Well I’ve managed to screw up my timing curve. I was trying to reduce the amount of mechanical advance in the distributor by having some of the advance come in before idle by stretching the spring. It worked. But now the first unforseen problem, hard cold starts (too much advance while cranking). To make it work I had to retard the timing to 12 deg from 16 deg initial. I could retard the initial some more to counter this problem but I don’t want to lose more performance. The second unforseen problem is that the curve slowed down. I don’t have that quick advance jump (2000-2200rpm) anymore. It looks like I need to buy another curve kit.

Anybody got recommendations for a decent HEI curve kit that has only 20 deg mechanical & that comes in by 2400 rpm or sooner? Anybody know of an aftermarket HEI that allows you to limit mechanical advance? TIA

 

nunus;

Best way to take mechanical advance out of the distributor is to close up the slot in the plate on the mainshaft. Check the article in Chevy High Performance, Sept. I believe. If you're interested, e-mail me and I'll give you the specifics of where to find the article.

 

nunus -
I've done quite a bit of playing around with the HEI curve system, and here are some thoughts...

First, the HEI curve length is not determined by the length of the slot on the cam plate like on the point-style distributors - the curve is determined by the geometry and leverage of the weights against the center cam. To change the length of the curve, you need to change the geometry of the weights, and this is difficult to do with the stock-type HEI weights. I have not seen any aftermarket curve kit that will properly modify the curve, and that's why I ended up building my own curve kits.

You can build a set of weights that will allow you to adjust the curve length. You can buy a Mr. Gasket curve kit for a standard distributor (part number 927) and use the standard distributor weights as a starting point. You'll need to shorten the "long" leg on the weights by about 1/8", and shorten the "short" leg by about 3/16" and then round off and blend all the radii. Then, you'll need to open up the inside radius slightly to clear the stock HEI center plate. A weight that is modified in this fashion will not run out of leverage at all, and you can get infinite advance out of it.

Obviously, you don't want infinite advance, but a controlled amount of advance. A weight modified as described will eventually hit the inside diameter of the rotor as it is flung outwards by centrifugal force. The rotor thus stops, and limits, the advance. By shaving a little material off of the outer lip of the weights, you can make the weights advance further and lengthen the curve. By adding a little dab of epoxy to the ID of the rotor, you can shorten the curve by limiting how far the weights are allowed to get thrown out. You can thus easily custom tailor the curve to the exact length you want. Once you have tailored the length, you can then swap around some springs (the springs in the 927 kit are pretty light, so the stiffest ones are about right) to bring the curve in right where you want it. When tailoring the curve, keep in mind that you don't have to have both of the springs the same: you can run any combination of springs to really tailor the curve to where you want it to come in.

Finally, keep in mind that if you create a curve that brings in full ignition advance of 36-or-so degrees at or below your cruise rpm (about 2500 - 3000), you need to pay attention to your vacuum advance, too. Most factory vacuum advance units bring in quite a bit of vac advance at cruise (about 18-22 degrees), and can do so since the factory centrifugal advance is never all the way in at this low rpm. If you get more than 52 - 54 degrees of TOTAL (full vac advance with full centrifugal) advance at cruise, the car will buck and jerk as you're cruising down the highway. You must, therefore, select a vacuum advance control unit that will not produce more than 16 degrees of vacuum advance (8 distributor degrees) maximum. 16 plus 36 is 52. Echlin part number VC1838 will pull in just over 14 degrees, thus allowing a max mechanical/centrifugal advance of up to 38 - perfect.

Contact me if you need any assistance.


[Modified by lars, 10:22 AM 8/14/2001]

 

noxqsz
Thanks for your reply. You got me thinking. I have a points-style distributor with the slots sitting on the shelf. I am now seriously considering using it instead of the HEI because of its advance limitation ability.

Lars
Thank you for the excellent information. Doing the epoxy-thing to limit the advance is something that I would want to do as a last resort. Because I have an old points-style distributor in excellent shape, an MSD 6a box, and MOST IMPORTANTLY the ability to still get your curve kit for points-style distributors, do you think it would be a good move to use the points dist to address my problem?

PS The qjet jet & rod info that you sent has really helped in making wise combination choices. Very easy to use & understand. The listing of stock settings for various years gives me a good sense of the proper sizing ranges for different applications. Thank you very much for sharing that info.

 

The point-stytle distributor works extremely well when used to trigger an MSD-6A in conjunction with a good coil. As you note, the point-style distributor is also very easy to tune in terms of advance curve length by altering the length of the advance slot. If the slot is a little too long, I like to weld just a little material into the end of the slot, and then use a swiss file to re-shape it to the desired length. This creates a reliable and permanent curve length. If the slot is too short, it's a simple matter of filing it a little longer. If you have a set of calipers, it's very easy to determine exactly how much to shorten or lengthen the slot: With the stop pin in the slot, measure how far the pin can move from end to end in the slot. Say it moves a total of .200". Then, run the distributor and determine how many degrees of centrifugal advance you're getting from beginning to end. Say you're getting 20 degrees. So .200" is equal to 20 degrees. Then divide .2 by 20 and you see that each degree is equal to .010". So to change the length of your advance curve 2 degrees, you would change the length of the slot by .020". Simple, eh? Obviously, your actual numbers will be different from these, but the process and simplicity is the same.

Glad to hear you found the Jet Sizing info interesting and useful. It is kinda' fun comparing some of the numbers and using the tables to make educated guesses in the jetting and tuning process.


[Modified by lars, 2:17 PM 8/14/2001]