[DoubleVision]

I`m not getting into the technical side or disagreement side of this. I`ll just say, Years ago I built a SBC 350, it had low compression around 8.75:1, decent 041 casting heads, a small crane cam around 210, 216 duration. a weiand dual plane intake, headers, a road demon jr 625 carb and decent gears. The thing was a complete slug. I tuned it until I was pulling my hair out and couldn`t figure out why it was so sluggish. From a dead stop on wet pavement it wouldn`t burn a tire. Then one day I was on here and BOBCRMAN told me to connect the vac advance to a manifold source. I had always plugged it in to a ported source since I asked myself, "why does it need vac advance at idle?" but since I knew of his wisdom and years in this line of work, I tried it. I disconnected it, plugged it, and reset the base timing to 12 degrees, locked it down and connected the hose to a full time manifold source then took it for a test drive. I was completely amazed at the difference it made. It felt like engine had picked up 50 horses. It had a nice crisp throttle response and it would leave from a stop sign in a hurry. So since then I`ve learned alot by looking into it all. lightly hot rodded engines rely on a lean idle and cruise is also somewhat lean, leaner mixtures burn slower than rich ones, so the extra timing advance is needed. So I fully agree with the logic, it depends on how the engine is set up. Manifold vac is only needed on combo`s that call for it.

[techron]

Originally Posted by DoubleVision

I`m not getting into the technical side or disagreement side of this. I`ll just say, Years ago I built a SBC 350, it had low compression around 8.75:1, decent 041 casting heads, a small crane cam around 210, 216 duration. a weiand dual plane intake, headers, a road demon jr 625 carb and decent gears. The thing was a complete slug. I tuned it until I was pulling my hair out and couldn`t figure out why it was so sluggish. From a dead stop on wet pavement it wouldn`t burn a tire. Then one day I was on here and BOBCRMAN told me to connect the vac advance to a manifold source. I had always plugged it in to a ported source since I asked myself, "why does it need vac advance at idle?" but since I knew of his wisdom and years in this line of work, I tried it. I disconnected it, plugged it, and reset the base timing to 12 degrees, locked it down and connected the hose to a full time manifold source then took it for a test drive. I was completely amazed at the difference it made. It felt like engine had picked up 50 horses. It had a nice crisp throttle response and it would leave from a stop sign in a hurry. So since then I`ve learned alot by looking into it all. lightly hot rodded engines rely on a lean idle and cruise is also somewhat lean, leaner mixtures burn slower than rich ones, so the extra timing advance is needed. So I fully agree with the logic, it depends on how the engine is set up. Manifold vac is only needed on combo`s that call for it.

doublevision, your post and rick WIs post are right on and very well put. i never intended to get into pissing match with curtis, but he started it. i was just giving a simple answer to the OPs question. i can't believe this thread is still at the top-but that's allright, the newer hotrodders are probably learning something.

and i still think curtis is all wet--->

[curtis73]

Originally Posted by techron

doublevision, your post and rick WIs post are right on and very well put. i never intended to get into pissing match with curtis, but he started it. i was just giving a simple answer to the OPs question. i can't believe this thread is still at the top-but that's allright, the newer hotrodders are probably learning something.

and i still think curtis is all wet--->

No pissing match inferred I just got off work so I'm a little sweaty, but I wouldn't say wet.

We just posted our experiences and disagree, but everyone builds engines differently. Good discussion and disagreement like this brings out little technical things that let the OP make a more informed decision.

[techron]

curtis, all is good, we just agree to disagree for the OPs benifit!! this thread has turned out better than i expected from my first post...CURTIS--->

[DoubleVision]

I have respect for you both and I can certainly see both sides of the story. I think other things play a role in how vac advance works as well.

[willys36@aol.com]

Port or Full time vacuum

Distributor Tuning and Theory - Part 1

Ported Vacuum or Manifold Vacuum?

[Carlos9]

Originally Posted by DoubleVision

I`m not getting into the technical side or disagreement side of this. I`ll just say, Years ago I built a SBC 350, it had low compression around 8.75:1, decent 041 casting heads, a small crane cam around 210, 216 duration. a weiand dual plane intake, headers, a road demon jr 625 carb and decent gears. The thing was a complete slug. I tuned it until I was pulling my hair out and couldn`t figure out why it was so sluggish. From a dead stop on wet pavement it wouldn`t burn a tire. Then one day I was on here and BOBCRMAN told me to connect the vac advance to a manifold source. I had always plugged it in to a ported source since I asked myself, "why does it need vac advance at idle?" but since I knew of his wisdom and years in this line of work, I tried it. I disconnected it, plugged it, and reset the base timing to 12 degrees, locked it down and connected the hose to a full time manifold source then took it for a test drive. I was completely amazed at the difference it made. It felt like engine had picked up 50 horses. It had a nice crisp throttle response and it would leave from a stop sign in a hurry. So since then I`ve learned alot by looking into it all. lightly hot rodded engines rely on a lean idle and cruise is also somewhat lean, leaner mixtures burn slower than rich ones, so the extra timing advance is needed. So I fully agree with the logic, it depends on how the engine is set up. Manifold vac is only needed on combo`s that call for it.

Hate to bring this up, but I have exact scenario in current motor. Are you speaking of the vac advance on distributor. I could not get my car running right. I had one carb that I replaced with another rebuilt one (quad). I was told it needed to be connected to a vac outlet that was not drawing vac. We did it and it worked. What's the difference if you connect to a line on carb that is drawing vac. Also when you mention connect to a full time manifold post.. are you meaning the carb or what exactly. I was intially doin search on 041 heads. and found this.

[red65mustang]

Carlos,
some vacuum units add SO much timing (up to 24*'s at about 17Hg at idle) that they have to be hooked to ported/no vacuum at idle....

or,,,, you would have to use minus degrees base timing with full time manifold vacuum and that means less centrifugal per rpms when you do mash the gas...

some dist's are designed/factory set up for ported only...

stick a 3/32 hex in the nipple and see if it is adjustable

[lg1969]

I got my motor running with no vacuum advance. All mechanical advance. I mine set at full advance of 38* at 3000 RPM. At idle it sits at 15*. Mine motor seem to run with no problem. Beside I never did like vacuum advance.

[oldbogie]

Originally Posted by KULTULZ

I was going to try and stay out of this misinformed topic, but if what you say is correct, why does one have to remove the vacuum source to set base timing at recommended curb idle?

Just in case the throttle blade is miss-adjusted exposing the timed port to manifold vacuum.

Researching the Venturi Effect will also answer a few misconceptions regarding vacuum signals.

Back in the older days up into the 1950 to 1960s on some cars a dual vacuum system was used with no centrifugal. One connection was made either directly to manifold vacuum or a timed port just above the idle positioned throttle blade. This was sensitive to cruise and light load advance requirements but like today's vacuum advance went away as the throttle was opened and pressure in the manifold became similar to the surrounding atmosphere, thus no vacuum.

For high RPMs with the throttle at WOT, what to do? They used a second vacuum connection into the venturi. At this point the vacuum is inverse to the manifold because as the incoming air must race thru the restriction of the venturi, it looses pressure , in other words forms a vacuum.

The vacuum cannister on these sometimes included dual diaphragms where the high manifold vacuum was applied to a smaller diaphragm and a larger one was used for the less strong signal developed in the venturi. So this system let the engineers tailor advance curves for idle/cruise and WOT, "sort of".

Bogie

[willys36@aol.com]

These ignition discussions go on forever w/ lots of theories and urban legends. I HIGHLY recommend getting a copy of this book by Dr. Jacobs. It goes into excruciating detail on the physics of ignition systems then more excruciating detail on how to optimize your system from a '23 T to a top fueler. Out of print since '91 but show up on ebaY pretty often. Takes all the argument out of the argument.



OOPS! Just found this site that sells the book. Thought it was out of print buy I guess not. GET IT!!!!!

And this more recent book by the doc is probably just as useful. I don't have a copy but if it is as good as the first it is a must-have.