Hey how much advance should a Vac. Advance give me. I am getting 38* at 1200 rpm and 50* at 2000 I keep getting noise from the motor and its all new.......I am just trying to learn what kind of advance I SHOULD be getting.



Thanks
Brian Felts

 

35* to 38* should be about right for a SBC. what's your base set at? vacumn ought to give you 10 or 12 degrees and mechanical around the same. you can find out what you have with a dial timing light.

 

Thanks for helping out Ede...........I used my dial back light and thats how I came up with those numbers........I am thinking that 50* of advance at 2000 is very extreme. I have the base set at 12* I have 32* of mechanical advance @ 3000rpm so we are fine there. Thanks for any input.



Brian

 

I agree, your numbers sound about in the ballpark... 12° static, plus 20° of mechanical, plus 12° of vacuum; that would be about 44° total high RPM no load, which is fairly normal. Like ede said, 35-38° is a typical full-load high RPM advance (no vacuum at that time) plus the 10 to 12° from the VA, so you're right there.

Timing usually is kind of quiet though. What kind of noise is it?

 

Does the vacuum advance, and mechanical advance set it self??
I only know one way to set the timing, which is turning the dist. housing with a timing light at the damper. Does this method set the base timing? mechanical? or both?

As you can tell, I am kind of lost on this setting the timing thing. And my timing light has no dial, but I have timing tape.

Can anyone help?
Thanks

 

Static advance (the one you set by turning the dist) determines where the dist shaft is in relation to the camshaft. Mechanical advance is controlled by some weights and springs that work by centrifugal force (actually, centripetal force, since there's no such thing as "centrifugal" force, that's just a figment of your imagination) like a governor against little cams on top of the dist shaft, that move the pickup in relation to the dist shaft. You can change it with different weights, springs, & cams. Vacuum advance is controlled by a little vacuum diaphragm. It adds advance by rotating the stationary side of the pickup system when the throttle is open beyond a certain amount (ported vacuum), and when there is enough vacuum to overcome the spring in the diaphragm.

Typically you want your base timing to be anywhere from 6 to 15° depending on compression, fuel, temp, gears, converter, etc. etc. etc.; you want mechanical advance to start advancing at around 1200-1400 RPM, and to reach its full advance (to around 36-38°, again a tuning thing) by about 2500RPM or so; and vacuum advance to be 12° or so on top of all that, for best gas mileage when cruising.

 

so the distributor decides vacuum and mechanical advance.

I have a Mallory Unilite dist. that says it comes with 24* mechanical. It said nothing of what the vacuum advance is, just that it has it. Should I just assume it will be about 12*?

If so, that would give me 36* before setting static right? Then I would just need to set the timing like you said, 6-15*, Is that btdc, or atdc???

my mods are in sig.

 

The noise I am hearing I think is a lazy lifter.........I hate it but it just seemed to go away when I have the advance line off driving the car..........but it will go away when the car warms up anyway......just in my head.


I have delt with the computers for so long I do not remember the numbers I should be getting so I really appreciate the help. I do need to recurve the dist. to get more advance at 3000 rpm I only have 32* and I would like 36*


Thanks
Brian

 

That would be BTDC.

Just as a WAG from your combo, I'd start at around 8° BTDC static, and go up if from there if it seems like it wants it. I don't know offhand what Mallory sets their vac advance to, it's kind of generic, so it's probably 10-12° more or less. 24° mech is reasonable for a performance (not stock) dist.

So, you'd end up with 8° or whatever of static; plus the 24° of mech at higher RPMs; plus 10-12° of vacuum at low load and above idle throttle opening.

 

Thanks again RG. I should be able to get it running fairly well from there. One last thing and I will stop with the timing questions already. Do I only need to set the timing at idle? I hear other people talk about their timing curve at different RPMS.

 

You can only set it at idle; below 700 RPM for instance. The weights & springs determine what it does at other RPMs.

 

Right on,

Thanks for all your responses.