Ronny

iTrader: (1)

seems a mechanical distributo would idle 6-10 deg A. yot in the tables i see in our cars are 20+ ded A at idle.

why are we running more at idle and off idle at tip in?

one more not related.

why does a mechanical distributor have a centrifugal cam /weight system in addition to vac assist? does the vac assist provide more A that the weights cannot provide?

some where along the way i thought some vac assist actually retarded timing at WOT or 100 MAP(min vacuum) ? in that it would prevent detonation?

kdrolt

You need an rpm-dependent way to allow timing to change, and a load-dependent way for timing to change.



Yes.

Mech advance, via springs and masses, is completely rpm-dependent. You need more advance, typically, at higher speed because combustion occurs slowly, so at higher rpms you have less time to get the job done. So you have to start burning earlier to get the optimum work from each combustion event.

Mech advance "knows" zero about how loaded the engine is (i.e. are we at part throttle, or WOT?)

Vac advance is a way to measure load so it makes up for what mech advance can't do. More vacuum means the throttle is more closed, so there is less load on the engine and hence less need for power. Less vacuum (higher MAP) means more power is needed.

In addition to these two, you also have the static timing set by fixing the distributor angular position w.r.t. #1 cylinder. HTH.

Ronny

iTrader: (1)

please comment as to why most timing tables for our TBI cars and TPI as well idle 20+A vs mechanical at 6-10A ?

ps i had my car at 10 at idle and off idle for a while trying to cure bucking. turned out was lean or car liked richer A/F ratios so returned to 20A for idle. what is the reason to run more adv at idle in our cars ?

Fast355

iTrader: (2)

Well lets see, my stock distributer from my van was set at 8* BTDC. It had a 20* vacuum advance canister connected to manifold vacuum. It then had 18* of centrifical advance starting at about 1,800 RPM and ending at about 4,400. That is about 28* at idle, 26* @ WOT and 4,400 RPM. At a typical cruise speed of 2,000 RPM the timing varied between 10-30* depending on engine vacuum.

Fast355

iTrader: (2)

Well I tried to add this but it wouldn't let me, trying again.

kdrolt

Are you sure the mechanical has only 6^o to 10^o of advance? IIRC there are small vacuum advance "cans" using an intake manifold ported vacuum line that cause the timing plate inside the distrib to rotate with a change in the vacuum. So isn't there both mech and vacuum portions that provide advance even at idle on a std distributor? And that btw would be on an engine with NO ECM whatsoever.

On an engine with an ECM, like as used on a CCC (or especially on TBI and TPI), things might be different as to how ignition advance is applied, because it would depend on what sensors are available and what could be stored in a lookup table in the EPROM (assuming there is one).

On a TBI and TPI, you have advance curves preprogrammed for change with rpm, as well as an adjustment based on MAP (which is an inverse way to measure vacuum).

The only real difference I can think of between two identical engines, one with EFI and an ECM, and the other that is controlled in an old-school manner (using centrifugal mech advance and vacuum advance) is that the ECM version can monitor and control (enforce) a desired engine rpm even when the AC is turned on. The mech system doesn't have any feedback means to do this. So if you loaded up the old-school engine at idle it might stall.

Really old engines had a timing adjustment that could be made from inside the car. It was a mechanical linkage on the dash that allowed different timing for starting, and then you moved it when you wanted to actually go somewhere (loading the engine and running above idle).

Because there is probably a crude feedback system that has a targeted (desired) engine rpm for idle (for example) that the mech old-school engine lacks. The ECM controlled engine can handle the advance at idle but a totally mech one cannot (you usually use less advance when loading the engine down). A mech design that has both rpm and vacuum can do a better job of it, but what's missing is that it has no provision (that I can think of) to sense that the idle rpm drops when the AC gets turned on.... unless the driver presses the gas down (which also reduces engine vacuum). I dunno if this helps. I haven't worked on an old-school engine in years.

Ronny

iTrader: (1)

thanks for the answers. let me see if i have this clear.

mechanical dist base timing is set say 8 deg A. at idle the vac canister(adjustible) provides more timing moving it to say 20 deg? the centifical system provides little or no timing input at idle. as rpms increase say 150-2000 rpms the vac canister provides less timing(less vac) and the centrifical adds timing. offset? at wot the vac is nil(100 map) and the certrifical advances as the rpms grow. eventally the centrifical is maxed out. please correct me or add to this.

now at 6000 rpms would you not want some retard? how would that dist handle that?

this sounds like engine therory 101.

kdrolt

I can only confirm if you understood what I think I wrote..... it doesn't mean, necessarily that *I* got it right myself.



Check.



Mostly correct. You could be driving on the highway at steady part-throttle cruise. So you might still have a lot of engine vacuum.... which means there really isn't much load on the engine. So the centrifugal springs/wgts will add advance in a linear fashion between two rpm set points, the set points being determined by the masses, the springs, and how far away from the dist shaft they are positioned.



Correct.



Do you mean that, if redline was 5500, that you would intentionally want some retardation on the ignition to prevent over-revving past 5500 rpm? If so then yes.

How was it implemented, if at all ? I can think of a secondary set of springs and masses that would rotate the timing plate in the opposite direction of advance at a high enough rpm but I don't know if any mfg ever implemented a caveman rev-limiter like that.

Yes, but circa 1962.... or 1955. Actually I think all of this was worked out by Charles Kettering in the 1920s or 30s.

Ronny

iTrader: (1)

no. i mean that engine pulls to 6000 where max HP resides. and can i assume it is 100 MAP? say in 3rd gear of 4 and a load on engine as mucho accellerating. i believe our SA tables take out some timing ,in fact quite a bit, at the 80-100 MAP areas. i assumed maybe falsely that was to reduce possibility of detonation? also i dont think engine needs 36 d A as load is lessening at higher rpm shift point(ran outta gear).

somewhere i read a mechanical dist with VAC can remove timing as well? maybe i am wrong on that. my buddy removed the vac can by sealing it off? that leaves only mechanical? not sure why he would do that.

kdrolt

Ahh. Well MAP is peaked so the vacuum-control is of no use. So it would have to be a secondary set of springs/wgts that back off some of the advance.



The mech aspect could, yes, for example in the manner I described. I doubt the vac could do much for you though. The vac will peg at 1 atm when you go WOT, no matter what the rpm. So only engine speed can help you to alter the timing.

If it's a race car, then you can assume he's always going WOT so he always has max loading on the engine. That would be a reason to retime the distrib using only centrifugal advance.... because there is never any load-dependence. It's either idle (when you start the engine to warm it up) or it's increase rpm to 2500 (at the tree) and then WOT to 6500 rpm.

It wouldn't be a good idea to do this on a street driven car, unless you have a really lumpy cam and the car is more suited for the track than street use.

kevm14

What is that curve for?

This was my stock one:



Between the adders (cool comp, PE and the slope), I'm looking at about 20° at 4800rpm. Do you think that's right? What a funny looking spark curve, too. The one in my Camaro actually looks like you'd expect. Smooth and gradual everywhere.

chesterfield

lars grimsrud is a legend amongst old school corvette enthusiasts.

here's a link to his hei paper.

http://members.shaw.ca/crussel/hei/HEIPrimer2.html

as i've learned from this forum, some of this doesn't apply to SP fast burn heads.

Fast355

iTrader: (2)

My big roller cammed, 187 swirl port headed 312.