The topic pretty much sums it up. I have ben running my 408 Motor with 18 initial and decided to mark the balancer at 36 deg total advance and time the motor for total advance. Have an 8" diameter balancer, so I marked it at 2.5". Upon running it found out the 36 Deg mark was not even on the timing tab [ revving motor until no more advance was evident]. Reset timing to 36 deg. total. If I had to guess I would say the timing was 30 deg max before. Now the intial is off the other side of the timing tab. The upside to this is I have a noticeable SOP power increase across the power band.

I had thought that these (Summit HEI) had 18 degrees built in, I now wander if it only has 12 - 14.

Most of the Chevy HEI's I've seen only have about 14-16 (crankshaft) degrees of mechanical advance. You have to do some grinding on the weights and weight cam to get more.

I have been running one of the Summit HEI's for 6 years and it did only have 12-14 mechanical advance initially. I flipped over the center weight and I use original GM HEI weights with a little grinding on them to get 18 mechanical out of it. I also have only one "light" spring attached, the other weight is held in place with an "e" clip. When running "all-out" I remove the spring and attach another "e" clip. My motor likes a lot of advance.

Far cry from the old points units I am used to playing with. I can remember having to play with progressively larger bushings to get the timing down to 20 degrees mechanical advance. I will have to adress this. Went out for a drive last night and with the motor at operating temp, it will kick back when you first hit the key. With 12 -14 that would put me at 22-24 initial. Surprising, though, I would think with the cam I am running and what I thought is supposed to be 9.5 CR, that this would not be an issue. I am now wondering if I made a mistake when CC'ing the heads and missed some fluid volume. Either way, thanks for the input, I will play with it this week - sure can't run it like I have it set up.

Usually on HEI's the bushing and slot isn't what limits the amount of mech advance, it's the springs and weights/cam. You could take the dist out and measure with a mm scale how much slot is left when the weights are fully extended or how far the weights sling out past a fixed point, then grind the weights and cam to get them to sling out more. Be careful not to get carried away and have them go too far as they can lock fully extended. You should be able to get it to around 20 degrees anyway. Some people, as previously mentioned, have had some success with just flipping the weight cam over so try that first.