[JC455]

Your timing will cease to increase- if it's showing a certain number(say 25) at 2800, then that number increases (maybe to 27) at 3000, but stay the same after 3000(still at 27), you've found the end of centrifugal advance.

[lethal-428-lemans]

I'll run it again tomorrow and see where the timing mark stops moving at

[Joe's Garage]

That's why I suggested removing ONE advance spring so you don't have to rev it up so high.

You could probably even take out BOTH advance springs and have maximum advance at idle, just to see where it actually is.

I've had Pontiac engines that only wanted 12 degrees initial timing and others that liked between 18 and 20 degrees at idle. And some liked direct vacuum advance while others liked ported vacuum advance.

The only issue with 20 degrees initial timing was/is that those engines sometimes start hard when hot.

Good luck with yours.

[Sun Tuned]

To add some info into this discussion...

On a point distributor one can remove one spring to not require revving it so high to check for total centrifugal advance, as has been mentioned.

You can cut to the chase even faster by removing both springs. This works on the point distributors as they rely solely on a pin / slot arrangement that limits total centrifugal advance.

However theHEI does NOT work in this fashion.

HEI relies completely off of the center can to limit the total. There is a problem with this idea though.

Only about 10-12 factory center cams limit properly. The vast majority of the other , at one time , GM cams all 121 of them, allowed for excess advance beyond 12-15 distributor degrees.

So on these setups, you can remove one spring and check for total centrifugal advance if you do it very carefully and watch for it to stop the first time like a hawk.

I caution anyone reading this ...
Do NOT try to remove both springs on an HEI and try to verify a total centrifugal number as you will not end up with accurate numbers.

The reason for this is the slots (there are 2) in the shaft plate of an HEI allows far more than the designed intended amount of centrifugal. Like upwards of some 60 crank degrees.

They were not designed to operate off of a fixed mechanical stop like the point distributors were.
Now a trained eye can pick up the point where the center cam stops on a few of the factory center cams but the vast majority of them which contain excessive override ( the point where the advance does not actually stop but keeps going) will not allow you to readily see the factory designed intent.

In other words let's take a #383 GM center cam and a set of 139 weights.

Now these are known to allow 12 distributor degrees
24 crank degrees.
You can remove 1 spring and gently rev engine and you will readily see it stop at 24 crank degrees.
If you keep revving it it will eventually override the centercam and continue advancing. It is at this point where the factory designed limit has been exceeded.
By removing both springs you will easily blow past 24 degrees and go right on up past 60 or so degrees.

So I do not recommend doing this on an HEI for this reason.

[4zpeed]

Thanks so much Sun Tuned for that very enlightening piece of information.

Thanks for posting the question Patrick, been following this one, my first HEI as well.

Don't want to see bam bam an rubble.

....
Frank

[Joe's Garage]

Because there was no mention of the distributor being an HEI. Or I missed it totally.

But I will still stick to my original suggestion: Remove ONE spring.

You will get maximum mechanical advance by around 2,000 rpms - that's just a really fast idle in my book.

And I will clarify in regards to my suggestion to remove BOTH advance springs:
I did not suggest revving the engine up with both springs off - just starting it and letting it idle. It should idle at the maximum mechanical advance.

I should have been clearer when I typed that. My apologies.

However, I will concede to Suntuned's distributor knowledge - and concur that you should NOT take both springs off on an HEI distributor. Just take off one and watch carefully for the advance to stop - do not over-rev with the spring off.

[Cliff R]

An HEI distributor requires a positive stop to be welded in as I've mentioned countless times in these threads. As Sun Tuned explained the stop for the advance is controlled when the weights roll over onto the "flats" of the center cam, the advance pin does NOT hit at this point. Once the weights reach the flats they will stop the total mechanical advance effectively ONLY if you use the stock springs and even then at really high rpms we've seen them continue to add timing as the centrifical force allows the weights to roll out on the tips a bit. I think this simply happens because the engineers never intended for the engines the HEI's were used in to attain the rpm's we send them to today.

Anyhow, we ALWAYS weld in a positive stop for the advance when setting up HEI's, and NEVER use any of the cheap POS aftermarket spring/weight kits. They are not only very poorly designed, the springs get weak quickly. Factory parts are much better and surprisingly many stock combinations have the advance coming in soon enough with a very smooth curve and not adding any timing at idle speed. Every single POS weight/spring kit I test will have some of the advance curve in at idle (below 1000 rpm's) making idle tuning difficult if not near impossible (sound familiar?).

To provide any specific recommendations for the OP put up all the engine/drivetain/vehicle specs, specifically the CID, true static compression ratio and camshaft specs, unless I missed them someplace?......Cliff

[Formulajones]

I've posted this before when the subject comes up.

If you can't weld or don't have the equipment, it's extremely easy to put a stop on the HEI's with just a 10/32 tap.

The hole is the perfect size, run the tap through, install the screw with a little lock tight, and you're set. This provides right at 16-18 degrees centrifugal. If you need more you can grind on the screw. Go too far, install a new screw.

It's easy peasy and works excellent. I've done dozens of them over the years like this.

To add to the "remove a spring" trick to see where total is, this works fine, but you still need to have both springs installed and run the total up if you want to find how fast or slow the curve is coming in. Which is just as important as where the curve stops.

[lethal-428-lemans]

Originally Posted by Cliff R

An HEI distributor requires a positive stop to be welded in as I've mentioned countless times in these threads. As Sun Tuned explained the stop for the advance is controlled when the weights roll over onto the "flats" of the center cam, the advance pin does NOT hit at this point. Once the weights reach the flats they will stop the total mechanical advance effectively ONLY if you use the stock springs and even then at really high rpms we've seen them continue to add timing as the centrifical force allows the weights to roll out on the tips a bit. I think this simply happens because the engineers never intended for the engines the HEI's were used in to attain the rpm's we send them to today.

Anyhow, we ALWAYS weld in a positive stop for the advance when setting up HEI's, and NEVER use any of the cheap POS aftermarket spring/weight kits. They are not only very poorly designed, the springs get weak quickly. Factory parts are much better and surprisingly many stock combinations have the advance coming in soon enough with a very smooth curve and not adding any timing at idle speed. Every single POS weight/spring kit I test will have some of the advance curve in at idle (below 1000 rpm's) making idle tuning difficult if not near impossible (sound familiar?).

To provide any specific recommendations for the OP put up all the engine/drivetain/vehicle specs, specifically the CID, true static compression ratio and camshaft specs, unless I missed them someplace?......Cliff

Cliff,
64 tempest 3700 lbs, 400 with 670 heads and headers. Compression was 210 to 220 on all cylinders. Running a 67 RA Qjet with HEI. Trans is a Tremec tko 5 sped 3.27 first od is .68, rear is 3:90, tires are 26.6 inches.

[lethal-428-lemans]

Originally Posted by Formulajones

I've posted this before when the subject comes up.

If you can't weld or don't have the equipment, it's extremely easy to put a stop on the HEI's with just a 10/32 tap.

The hole is the perfect size, run the tap through, install the screw with a little lock tight, and you're set. This provides right at 16-18 degrees centrifugal. If you need more you can grind on the screw. Go too far, install a new screw.

It's easy peasy and works excellent. I've done dozens of them over the years like this.

To add to the "remove a spring" trick to see where total is, this works fine, but you still need to have both springs installed and run the total up if you want to find how fast or slow the curve is coming in. Which is just as important as where the curve stops.

Is the screw you put in right above your thumb ?

[Cliff R]

With that much cranking compression the engine would have a pretty small cam in it. It's not going to want a lot of timing anyplace, initial, total mechanical, or much from the vacuum unit. Won't like, want or need a quick mechanical curve either.

I'd set the distributor up for 9 to 10 degrees, or 18-20 at the crank, initial timing at 8-10 degrees, and modify the vacuum unit for another 8-10 degrees. Start out there and advance the timing if/as needed until it either bucks the starter on hot restart, or pings at heavy/WOT.

If it pings anywhere along the way at light throttle, reduce how much vacuum advance is being added......Cliff

[rohrt]

Originally Posted by Formulajones

I've posted this before when the subject comes up.

If you can't weld or don't have the equipment, it's extremely easy to put a stop on the HEI's with just a 10/32 tap.

The hole is the perfect size, run the tap through, install the screw with a little lock tight, and you're set. This provides right at 16-18 degrees centrifugal. If you need more you can grind on the screw. Go too far, install a new screw.

It's easy peasy and works excellent. I've done dozens of them over the years like this.

To add to the "remove a spring" trick to see where total is, this works fine, but you still need to have both springs installed and run the total up if you want to find how fast or slow the curve is coming in. Which is just as important as where the curve stops.

That is pretty slick.

Did you run the screw up from the bottom?

["QUICK-SILVER"]

Originally Posted by lethal-428-lemans

400 with 670 heads and headers. Compression was 210 to 220 on all cylinders

Originally Posted by Cliff R

With that much cranking compression the engine would have a pretty small cam in it. It's not going to want a lot of timing anyplace

Small cam OR static compression well over 10:1?

The more you try to "de-tune" for too much compression for the fuel being used.....THE WORSE THINGS GET. Bigger cam or retarding this cam, to reduce effective compression, will just move the chatter/ping (detonation) to a higher RPM where it could potentially do more damage. Because of the higher VE and/or dynamic compression at higher RPM. Not to leave out low RPM ping/chatter at lower RPM's due too hotter chamber temps from timing set too low for what it is.

Need to try some race fuel in this thing.

IMO Clay

[Formulajones]

Originally Posted by rohrt

That is pretty slick.

Did you run the screw up from the bottom?

Yes, up from the bottom.

[Formulajones]

Originally Posted by lethal-428-lemans

Is the screw you put in right above your thumb ?

Yes sir, hole closest to the breaker plate. Can do this on either side, doesn't matter.

[FULZGOLD]

And now I think back on all those younger years when I just did it by feel. Made sure it started good, idled good and ran good. lol

[lethal-428-lemans]

Originally Posted by "QUICK-SILVER"

Small cam OR static compression well over 10:1?

The more you try to "de-tune" for too much compression for the fuel being used.....THE WORSE THINGS GET. Bigger cam or retarding this cam, to reduce effective compression, will just move the chatter/ping (detonation) to a higher RPM where it could potentially do more damage. Because of the higher VE and/or dynamic compression at higher RPM. Not to leave out low RPM ping/chatter at lower RPM's due too hotter chamber temps from timing set too low for what it is.

Need to try some race fuel in this thing.

IMO Clay

Comp 270h
470 gross lift
224 @50 duration

[lethal-428-lemans]

Originally Posted by Formulajones

Yes sir, hole closest to the breaker plate. Can do this on either side, doesn't matter.

I'll have to look in to doing that.
Thanks

[Kenth]

Originally Posted by "QUICK-SILVER"

Small cam OR static compression well over 10:1?

The more you try to "de-tune" for too much compression for the fuel being used.....THE WORSE THINGS GET. Bigger cam or retarding this cam, to reduce effective compression, will just move the chatter/ping (detonation) to a higher RPM where it could potentially do more damage. Because of the higher VE and/or dynamic compression at higher RPM. Not to leave out low RPM ping/chatter at lower RPM's due too hotter chamber temps from timing set too low for what it is.

Need to try some race fuel in this thing.

IMO Clay

Two important points to remember:
The DCR is always lower than the SCR
The DCR does not change at any time during the operation of the engine

[Sun Tuned]

Not picking on anyone especially @ joes garage, I had to scroll back to the first few posts to see what he had myself.

At the end of the day, at the single enthusiast level, use whatever you have at your disposal to get the job done cleanly.

Cliff prefers to weld his up, Formulajones uses the screw, I prefer a combination of the two and gas silver braze or TIG mine.

The screw is just as effective as just about any other method Ive seen, and Ive been around some damn smart sneakey folks. Plus its fairly simple and whatever you do dont forget the red loctite!

On mine when my 12 degree die was not missing a sizeable chunk out of it, I just stamped a bunch out and had them heat treated and simply reached in the box and pulled one out and installed it. Presto! no worries. Now I use existing stock and when that is depleted, as I have no plans to spend the $$$ to redo my 12 degree die again. Ill just incorporate a stop on the units that need 12 degree distributor curve. These cams and weights are getting pretty expensive to stamp out and then have heat treated. The 10 and 11 degree cams I have covered but since im missing a piece out of my 12 degree cam die I like everyone else will have to do something else after my supply is gone. At that time Ill just likely do the stop deal.

Ive tried the screw deal on circle track cars before and the guys are damn hard on equipment. On a street driven type drag oriented car the screw deal works well and is quick to incorporate. On a dirt car those guys will sling and shake the screw out in like 3 races no matter how much loctite you use. Plus most screws are somewhat soft and they end up mushrooming the little threads. So what I do is a touch different and my dad has been doing it this way for 40 years. At my place we take a 3/16 X 5/16 alloy steel dowel pin and lightly peen the hole and press the dowel into the hole. Then I just TIG the pin to the bottom of the reluctor assy. I used to gas silver braze them in but they shook a few of those loose, so I Tigged the rest and havent had any issue so far. I prefer to use the right center cam to limit the HEI but there are times when that just isnt the easiest thing to do. Like when someone wants/needs something strange like 8 dist degrees. Its just easier and quicker to do the stop pin and move on.

As Formulajones said when the shaft plate just gets to the edge of the inboard hole you will be looking at 8-10 distributor degrees dependant upon the center cam you are using. For most Pontiacs you will find 9-10 dist degrees. So definitely I would reassemble and test to see where you are before proceeding further. If you need more I prefer to mark on shaft plate where the pin hits and then take a chainsaw file and get the extra degree or two you need. some others may prefer to gringd the screw or pin but it is up to you. I do the shaft plate as its real easy to rebuild that surface and file flat if you wish to go back to your original start spot. Especially this is true in my case after i just welded in a alloy steel solid dowel pin. Much easier to leave that alone and modify shaft plate.

So when you get done you can eliminate searching all over the planet trying to find salvage yards to pirate center cams from and you dont have to do like i did and spend 7500.00 on having dies made to end up with the same result when you get finished. I found that out the hard way after thinking I had everyone covered with a 10,11,12 degree cam set and then a guy calls wanting 8 degrees for a special application. Hmmm... plan B. Build a stop. If it functions same and gets the same job done then you've done what you set out to do.

Now I will say if you do incorporate a mechanical stop, dont be tempted to put light springs on it. Ive seen the light setups cause the advance to swing around and smack hell out of the stop pin or welded stop and bounce, or chatter, and cause a wierd timing variation as a result. Once you get the stop degrees sorted out you can actually find that you can spring the deal rather aggressively and still be fine.

And I can assure you, no one will know or see it in there spinning at 5000 rpm to care.