Taming HEI Centrifugal Advance
 

Folks,

This is a particular problem I have had forever on this engine. I'm using all factory mechanical advance stuff with the strongest springs I can find but I still get all my timing in faster than the engine has ever wanted (new heads or old heads, new cam or old cam). Specifically, I get ping between like 1800rpm and 2600rpm way before I get it anywhere else. If I could slow the curve I could run more total and have more early. I could also run more vac advance. Just not sure how to slow it down more. It's not too much total, it comes in too quickly with RPM. 394 centre plate, 139 weights, numbers down (if I remember correctly) which I believe is correct for Pontiac rotation direction.

I think I have a 406 plate as well, also a 345. But the 394 seems my slowest. I am half tempted to take a bit of material off the 139 weights but I hate to butcher good old factory stuff.

Again, I'm not trying to limit the total. I'm trying to get it coming in slower..

Sam

http://www.daytona-sensors.com/tcs-1...ol-system.html

I like things exact.

Greetings Sam! I see you have the T/A running again. Outstanding! You have really made a lot of things happen to get this far.

I see you have some issues your trying to hash out. I don’t know if any of my ideas will help. But I will try. I went through the other treads you started too. I may get something confused, feel free to correct me. Maybe you can answer some questions:

1. What engine, cam, and compression combos has this distributer been in that you have ran? Has this last combo aggravated it, you indicate it is not totally a new issue?

2. What is your altitude where you are at and normal ambient temp, fuel rating (where your at, i’ll cross reference it.)

3) Which dizzy are you running. It has a pertronix point eliminator in it? What is the initial timing? How much mech advance are you getting and at what rpm does it appear to be all in? Your best guess anyway.

4) What spark plugs/heat range do you have? Maybe a photo of a plug would help to make sure we are not working on a distr problem and not fuel related.

The engine shouldn’t be pinging at all with no load in the mid range for your current combo. That appears to be something you made manageable with the vacuum advance. But that sounds like it is masking another problem.

That 703 cam with the eddy alum heads in a 455 should work at 10:1. It is kind of in the gray area were issues start too occur and would be happier with more cam. But it should be manageable. The issues should be mid range under a load, not at no load or a light load. It sounds like a issue with the dizzy. I am wondering if it is something in the pertronix? Have these issues followed the pertronix around? The advance curve for the parts that are in right now should work. We have done about a dozen Pertronix. Some have been absolutely great, but some had issues also. Do you have a set of points you could try? What year is your pertronix built?



Jay

Jay, good to see you. Thanks for all the help!

455HO .030 over
Edelbrock CNC chamber 87cc round port
1971 HO intake
It’s an HEI. The Pertronix is the adjustable vacuum advance unit. Honestly I think it is just a rebrand of the Crane vacuum advance.
Plugs are Autolite 3923 which are listed as compatible with the Edelbrock heads.
Zero altitude. Lots of heat and humidity.

Definitely likes to ping in the midrange light to no load. I can make it go away with more throttle.

So no points. No points replacement. Just plain ol’ HEI

Sam

either jet up or science -out the ilde bleeds. Put a
q-JET on and see if the problem goes away.

Ok, HEI, and a stock gm module with a qjet.


So you are near sea level, with heat and humidity? Normally the 3923 should be fine, it is already a pretty cold plug. Might try a colder plug though. If anything it would give you some hints what the engine is wanting, The vacuum advance isn’t doing you any good if it makes it ping with no load, your better off not running it until you get farther in your tuning..Maybe a 3932 autolight or a 3922 plug would be worth trying. If it is doing this with partial or no load it probably won’t show up reading the plugs. I think I would start there before I spent a bunch of time recurving the distributer.

Are you working with a GM HEI or aftermarket HEI?

Makes me think the distributor problem is in the carburetor.

You're too lean in the midrange.

GM. It’s in good shape. I try to avoid aftermarket parts where possible.

Sam

Try a thinner set of primary rods?

Sam

Is this happening with or without vac advance?

^^^ really good question.

"I" would like to see a chart of advance vs. RPM, and vacuum advance vs. vacuum level. And then a description of vacuum produced by the engine during the detonation.

So.. This morning I found the little limiter cam plate and I stuck it in with the factory 14 degree can I put in yesterday. I did three clicks and confirmed that I am now at the following:
base 12
with vac 20
vac adding 8

This setup is now mostly ping free but has remaining ping in the aformentioned midrange (like 1800-2800rpm) on light throttle. I actually might have heard a little rattle around 3000rpm at full throttle so I'm thinking my next step is going to be dropping base to 10 for:
base 10
with vac 18

I will try this later today and see what it is like on the drive home.

My notes on my current centrifugal curve (394 centre plate numbers down, 139 weights, stiff OEM springs) is (from my notes):
+8 at 1500
+12 at 2000
+13 at 2500
+14 at 3000

I haven't been able to get a slower centrifugal advance curve than that.

Sam

Midrange light throttle ping is a classic symptom of a too light spring in the vacuum advance.
Using an aftermarket adjustable vacuum advance only gives new gray hairs and a new vocabulary, i never got one working properly.
I would try to find a 20° Standard VC389 replacement vacuum advance unit, connect it to a ported vacuum source and forget it.
However, if centrifugal comes in full at 3000 rpms or earlier i would shorten the advance slot on the vacuum advance bracket by a quarter of the distance to have 15°.
You may have an original vacuum advance with 25° advance, the amount is stamped on the bracket along with the three last digits of the part #. For this, shorten the advance slot one fifth of the distance for 20° or 2 fifth´s for 15° advance.
The OEM vacuum advance or the VC389 will drop off advance quicker (properly) at light throttle reducing or eliminate ping.
Also, try to find some springs that holds back the max centrifugal advance to at least 3800 rpm´s. Original springs are hard to find, but are the best, should never be replaced. IMHO.

It's a factory 20 degree can limited at the "back" of the slot already to 14 degrees and now limited at the "front" of the slot with the cam plate to 8 degrees. Limiting at the "front" like that should also raise the vacuum required to start advance.

I have strong original springs. Sounds like I need stronger ones but no idea where to get them..

Sam

What is the deal with the Pertronix adjustable vacuum advance?

It's what was on the car when I started troubleshooting. It's basically, I think, a rebrand of the Crane unit (PerTronix D9006). Looks the same. Acts the same. 3/32 or whatever allen key to adjust the tension.

Anyway, yesterday when I was troubleshooting I replaced it with the above-mentioned factory advance can (I have like three HEI factory cans from old distributors and this is the stiffest. It is stamped with "20"). So I now have the factory can. It is limited on the back end to 14 degrees with a travel limiter bushing I made. It is now also limited on the front end to 8 degrees by installing the little cam plate thing on the front mounting screw and clicking it three clicks (it takes out 2º per click). I verified this at 12 base, 20 with vac at idle. So it really is adding 8 degrees at present.

Sam

So at idle, you have 8 degrees of vacuum advance...which means manifold (not ported) vacuum.

How high above curb idle do you have to rev the engine to get ONE degree of centrifugal advance? In other words, are you sure there's no centrifugal advance at idle?

1. Is the advance DONE at 3000 rpm, or does it continue to advance as RPM increases?

2. Are we talking about crankshaft degrees, or distributor degrees? With 12 degrees initial read off the crank damper, and +14 crank degrees from the centrifugal advance, your total timing at 3K is 26 degrees. Likely not enough. With 12 degrees initial, plus 14 distributor degrees of centrifugal advance, you have total timing of 40 degrees at 3K--very likely too much. EITHER WAY you need to straighten out your centrifugal advance.

3. The vacuum can is limited on both the "front" and the "back", but you've never said how much vacuum it takes to activate, and how much vacuum it takes to fully advance. Similarly, you haven't said how much vacuum the engine is developing when you hear the pinging.

4. I still have a hunch that you need more fuel. But what do I know? I'm ten thousand miles away.

Do you have the extra-heavy weights? Perhaps lighter weights are in order.

There is nothing wrong with your HEI's mechanical advance and it's not the reason you are getting light throttle ping. Pontiac HEI's actually have set-ups in them that bring the timing curve in pretty quick, but not all-in right off idle with some of it in at idle like the junk aftermarket weight kits.

My HEI starts adding timing right off idle around 900-1000rpms and all in by 3000, with most of that all-in by 2500rpms. It uses stock weights, center cam and strong stock springs on it. The last little bit of it sneaks in a few RPM's later. It's been on 5 Pontiac engines to date and all of those engines have been flawless for performance and no ping whatsoever.

My vacuum unit is a very early "adjustable" unit that adds or takes out how much is added by turning the screw inside of it. It was purchased and installed back in the 1980's and still working fine. I've never even tested the spring tension on it and don't care, it's adding what I want at light part throttle as I use ported vacuum to it.

You simply don't have enough camshaft in the new engine and LSA is tight so it's not going to want a lot of timing anyplace. I saw that coming and mentioned it at least once thru all of this.

It will still work fine with correct tuning. Start out by doing this:

Get the engine fully warmed up and heat soaked. Set the idle mixture screws and idle speed best you can, down around 700-750 rpms. Loosen the distributor slightly and advance the timing well up off the scale till the engine smooths completely out. Then retard the timing until it just starts to slow down some and listen to the engine's idle quality. When you reach a point where it just starts to slow with a "deep/heavy" sound plus it sounds happy and still making good vacuum stop and see where it is at. Hopefully it's not up around 20 degrees but down around 4-12 degrees. Search for the sweet-spot and lock it down. Hook up the vacuum advance to ported vacuum and do some testing.

With 455 CID and roughly 10 to 1 compression with relatively tight squish plus your 703 camshaft I doubt if it will like, want or need much more than 26-30 degrees total timing plus another 8-10 from the vacuum unit. It may not even want quite that much, but whatever happens tune it for ZERO ping anyplace.

I doubt if the carb is the problem, if it were too lean at light part throttle you'd feel it surging, or at least the engine would be anemic, "flat" and down on power. If it runs good and not surging of down on power it has enough fuel in that range.........Cliff

I have the 139 weights which are the ones that you see used in all those charts that compare curves on the different centre cams. The only other ones I have are the junk ones you get in parts store advance kits.

I don't have vacuum readings because I don't know. But the centrifugal advance doesn't come in at idle and the vacuum advance isn't wavering at idle. So the centrifugal starts higher than my idle speed and the vac is all in by my idle vacuum level.

I don't have a vacuum gauge in the car nor a handheld tach. The numbers I have written down are from revving the engine with the light in hand, remembering the pitch, then getting into the car and revving to the same pitch and consulting the tachometer. I'm a musician so that works for me. Hence not so many detailed numbers..

ALL the degree readings are off the front balancer. That's my only source for any number I give. It's a newish SFI balancer and has a long numbered scale so I can read high numbers directly off it.

Sam