vacuum advance???

[RJRENTON]

Bill Monk,
Your premise is partially correct. Most or all of the new cars are computer controlled and measure manifold vacuum as MAP or manifold absolute pressure. This is an inverse function of vacuum...high MAP (760 mm mercury) equals low vacuum. In addition, RPM, inlet air temperature, throttle position, engine temperature, transmission gear selection, fuel density (flex fuel cars), and most importantly, AIR FLOW into the engine, measured in grams per second, which includes the air density factor, are all inputs to the computer which calculates both the degrees of spark advance AND the fuel flow on time to the injectors as a function of the pulse width modulated on time (open). The result of the combustion event, the oxygen sensors keep the fuel calculation at the ideal 14:1 ratio. The newest technology is gasoline direct injection where the fuel is directly injected into the combustion chamber, slightly ahead of the spark occurance . There are knock sensors listening for pinging that retards the spark slightly. Is this better than just the distributor advance system of old....YES absolutely.
Resulting in lower emissions, better performance, better fuel economy, and drivability.
Bob Renton

 

[Ranger16]

This paragraph pretty much answers the OP's initial question and is a better explanation of what I said in post #16. It was taken from this much more lengthy, but excellent explanation of timing & advance.
http://chevellestuff.net/tech/articles/vacuum/port_or_manifold.htm

"Now, to the widely-misunderstood manifold-vs.-ported vacuum aberration. After 30-40 years of controlling vacuum advance with full manifold vacuum, along came emissions requirements, years before catalytic converter technology had been developed, and all manner of crude band-aid systems were developed to try and reduce hydrocarbons and oxides of nitrogen in the exhaust stream. One of these band-aids was "ported spark", which moved the vacuum pickup orifice in the carburetor venturi from below the throttle plate (where it was exposed to full manifold vacuum at idle) to above the throttle plate, where it saw no manifold vacuum at all at idle. This meant the vacuum advance was inoperative at idle (retarding spark timing from its optimum value), and these applications also had VERY low initial static timing (usually 4 degrees or less, and some actually were set at 2 degrees AFTER TDC). This was done in order to increase exhaust gas temperature (due to "lighting the fire late") to improve the effectiveness of the "afterburning" of hydrocarbons by the air injected into the exhaust manifolds by the A.I.R. system; as a result, these engines ran like crap, and an enormous amount of wasted heat energy was transferred through the exhaust port walls into the coolant, causing them to run hot at idle - cylinder pressure fell off, engine temperatures went up, combustion efficiency went down the drain, and fuel economy went down with it."

 

[Bill Monk]

Bill Monk,
Your premise is partially correct. Most or all of the new cars are computer controlled and measure manifold vacuum as MAP or manifold absolute pressure. This is an inverse function of vacuum...high MAP (760 mm mercury) equals low vacuum. In addition, RPM, inlet air temperature, throttle position, engine temperature, transmission gear selection, fuel density (flex fuel cars), and most importantly, AIR FLOW into the engine, measured in grams per second, which includes the air density factor, are all inputs to the computer which calculates both the degrees of spark advance AND the fuel flow on time to the injectors as a function of the pulse width modulated on time (open). The result of the combustion event, the oxygen sensors keep the fuel calculation at the ideal 14:1 ratio. The newest technology is gasoline direct injection where the fuel is directly injected into the combustion chamber, slightly ahead of the spark occurance . There are knock sensors listening for pinging that retards the spark slightly. Is this better than just the distributor advance system of old....YES absolutely.
Resulting in lower emissions, better performance, better fuel economy, and drivability.
Bob Renton

I appreciate the detail Bob, there is no doubt modern technology is far superior. With that said, I don't see my wifes 2015 Hyundai being something people will want to own in 48 years but who knows. I enjoy the convenience and efficiency of the modern car but of coarse the roadrunner isn't a daily driver and I want to retain the heritage of the car and still run as efficiently as possible. I guess I want it all, huh? This has been a good thread, I've learned a lot! Thanks to everybody for the input

 

[Bill Monk]

This paragraph pretty much answers the OP's initial question and is a better explanation of what I said in post #16. It was taken from this much more lengthy, but excellent explanation of timing & advance.
http://chevellestuff.net/tech/articles/vacuum/port_or_manifold.htm

"Now, to the widely-misunderstood manifold-vs.-ported vacuum aberration. After 30-40 years of controlling vacuum advance with full manifold vacuum, along came emissions requirements, years before catalytic converter technology had been developed, and all manner of crude band-aid systems were developed to try and reduce hydrocarbons and oxides of nitrogen in the exhaust stream. One of these band-aids was "ported spark", which moved the vacuum pickup orifice in the carburetor venturi from below the throttle plate (where it was exposed to full manifold vacuum at idle) to above the throttle plate, where it saw no manifold vacuum at all at idle. This meant the vacuum advance was inoperative at idle (retarding spark timing from its optimum value), and these applications also had VERY low initial static timing (usually 4 degrees or less, and some actually were set at 2 degrees AFTER TDC). This was done in order to increase exhaust gas temperature (due to "lighting the fire late") to improve the effectiveness of the "afterburning" of hydrocarbons by the air injected into the exhaust manifolds by the A.I.R. system; as a result, these engines ran like crap, and an enormous amount of wasted heat energy was transferred through the exhaust port walls into the coolant, causing them to run hot at idle - cylinder pressure fell off, engine temperatures went up, combustion efficiency went down the drain, and fuel economy went down with it."

Good stuff! I stand corrected and educated, thanks

 

[747mopar]

Bill Monk,
Your premise is partially correct. Most or all of the new cars are computer controlled and measure manifold vacuum as MAP or manifold absolute pressure. This is an inverse function of vacuum...high MAP (760 mm mercury) equals low vacuum. In addition, RPM, inlet air temperature, throttle position, engine temperature, transmission gear selection, fuel density (flex fuel cars), and most importantly, AIR FLOW into the engine, measured in grams per second, which includes the air density factor, are all inputs to the computer which calculates both the degrees of spark advance AND the fuel flow on time to the injectors as a function of the pulse width modulated on time (open). The result of the combustion event, the oxygen sensors keep the fuel calculation at the ideal 14:1 ratio. The newest technology is gasoline direct injection where the fuel is directly injected into the combustion chamber, slightly ahead of the spark occurance . There are knock sensors listening for pinging that retards the spark slightly. Is this better than just the distributor advance system of old....YES absolutely.
Resulting in lower emissions, better performance, better fuel economy, and drivability.
Bob Renton

There's no doubt which is better but part of the fun in these old cars is the tinkering and lack of sophisticated electronics.

 

[Bill Monk]

There's no doubt which is better but part of the fun in these old cars is the tinkering and lack of sophisticated electronics.

I appreciate you man!

 

[RJRENTON]

With regard to the Chevelle stuff link, and looking back at those simpler times, If manifold vacuum source is/was best, WHY did other GM divisions not subscribe to that premise? For example, Buick used ported advance, so did Cadillac but Chevrolet, Pontiac Oldsmobile used manifold vacuum. If manifold vacuum source is best (according to the link) why did Mopar use ported vacuum source? Ford used ported vacuum source on most of their engines except those that used both manifold vacuum retard as well as ported vacuum advance source on the ssme distributor (dual diaphragm) and Mopar using Electric retard AND ported advance distributors. Some of these functions were temperature controlled some were speed controlled, tutning off or on the devices to yield desired results. As far as the old AIR (air injection reactor) injection systems operate, we were told that they resulted in burning any unburned hydrocarbons by injection air in the exhaust manifold.....what that REALLY did was DILUTE the exhaust stream so the down stream sampling (EPA) would show acceptable results. With regard to the comment of NOx formation, NOx is formed in the combustion chamber at or sbove 1600 degrees F. To control the temperature, exhaust gas was introduced into the fuel charge, via the EGR system (exhaust gas recirculation) to reduce the combustion chamber TEMPERATURE to below 1600 degrees F, to prevent its formation. In today's engines, this done using variable valve timing at different operating conditions (RPM, load, vehicle speeds). In the old days, too much EGR resulted in poor performance, fuel economy...now you don't know its occurring. In the old days, you just disconnected the EGR valve....which way is better... ported vacuum advance or manifold vacuum source ....its a case of Mom loves me best...... for my 70 Plymouth GTX, i don't use EITHER....i just recurved the distributor advance rate to the yield the results that I want.
Bob Renton