[hisgto]

In Sep. 2005 I decided to address the pinging during part throttle acceleration. After reading many posts, I removed my vacuum advance can to find out it was delivering about 20* of advance.

So I purchased and installed a Crane adjustable can. Before connecting it, I set the initial timing to where I would not get any pinging on WOT test runs up a long hill. Then I connected the vac adv can and adjusted it to where I would not get any pinging during part throttle acceleration. Performance was OK.

Fast forward to May 2006. After reading a ton more on timing (just a bit addicted to this forum ), I realized that maybe the better adjustment would be to limit the total vac advance with the provided cam plate instead of what vacuum level it comes in at. So I decided to start by mapping my mechanical timing curve. With a digital inductive tach, vacuum gauge and timing light, this is what I found.

Rpm...............Timing...............Vac
550 (initial).......5-6................11-12
700..................8...................17
825.................12...................18
1050................14...................20
1450................16...................22
1900................18
2150................20
2350................22
2500................24
2800................26
3000................26

So it looks like I'm getting about 20* of mechanical advance. Reading so many post that say about 34-38* all in by about 2800prm is best, I decided to set the initial to 15* to get a total of 35*. I also install the vacuum advance limiting plate and limited the advance to 0* as a starting point. Go for a ride and it runs pretty good, except when I floor it, it pings pretty badly. Hmmm....

So today I decided to remove the mechanical advance springs to allow full timing advance at low rpm to see what I was really getting. To my amazement, I am getting 40* of mechanical advance. With 15* initial, I read 55* at idle.

- Could this be correct?
- Is this a valid way to check for maximum mechanical advance?
- Do I have the wrong center plate and weight combo?
- Is my testing methodology valid?

I need to get my mechanical advance dialed in before I can even mess with vac advance.

After all the hours surfing this board, I know I can't forget the specifics.
1966 GTO
'72 455 stock long block (155,000+ miles)
Summit 2800 cam
Performer intake
Holley Avenger 670 carb
HEI ignition - 389 center plate - 139 weights - Crane adj vac can
Muncie 4 speed

[Malky]

I think you are headed in the right direction. Yeah, you definitely need to get the centrifugal advance dialed in before the vacuum advance. Then you can measure the cruise vacuum and tune the vacuum advance so it only operates while cruising and drops out when you accelerate.
The 389 centreplate with 139 weights gives a total of 21 deg advance according to Rocky Rotella's article in the May 2004 HPP, which matches your test results. Your cam is pretty mild so you can't get too aggressive with the timing. Normally you don't want advance starting until about 1000 rpm so you must have quite weak springs since it starts at ~600 rpm. Stronger springs will slow down the advance.
Running hot or a lean carb setup will also cause pinging, and those have to be fixed first before the timing can be optimized.

[Sun Tuned]

Well, let's do it this way. If you had installed the center cam with the numbers facing down... you should see 10 distributor degrees of centrifugal advance (20 crankshaft degrees). Couple of things will alter this, center cam worn or pins in shaft assy loose, weights worn, really light springs, etc. These are concrete numbers as long as everything is healthy you'll get the same, unless your timing light is questionable, thats another variable. We'll assume it's fine as I believe you said you got about 20 degrees before removing the springs completely. I hope I understood that part right (springs were later removed????).

Removing the springs entirely on an HEI is not the way to measure total centrifugal advance. Why?? The plate assy was not designed for this purpose as were the old point dist. were. I mean on them the slot was a specific length and varied for different models of dist. HEI's all are the same hole length. They are made this way to allow 1 shaft assy for all models with the center cam being the limiting variable/factor or method for the different models involved. That being the case the slots in the top plate assy are much longer than needed for a certain given amount of advance. So what to do??? Well many prefer to braze up and shorten the slot to the desired amount of advance needed like on the old point dist. This will work, however ther are certain applications I would use this method on and quite frankly more that I would not. Circle track applications jump to mind immediately. I have limited those on occasion. Those guys can seriously test the mettle of what a dist is made of in just a few races. If it can be broken I have circle track guys here in town that can literally destroy just about anybodys dist in a few races. These guys get over here at Riverside Raceway and proceed to wipe out everything on a car that isn't built to unbelievable standards. If you locked a few of them up in a rubber room they could destroy a cannon ball with a handfull of tooth picks and Q-tips in under 5 mins. One of the things that will happen is the center cam leverage designed into the center cam will be overcome due to the constant on/off full throttle application on the dirt. These things will go from 4000 rpm to 8800 rpm in a blink many, many times during a race. A real good set of springs is required that will not heat fatigue and lose ternsion is needed.So as a safety precaution, just in case,we usually fillister head screw the bottom plate as a stop for the top plate assy. Still ya gotta be careful of this, as there exists the tendency for the advance to bounce off the mechanical stop and i don't like that. Gotta watch the springs so they as gently as possible "ease" the advance assy to a stop on the screw stop.



Street cars usually never see this type of abuse, so I like to not provide a means for a positive stop. This is the way the factory designed the advance cams, to be the mechanical stop for the deal. In most cases the better advance cams were designed as gently as a camshaft lobe and these things are just as particular. Just as you don't want a cam lobe to snap a valve open to full lift and then slam it back down on the seat... you don't want the dist advance cam to do the same thing. You want it fast, but you also want it smooth and gentle. Erratic is no good. Before everyone goes ape telling me different cams do this and different cams do that, I know they do. Given their intended original application, they probably did well. When we go through the salvage yard and gather up 20-30 different ones and try to press them into a workable combo for a specific dist, only then do we find some are good and others just plain suck.

That "389" cam is a good center cam. No "override". Unless you run hair thin springs and try to wind the thing 12,000 rpm, then of course it will. You try that and they all will. I have always been an advocate of the smoother is better advance mentality, and you should always run the stiffest springs you can find to allow the advance rate you need. Fatigue issues are minimized at this point in the game by doing this. You want the thing to start to advance at a certain rpm and be finished by a certain rpm and above all be "CONSISTENTLY REPEATABLE" at these settings.

There is another chart on this site that lists some different values of center cams and it is important to remember that it's values are derived with the advance cam turned numbers facing "DOWN".

Rocky's article will give different values than mine as his tests were done with the numbers facing "UP". I am not sure of any of the accuracy of the charts with numbers facing "UP", I can only tell you what they are with numbers facing "DOWN", as that is the only way I have tested the Pontiac dist.

I do not make mention of this other than to illustrate that there is a difference in the way the two charts derive their numbers, and that the reader of these charts needs to be absolutely sure of what he is looking at so as no confusion exists. I can only guarantee the numbers I give. However, i am quite positive the "numbers up" chart is likely accurate as well if tested in the manner in which it was derived.

Still pinging?? Maybe verify fuel system is working for you and not against you. And whatever you do don't try to get greedy with the timing. You'll ultimately lose.

[hisgto]

Sun Tuned,

I was hoping you would be out there surfin' the boards. I have read a ton of your posts and printed a few for the garage, so thanks for sharing your test info and knowledge.

You understood me correctly that the springs were later removed. I only did this in an attempt to see if I was truly reaching full mechanical advance. It is comforting to hear this is inaccurate and the wrong way to measure, because that means I'm not getting 40* of advance at the crank.

The springs I used during my tests were from the Crane kit. One heavy (blue) and one medium (silver) per the instructions. After the testing I put on both heavy springs. Also, center cam IS numbers facing DOWN.

So with 15* initial and 20* mechanical I should be good at 35 total. But it pings when I jump on it. When I originally installed the Crane vac can, and followed the instructions to reduce initial timing until the WOT pinging stopped, I got down to 5* for a total of 25*. Seems low, but do I just set it where it likes it?

As far as vacuum advance is concerned, what should be my max vac advance?
When should it start and when should it be all in by?

Am I correct in understanding that a lean fuel mixture can also contribute to pinging?
Carb is Holley Avenger 670cfm out of the box stock. I'm a carb neophyte. Beyond float and idle mixture settings, I lack the bravado to make changes let alone understand it all.

Thanks again for your wisdom!

.

[Sun Tuned]

Couple things that stand out... for me anyway. 155,000 miles really is not a problem. How much carbon is in there would be my concern. Skip that for a minute. Don't overlook the fact that coming up on sept when you worked on it that the winter/summer blend fuel will have an effect on just how much timing you can get by with. Also 10-15 crank degree vac advances work well. I am not nearly about to get into the ported/manifold vac wars with everyone, but there are very specific reasons for using both.

I will tell you if you bring this dist to me to be worked, or if you bring it to the guy who taught me, the aftermarket adjustable vac advance will get handed back to you by me, or thrown in a Berryman carb cleaner bucket/trash can by Mr. Bonner. They are in my opinion junk! Made back in the day when EGR was the norm for cars and not changed since. They are built cheap and sold the same way and they almost will always break in the same tired, old, weak, cheap way too. You cats out there that are fixing to blast me for saying that .... show me one that has been fully operational just as it was originally set and untouched since for 150,000 miles and Ill think about changing my opinion. I've seen new stockers that broke/malfunctioned within 1000 miles too, so, all mechanical stuff breaks sooner or later. The adjustable ones will eventually creep around and not hold adjustment well, some lag pretty good some don't (my experience/opinion). They are cheap, millions have been sold , I'm sure. Anyone ever get one replaced under warranty???LOL. Bonner has thrown probably half that number in the garbage followed closely behind by the infamous Mr. ****et/morocco curve kit, in the same trash can. I just don't like em...can you tell?? They work... sort of, if ya got a bout a week to actually make them work properly. But then as I have said in the past...Your underwear works if you put it on backwards too; just not very well. So there is that.

Keep the vac advance non operational below 6-8 in.hg. and you'll be good. If these things are done then you should be good, if not, then we need to be looking at other things... carb,fuel system, filter, etc. Going over the basics from square one will do absolute wonders for you, so be sure to check that first, it'll be your first, best, and last only line of defense needed.

Remember with a motor thats got that many miles what you see that works in other motors may not gee-haw real keen in yours. They are all different, most will work great with general guidelines, but many will need a little extra TLC. Get the basics nailed first then modify from there.

[Malky]

I know I am no Sun Tuned, but if you are only getting 20 deg, the amount of cent. advance is not your problem. If you still have WOT pinging with say 12 deg initial AND VACUUM ADVANCE DISCONECTED, your problem is either weak springs letting the advance happen too soon (which is a definite possiblity) or lean mixture.
As I said above, the very early start of cent. advance at 700 rpm indicates weak springs.

[Sun Tuned]

Hey man I ain't nobody. Just a reg guy like everybody else. You're right though, but I think the basics plus maybe some carbon buildup, oil stem seals hard, little lean on the mixture maybe. All added up will make it ping easier. I agree most centrifugal starts too soon and it has got to start at least a couple hundred rpm higher than highest idle speed to work good.

A 389 center cam will allow 20 crank degrees advance with numbers down. So figure the initial setting plus the cent and you'll have the total down... just got to see what rpm it starts and where its done. Easily verified. Now... and I'm sure the carb guys will agree... don't go changin the guts in the carb till you got the basics in order. Give it a fair chance then change it up if necessary.

[hisgto]

Malky,

I meant no disrespect if I came across that way. I value the help I get from the members of this board.
Your info and recommendations are well received.

As of Sunday, I'm using a heavy/heavy combo of springs from the crane kit. Later this week or this weekend I will remap the timing curve with the heavier springs. I'll post the results to compare to the medium/heavy spring combo.

Temps aren't a problem. I typically run between 165-185*.
Carb may be lean but I don't have a way to tell.
Carbon build up could definitely be a factor with the high miles.

Is my 670cfm carb too small for the 455?
Would it be worth while to use a product like RxP fuel treatment to reduce carbon buildup?


.

[Sun Tuned]

I don't think the 670 carb is too small unless your trying to do something wierd like haul a full size car on a 16 ft trailer, which I'm sure your not.

They built plenty of 455's with regular old 2bbl carbs, so no, I think thats a non issue.

Might be a little or a lot lean on the idle fuel part of the deal as Cliff has suggested and that could be a problem, but I'm not up on the carb circuit stuff though like he would be.

Anything you can do to reduce carbon would be good, but knowing how much there is or isn't would be hard to guage.

Check over the stuff this weekend and let us know what you find.