[389]

Originally Posted by Cliff R

The distance between the eyes is incorrect as well and requires that they be "stretched" out too much to get them over the pins further adding to their early demise........FWIW......Cliff

My buddy works for Diamond Wire and Spring company. He says this is the issue with springs wearing out quickly. If they have to be stretched past where they were designed to be parked they quickly loose there tension.. There's actually a lot of engineering that goes into every little part that we take for granted..

[kyle_blake]

Quick question:

The center plate is NOT REMOVABLE in a points distributor, right? And most after market spring / weigh kits for points DISTRIBUTORS do not come with a center plate? Right?

The HEI does have a centerplate that can be replaced with after market kits, right?

Thanks

[Cliff R]

Correct, the center cam is part of the shaft. When a troubled points distributor comes in here with the vehicle we remove the aftermarket parts and put stock weights and springs back on it.

If it's using a Petronix we put points back in it.

99 times out of 100 the sleeve is missing on the advance pin so it gets a stainless steel one installed. I check the amount of timing being added and use the MIG and a round file if/as needed to get it where it needs to be. For most points distributors this step isn't needed with the factory parts and a sleeve on the pin.

We remove the Petronix because we've been running into a LOT of them lately that aren't making the grade at higher RPM's.

Just recently we built a custom 1970 LS5 Q-jet for a "stealth" 496cid "factory appearing" set-up. We shipped the carb to the engine builder and they called us right form the dyno room claiming the carb wasn't up to par for the power of their engine. It did fine to about 4500rpm's but at that point the engine only made 424hp and would not make any more. After some back and forth with fuel delivery and several long phone calls I asked about what the timing was doing. He said that they "upgraded" to a Petronix and before he could finish the statement I told him to remove it and go back to points.

The next pull was 565HP at 5800rpm's!!! Right where they targeted the power to be and it made nearly 600ft lbs torque with over 500 clear across the pull........Cliff

[David Ray]

Although the points and large HEI distributors use the same basic premise for mechanical advance, there is a giant difference between them.

The points distributors use various center plates to set the actual start to limit curve, and, the slot length is set to set those start and limit points for the curve.

At one point, I was going to modify the way I do my conversions to add a threaded hole in the top of the shaft, a locating dowel, and removable center plate. Never had the time to do it.

There are 4 different weight profiles for the points distributors, one is mostly used, for all the various points curve centers.

The large HEI takes the positive start and limit points of the curve, and sets them into the curvature of the center plate. This way, changing the mechanical start. limit, curve parameters is easily changed, FOR EMISSIONS use, which is what the large HEI was designed for. ONE distributor, 437 different curve packages to match all those different emissions packages for all those different engines.

We designed the large HEI to be able to be modified for the correct application by changing a few parts, and not extensive mods, so, center plate, weights and springs are easy to change. No center plate welded to the top of the main shaft. ONE basic distributor for YOUR engine, changeable parts to adapt it.

To properly change the curve in a large HEI, you have to use the weights/center package and spring set that fits the application. Some few curve packages do work for a very few performance engines, but, there is NO scale, chart, anything to show one curve parameter package from another, and YES, there are 437 curves GM used on their emissions engine applications.

Only ONE problem with that, ONLY performance HEI GM ever built was the ZZ series Chevy crate engine units. EVERYTHING else is emissions parameters.

The ONLY performane application GM ever produced for the large HEI is the 41 weights, 375 center plate set from the ZZ Chevy crate engines, period. MOST of today's large HEI manufacturers have adopted those parts for their over the counter distributors, so, those parts are available, but not marked as to what they are.

Jamming a screw into the thing, welding, brazing, hacking, whacking, beating with a hammer is NOT the correct way to change the curves in the large HEI, selecting the correct set of weights and center are, and that really doesn't exist. Ask the aftermarket for their total degrees their parts give, shoot for the ones that mimic the 41/375 combo, they give 22 crankshaft degrees of mechanical advance timing. Work from there.

Setting up the vacuum advance to give the right number of degrees, and running on full manifold vacuum, is a completely different subject for both points and large HEI setups.

To do it right, is a simple one screw, one washer, drill/tap hole, mount plate situation.

For a FREE set of pictures and instructions on how to do it with the Crane/FAST 99619-1 scroll stop plate (the easiest way to go), ask for them, works on both points and HEI distributors, and, they are FREE.

All the items to do this are less than 5 bucks to get them right from your local hardware store and local/online speed shop, and you can do it all by yourself.

Ask for them at this email address:

gmvacuumadvancemodifications@gmail.com

Decades ago, I devised a drop in plate with two screws and nuts that would add a start and limit stop to the large HEI, from under the rotor, NOBODY WANTED THEM. They hacked, whacked, welded, beat on, and messed up countless large HEI's from there.

[kyle_blake]

Thanks for confirming the answer on my center plate question. That is very valuable , I do have one interesting question for you.

I notice one of my points distributor has some sort of assembly lube on the points ramp section of the shaft. When shops install a hall affect sensor do they clean off all the paste so it is dry?

The instructions for crane xr-i or fast / petronix don't mention anything about that.

[Cliff R]

Although the "perfect" weights and center cam don't exist, surprisingly many of the factory combinations do quite well for performance applications.

This simply happens because many factory HEI's were designed for the timing curve to start right off idle and come in smooth, steady, and increase with engine RPM's. Actually many are "all-in" at or not too far above where many engine builders consider "ideal".

The biggest enemy of the HEI is the cheap aftermarket advance curve kits, and/or using lighter springs from those kits with the stock parts. By design the advance curve is suppose to stop when the "flats" on the weights reach the "flats" on the center cam. They work as designed provided you are using factory springs on them. If you put some of these really light (especially the "gold" and "silver" ones) springs from the cheap advance kits in place or the factory springs they will for sure get speed up the advance curve getting it "all-in" much sooner. The big negative is that once the unit reaches full advance the weights can roll out on the tips and add more advance that the parts were designed to.

This is why I fill in enough of the upper end of the slot for the pin so that we can use a round file and provide a POSITIVE STOP to prevent this problem from occurring. It's not "hacking", "whacking" or "beating it with a hammer", just a very good way to make sure you have FULL CONTROL of the advance curve and that it stops exactly where the factory designed it to. With a positive stop welded in you can then use pretty much any springs you want to and the unit will still start and stop at the same point adding the same amount of advance, you'll just be able to get it in sooner, or later, depending on what combinations of springs you end up using.......Cliff

[David Ray]

ZZ4 and all Chevrolet crat engines come with a 22 degree mechanical advance weights and center set, weights 41, center 375.

MOST of the aftermarket large HEI builders have started to use that weights and center combo, not marked, but it is there. Ask them for the weights and center from their over the counter distributors.

These parts from GM are insane expensive, comes with springs, shaft and reluctor.

Now, spinning large HEI and/or points distributors with no springs on them on my distributor machine, at idle rpm's, doesn't read full advance. It takes adding rpm's to get the weights out all the way, but you guys go right ahead and do it, nice to know yet another way to not do it right.

Oddly enough it worked the same trying that with a running engine as well. Of course, most engines will have a heck of a bad time trying to start at full curve engagement.

Good luck with it all, should be great fun to watch.

[Sun Tuned]

Well...
What the hell.

Speaking of a way to not do it right. On a factory Pontiac HEI the 41/375 deal ain't gonna work right without some modification. You see the center cam fits well enough, but the real issue is the weights. The 41 weights are the last version that GM made. Yep they were on all these ZZ crate engines. And they also are of powdered metal design.

They also have the nifty little triangulated raised boss at the weight hole. The one I have talked about before. The stock Pontiac weight pins on the factory HEI will have a flange at the base of the pin. That automatically spaces the weight up just high enough that the opposite side sits dead level with the nylon weight pad. Take one or the other away and the weight rides crooked. Or in this case add a 41 weight with a triangulated raised boss and now you end up with a weight that sits crooked as hell at the pin end.
So to fix that you gotta grind off the raised boss. The later style HEI that uses this raised boss weight will not utilize a nylon set of pads as they did away with that in lieu of another raised boss that took the place of the pad. And to make all that work the pins were of the non flanged variety.

So technically speaking the factory Pontiac HEI used flanged weight pins that won't work without modification of the 41 weights. In which case the correct part would be the easiest route by far.

But since you helped design this deal supposedly, I'm guessing you might have forgotten about that. You know, engineers do things for reasons and such.

As far as the 375 center cam. Yes a lot of people have copied it. Although I'm not sure why because it's not a very well thought out choice at all. There are far better choices hands down for sure. Plus the 375 exhibits override. Not a good choice at all in my opinion. But it's cheap, and available I guess.