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.

 

[Ricko1966]

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

Okay as long as no one has changed anything with the factory heads that thing is going to want approximately 36 degrees total advance and manifold vacuum. To set total you either need a timing tape,or a dial back timing light. Put your advance on manifold vacuum and start your car.check the timing at idle,write down that number. Now disconnect the vacuum advance check the number again,write it down. Okay now Rev the engine while watching the timing tab, when it quits moving write down this number,twist the distributor until your 30 mark lines up. Lock it down check degrees of advance at timing at idle.reconnect vacuum advance. Find a stretch of road with two land marks,that will let you accelerate hard as you can to get 1/2 or more through second gear. Run it a couple times,track your speed when you pass the second landmark. Start bumping base timing 2 degrees at a time,and tracking your trap speed,eventually speed is going to peak,and then drop. Go back to the lowest setting with the highest trap speed. That is MBT minnimum best timing,minnimum best torque. Let us know all these numbers and we can advise you about your vacuum can,but your base timing and total will be best for your engine.

 

[13matsc]

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.

8 degrees were set with the vacuum advance pod disconnected and plugged.
I put vacuum to the vacuum advance pod and watched the inside of the distributor move. Not sure if this verify a working mechanical advance?


The idle was set down to about 800 rpm as this sounded the best to our ears. I had the help of a buddy with more knowledge than me, but not much with old american cars. Cannot remember if we verified the mechanical advance… The timing pointer has not been verified according to TDC. The timing label on the harmonic balancer maxed out at 10 if I recall correctly, thats why we didn't push it any further. The improvement was massive anyways, so

 

[13matsc]

Okay as long as no one has changed anything with the factory heads that thing is going to want approximately 36 degrees total advance and manifold vacuum. To set total you either need a timing tape,or a dial back timing light. Put your advance on manifold vacuum and start your car.check the timing at idle,write down that number. Now disconnect the vacuum advance check the number again,write it down. Okay now Rev the engine while watching the timing tab, when it quits moving write down this number,twist the distributor until your 30 mark lines up. Lock it down check degrees of advance at timing at idle.reconnect vacuum advance. Find a stretch of road with two land marks,that will let you accelerate hard as you can to get 1/2 or more through second gear. Run it a couple times,track your speed when you pass the second landmark. Start bumping base timing 2 degrees at a time,and tracking your trap speed,eventually speed is going to peak,and then drop. Go back to the lowest setting with the highest trap speed. That is MBT minnimum best timing,minnimum best torque. Let us know all these numbers and we can advise you about your vacuum can,but your base timing and total will be best for your engine.

This seems like a bit out of my knowledge and equipment base. I was hoping for some more basic verification method applicable inside the garage

 

[Ricko1966]

This seems like a bit out of my knowledge and equipment base. I was hoping for some more basic verification method applicable inside the garage

Okay disconnect the vacuum advance rev the engine until the timing quits moving,rotate and lock the distributor down at 36 degrees.Reconnect vacuum advance. Call it a day. If it doesn't ping ever this is as good as your going to get without more work. You will need to make a timing tape,or buy a dial back timing light.