What is the purpose of locking out the distributor and advancing the distributor to full advance at idle?

Performance I guess. Typically on a decent build you'd want all timing in around 2400 and set. I usually start at 36*. No vac-advance either.

For us it's idle quality and off idle response (footbraking). Just under .750" Roller T/R with2 1150 Dominators. Flash stall is around 5500. We also run a grid and crank trigger, so we can put any curve in we want. At the moment we are trying hi speed timing retards but we haven't had any really useful data yet.

Most common reasons mentioned:
1. one less thing to go wrong
2. eliminate a variable
3. useful RPM is above the end of the advance curve
4. don't understand what it does

5. don't know how to set up a centrifugal curve,....lol!

What would be a good advance curve?

Most common reasons mentioned:
1. one less thing to go wrong
2. eliminate a variable
3. useful RPM is above the end of the advance curve
4. don't understand what it does
16-18 initial. 35-6 all in by 2400 each engine will have its limits and needs with timing . if its racing, it never sees the low rpm's so no curve is needed. when locked out, it is one less issue to have to deal with and, it helps keep timing rock steady....

It is cheap and easy and it kinda works okay. It takes a bunch more money to build a sophisticated timing curve and the car probably won't go any faster. It will start easier and the plugs will stay cleaner with a fully developed timing curve but going down the track performance won't change much.

Lots of cam overlap (duration) can idle cleaner with more timing.
From experience with footbraking, it helps to get full advance several hundred RPM below the staging (footbrake) RPM. If it is too close, it can be inconsistent off the line.
I have seen a big difference in the response and sixty foot if the curve isn't all in. Say footbraking at 2000 rpm and the curve is all in at 2500.

I've used the Mr. Gasket 95B spring kits in both BB and SB distributors and welded up the advance slots so it would idle between 14 to 18 BTDC at idle, under1200 RPM and be all in by 1600 to 1800 RPM with 34 to 36 BTDC max advance, works good, last a long time on both street and street and strip motors, pump gas or race gas

Usually with a big cam you have to make the thing idle so fast so it will run, that you are well up into the curve anyway. All in at 1500? When nothing happens below 3500? Or a curve that's 12 degrees from base to all in? Why bother?

Big roller, timing locked, idles in gear at 750. Nice and smooth. With the throttle plates set where they should be. Clean. Nice, sharp response.

For me it was better response, better ETs, better idle and mine are all in (not locked) by 500 rpm. Why do you need a curve when engine is on the step at 3800 then 5000 to 7000 between shifts? Street car is a 180 degree different approach.

Yep, way back when I was even less experienced, I can remember chasing an off the line hesitation with squirters and cams at a test and tune day. An old timer came over and after a short discussion told me to put some lighter springs in the distributer. That cured it.

Track only experiences may be different, but on the street, our big blocks respond dramatically to rapid early timing advance. In my experience, as soon as a cam has enough overlap to be heard at idle, the engine will want more initial. And total as quick as it will take it. I always put as much initial as the engine will take without hitting the starter at hot start. Or maybe even hitting the starter some. It really improves the idle quality. Off idle response, too. I've had good results with 20* to 28*initial with pump gas.

Then, shorten the slots and install springs to get the advance in as soon as the engine will take it. On mine, it's usually all in by 2,000. Sometimes, 1,500. Although it would be much easier, I have not had an engine that would start with the distributor locked. Starting a street engine hot with 32* +/- initial timing is not going to happen. Well, not for my combos. Again, track experiences may be different.

I should have added...yes get the spring kit at least.

I used one light spring to make it close on start up, then bang it’s wide open. Didn’t need a spring with points because friction on the rubbing block would pull it back, electronic needs the spring. Never bothered with the slots, just the stock dist in every race car.

Trying to set idle with light spring is tough. In my experience they will move your idle timing around, causing erratic idle rpm.

Would your(street) recommendations change when running vacuum advance? and a four-speed?
My cam likes to idle around 1100-1200. thanks.

Haven't had the opportunity to mess with as many 4 speed cars as automatic cars. The reasons behind the recommendations on the initial timing would be the same but then obviously bringing the timing in that soon could reap less benefits with the stick.

As for chasing the timing at idle, with the initial as high as the engine likes, the additional advance that comes in and out with RPM changes has less impact. The farther the initial is from what the engine wants, then the more the engine responds when it changes. The closer the initial timing is, the less impact timing changes have.

As an example, I have about 28* initial and all in to 34* around 1,600 on my current combo. In and out of gear means a change of a couple of degrees in advance. Since the engine is already pretty happy with the timing, that additional timing doesn't have a big impact. Some of course, but not enough to hardly notice. It does hit the starter when hot. But not too bad.

Vacuum advance is a different thing in and of itself. So, in my humble opinion, it has no impact on any of this. Set each of these timing events to the way that the engine likes it. On my cars, the timing is all in before 2,000 and I usually cruise above 2,000, so each system can be set to it's own preference. Tall rear gears would be the only curve ball here and that would be on how early the total could be in when in the higher gears.