Hi guys!! Along the lines of "the more I learn the less I know". I recurved my HEI this weekend. I would like to share the procedure for evaluation and criticism. I clamped the dist gear in my vise (wraped in a rag) so the dist was vertical. I then began marking the dist at 180* then 90* a and kept dividing a measured section in half untill I had 2* marks. I then applied outward pressure on the centrifugal weights untill the reached max travel against the center plate. and recorded the advance Degrees as distributor degrees and multiplied by 2 to get crank degrees. I then looked under the center plate to make sure the pin slots were capable of additional advance (they were). The I studied what limited the mech advance. It appeard that the thinner "arm" of the weight limits the advance By virtue of the length, and the rate of advance by its shape. In addition The center plate also contacts the "arm pit" of the weight. next I notices that I could back off the "zero advance point" by grinding material off the weights where thay contact one another at rest, there by allowing additional advance. That Is the way I understood it to work! Next Here is what I did. I screwed a wire in place of the rotor. I first removed a little material from the at rest contact points to allow them to"retard" allowing for more travel. Them I began to reshape the thinner arms of the weights by shortening and producing a gentle radius. This increases the mechanical leverage that the weight applies to the center plate and allows more advance. I then found some plastic bushings from all those carb kits laying in my box and fit one to the limiting pins and slots to limit the total. It also seems that one could modify the end of the thin arms to creare a flat end to limit the mech advance. the bushing thing seemed better to me because I can adjust the thickness to tailor the curve. I will be installing an adjustable vac can and hope to try it out this weekend in the combat rat. WHEW that was a lot sorry so long!

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67 prostreet BB Camaro The Combat Rat
80 z28 Camaro 355ci
71 BB SS Chevelle (clone)
2002 35th anniv SS Z28 conv.

Milan,
Sounds like you are getting your hands dirty! The only thing I would offer is that with a typical HEI, you will want to LOWER the amount of mechanical advance in the distributor rather than increase it. The bushing is a good way to reduce the amount of advance. Hopefully the bushing will take care of it by itself. If not, you may need to shorten the length of the slot that the bushing rides in the autocam plate to get the mechanical advance down where you want it.

-Mark.

With my HEI, I also had to increase the amount of timing. Weird.

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1969 Base Camaro
Vortec 355, Perf. RPM, Demon Carb., TH-400
All sheetmetal is NOS GM
See my webpage at: http://www.geocities.com/compuboy007/

Mark I used the stock weights and center plate I think that is why I had to increase my mechanical. It only offered 10-12* advance. If this works as I hope it will, the HEI in my chevelle is next. I ran that with 15* base and 45 total with vac advance. Assuming the vac applies 15* that would leave 10-15 in the weights. and occasionally she cranks over slow after a brief heat soak so the reduction in the initiall timing would be welcome here as well. I hope?!? I'm finally hoping to get the camaro to the track if this cures my startin problem. http://www.camaros.net/forum/smile.gif

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67 prostreet BB Camaro The Combat Rat
80 z28 Camaro 355ci
71 BB SS Chevelle (clone)
2002 35th anniv SS Z28 conv.

Sounds like you've thought your way through this pretty well. Fine example of how using your head will allow you to accomplish all sorts of things without several grand worth of special equipment.

The only thing your going to have trouble modeling is when the maximum advance kicks in, but trial and error should get it. Good thing the springs are easy to replace.

An old timer Engine Builder friend of mine cusses HEIs because of the bushing wearing out and that they give 'way too much advance'. He does essentially whay you have done. He also replaced those bushings with a bronze piece, but I don't know where he got them.

-dnult

The magnetic triggering of the HEI tends to retard the timing at higher speeds, so you need more advance measured on the bench (static) than will show up running (dynamic) in the car.
There is enough slot to go a bit retarded in the advance mechanisim, but the springs may not actually pull the weights that far.

I once made a collar to attach an engine degree wheel to the distributor and spun it with a variable speed drill.
You can hook up the ignition and run a timing light on the degree wheel.

I highly recomend running it on a distributor machine.
Hope this helps.

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Check my web page for First Gen Camaro suspension info:
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67 RS 327 original owner. 69 Camaro Vintage Racer, 65 Lola T-70 Chev SB Can-Am Vintage Racer

I probably will run it on a dist machine Just to evaluate how my work rates. For now That will have to wait. I set up my vac can for 12* of advance. so my dist specs are as follows total of 28 crank degrees mech and 12 crank degrees vac advance. This will allow me to have 6-8 degrees cranking and base advanve 18-20* at idle and 36-38 under load. I will have to see how much the module retard effects this But I feel that I should be able to crank this compression monster over have a reasonable idle and good load advance. Sure hope it works http://www.camaros.net/forum/smile.gif as always will keep ya posted!

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67 prostreet BB Camaro The Combat Rat
80 z28 Camaro 355ci
71 BB SS Chevelle (clone)
2002 35th anniv SS Z28 conv.

Well for the first time It started and idles in my drive with no coaxing to keep it running. It idled at 1200-1500 RPM. The base was at 5 degrees 25 Degrees vac advance. I had started with medium "kit" springs and the mechanical timing was all in at idle. I then installed one stock spring and one "kit" heavy spring. I have yet to check for total timing as well as rate. It was getting too dark to read the dvom/tach so I packed it up for the night. But man It idles smooth. Thats a first!!!!!! I have more tuning to do before fridays test and tune. Hope the rain holds off.

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67 prostreet BB Camaro The Combat Rat
80 z28 Camaro 355ci
71 BB SS Chevelle (clone)
2002 35th anniv SS Z28 conv.

It sounds promising! Keep up with the updates. Vacuum advance is a good thing! http://www.camaros.net/forum/cool.gif (I am starting to sound like Martha Stewart.)

-Mark.