1968 Riviera 430 low vacuum reading on gauge

I understand all that. Whats the real difference between leaving the initial set to zero and hooking up the vacuum advance to manifold or advancing the initial to 12 degrees and leaving it set to ported?

 

The distributor in that application has between 30 and 34* of mechanical advance at 4600 RPM. That is why it is initially timed to 0* (TDC). If you bump the initial timing to 12*, that is where you start, and the 30-34* of mechanical timing comes in addition to that. That means at WOT, you will have 12* + 30-34*= 42-46* at 4600 RPM and above. Ported vacuum, or manifold vacuum will be 0 at wide open throttle, so there will be no added advance from the vacuum canister, except at part throttle.

Using manifold vacuum to the vacuum advance canister results in that advance being available at idle while the car is stopped in traffic. The engine will idle better, and run cooler. There will be more vacuum at idle. Those are the only differences.

 

Still not sure of the advantage of what Larry suggests

 

More ignition advance at idle that goes away when you punch it. An engine that idles at 16-20* BTDC will run cooler than one that idles at TDC. Have a look at the Buick V8 engines in 1969. Initial advance is specified as 0* (TDC). The vacuum advance was hooked to manifold vacuum, and the vacuum canister provided another 14-18* of advance at idle speed, so the engine would idle at 14-18* BTDC.



Later emissions engines used ported vacuum to the advance (no vacuum at closed throttle). Take the 1972 engines. Initial timing was 4*BTDC. No vacuum advance at idle meant they idled at 4* BTDC. That's retarded timing at idle. An engine will have a tendency to heat up at idle speeds with retarded ignition timing. That is why Buick used a thermovacuum switch (part of the TCS emissions) that switched the vacuum advance from ported to manifold when the engine coolant temperature rose beyond 220*.

 

Maybe I'm dense, but why not just run 10-12 degrees initial and have the mechanical make up the rest?

 

I get the additional advance at cruise from my vacuum advance which is connected to ported vacuum. I think the outcome is the same.

 

It is at part throttle. Manifold or ported gives you the same advance from the canister at part throttle. Connecting it to manifold gives you the extra advance at closed throttle and idle. That's the way it was pre emissions devices. Ported vacuum advance was a 70's emissions thing.

 

Because you can't just run whatever you want for initial. With a distributor like the 1969 ones, you'll over advance at higher RPM. If you have 30-34* of mechanical advance, you need to reduce that to run more initial. Either modify the advance slot or use a bushing.

And yeah, you are dense.

 

Ok, so by reducing the amount of mechanical and increasing the initial, what's the difference? The end result is the same.

I have 10 degrees initial in the beater. Plus the mechanical, gives me 32 total. I can tell you the distributor casting but I made it out of three different distributors

 

The Stage 1 distributor has 20-24* of mechanical advance. So, yes, 10* of initial works perfectly. If everything is working well, and you aren't having problems, that's fine. Not all distributors have those kind of specs. If you need more initial advance as someone with a hotter cam would, you may have to modify the mechanical advance. That's all I am saying.

Do you have stock advance springs in the beater? When do you have 32* in at?

 

I used either the Mr gasket kit or some MSD advance springs. I mixed and matched them to get a nice steep advance curve. I think I'm all in at around 2500.

 

Did you limit the vacuum advance canister? The stock one is 14-18* That means cruising down the highway at 60MPH, you might have as much as 50* total timing. Does it ever surge? Any part throttle ping?

 

Yes, it's limited.

 

Very Good Grasshopper.

 

Despite popular opinion, I actually do know what Im doing

 

Absolutely!