Manifold and ported vacuum

[13matsc]

The old question. I switched to full manifold vacuum about a year ago, after some late night reads..
Still not sure of this is best/most correct.

How many of you use full manifold vacuum for the advanced timing?

Im not talking stock engines with emissions etc. This is for engines with aftermarket intake manifold and carbs.

 

[brooksman9]

Yep

 

[Sad Sack]

Understand your carbs timing port or manifold port will do as well.

 

[13matsc]

Understand your carbs timing port or manifold port will do as well.

Yes, of course. Im talking about which of the ports people actually use. I’ve never seen one in real life use manifold vacuum, but alot of guys online sure does.

 

[Sad Sack]

Yes, of course. Im talking about which of the ports people actually use. I’ve never seen one in real life use manifold vacuum, but alot of guys online sure does.

Most choose manifold full vacuum. I've just used what carb instructions state.

 

[Edelbrock]

I looked into this a lot! I read lots of info on various forums.

Answer:

Try one, then try the other. use which ever one you like the best. If they both seem the same, then just leave it where ever it landed and move on.

 

[Ricko1966]

You should use manifold vacuum to vacuum advance. Ported vacuum is generally an emissions deal and even when new from the factory there was usually a thermo vacuum switch which over rides the ported vacuum switching to manifold vacuum if the engine gets beyond a certain temperature. Manifold vacuum and ported vacuum are the same after you start to open the throttle,retarding the timing at idle(ported vacuum) makes the air fuel charge finish burning in the exhaust,cleaning up the exhaust at idle. You haven't given enough information to give you much advice on timing. What engine and what heads are you running?

 

[13matsc]

You should use manifold vacuum to vacuum advance. Ported vacuum is generally an emissions deal and even when new from the factory there was usually a thermo vacuum switch which over rides the ported vacuum switching to manifold vacuum if the engine gets beyond a certain temperature. Manifold vacuum and ported vacuum are the same after you start to open the throttle,retardingbthe timing at idle(ported vacuum) makes the air fuel charge finish burning in the exhaust,cleaning up the exhaust at idle. You haven't given enough information to give you much advice on timing. What engine and what heads are you running?

Stock 350 engine with edelbrock 2101 intake manifold and 1406 carb. Adjusted last year to 8* btdc with full manifold vacuum.

The distributor is stock - should the vacuum pod be replaced? All the emission stuff has been removed.

 

[Ricko1966]

Stock 350 engine with edelbrock 2101 intake manifold and 1406 carb. Adjusted last year to 8* btdc with full manifold vacuum.

The distributor is stock - should the vacuum pod be replaced? All the emission stuff has been removed.

What year stock 350 engine? What heads?

 

[13matsc]

What year stock 350 engine? What heads?

Its a 1985 modell C25. Do not know the type of heads or have any knowledge of the different types.

 

[75gmck25]

I have aluminum heads and I run manifold vacuum with 15 degrees base timing and 20 degrees mechanical (in by 2000 rpm). My vacuum advance adds 17 degrees. In general, aluminum heads tolerate more timing without pinging because they dissipate heat better.

I want to make sure of your timing process.
- did you set it to 8 degrees BTDC with the vacuum advance line disconnected from the distributor and plugged? Did you also set the idle down to about 650 so there was no mechanical advance? Did you verify the mechanical advance works as you rev up the engine?
- have you ever verified your timing pointer and balancer are accurate?

Typical timing for a smog-era SBC is base timing of about 8-12 degrees, mechanical advance of about 18-20 degrees (preferable all in by about 2500-3000 rpm), and vacuum advance of about 18-20 degrees. Do not exceed about 52-56 total.

Many stock HEI distributors have quite a bit of vacuum and mechanical advance (each may exceed 20 degrees), so it’s always good to check.