Maximum total advance (with vac) DSII

[Anonymous]

Hey guys!

I just got a new Craftsman dial-back timing light that does a much better job reading at high RPM's.

I'm know that most people here say to get about 12 - 15 deg advance with the car at low ldle (about 650 RPMs)

My question is -- what is the MAXIMUM advance I want to be at with my vac advance connected. I haven't checked it yet, but I'd like to know about what it should be.

Thanks!

 

[Anonymous]

You'll want to run somewhere between 34-38 degrees total depending on your car

Later,

Doug

 

[CZLN6]

Howdy CC:

Do you nave a good vacuum guage too? If no, put that on your Christmas wish list. Tune you low speed air screw for max vacuum at an idle. As idle speed increases reset the curb idle.

Vacuum cannisters are designed for a specific application from the factory. Cannisters on engines with an Auto trans typically have more than stick engines.

Total advance is a combination of initial, vacuum and centrifugal. Initial is pre-set. Vacuum advance is determined by the engine vacuum and the mechanics built into the cannister- high vacuum= max advance. An engine can use more advance at low load/high vacuum conditions. Centrifugal advance is preset with springs and weights and built into the mechanics in the bottom of your distributor. Low rpm=no advance, higher rpm= more advance, until max is reached at about 3,600 plus rpms. An engine needs more advance as rpms increase.

A typical factory combination might be; initial- 8, vacuum 11 to 16 degrees, centrifugal- 10 - 14 degrees. If you total these up you will get about 30 to 38 degrees. BUT, in actual engine conditions, it just doesn't work like that. For instance, at an idle you will always have the amount of initial, max vacuum and no centrifugal. but at WOT acceleration, you have should have no vacuum and a certain amount of centrifugal advance depending on rpm. At 65 mph, an amount of vacuum and an amount of centrifugal depending on the vacuum signal and rpms. In certain conditions it is possible to achieve max vacuum and centrifugal advance at the same time, but in actuality, it's seldom and unlikely.

Now to your question; Doug covered total advance. To set the initial advance, start by disconnecting the vacuum advance tube and plug it with something like a golf tee. Now point your light at the timing marks. Reset by loosening the set bolt and rotating the distributor body to achieve your desired setting. You're numbers of 12-14 degrees of initial is a good ball park to be in. Each engine will have it's own "best" setting. In this case, best means- most without knock. Whatever your initial setting is, write it down. and dial the timing light back to zero.

Now reconnect the vacuum tube to the distributor to determine the amount of vacuum advance. Typically, the mark will dissappear over the top of the damper. Dial the mark back to zero and read the timing light. Subtract the initial setting amount and the difference will be the amount of vacuum advance your system has.

The only thing left to check is centrifugal advance. This requires a tach, a friend with a steady foot and a graph. Let me know when you're ready for this one.

I do go on. I hope this is what you wanted.

Adios, David

 

[Anonymous]

OK let's ty these numbers on for size and let me know what you guys think. It looks like many readings are in the range David outlined above -- but my total is much higher than 38 deg?

Advance at idle 550 rpms (vac disconnected) = 15 deg.
Maximum initial + centrifigal (vac disconnected) = 39 deg
Max initial + centrifigal + vac connected = 57 deg

The car doesn't like to rev that high. I was unable to check the RPM readings for these numbers, but my guess is that these readings (particularly the max with vac connected) are at about 2000 - 2200 rpms - right in freeway cruising speed.

Any suggestions on these numbers? Are they about right? Anything to be gained by adjusting up or down?

For instance -- would there be any benfit to bringing my total mechanical advance down with, say, ligther springs so I hit my max earlier?

Thanks!

 

[CZLN6]

Howdy Back Cc:

My, You have been a busy boy.

It would be a good idea to go for a drive with the vacuum guage hooked up, to ascertain the vacuum reading at cruise speed, or 2,200 rpms. Then add the advance at that vacuum pull to the centrifugal advance at that engine speed, and the initial setting to know your advance at that speed. At that speed you will likely not have the same vacuum reading you got at an idle and, consequently, not the same advance.

Your engine can likely handle 39 degrees under WOT, given a correct A/F mixture, octane rating, ambient temp and engine temp. What did your rpm curve look like? At WOT you will have very low, if any vacuum, so consequently, little or no vacuum advance.

You would likely improve performance if you were to reprogram your centrifugal advance curve. Say, all in by 2,800/3,000 rpm, depending on trans type and cam. Recurving the centrifugal advance on a DSII is not easy, as the weights and springs are under the stator plate in the very bottom of the distributor. It is best done on an old fashion Sun Distributor machine. Those machines are few and far between any more.

Are you getting any rattle with these setting? What do your plugs say? What plug are you using? What rpm is your max centrifugal advance occurring?

You might improve your low speed power with more initial, but you would likely need to find a way to limit vacuum advance.

Fun, ain't it?

Adios, David

 

[Anonymous]

David

As always -- you're a wealth of information.

At this point, I must say, I think I'm in over my head.

I'm pretty sure I understand what you're saying, but I don't know how much more fiddling is worth it at this point if you seem to think I'm in a good range. I'm running a standart set of autolite plugs gapped to .040. I haven't checked them lately but at the last replacement they looked good and tan. Right now I'm getting no ping or detonation, but interestingly I'm not really sure what that sounds like. The damn thing is so loud, but there is no "ratteling" per-se that I can hear.

Basically, I don't think this car has ever SEEN 3000+ RPM's. Even at "WOT" it's shifting at about 2500. The thing sounds like it's going to explode at anything faster than that, so ALL my advance (39 w/o vac, 57 with) is coming in about 2200 or less now. I adjusted the fast idle screw to about 2000 rpms while checking the timing, and I think I was at 55 - 56.

What I'm learning that is interesting about timing is that the theory is similar, but in practice it's different for each engine. For instance, my total timing on my Corvette Stingray 350 V8 is 52 deg, all coming in at about 2800 rpms. I have 36 deg of total mechanical coming in at the same speed (w.o vac adv.). There is a pretty good history of performance characteristics for those engines, so I was able to get a very specific set of guidlines for adjusting timing -- "get a set of weak springs so that 36 deg comes in at 2800 rpms. Get a adjustable vac canister and set for 16 deg) and that is supposed to give me optimum performance.

Anyway, I much appreciate the help. It's funny that this I6 can handle so much more advance than my chevy V8. Not sure why that is (maybe compression?). :idea:

 

[CZLN6]

Howdy Back CC:

And, you might check your vacuum cannister to see if it is the type that can be adjusted externally by inserting an Allen wrench in through the tube nipple. I don't recall the correct size of Allen wrench OTTOMH. Tightening reduces the amount and when it come in, loosening increases the amount and allow it to advance sooner.

When you get to be an OF (Ol'Fart), like me, and your hearing goes, you have to rely on other senses to detect knock. Next time you pull a plug look for a condition called "peppering". It's black or dark pepper like specks on the porcelien. That is a visual indication of pre-ignition.

You might also help yourself by opening the gap on the plugs to .045" or .050" with the DSII system in place. That will creat a bigger, fatter spark kernal to start the burn. That's one of the advantages of the DSII.

Several things contribute to the anti-knock characteristics of our engines. first and probably foremost is the relatively small bore and central plug location. That translates into-once the spark sets off the flame front for the fast burn characteristics of a typical A/F mixture, it doesn't have far to go. Things like a dished piston, a relatively tight quench and a rich A/F mixture also help anti-knock qualities. I don't know which 350 engine your 'Vette has for comparison, but CR is diffinately a factor.

Adios, David

 

[Anonymous]

Well, thanks again and now the PLOT THICKENS!

At your suggestion, I pulled my #1 and #3 plugs. I definitely see what appears to be tiny black specs on the plugs - "peppering" as you say.

Here are pictures - I'd love if you could take a look and let me know if you think that's what it is.

http://www.albertpenello.com/mustang/plug1.jpg
http://www.albertpenello.com/mustang/plug2.jpg

Now, if it IS pre-ignition, what should I do to fix it? I never heard anything - should I reduce the timing back a bit?

OK, then there is this -- I pulled the valve cover. ALL MY PUSHRODS ARE LOOSE! The adjustable rocker arms adjusters are bottomed all the way down, and the pushrods still move freely between my fingers! Shouldn't there be 0 movement of these pushrods? I took a measurement and they could stand to be AT LEAST 11/32 LONGER!

Could this be contributing to the problem?

Thanks much!

 

[CZLN6]

Howdy baccCC:

It's hard to tell from the photos, but I don't think your plugs show any sign of peppering or Pre-ignition. You might be showing signs of high speed glazing, but, again, it's hard to tell from the photos. Glazing occurs under hard, fast acceleration. Plug temp rises suddenly and normal combustion deposits have no chance to form. Instead they melt onto the insulator forming a shiny, glazed surface. Once plugs are glazed over it is almost impossible to clean and rejuvinate them.

My advice is to leave the ignition alone, install a fresh set of plugs- you might want to try one step colder and keep an eye on them.

Be sure that your oil pressure is up when you check the rocker clearance. It won't hurt to run your engine with the valve cover off. Make sure it is at operating temp and that oil is getting all the way to the front rocker. No turn off the engine and check again. Once you've check a rocker, don't check it again until you've started the engine again to get pressure back up in all the lifters. If you keep tightening a rocker, it will continue to leak off pressure out of that lifter until pressure is reestablished.

I don't recall the machining that you have done with this engine, but usually that takes some clearance out of the rocker/push rod assembly. Hummmm...

Doncha just love a good mystery?

Adios, David

 

[Hot 6t Falcon]

A thought from the other brother. I've been following this conversation between you and Dave. I remember your saying previously, and here again, that your engine doesn't rev to more than 2500 rpm and then it sounds like it is coming apart.

I've got a Summit 0-8000 tach and that has an adjustable switch for 4-6-8 cylinders, When I first stuck it in it was preset to 8 cyl. and gave very low readings.

Hmmm! Just something for you to think on.

Good Luck

 

[Anonymous]

Thanks Dennis!

I just e-mailed David a better picture of the spark plugs (hopefully) to confirm the glazing. I just replaced those plugs less than 100 miles ago and they have bad buildup already. Also this pushrod thing has me worried as they are REALLY loose.

I'm pretty sure the tach is set for 6 cyl - because I checked it with my digital timing light and they match up pretty closely. Also, I know at cruise I'm about 2200 RPM which is about what everyone says it should be.

I will definitely double-check on Monday!