Locking the D is for drag-race cars that just operate in pretty much two modes, namely idle and WOT, and normally will not operate under load in the rpm below say 3000/3500.
But for Street-cars;
The Mechanical system is strictly there for two reasons;
1) to reduce the ignition timing below that rpm (3000 to 3500), to make sure that the engine is never over-advanced in that lower-rpm zone,which costs both power and fuel. and
2) to start the fire at the correct time so that peak cylinder pressure will occur at the right crank-position, such that the maximum amount of that pressure can be delivered to the crank at the best time for it to be transmitted to the flywheel.
Too early and the engine fights itself as the peak pressure occurs before the crank is ready to receive it.
Too late and the potential for peak pressure is lowered, as the expanding gasses now chase after the falling piston.
This crank position window is very small, perhaps as small as 3 degrees. This is why we hunt so long and hard to find it. Well I guess some of us, lol.
The vacuum advance is a load-sensitive device, that is there to make up the difference between what the engine wants at Part Throttle, and what the Mechanical is able to deliver. If these systems are or can be, coordinated and synchronized correctly, for as many load and rpm settings as possible, this gives you the best potential for maximum torque and fuel-economy.
In some to many cases, the Idle-timing has to be retarded, either to make the engine less powerful at low-rpm, or so that an effective Power-timing curve can be built, or an all-in number can be achieved at the right time.
If you have a streeter with a hi-stall say over 3000/3500, you can get away with a pretty sloppy advance-tune, on account of the engine is like a race car now, with very little load ever going into the crank in those lower rpms.
If you have a streeter with a manual trans, and
you do not have a distributor machine, and
you do not have a way of changing the timing while driving, and/or
no access to a chassis-dyno, and
you really have a strong desire to clean up your low rpm ignition timing, then;
be prepared to spend many many hours changing the distributor guts, many of them at the side of the road.
And if your goal is Power with Fuel-Economy, the reward with close to perfect timing, will initially be substantial. Then you can spend the rest of your days chasing the last few degrees.
I'll tell you right off; if you are currently cruising at an rpm of about 2350,plus/minus 100, and you expect fuel economy to increase dramatically at less rpm, good luck with that. Your manual-trans car, depending on your cam and available cylinder pressure, will want a boatload of timing advance; Far more that the factory-type distributor can ever deliver. You will need a stand-alone timing computer, to even get close.
>Typically, your Mechanical curve will deliver .8degree per 100 rpm, and hopefully it does not start before 900rpm. This means that by 2250, your mechanical is likely to be ~10.5 degrees; rounds to 11.
> if you want your street-cammed SBM to run half-decent below 750 rpm, your Base Idle-Timing will be ~14 degrees, for a total of 11 +14=25 at 2250.
> If you correctly modify your Vcan, you can get 22 to 24 degrees out of it. I got 22*. Trailbeast got 24*. So say you get 22*, adding that to the other at cruise Rpm you get 25 +22=47
> I guarantee you that this is NOT enough already! Your SBM will want about 7 to 10 more degrees for best fuel-economy. The slower the cruise-rpm, the worse this becomes. So then without a timing computer, your only option is to increase the Idle-timing. Adding 8 to the 14 mentioned, you are now up to 47+8=55.. Which is close enough. However, this mucks up ALL the rest of your timing systems!..... so now you are back inside the D swapping out parts. And this is how it goes...........
> If these things are important to you, then I highly recommend to install a dash-mounted, dial-back, adjustable timing module, so you can adjust the timing electronically while driving. With it, I was able to achieve phenomenal fuel-economy even geared down to 75=1850/ 65=1600. My device has a range of 15 degrees.
If you get this right, your hi-compression engine/ manual-trans / hiway friendly geared car, will be a dream to drive, and you will want to drive it all the time.
I chose a 360 to have the low-rpm torque to run any hi-way gear I chose to. That, of course, reduces my hi-way rpm. If you have a smaller engine, this just means you will likely have to run more gear and cruise at a higher rpm. By the time you get to about 2800, the Total Cruise-timing can be up to say 54 degrees without the help of a timing computer, which should be close enough. Whether or not you want to cruise at 2800 is another story. Plus the biggest power-thieves to steady-state fuel-economy are; road-speed, vehicle-weight, and engine-rpm.
Inside the engine, the biggest thieves are valve spring pressure and ring-friction. In a performance street-engine, you can't do much about those, but you can reduce your engine rpm pretty easily.
