|01-29-2009 06:05 PM|
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.
|01-29-2009 05:06 PM|
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".
|01-29-2009 02:59 PM|
|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.|
|01-29-2009 02:13 PM|
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
|01-29-2009 01:10 PM|
|01-13-2009 10:12 AM|
|01-13-2009 08:16 AM|
|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.|
|01-13-2009 06:49 AM|
|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--->|
|01-13-2009 04:04 AM|
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.
|01-13-2009 01:19 AM|
and i still think curtis is all wet--->
|01-13-2009 12:18 AM|
|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.|
|01-12-2009 07:41 PM|
|Cousin_Joe||Thanks for all the feedback. I wasn't really trying to get into a discussion my specific setup, but just trying to understand more about timing.|
|01-12-2009 06:30 PM|
Joe, your distributor should be setup to give you 20 degrees advance with 32/33 total mechical degrees in the 2500-3000 RPM range, assuming 12 degrees advance dialed in at idle. Your advance should start to come in at about 1000 RPM, as the engine will idle lower than this. That will be very close when setting up the mechanical curve.
The vacuum can should have around 8-10 degrees of additional advance and be plugged into the manifold port. This should give you a smooth idle assuming the carb is adjusted correctly and good part throttle performance.
You can not crutch one type of advance curve with the other, vacuum versus mechanical, they perform totally different functions. Vacuum advance is load based and mechanical is RPM based.
As to your question about cruise RPM, the only way you can have the best of all worlds with optimum timing at 1800 RPM is via a totally programmable ignition system. Not worth the expense on an application of your type.
|01-12-2009 05:43 PM|
That would have to be some engine to achieve atmospheric pressure within the intake....it is negative pressure( vacuum), by the way.....and the reason manifold vacuum drops IS the fact that it is below the throttle plates. It is the spring rate within the can that over powers the signal as it drops and returns it to it's rest position. If it became the same as atmospheric...how would the fuel enter the venturis??
Now you can research the Venturi Effect as Gary suggested....
|01-12-2009 05:41 PM|
But on anything I do (street torque stuff mostly) the ported source is safely covered and showing 0-.5"
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