Technical SBC 350 distributor advance vacuum question

Had it hooked to venturi vacuum. Same distributor we were talking about in the other thread. I hooked it to manifold vacuum and checked it with a light. I adjusted it until I got 8 degrees advance, then hooked it back to venturi. So that was giving me 48⁰ by my calculations. I could hear it spark knocking out on the road, so I pulled the hose off the distributor and plugged it. No more detonation. I could drop my initial and hook it back up, but it runs (ran) good where it's at, so I've left it unhooked.

 

It might work better if it was left on manifold vacuum.

 

But if it's left on manifold vacuum, wouldn't it just advance an additional 8⁰ as soon as the engine starts?

 

Yep. It'll be more responsive down low, too. As you give it throttle, the vacuum and the advance goes away and so should the pinging.

 

The manifold vacuum advance works when vacuum is HIGH.
When is vacuum high?
It's high at idle, but goes away very quickly when you touch the pedal.

Vacuum is high when cruising with light throttle. Again, increase pedal and vacuum advance goes away very quickly.

Manifold vacuum advance has little effect the rest of the time

 

No you didn’t. It’s opinions….try both ways as see what you like. I can get a lower idle with ported vacuum and most cars came from the factory that way.

 

It does!
And as soon as you touch the gas it would retard back 8° [ causing "slight throttle" stumble ]

You are better off setting the timing as if you had no vacuum advance, Then use ported vacuum [for economy / cruising]

 

Thanks, just getting to this now. So the truck with the SBC 350 and rochester carb ran way better off port, no stumble like before! Thanks for helping me get that fixed.

This SBC 350 with Holley is in my Shoebox Ford. Just noticed the distributor had no vacuum running to it and no PCV. I bought an AC/Delco PCV and grommet and will run that along with one of the breathers. I will run the distributor to port on the Holley carb as you suggested.

 

Yes but that advance is when there is light load and as load is applied manifold vacuum drops and advance when detonation is more likely to happen

 

Doubt it. I started out using the vacuum advance, then drove it without, and it runs better. Not changing it after trying both, and having it this way for over a decade. I'm sure if it was a heavier car, and not 2300 lbs. the vacuum advance would work better. On my heavy '39 Chev I run the vacuum advance. But on a light car with a lot of power it's not needed, nor does it run better with it.

 

So, here's my 2 cents. First, you need to know what else the combo has. Compression, cam specs, carb size, vehicle weight and trans/rear/tire gearing. All of this affects timing. The specs for a fairly light shoebox with a mild 350 are going to be different from a 327 in a heavy tow truck geared to top out at 55 MPH.
Typical starting point for SBCs are 0-10 BTDC initial, initial and centrifugal top out at 35 degrees. When it 'comes all in' is load and intended use variable. Typically 1800 to 2500 RPM.
A bigger cam often needs more initial due to overlap, more compression often needs less total due to quicker combustion flame front travel. Vacuum advance is weight/load dependent, less vacuum timing for heavier or high load situations. You can use a vacuum gauge for tuning, along with listening for and inspecting the plugs for preignition. Having the most timing does not mean most power!
There have been volumes written about this, but to start, make sure the engine is healthy, ignition and fuel are working well, then try different settings. Racers will adjust for best performance, street guys will ensure best driveability and MPG.

Here is a modern generic timing table which is controlled by more than weights, springs and a vacuum signal.
It does provide you with the various drive modes and how load and RPM require different amounts of timing. This is what the engine 'wants' and is what you are trying to tune for with the weights, springs and a vacuum signal.