Need Help With Timing Curve (I have Numbers)
 

Hello everybody. I haven't been on here in awhile.
I know this subject has been talked to death, so I will try to keep it straight forward and simple.

I know it needs to be 32-36 degrees in by 3,000.
My initial timing is set at 12. My distributor has 20 degrees of mechanical advance in it, with the stock springs it totals out at 4,500rpm. My all in without vacuum is 22 degrees at 3,000rpm. So theroticly I just need lighter springs to gain that last 10 degrees of mechanical advance earlier. The problem (I think) is going to be the vacuum advance. At 3,000rpm with vacuum connected I am at 47 degrees. If I add the extra 10 without limiting the vacuum advance that will put me at 57 degrees at 3,000rpm and that just seems like too much.

I drive this truck everywhere. It has a TH350 and 3.73 gears with the factory locker. This truck see's a lot of highway trips so my cruise rpm is about 3,000 at 60. I'm afraid if it is going to ping it's brains out it's going to be here. According to my calculations my vacuum advance can seems to have 28 degrees of movement in it. That also seems like too much.
So do I limit the vacuum advance, and add the lighter springs?
Or just add the lighter springs?

For what it's worth, the truck drives great. It doesn't lack power, it has plenty of torque, Idles smooth, and I've never heard it ping. Sea level, high altitude, 100+ degrees outside climbing a hill, It doesn't care.

Your input and wisdom is really appreciated. I like to think I know a lot about cars, but I'm still young. Thanks!

Re: Need Help With Timing Curve (I have Numbers)
 

Use the blue springs in this kit with the factory center plate and advance weights. Tack weld a stop plate that will only allow the pull arm on the vacuum advance to move .125(1/8") this will give you pretty close to 12 degrees advance provided by the can for a cruise of about 46-48 assuming your provided numbers are correct.

https://www.summitracing.com/parts/c...00-1/overview/

Re: Need Help With Timing Curve (I have Numbers)
 

You don’t need to tack weld.
Install an adjustable limiter like this. Easier to fine tune.
https://www.summitracing.com/int/par...19-1/overview/

How it’s installed along with some hei tips in here.
http://www.crankshaftcoalition.com/w...EI_distributor

Re: Need Help With Timing Curve (I have Numbers)
 

Vacuum advance is only a part throttle thing. Assuming a daily driver street setup, part throttle is a leaner mix at lower cylinder pressure due to the high vacuum which burns slower and needs to be lit earlier. As you put your foot into it, vacuum goes down, cylinder pressure rises, burn rate speeds up but vacuum advance is less now because of lower vacuum so the fire get lit later.

Adjust your mechanical first to be full in at 3000~3500 and then worry about vacuum advance if it pings at part throttle.

There are several articles on timing and the 50+ advance at part throttle is what you want, again assuming a relatively normal street engine.

There are also charts on the various AC vacuum pots available and when they come in and how much.