Manifold vs ported vacuum advance in theory
 

OK, I hesitate to even post this as I've done a lot of searches and I know that there are pretty strong opinions for both options. My reason for this post is not to necessarily figure out which one is "better" as "better" is going to be determined on a combination to combination basis. I'm simply looking to see if my feeble grasp of the theory is remotely accurate so please don't blast me to hard.
Again I'm only looking at theory, at least to start out with, so even though I will be using numbers in some examples its only for illustration purposes. I'll use some basic numbers I've found based around my combo for the illustration. My combo is a 355, vortec heads, Summit SUM 8802 cam (similar to HOT cam), 9.6-9.7 CR, speed demon 625 carb, HEI, auto, 2500ish stall, 3.73 gears just to give an idea. From everything I've read the vortec heads for the most part don't need or want a lot of total advance so thats what I'm going with.
From what I've gathered there are a few things you need to know when attempting to dial in the correct timing curve:
1. How much initial advance the combo "wants" at idle
2. How much total advance the combo "wants" at WOT
3. How much centrifugal advance is built into the HEI (on mine like many the centrifugal advance is not adjustable, only the rate at which it comes in)
So for this engine lets say it wants 16* initial and 34* total which from what I've read is not out of line for Vortec heads. Two scenarios:
1. HEI has 26* cent advance built in. In this case would it make more sense to use manifold vacuum? Set initial at 8* which plus the 26* would give you the 34* total it wants, but then set the vac advance to 8* to get the initial up to the 16* it wants.
2. HEI has 18* cent advance built in. Maybe use ported vacuum for this one and set initial to the full 16* at idle and the 34* at WOT it wants and the extra advance from the ported vacuum advance at part throttle and adjust the vacuum advance to get more MPGs.
I understand this is a very simplistic example and you can always just get a distributor that has adjustable advance for nearly infinite possibilities but am I even close to being on the right track with my thoughts? If you had the choice of the above options which would you think would be better and why?

Re: Manifold vs ported vacuum advance in theory
 

See post #10 for a bit of an explanation of ported vs. manifold vacuum.
https://www.jalopyjournal.com/forum/...or-not.706544/

Re: Manifold vs ported vacuum advance in theory
 

2 but get a vacuum 16° vacuum advance canister (78ish 305) and connect it to manifold vacuum. That cam generally likes 30-34° of timing at idle. You will probably find your vortec heads like 30-32 total at WOT which will put you at 12-14 initial.

Re: Manifold vs ported vacuum advance in theory
 

OK so let's say it's 12 initial, wouldn't hooking it up to 16* of vac adv put it at 28* at idle? Sorry if I'm not grasping it correctly.

Re: Manifold vs ported vacuum advance in theory
 

It would but better to sacrifice a few degrees of spark at idle than over advanced WOT. 28 is alot closer to 30-34 than 14-16 running ported vacuum.

Re: Manifold vs ported vacuum advance in theory
 

This is where an MSD Distributor with a Mech. Advance Limiting Bushing (regardless of Initial Advance) makes for a really nice Ignition System feature.
:nod:

Re: Manifold vs ported vacuum advance in theory
 

And at part-throttle cruise with close to 24+" Hg you might want closer to 45° advance for best economy while at only 1,500-1,800 RPM (e.g., not much mechanical advance) and still have some vacuum advance left down at 15" Hg for WOT operation. Experimenting with the vacuum servo springs could get you close to that in addition to tweaking the centrifugal weights, springs, and rate of apply ground into the cam on the weights.

Re: Manifold vs ported vacuum advance in theory
 

When I had the 24x setup on the cammed L31 in my Express it liked up to 52° of timing cruising along at 2,800 rpm.

Re: Manifold vs ported vacuum advance in theory
 

Ok so if I set it up this way then I'd in theory have:
28* at idle (12* initial + 0 mechanical+ 16* vacuum)
30* at WOT (12* initial + 18* mechanical + 0* vacuum)
46* at part throttle (12* initial + 18* mechanical + 16" vacuum)
Is that about right?
Thanks again for the help!

Re: Manifold vs ported vacuum advance in theory
 

This is a big reason I want to understand this better, to see if stepping up to a better ignition system is a good idea for my combo.
Thanks!

Re: Manifold vs ported vacuum advance in theory
 

The MSD Distributors allow for a Mechanical Advance Limiter (Bushing) to be installed.

For example:
-I want 30* Initial Advance, but the Combustion Chamber will ONLY tolerate a maximum of 40* Total Advance... I can install a 10* Max Advance Bushing to Attain this.
-Prevents the Mech. Advance from going higher to 45*, 50*, or higher causing detonation.

There are multiple Limiting Bushings in different Timing Increments, that can be used to set a Maximum amount of MECH. Advance.

Re: Manifold vs ported vacuum advance in theory
 

https://www.thirdgen.org/forums/engi...arb-350-a.html

Read post #6

Re: Manifold vs ported vacuum advance in theory
 

Thanks a ton for all of the help in this thread. When I changed my gauges a while back I had an extra spot that I put a vac gauge into. It's been a big help getting small adjustments done. As for what I've done so far:
I played around with the timing as it was set up by the guy that did the install (he put the vac adv on a ported source). I absolutely couldn't get the idle, WOT, and cruise timing to work together. If I set the idle to where it was happiest (stable idle and highest vacuum) it would ping almost everywhere else even when I minimized the vac adv. When I backed the initial down to eliminate the pinging the idle was awful. Remember at this point I can't change the Mech adv. Next just for test purposes I plugged the vac adv and set the initial to where it was happy and took it out. Idled OK and no pinging but was just a slug as I expected. Next step I backed down the initial and hooked the vac adv to manifold, restarted it and set the timing to most vacuum and a nice smooth idle. Took her out and detected just a hint of pinging so backed it off just a bit. Now she idles better than ever, has nice response, and no pinging that I can hear so I think it at least as close as I can get it to its sweet spot. The fact that I had to back off the initial a touch from its happy spot to keep it from pinging I THINK means it has a bit more Mech adv than the engine wants. Thinking of getting an MSD billet to be able to limit the Mech adv so I can bump the initial back up and maybe be able to run more vac adv for cruising. Thoughts on this idea?

Re: Manifold vs ported vacuum advance in theory
 

Forgot to mention, my timing light has decided to take a crap so I actually have no idea what the timing curve looks like LOL. I just know it runs pretty good now and Much better than before. When I get a new one I'll take down the numbers for my reference as a starting point and post them here as well.