Which way? Advance or retard? For better mpg . . . which way to go? How many degrees? Have modern ignition systems negated this by advancing/retarding on their own? Rocketman
I’ve been wrong before, but I don’t believe the timing will affect the MPG. This is done mostly by the fuel control (carb or fuel injection unit), right?
Adjusting the timing will affect the power output.
And yes, modern ignition has negated your ability to change timing. It has on my cars anyway.
Modern ignition systems have made this obsolete.
In the old days if ane engine was lugging or pinging timing could be adjusted to correct it. As a matter of fact, as points wore down from normal zapping ignition timing would change and it needed to be corrected with every tuneup. But those days are gone.
“Have modern ignition systems negated this by advancing/retarding on their own?”
Yes, since the '20s I think. Before computers the distributor had at least a mechanism to advance the spark with increasing RPMs.
Most also have one or two vacuum advance mechanisms.
From experience it seemed some of the cars from ~'73 to '85 would run better with the base timing advanced by 2-4 degrees over spec.
Emission systems weren’t so sophisticated and retarding spark was one way to reduce hydrocarbon emissions.
At any given engine speed, throttle position and other conditions there is a value of spark advance that gives the highest torque and power.
Efficiency generally peaks at this value too.
Engine knock, emissions or other constraints will call for the timing to be retarded from this peak value.
The modern computer controlled spark systems do a much better job of keeping the timing near optimum than the mechanical distributors.
Henry Ford had the right idea on his Model T. There was a control on the steering column to advance and retard the timing. As you got underway, you could advance the timing for better mileage and performance. If you created your own blend of premium fuel by adding moth balls to the gas tank, you could advance the spark even farther to take advantage of the higher octane the mothballs (alledgedly) provided.
Of course, you had to be certain to retard the spark before attempting to start the engine with the hand crank or the crank might kick back and break your arm. Of course, real men who bought the Model T didn’t opt for the electric starter–electric starters were for wimps.
If one was really attuned to the engine of the Model T, one knew instinctively how to set the spark timing for best mileage and performance. We didn’t need any stinking centrifugal, vacuum or computerized timing advance mechanisms.
For better mpg the ignition must be timed to just short of detonation but you can’t touch it now unless you could reprogram the ignition timing map stored in the ECM. The knock sensor or two of them are mounted on the engine. They listen for detonation and send an appropriate electrical signal to the ECM to optimize spark timing consistent with the map including emission constraints.
Before computer controlled engine spark timing, a distributor rotor shaft had two weights equipped with return springs that would move inward or outward according to the shaft rotation speed, retarding or advancing spark timing as engine RPM fell or rose. If you stepped hard on the throttle, a vacuum controlled diaphragm would actuate that would temporarily retard the spark timing to prevent detonation due to temporarily higher engine compression until the low vacuum condition caused by the larger throttle opening subsided. The diaphragm retard was speed sensitive in an indirect way which was ok as WOT at a higher speed would result in less vacuum than at low speed but a higher speed would permit more advance anyhow.
Then, spark timing was likely not as well controlled under all conditions as now but then you could attempt a little more advance if the engine did not detonate. Spark timing was set at engine idle with a timing light triggered by one of the spark plug wires to flash at an arrow pointed at marks on the harmonic balancer on the front of the crank and rarely on the flywheel viewed through a hole in the bell housing. More could be done to make more spark advance such as weaker advance weight return springs or increased advance weight movement effect on distributor timing done by modifying the shapes of the advance weights. The spark timing could be more advanced if you consistently used premium fuel.
In the very early days there was no distributor centrifugal weight or vacuum control on ignition timing until engineers decided that they could spend a little money to get more power from an engine with better control of ignition timing.
If you don’t have a laptop, you’ll need to buy one, download/buy a program that’ll allow you to mess with the ECU, then buy an OBD2 cable to hook into the diagnostic port, map all the various fuel system ranges, fine tune the map for best economy(the hardest part).
I’m sure that’s not all you’d have to do, but you can tell it’d be an involved project to say the least.
On a distributor equipped engine you can usually get away with a couple of degrees extra advance and this would improve performance and fuel economy a small amount.
With older cars in the days of leaded gasoline you could get away with more than a few degrees extra. Unleaded gas vehicles are touchier so a couple of degrees is huge, especially if the EGR system is inoperative.
An older Harley I used to own (75 Shovelhead) was designed to run on leaded gasoline. After that was phased out this led to a motorcycle that had a certain amount of clatter no matter what. Eventually I got this cured by removing both cylinder heads, milling some fin out between the pushrod tubes, and drilling/tapping an extra hole for a second spark plug followed by the addition of a second coil.
This meant the flame fronts of the explosion in the cylinder now started on each side of the bore, met in the middle, and it totally eliminated all pinging no matter how hot the temperatures or how hard the throttle was hit. As a bonus, spark plugs lasted much longer, ran far cleaner, and fuel economy went up by almost 5 MPG.
My 2 antique Harleys ('44 and '50 year models) not only have the twist grip throttle on the right but also the stock twist grip on the left which operates the distributor by cable.
Roll it forward to retard for kick starting and twist it back to bring on the advance.
(It’s a hoot at night to back off the throttle and roll the left side grip forward. It’s the 2 wheel equivalent of the Air Force F-111 torching maneuver.) 
Thanks guys! It’s just too much time on my hands I guess. My '89 Accord has a pretty simple computer and still has a distributor. I drive pretty much the same routes with this car right now . . . about 75% highway - 25% town and get 30 mpg, which is pretty good. Just tuned-up, stick shift, about 2 pounds over tire pressure, front air dam, careful not to put my foot in it too much. I think I’ll try 2 degrees or so a week until I hear it ping. Thanks again! Rocketman
I ran my '88 fuel injected Accord 2 degrees advanced from stock.
That made it a little peppier without ping.
You could go a little further if you used higher than 87 octane.