The Money Pit

I've played with the initial timing a bit and feel there's more to be gained in tweaking the advance curve. So I ordered a kit from Summit,and the lightest springs in the kit supposedly pull a curve from 1000rpms to a full 24 degrees in at 5000 rpms. This is the lightest set of springs. Seems like the norm is all in by 3000,and this is no where near. I also ordered a balancer cover that's degreed,so when it comes in,I'll be able to better map the curve.This is a 406,and from asking around,the bigger engines don't seem to require as much advance to make peak power.I just read an article on a 400 similar to mine that pulled 554 hp and ran 35 degrees total advance.Anyone that has tinkered in this area please feel free to chime in.

Jvette73

I would bet that the lightest springs will bring full mechanical advance in long before 5k. When I went with the lightest springs I had a problem with the advance begining even at idle which is 900 to 1000 on my engine. It also makes a difference what weights yur using. If the weights in the kit are lighter they will come in later. I use lighter springs but stayed with the heavier stock weights. I rested with the lightest spring on one side and a medium one on the other. Trial and error will tell the truth. Dont count too much on whats printed. Are you runnin HEI?

Jvette73

The shape of the weights can also have an effect on the length of the curve. Stock weights may only provide 16 in advance whereas the kit weights my give you like 24. So if your initial was set at 14 then with stock weights the total would only reach 30. or with the others you could set at 12 initial and get 24 from advance giving you a total of 36. So again it all comes down to trial and error. Be sure to take your readings with vacuum advance disconneted. Set the timing to 36 total at higher rpms with full mechanical advance achieved. The tension of the springs will determine at what rpm its all in. Dont worry too much about yur initial setting. Set it on the top end. The initial will be the 36 minus whatever the length of the curve is.

The Money Pit

I forgot to mention this is a HEI,and the kit has a top plate and wieghts. I'm going to wait till the degreed balancer cover is on,so I can T/E with a bit more precision.Should I try to hit full advance by 3000 rpms,or is it more streetable if it come in later?

Jvette73

Full in by 3000 is best. Again dont waste too much time setting initial at idle. Its the full advance at the top thats important. The initial timing will be pre-determined by the length of the curve when the engine falls to idle and the springs retract the advance weights. (example) I have the stock points type dist with pertronix ignitor II. The spring combo I selected brought my full advance of 36 in by 2500. When the engine fell to idle my initial was 16, indicating the length of my curve is 20. Like I stated before, if the springs are too light, the weights can be in partial advance even at idle. When setting the timing at the top side with higher rpms, idle speed and initial timing setting is irrelevant. Just be sure that the springs arent too light as to cause the weights to be in advance at idle. The shape of the top plate and weights is what will determine the curve length. Check the length of your stock curve prior to to swapping the other stuff in. Then you can compare after. You can do this after you get the balancer cover on. Cut your idle way down to be sure yur weights are not in advance and note the timing setting. Then rev the engine to get as much advance as you can and note that reading. The difference between the two will be the length of the curve. Curve length is important since yur trying to get 36 total. If yur curve length is only 14, then yur initial would only fall to 22 to equal the 36. Thats why its important to create a longer curve. A 20 curve would allow yur initial to fall at 16. I had this problem when curving my dist. My curve was too short at 14 and my initial needed to be 22 in order to achieve the total 36. I modified the stock plate to lengthen the curve to 20. Now my initial falls to 16, which is more acceptable than 22. Hope im not confusing you. i got it in my head, but putting it in writing is another thing. :D

PS do all this with vacuum advance dis-connected. then re-connected to a ported vacuum source after done.


[Modified by Jvette73, 1:27 PM 1/1/2003]

Vetterodder

Generally, the sooner you can bring the advance in the better but too much too soon can result in pinging and drivability problems. Car weight and gearing can make a big difference. A light car with low gears will tolerate an earlier curve than would a heavier car will higher gearing due to less load on the engine at a given rpm. The only way to know what works best for your particular application is to try different combinations and see how they work.

As for the instructions saying that the lightest springs won't give full advance until 5,000 rpm, I think they probably goofed and they actually meant the heaviest springs. If your goal is 36º total, there's an easy way to set it. Measure your balancer's circumference and divide that figure by 10. Put a mark that distance from the TDC mark on the balancer and you will have a mark at 36º. With vacuum disconnected and plugged, rev the engine past where it quits advancing and align the new mark with 0º.

73 LS-4

Actually from what I have read and it makes sense to me, the larger the bore the more timing adavance you can run. This makes sense if you think about it. A flame front will travel at a given speed. The time it takes this flame front to cross a smaller bore is less then it is to cross a bigger bore. therefore to have the entire cumbustion process done at the same point of time (optimum is 12.5 degrees atdc), you have to give the bigger bore engine a little more of a head start, therefore more advance. Of course there's a lot more variables involved here then just the bore, such as cumbustion chamber size and design, but this is normally the case if all other variables are the same except the bore.

:cheers:
Pat Kunz