Ignition timing adjustment

[SwissMonaco]

Good day to all. I am currently in the process of checking my ignition. To be precise, I am checking the adjustment mechanically and with vacuum. I can not find any values which are correct for this. It is a 440 cui from 1977, at idle (780rpm) I have the 12° before TDC. Without vacuum, the mechanical adjustment goes up to 35° at about 3000rpm. What values are correct with vacuum when I have this connected to the timed vacuum port on the carburetor? Or in other words do I even need the vacuum port? I bought the car without the vacuum port. The engine is completely original.

 

[CBODY67]

There are distributor specs in the factory service manual. Only thing is that the manuals for free download at www.mymopar.com do not go quite to that newness. But those older FSMs can be used as a guide, usually.

The only specs for the vac advance are related to inches of Hg, not engine rpm. You DO need vacuum advance for highway fuel economy and better drivability. On the '77 engine, there are some vacuum delay valves to modulate how the vac advance works for better exhaust emissions.

In general, the mechanical advance figures you mention seem to be "in the ballpark" for Chrysler B/RB engines. It does not seem that a big lot of improvement in power can result with more than what you now have, so I would suspect everything is good. As long as the engine does not ping on slight part-throttle acceleration for the fuel being used.

The only mechanical + vacuum advance totals were some which I saw in an Exxon Tune-Up Manual (not available to the general public, but only to Exxon station owners, which I saw back in about 1975). It used 2500rpm for a quick check to see if everything was working well. So basically, it was the mechanical advance from the factory service manual plus the maximum vac advance figures quoted in the factory service manual. These figures were usually in the 50-54 degrees BTDC range. I know, that seems a bit too high considering that we are only usually seeing mechanical advance figures at WOT only, but for a part-throttle load at highway speeds on level roads, that higher figure is good.

Usually, the mechanical and vac advance systems are looked at separately for diagnosis and checking them. The combined figures in the Exxon manual allow for a quick check of both of them as they operate together AND a quick check of such.

As the air/fuel ratio is "thinner" at that cruise situation, the mixture is harder to get ignited, so more ignition "lead" is needed so the full combustion happens when it needs to happen at or near TDC. Whereas, at WOT, the mixture is "thicker"/richer and easier to get ignited.

An inoperable or disconnected vac advance will usually result in a slightly less-responsive to throttle engine and decreased fuel economy (close to 10mpg or thereabouts) on the highway. So it is needed. All things considered, "fuel economy" needs all the help it can get.

Always look for specs for the original vehicle the engine was installed in at the factory. Unless there have been some changes to it (related to the distributor and/or carburetor), especially if the distributor is not the original one, being replaced with a more-generic-spec rebuilt item. THEN, you get to be more "on your own", using what seems to work for good performance and not pinging/detonation sounds from the engine "under throttle". But what you quote for current specs seems good, to me.

Sorry for the length,
CBODY67

 

[SwissMonaco]

There are distributor specs in the factory service manual. Only thing is that the manuals for free download at www.mymopar.com do not go quite to that newness. But those older FSMs can be used as a guide, usually.

The only specs for the vac advance are related to inches of Hg, not engine rpm. You DO need vacuum advance for highway fuel economy and better drivability. On the '77 engine, there are some vacuum delay valves to modulate how the vac advance works for better exhaust emissions.

In general, the mechanical advance figures you mention seem to be "in the ballpark" for Chrysler B/RB engines. It does not seem that a big lot of improvement in power can result with more than what you now have, so I would suspect everything is good. As long as the engine does not ping on slight part-throttle acceleration for the fuel being used.

The only mechanical + vacuum advance totals were some which I saw in an Exxon Tune-Up Manual (not available to the general public, but only to Exxon station owners, which I saw back in about 1975). It used 2500rpm for a quick check to see if everything was working well. So basically, it was the mechanical advance from the factory service manual plus the maximum vac advance figures quoted in the factory service manual. These figures were usually in the 50-54 degrees BTDC range. I know, that seems a bit too high considering that we are only usually seeing mechanical advance figures at WOT only, but for a part-throttle load at highway speeds on level roads, that higher figure is good.

Usually, the mechanical and vac advance systems are looked at separately for diagnosis and checking them. The combined figures in the Exxon manual allow for a quick check of both of them as they operate together AND a quick check of such.

As the air/fuel ratio is "thinner" at that cruise situation, the mixture is harder to get ignited, so more ignition "lead" is needed so the full combustion happens when it needs to happen at or near TDC. Whereas, at WOT, the mixture is "thicker"/richer and easier to get ignited.

An inoperable or disconnected vac advance will usually result in a slightly less-responsive to throttle engine and decreased fuel economy (close to 10mpg or thereabouts) on the highway. So it is needed. All things considered, "fuel economy" needs all the help it can get.

Always look for specs for the original vehicle the engine was installed in at the factory. Unless there have been some changes to it (related to the distributor and/or carburetor), especially if the distributor is not the original one, being replaced with a more-generic-spec rebuilt item. THEN, you get to be more "on your own", using what seems to work for good performance and not pinging/detonation sounds from the engine "under throttle". But what you quote for current specs seems good, to me.

Sorry for the length,
CBODY67

Hello

Thank you very much for the detailed answer. I have already downloaded the manual and also brought here. There is just the angle for the idle. I would like to have an ignition curve so that I can also check whether the mechanical adjustment at the speeds is correct. I would like to check whether the springs of the centrifugal adjustment are still in order and are not lame and the distributor too much adjusted. With the vacuum adjustment connected I think the adjustment goes too far since I am at 3000 ready at 58 °. That means I could drive the engine without problems without vacuum connection?

 

[halifaxhops]

Section 8-27 should have all the info you need at say 500, 800, 1200, and 2100 distributor deegrees which is half of crank degrees. Vacuum you need a guage to see what it is, sounds like it is going full advanced when connected at idle. Make sure it is on ported vacuum and there is 0 inhg at idle. Also is there still a number tag on the distributor? If not probably a rebuilt one and will just have a "general" curve in it.

 

[CBODY67]

The vac advance will vary with vac level and engine load. Mechanical varies with engine speed only. As I recall, it takes about 9" Hg for the vac advance to START and about 14" Hg for it to max out, in general. The only figure you really need to be concerned with is the "max advance" number, which is at about 4500 engine rpm (although some performance people want it maximized at 3000rpm, a bit sooner), plus the base timing amount, which will usually be about 36-40 degrees BTDC.

Some people can get all concerned about "the specs" of things, wanting everything correct down to the n-th degree, BUT as long as everything works well, with no problems starting when hot, or "clatter" on mild or WOT acceleration, for the fuel being run, things can be good. And with the provided information, it sounds like you are there. Changing things by a few degrees will not make a significant difference, but you might try that for experimentation purposes, just to see. But from the numbers, it sounds like things are pretty much where they need to be, to me.

Are the distributor and carburetor numbers correct for a 1977 440?

Absent any other issues, drive and enjoy the car!

Enjoy!
CBODY67

 

[SwissMonaco]

The vac advance will vary with vac level and engine load. Mechanical varies with engine speed only. As I recall, it takes about 9" Hg for the vac advance to START and about 14" Hg for it to max out, in general. The only figure you really need to be concerned with is the "max advance" number, which is at about 4500 engine rpm (although some performance people want it maximized at 3000rpm, a bit sooner), plus the base timing amount, which will usually be about 36-40 degrees BTDC.

Some people can get all concerned about "the specs" of things, wanting everything correct down to the n-th degree, BUT as long as everything works well, with no problems starting when hot, or "clatter" on mild or WOT acceleration, for the fuel being run, things can be good. And with the provided information, it sounds like you are there. Changing things by a few degrees will not make a significant difference, but you might try that for experimentation purposes, just to see. But from the numbers, it sounds like things are pretty much where they need to be, to me.

Are the distributor and carburetor numbers correct for a 1977 440?

Absent any other issues, drive and enjoy the car!

Enjoy!
CBODY67

Thanks for your reassuring words Just one more question then I will put the experiment aside. Can you tell me the values which you call okay?
Ignition timing idle without vacuum:
Ignition timing at 3000 without vacuum:
Ignition timing with vacuum at 3000:
Thank you

 

[78newport]

Good day to all. I am currently in the process of checking my ignition. To be precise, I am checking the adjustment mechanically and with vacuum. I can not find any values which are correct for this. It is a 440 cui from 1977, at idle (780rpm) I have the 12° before TDC. Without vacuum, the mechanical adjustment goes up to 35° at about 3000rpm. What values are correct with vacuum when I have this connected to the timed vacuum port on the carburetor? Or in other words do I even need the vacuum port? I bought the car without the vacuum port. The engine is completely original.

My 400 77 newport runs at about 15 tdc and ded reccomend the vaccum advance or it will loose power down the road just falls on the face

 

[CBODY67]

Thanks for your reassuring words Just one more question then I will put the experiment aside. Can you tell me the values which you call okay?
Ignition timing idle without vacuum:
Ignition timing at 3000 without vacuum:
Ignition timing with vacuum at 3000:
Thank you

For the base timing, that depends on how much mechanical advance is in the distributor, as it is the TOTAL mechanical advance + base timing which makes the 36-40 degrees BTDC that is good for Chrysler B/RB engines. But if you want numbers to look at, the distributor specs for a '66 Chrysler 383 2bbl seem to be pretty good to me, for a normal street-driving car. The base timing is 12.5 BTDC and I usually ran it at 15 degrees BTDC base timing. For a more emissions-era car, the 1970 383 4bbl in my '70 Dodge Monaco. Less initial timing for better lower-rpm emissions, yet still had enough mechanical advance for good WOT power. Base initial timing for that car is 5 degrees BTDC, IIRC. Completely stock, it would do 20mph to 100mph in .4 miles with no hesitation, and it would keep on climbing had it not been for a driveline harmonic which started at 105mph.

Some like the mechanical advance to be complete by 3000rpm, some 2500rpm, and some racers at 2000rpm. In some cases, it would take putting the engine on a dyno to see what was best.

KEY THING, to me, is that if you tromp it 1/2 throttle from idle, the engine responds quickly and eagerly, with no hesitation. And does not clatter while driving, plus getting good fuel economy on the highway cruises.

Enjoy!
CBODY67