines textbook where I am.) Most of the sources were consistent, but this one is the most succinct with respect to detonation and pre-ignition. It comes from an article in Contact Magazine, which is a forum for experimental aircraft and powerplants:

Detonation: Detonation is the spontaneous combustion of the end-gas (remaining fuel/air mixture) in the chamber. It always occurs after normal combustion is initiated by the spark plug. The initial combustion at the spark plug is followed by a normal combustion burn. For some reason, likely heat and pressure, the end gas in the chamber spontaneously combusts. The key point here is that detonation occurs after you have initiated the normal combustion with the spark plug.

Pre-ignition: Pre-ignition is defined as the ignition of the mixture prior to the spark plug firing. Anytime something causes the mixture in the chamber to ignite prior to the spark plug event it is classified as pre-ignition. The two are completely different and abnormal phenomenon.

There is another discussion in the article about optimum ignition timing, and helps explain why we need ignition advance:

There is another factor that engineers look for to quantify combustion. It is called "location of peak pressure (LPP)." It is measured by an in-cylinder pressure transducer. Ideally, the LPP should occur at 14 degrees after top dead center. Depending on the chamber design and the burn rate, if one would initiate the spark at its optimum timing (20 degrees BTDC, for example) the burn would progress through the chamber and reach LPP, or peak pressure at 14 degrees after top dead center. LPP is a mechanical factor just as an engine is a mechanical device. The piston can only go up and down so fast. If you peak the pressure too soon or too late in the cycle, you won't have optimum work. Therefore, LPP is always 14 degrees ATDC for any engine.

So, if we need LPP at 14 degrees ATDC then we need to light off the controlled-burn well ahead of that. But, "well ahead" is a function of a number of things, including the load on the engine and the RPM of the engine. The load on the engine is determined by the vacuum, hence the use of vacuum advance. And the RPM factor is applied by use of the mechanical advance.

As for EGR and "dilution", I think the intention of the comment was that exhaust gas "dilutes" the air/fuel mix. Not that it "leans" the mix by reducing the ratio of air to fuel, but that there is less air & fuel in the incoming charge - but the air/fuel ratio didn't change. However, I think the issue is really what EGR does to ignition timing. Here's a quote from Wikipedia: "Although EGR does measurably slow combustion, this can largely be compensated for by advancing spark timing."

In other words, distributors on engines with EGR will have more advance built in than a comparable engine w/o EGR. So, if we block off the EGR then we are going to have too much advance - like Big Blue when I add EFI but delete the EGR.

I'm just taking a chapter from my head textbook, as I used to teach this very subject ;)

[Gary Lewis]

Ok, first item, on the carburetor, look at the flange at the base of the float bowl section, there may be a Carter number stamped into the side just above the throttle body. This may help in identifying at least the year. One fast identification is the choke design, if it has only the internal piston it is definitely an older model. One other, the older carbs had a taller throttle body. The pictures I saw, look like it is probably mid 70s as it has the tab on the front of the throttle shaft to operated the EGR kill valve.

On timing, distributors, etc. the 300 is known to "slip" the balancer and will result in a real problem setting the initial or static timing. The other issue which Pete referenced, almost every 300 built still has the old cast in timing scale on the timing cover. Now is where it gets interesting, some engines have a scale like the V8s do on the balancer, others have a single TDC mark and a bolted on metal scale. The thing to remember is the engine turns clockwise viewed from the front. On the balancer scale version, the advance side is clockwise from 0° TDC (Ford scales usually go from 10° ATDC to 30° BTDC), on the metal scale, advance is counterclockwise from 0° TDC.

If you have access to a timing light that can be used to read spark advance, you can verify how much total mechanical advance you have by running the engine up till the advance stops, then use the light to get it back to TDC on the scale. Ford distributors have an advance cam with a notch that sets the total mechanical advance, example would be a stamp of 24L, this will give a total mechanical advance of 24° which is added to the initial of say 10° for a total of 34° timing. Unfortunately on a Dura Spark distributor, to get to the cam where you can see it, the reluctor and pickup plate have to come out.

Gary is correct in his post on detonation or "pinging" it is caused by a second flame front starting on the side opposite the spark plug (one of the reasons many newer engines have the plug centered), pre-ignition and run-on or after-run are a separate cause, generally from carbon deposits getting red hot and igniting the mixture before the plug fires or continuing to ignite the mixture after the ignition is off.

EGR, EGR was designed to reduce the combustion temperature be diluting the incoming fuel/air mixture with burned gases and as a result, can reduce light throttle pinging or spark knock since the mixture burns slower (it also killed gas mileage),

Bill - That's a very good point about the balancer slipping. It is not unusual at all with these old engines. It was either Dad's or Rusty's 351M that had the balancer's outer ring slipped and cocked slightly sideways. But they don't always give any external indications of having slipped, so need to be checked.