sorry for the dumb ass questions... but i totally forgot about everyhting i learned lastyear when working on my camaro...

ok setting inital timing i want it set at about 8-12* BTDC correct?? (higher up on the timing tab is more BTDC right?)

now "advance" means MORE * BTDC or less * BTDC??

i hear mechanical "advance" adds about 28*
and vacume "advance" add's like 10* or something and that vacume advance is only on at part throttle

say i set initial timing at 10* BTDC

what does the rest do??

i feel like such a dumbass because i should know this.. but everwhere i look i cant find answers explained in plain english...

(only the stupid people dont ask questions,the only way to learn to is ask,and to ask is to know)

BTDC = Before Top Dead Center

More advance means the spark plug fires sooner in relation to top dead center. IOW, 18 degreees BTDC is more advanced than ,say, 12.

Total advance is always equal to initial advance + mechanical advance + vaccum advance. The timing mark will move towards the passenger side of the car as seen with a timing light as advance increases.

Mechanical advance is zero at idle and grows to around 20 degrees as the engine rpm increases to about 3,000 rpm. The total amount of mechanical advance and the rate at which it "comes in" is adjustable via the weights and springs in the distributor.

Vacuum advance also varies with manifold vacuum and ranges from zero degrees at wide open throttle to somewhere in the 8 to 10 degree range during part throttle cruising. Again, this is adjustable using an aftermarket vacuum advance can.

So, to review two typical situations:

At idle, say 800 rpm. You will have 12 degrees of initial (again, this is adjustable) plus zero degrees of mechanical plus 10 degrees of vacuum for a total of 22 degrees advance.

At 70 mph on the highway at say 2,700 rpm you will have 12 degrees of initial plus 10 degrees of vacuum plus 20 degrees of mechanical for a total advance of 42 degrees.

These figures are just typical examples. Some cars like as much as 50 degrees on the highway. There are many variables such as altitude, octane, engine design, etc. that will determine an optimal advance curve.

HTH.

alrighty.. its all coming back to me now...

thanks...

also when changing the curve "springs" in the distributor should i go with the lightest or the medium ones?? i was told the medium ones should be fine?

engine is a 327(bored .030) with single plane torker2 intake 780 holley carb and Melling 22301 cam Duration .050-Int. 230, Exh, 230. Adv. duration Int. 287, Exh. 287. Valve lift Int. 480, Exh. 480. Center Lobe Int. 107, Exh. 111. street and strip, rough idle, lower vacuum, power range 2500-6000 rpm-hyd.


rear is a 3.73 trans is a TH-350 with a 2600 or so stall.. if that makes a difference

Depending on what type or brand of distributor you have, you should have gotten some "data" on what the advance would be with the different sets of springs. For example: Medium = 0* @ 1k rpm Light = 8* @ 1k rpm The lighter springs will give you more mechanical advance sooner, and will provide for total advance at a lower rpm than the medium springs.
I would guess, with your engine set up, you are going to want to use the lighter springs to make sure that you get full mechanical advance by 3k rpm. Most of the advise I've seen from others in various threads on this subject agree.
I have a Petronix Flamethrower and I used the lighter springs to get more mechanical advance sooner and to have full by 3k rpm. The difference was very noticeable after changing from the mediums. I've got a little less cam than yours (comp268), so I would think you may see and feel the difference even more.

A SB 350 likes 38* total advance at 3000 rpm. If you don't have one of these SUN-CP7519 http://static.summitracing.com/global/images/prod/large/sun-cp7519.jpg
get one of these,
-set the dial to 38*
-with the car fully warmed up
-loosen the dist hold down bolt
-plug the vacuum advance
-have a friend rev the car and hold it at 3000 rpm
-turn the dist so your timing mark hits zero (0).

Go for a drive and hammer it through the gears, if it pings then backoff timing 2*.

------------------
Mark D
1970 Z28
http://http://ca.geocities.com/mlvd@rogers.com/ (http://ca.geocities.com/mlvd@rogers.com/)
WTB: 70 SS396 Chevelle 4-spd, flapper hood car, no rag tops, 1-2 owner car.