Grading On The Timing Curve....

[Good Wrench]

Hiyall,

I'm wanting to re adress my centrifugal advance on my Chevy HEI distributor.

Does anone know the optimum curve? When they should start advancing? How far should it advance? I been just changing springs around and testing it out but i'm tired of guessing, I guess.

Static advance is 12 degrees BTDC. Stock cam n crank timing. Stock vaccum advance. Car is red

[Good Wrench]

Ooops yeah,


It's the 76 Monte, 350 4bbl THM 350 tranny with a stage 1 shift kit.

[65ny]

I recently did the same thing on my '76 Monte. I got the best results from this combo: 16 deg base timing, 20 degrees mechanical starting around 1000 rpm, all in by 2800, and 15 degrees vacuum advance.

Try to shoot for 52 degrees of total timing (base + cent. + vacuum advance)
The trick is to get all your timing in at an RPM that is reasonable to you, and not advanced to the point that you have pinging and hot-start problems. I spent an entire weekend tuning and tweaking to get my car where I wanted it. Somewhere I have a link that gave me all the information I needed. I will look for it. I don't think I can post it here since it is from another forum. (I think this violates forum rules, someone correct me if I am wrong) If I dig it up, I will send you a PM

Making these changes made a HUGE difference.

[Good Wrench]

Dang! 52 degrees total timing? I think i'm sitting at 45 now and it makes people cringe.

However, i'm gonna try it and let them try to stop me.

Thanks for the link!

[Dave Severson]

52 degrees on an sbc????? Better recheck that. I would think 8-12 degrees base timing, then all the advance in somewhere around 2800. If you check it with a light that has the advance knob on the back I think you'll discover it's around 32 to 36 degrees total...

Base timing is set with a light with the vacuum unhooked, then reconnect the vacuum and the vacuum advance and the mechanical advance combined will pull around 24 to 26 degrees for a total of 32 to 36 degrees. At 52 degrees of timing as measured with a light, don't think you'd even be able to keep it running, but if it did it wouldn't run for long.......

[65cayne]

52º..??? ... total should not be over 36º

Do you mean "TOTAL" total? Or just "initial and mech" total?

I dont think that sounds unreasonable (52 deg). I think what he is alluding to is cruise/no load timing such as the highway where he might be running @ 2900rpm or so, initial + mechanical +vac adv puts you right in that ballpark wouldnt it?

Case in point, and tell me where this goes wrong....

'69 350cid, 255hp (extracted from a MOTOR's Manual)

Initial timing is 4deg BTDC
Mech is 32deg @4400rpm (crank degrees, advance starts around 1000)
Vacuum advance is 24deg at 13" vacuum

So at 4400rpm cruise, it would be 4+32+24 = 60 degrees assuming 13" of vacuum

Backing down to 3000rpm...you have 4+18+24 = 46 degrees total timing.

Okay, not quite 52 degrees , but more than 36 degrees. Point being, if the car is tuned to run lean and clean, should it not be able to handle the extra timing at cruise?

[65cayne]

Just to clarify what you said Denny,

With what you have as far as settings go, as I understand it, 8º is base timing. Easy enough to see

Intermediate advance is 3 1/2 at 800, that's centrifugal at rpm of distributor.

Full advance is 10 at 2100. full mechanical? crank or dist deg?

Takes 4 of vacuum to start the plunger.

Max advance distributor degrees at 15" is 12º. is this max vacuum advance?

That's almost half (more than half ?...8+10+24 = 36)of what you are saying you are going to try. total should not be over 36º

Trying to clarify where you are talking crank degrees and where you are talking distributor degrees...thanks

[65cayne]

Re check that manual. Distributor timing is half of the timing degrees. So that would be 16, instead of 32, and 12 instead of 24. So you have 4 + 16 + 12 = 32º on the balancer, damper.

The mechanical and vacuum advance was given in distributor degrees which I doubled to give the "as seen at the damper" degrees.

4 + (16x2) + (12x2) = 60 (assuming at 13" of vacuum is present)

BTW: thanks for the clarification in your second post. I think maybe the gorilla in the picture typed the first post? kidding...

[Good Wrench]

Dang, I think i'm getting it now. Well, at least i'm not going to yank the stinking cap off again for a while.

Now, I got my crank timing set at 12 degrees BTDC with the vaccum advance unplugged. The weakest springs I got will start the centrifigual advance a 900 rpm and advance 20 degrees to 8000 rpm.

I'm using one of those.

The next spring starts at 1600 rpm and advances 15 degrees total at 6000 rpm.

I'm supposing my real advance is somewhere in between then.

I donnou if I even see 6000 much less 8000 rpm.

So, ballpark advance on the weights would be 16 degrees starting at 1100 rpm? With the springs I got right now?

Sooo, mine would be 12+16+15= 43 degrees? Am I right?

[Dave Severson]

Quit the math games and either put a timing tape on your balancer or borrow somebody's light that has the timing advance knob on the back.... I seriously doubt your springs are advancing near that much. You will find that the springs advance simultaneously, not separately. Read your real timing at the balancer and the timing mark to see what the actual advance is....

[Good Wrench]

Yes, one spring starts advancing at 900 rpm at 20 degrees at 8000 rpm and the other starts advancing at 1600 rpm at 15 degrees at 6000 rpm. I'm just splitting the difference. At least this chart I got says so.

All I could find this week was a single action point and shoot timing light as my other one is in Ireland.

Tape won't do me good cause I got one of them fully illuminated genuine chromium plated accessory timing plates that cover the whole dang harmonic balancer surface right to the very end an all you see is a teeny speck of the timing mark.

Now, I know i'm just making excuses and of course, Mr Severson is elevated and correct and of course would end all the guesswork out of total timing.

But I can't help feeling deflated like a total failure so if you'll excuse me I will now go ask my wife to beat me senseless with my new useless timing light. I will return if I survive.

[Dave Severson]

Not putting you down, just hate to see someone get stuck on the math and book stuff instead of the "real" stuff....

[erik erikson]

I would say have all your advance out by 2,800 rpm's.
I would try total timing of 33-36 degree's

[Dave Severson]

Ah O, did you hear that Tech...???? No more book Tech stuff....

Some days your reading comprehension is really off....New Meds????

[erik erikson]

Some days your reading comprehension is really off....New Meds????

MEDICATION TIME!!!