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Discussion Starter #1
Hey all....My first time posting here and I know this subject has been beat to death. I've read all available post on this subject but still am confused about the numbers I see. Some pity help would be appreciated. I used to do it by "get it to ping and back it off, done". Now I want to do it right. I just bought a advance timing light from Sears. Here are the number I am getting with the vacuum line removed and plugged. Chevy 383, 10:1, Mild cam, HEI. Every piecesparts are new in/on the engine. 500mi.

RPM / dial reading @ initial on tab / Dial reading @ zero on tab

700 / 0* / 14*
1000 / 3.5* / 17*
1500 / 10* / 24*
2000 / 15* / 28*
2500 / 16* / 30*
3000 / 16* / 30*

1. It appears I have 30* total advance. By what I've read I should have about 35-36*. Do I have to enlongate some slots in the flyweights to get more advance. Or should I move the initial up to 19 or 20* to end up w/ 35-36*

2. Also my mechanical advance is all in right at 2100 or so. I put in 1 heavy spring and left one original and no change in full in RPM. What gives?

3. Once I get this part straight I can proceed on to trying to figure out how to adjust the vacuum advance once I install my new adjustable can.

Thanks
 

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wind & fire = guides to power
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You springs are too stiff(or the weights are sticky..make sure the move freely by hand)
 

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Discussion Starter #3
I just had it apart and the weights are as free as can be.

Would'nt I want stiff springs to keep the advance in longer? lighter ones would allow it to start earlier and finish earlier, right?
Or am I thinking Bass Ackwards.
 

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Beyond the Sea
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My HEI works exactly the same, I also only get 15 degrees all-in at 2100 or so. I just set my initial to around 21 degrees. It still starts fine and has lots of get-up-and-go (for a 283 that is). The problem is in trying to maximise gas mileage via the advance can with such a set up. Assuming you set it to 20 intial + 16 mech. = 36 all-in then you need about 15 more for 51 degrees advance at cruise. These 15 will be too much at idle, giving you 35 degrees (20 initial + 15 vac. can). This causes my 283 to run a little roughly. It runs smoothest with around 27-30 degrees at idle. So my simple solution is to run ported vacuum instead of manifold vacuum which I normally would prefer. Hope that helps.

If anyone has a better idea of how to set it up given these paramters I would like to hear it too.
 

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Discussion Starter #5
Well I'm not sure how your adding an extra 15 to get 51, vacuum advance? I would really like to get there using my mecanical advance. My guess is to enlongate the slots somehow. Seems very time consuming and hit and miss. Just seems strange that it's so low, and the fact that it's all in around 2100. At least there is someone else out there that is getting the same readings I am.
 

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Beyond the Sea
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Yes, I meant the vacuum advance adds another 15 degrees at cruise (depending on your vacuum can of course, you can increase the advance by filing the slot in the can bracket, I did that today on mine). So if your starter is up to the task set your intial to 20, your mechanical will pull in another 16 to give you 36 at WOT at above 2100 RPM in your case. Thats pretty quick for the advance to come in, if your engine doesn't ping it's OK that way.
 

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Discussion Starter #8
well today I set my initial to 19*, w/ the vacuum plugged I drove it and it pinged so I had to back it down to 12*. I didnt have the time today to mess with setting the vacuum advance(its adjustable). that will be done tomarrow.
Could it be that my mechanical is coming in too soon and thats why it pings when I set it up above 12*
 

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Good topic! I built a '71 Firebird 350 down in Brazil 4 years ago with 10:1 CR and a mild cam.Back then all we had was 87 octane.I installed a new HEI and had to back timing to as low as 4 degrees advance and almost no vacuum advance or it would detonate furiously.Engine ran poorly with only 14 "hg.Then a new 91 oct gasoline became available and I could advance to 12 degrees.Vacuum signal rose to 17"hg and I even had to close the throttles a little.Now it idles well at 750 rpm.max advance is 38 degrees. I am still exploring the vacuum curve. Paulo
 

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Discussion Starter #10
Someday I'm going to Rio. Yeah I'm into Firebirds pretty thick. I have 3, two '67's and a '68. Would like a '70-'71 Formula or T/A. They are a better handling car w/ a few mods.
I'm see 12* and 17hg but alot less advance at 28*runs pretty decent though.
 

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Highstick, I have a 383 with 200 psi of cranking pressure that likes 36* of timing. I would say 30 is not enough but my cars mph is the same with 32* as with 36* except the low end likes 36. If your timing is coming in at 2100 thats pretty quick, I have ran mine with a curve like that and it would ping sometimes. Ive tried many curves with my 383 and found that its best when initial timing is pretty high, about 16-18 and comes in at 2400-2800. I know all engines like different stuff but we are talking 383s here.

If you want to recurve your distributor its fairly easy to do, I have found alot of tricks. Its a good idea to get various wieghts to try, and the center pieces. Alot of GM hei's have different shaped wieghts,bushings and springs that can be mixed for alot of different curves. You can also shave metal off the wieghts or drill little holes to slow the curve, but dont do too much. On my hei I put a small screw in one of the little holes that are right next to the ones for the rotor, that way I have less total so I can bump up my intial. Theres a few other ways to do that but would take forever to explain. You say you got detonation with 19 intial? are you running iron heads? 19* shouldnt be a problem, its probably because or the quick timing curve or you have too much vacuum advance. I would say shoot for atleast 32 total, your intial should be where the car isnt hard to start. If you have more questions let me know Id be glad to share some things Ive learned.
 

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Discussion Starter #12
Hey Nitrous...I have the SR Torquer iron heads. The engine has no problems starting between 10*- 20* initial. I drove it at 19* w/ the vac line plugged thinking it shouldnt be influenced by the vacuum advance, vac signal goes away anyway @ WFO, was this correct thinking? Thats when it pinged so I had to back it back down to 12*. I know I need to slow the rate in which it advances. Now if I increase the max mechanical (currently 16*) by increasing initial to say 19* and adding heavier weights and/or springs wouldnt that (weights and springs) raise where it cuts in at to. Unless there is a weight that is shaped in a way that it will still cut in early but be fully extended later. where do i get all these various shaped weights, seems like all the kits have the same weights. Thanks Nitro
 

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Adding heavier wieghts will move the whole timing curve upward. Although some of the spring Ive taken out of various HEIs are heavy springs but are longer at rest so they may have the same tension as a lighter spring at rest but have more tension once the timing moves.

The wieghts that Im talking about are from older HEI units that have all sorts of different curves. They not only had different wieghts but different center pieces AKA bushings. Some bushings relocate the pins that your springs connect to causing more or less initial tension on the springs. What I mean is, they have the 2 holes in a different spot in the little plate, which is essentially the same as adding heavier springs. Its not too hard to find some of these other pieces, if you really want to customize your timing. I got alot of HEIs dirt cheap that were no good just for the advance stuff.

I started out with a Mr.Gasket curve kit and used the bushing from the kit and stock HEI wieghts. When I installed the center piece(bushing) and started the car the initial timing had moved up a bit, and the starting point of my curve was later as well. Thats because the holes for the pins were moved away from the pins that hold the wieghts, adding more tension and advancing the rotor. This was actually good for me because my engine idles at 1050 and I need the timing to start later. The only problem was that it also moved my total to a higher rpm. To fix that I ground one of the wieghts on the "finger" we will call it, and increased its tork against the center piece aiding in quicker advance at the end of the curve but not effecting the starting point of the curve. Oh boy, I hope that makes sense, but anyway thats just to give you some ideas I hope it helps. You can make any curve using these and some other techniques, I have made alot of different timing curves just by being creative.
 

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Ok Highstick.Welcome down.But since you are from FL the climate is almost the same. Distributor curve is indeed a science.It can make a huge difference on your car.The variables are endless.For the computerized cars there is a kit from South Africa called Unichip that allows you to do anything you want with ignition curve and fuel curve.And you don't even have to open your computer.Magnificent.I am lucky enough to be at the only shop with a chassis dyno in Rio and we install these kits on cars from 911s to Golfs.Sometimes we gain 20% more power changing no motor parts whatsoever.And usually leaning out on the fuel curve to boot.But the spark curve is detrimental. Paulo
 

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Discussion Starter #15
The spring length makes sense...I didn't know they had different length springs. The longer the the spring as you described would delay the cut-in and would be full in later.
Cool, I will experiment with all these different kinds of weights and springs. I thought the designs of these were limited.
I just wonder now how much more performance I could have gotten out of my previous engines that I just got it to ping and backed it off some.
Thanks for your time and explaining this...I have a good grasp on what I need to do now.
Too bad 67's didn't have computer controlled engine controls..The problem is the computer would have taken up the whole trunk.
 
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