Full advance

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.

Are you idling below 400 rpm? Just guessing on that rpm because when I check at 600 rpm it’s at 37 degrees and steady. Maybe we talking different springs?

Must be talking different springs. Most of the light springs I've messed with are full in around 1200rpm. So when you set the timing at say 23* at 900 idling in neutral then drop it in gear it takes timing away making it want to stall, at the timing is all over the place.I guess if your idling at or near 1200 in this case it would be a wash. So you are saying with light springs installed you have no curve? From 600 all the way up?

My curve starts at 1700 @ 30*, and 3500 @ 40*total. I have a 400 with stock stroke. I’m afraid too much too fast @idle puts things in a bind . The idle and first gear idle seems better!

all the light race springs i've ever used will bring the full centrifugal in at 1100-1200rpm using a 9 degree plate. so if you set the total centrifugal at 36 degrees there will be 36 degrees at 1100-1200rpm. naturally below that rpm advance will reduce. there is a point were the springs can be too light and not let the weights fully retard when the engine is shut off but that would only happen if someone tries to use only one of the race springs. i've found erratic timing to be other issues than springs.

Spring was made for the Chevy dist, looked like the size of hair, almost to light to rotate it back to zero. Also no grease on those pivots or slots (center shaft too) dry lube only. We used one of those Sun machines to set it up but lost track of the dude, don’t know where him or machine moved to.

I do the same with my 511. Initial at 29* and all in 36*@2500rpm. I launch the car at 2500rpm foot brake. Runs very clean and reliable like this. I can let the car sit for hours and just hit the start button and it will fire and idle by itself. This is with a Indy T ram with 2 of Dom's 650's and did the same with a single 1100 Dominator. No hiccups bogs or stumbles. Works very nice like this.

After sitting for hours, I can see it being able to start at 29 degrees. But will it start hot?
My 451 starts to buck at 26 hot.

The current setup I'm running to avoid the hot start problem is one light spring, one heavy spring with loop, and a 22/11 degree plate. This allows me to crank at 16 degrees, the light spring is all in at 27 degrees (the loop on the heavy spring holds it from going higher) as soon as it starts and idles at 1100-1200. The advance starts coming in around 1500 or so. I've measured 35 degrees at 4000 but I won't stand next to it and rev it any higher! The plate will limit the advance at any speed to 38, less the inevitable retard in the ignition box, so that should be about 35-36 at high rpm.

Plus vacuum advance that does nothing at idle (8" Hg.) but is fully engaged at cruise (15" Hg.) Don't remember how many degrees. Haven't heard it ping yet on pump premium, 10.3:1, iron heads for now. 272@.050, .652 gross lift, .018 lash.

After reading this thread, I'm tempted to just put two light springs in so it's all-in at idle, and try it out...

I think that the engine has a hard time with 38* anytime, except wide open. It just makes sense to take away timing when it isn’t wide open.

But - at partial throttle and low load, the lean, less-dense mixture requires MORE time to burn and reach maximum pressure at 15(?) ATDC. Sometimes much more time. That's why vacuum advance is so useful on an engine that doesn't run only at wide-open... light throttle cruise well into the 50's advance is not unusual.

The reason for having variable centrifugal advance is that at low rpm there is already more time for the mixture to burn so less total advance is needed. As the rpm climbs, so does the need for advance - up to a point... that point is usually 34-36 degrees for most big-blocks, maybe a few degrees more for small-blocks. Depends on the combustion chamber design, of course. You're right that 38 may be too much at low rpm and wide-open throttle. Except that my cam has a lot of overlap so it needs more low-speed advance...

My engine won't see 38 degrees. Every ignition box has some retard, because a finite amount of time is required to read the trigger signal and fire the coil. At higher RPM the crank turns more degrees in that same time interval. Somewhere I saw (maybe on FABO) a test of several boxes and many were retarding 1 degree per 1000 rpm, or even more

Thanks for the reply’s! I might try some your timing curves.

What ever works is best, we all have preferences. All my race stuff is locked out Even my 14 to 1 good engine, a little tough on the start. but it works.

Yes, will start and idle hot or warm. cold, 1 pump of the throttle and fires up and idles without being too rich once warmed up. Idles at 13.5-14:1 AFR once up to temp. I think alot has to do with the Dragon fuel i use. Good stuff. I think i used 1 blue and 1 silver spring and a large bushing from Don at FBO.

All ignition boxes retard with RPM. If you lock out a distributor you haven’t a clue where the timing is once the box starts retarding. And it WILL retard.

i think the only way to properly set up a locked ignition is with a start retard box. at least with a start retard box there's less of a chance of breaking the snout off a starter.

Yes, they will and i am using the Ice Ignition box which has 10* start retard.

I know and work on engines that acquaintances race and all are locked ( usually crank trigger). Some are mid 600 cube at close to 16-1 and no-one uses start retards. I have never heard one of these engines kick back. Heck they wing around with ease, but they have good well planned starting circuits/parts (even 16v on some). Go to a racetrack and ask. I think the most common answer is no on retards. I personally have never seen a broken starter nose from a hard to turn over engine. If your car is kicking back turn off your start retard has always been the consensus around here. Again I am not saying, one that works for someone else is no good. I'm just giving what my experience has been. The only reason I responded to this is because , I got the impression some are saying locked out timing, is a intolerable condition (broken parts/ won't work).

Crank trigger is the way to go. I ran one on my 408 Indy headed small block and never had a kick back and no loss of timing. Rock steady timing. I do not have a crank trigger on the 511 BB, just didn't want the expense, but i will say again i think it is the way to go. What i noticed with my current set up, Ice Ignition box and billet MSD dist. is when i try to run it locked out, the timing will retard as i increase the rpm of the engine. I was losing 3 or 4* by 3-4000rpm and had problems with kick back hot locked at 35*. The retard may have had something to do with that as there was a post i read about that awhile back. When i run a curve as i do now 29*/36* it's almost like having it locked out as far as performance that i have seen. No timing loss with rpm and no kick back at all. Thats why i use a curve for now.

Big cams always idle better w/ full timing in my experience. The only thing I wouldn't lock out is a near stock build.

Like some others have said, we don't use a start retard on anything.

Based on this thread I bought the Mr. Gasket 925B spring kit and put it in my .590 cammed 440. The distributor had one heavier spring in it already but changing it out was a noticeable improvement in idle quality and off idle response, definitely snappier/crisper now and the idle speed picked up by a couple hundred rpm with no other changes. My drive idle is just below full advance so it still might lead to some hunting but I'll live with it.