I see dorfs is trying to get something started with out knowing the subject again...
Too bad.
He's always trying to stir the stink pot instead of learning something... and he seems to have a real problem with me.
I guess he feels threatened.
I just wish he would learn the subject, instead of post misinformation, dis-information, and outright false hoods.
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Someone at Holley has been watching the posts here, and the fine people at Holley and provided written permission for me to use their drawings and graphics in my tech articles. This is the second place to do so. (I must be doing something right!)
Thanks to Steve C. at Holley! The disk will come in handy here, and for my upcoming Jeep Tech Upgrades Disk...
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*TeamRush,*
*Help me out here...the way I understood how it was supposed to work is as follows...*
*1. The mechanical advance increases the ignition advance as engine RPM increases.*
Correct. Centrifugal advance is directly tied to engine rpm by a set of swinging weights that add advance as the distributor shaft rotation increases and slings the weights out.
The weights have springs attached to them to bring them back in when the engine slows.
By running lighter springs, the advance comes in sooner because the weights can be spun out faster.
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*2. The vacuum advance adds additional advance based on engine loading...i.e., at low throttle, manifold vacuum is greatest, hence, low loading.*
Incorrect assumption.
Manifold vacuum has very little to do with ported vacuum for the vacuum advance.
This is not recognized by the average Joe, and that's why so many vacuum advance sources incorrectly come from the intake.
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*As you step on the gas, vacuum drops, so does advance.*
Incorrect assumption.
As you step on the gas, manifold vacuum drops.
Ported vacuum draws it's vacuum source from the carburetors venturi body, where vacuum is just starting to really build.
If you want to prove it to yourself, hook a vacuum gauge and a timing light to your engine.
Start the vehicle.
Throw the throttle blades open, watch what happens to manifold vacuum (It drops), and watch what happens to the ignition (it advances).
The ignition will stay advanced no matter if the throttle is held in an half open position or revved up more to full throttle.
That is because the more air flow through the carburetor is increased, and the ported vacuum is increased with the airflow.
If you let the engine return to normal idle, or close the throttle blades suddenly, the air flow will all but stop, and the ignition timing will retard.
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*This helps prevent some of the pre-ignition.*
'Pre-Ignition' is a term that was much over used a few years ago, and simply means the charge mixture burned too soon.
'Pre-Ignition' is a misnomer anyway. Either you have ignition or you don't. There is no 'Pre' about it...
It may be too soon, but it's not 'Pre'.
Most of what is called 'Pre-Ignition' is good old fashion Detonation.
The only time you can use the term 'Pre-Ignition is when a sharp edge, or a burr left over from machining, or something like that protrudes into the cylinder and becomes red hot. When the charge mixture enters the cylinder-- BOOM- the hot spot sets off the charge.
That is the ONLY correct use of 'Pre-Ignition'.
You can fire the cylinder too soon with the ignition, but thats not 'Pre'. That is the ignition working correctly, and firing at the time it was supposed to, just a nit-wit adding too much advance too soon. Thats human error, not 'Pre Ignition'...
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*In other words, we want as much advance as possible*
Correct, with a reservation...
The sooner we can fire the cylinder, the longer the cylinder has to burn, heat the gasses, the gasses expand, push down on the piston, and make power...
If we fire the cylinder too soon, the piston and connecting rod get shoved back the way they came instead of breaking over the crankshaft.
When that happens, we call it detonation. Detonation will hammer the pistons right into the oil pan in a hurry!
We want as much advance as we can get with out detonation.
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*and as we step on the gas, we want to retard that advance to prevent pinging.*
No. We want the advance to ride that fine line just before detonation.
If you retard, you will loose power when you try to 'step on the gas' or move off the line as we say in the business. Advance isn't all the way in yet, because the air flow through the venturies hasn't picked up enough yet...
The timing isn't retarded, it just hasn't advanced fully yet...
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*If we use ported vacuum, advance would increase under heavy accel.*
Yes it would, but not the way you are thinking. The engine uses X amount of air per revolution. It will use a little more when the intake isn't restricted by the throttle blades.
The amount of advance is linked to the amount of air passing through the venturies.
If the engine is just starting out under the worst of the load, trying to get it moving at low engine RPM, then the air flow is going to be slow (low horse power to weight ratio), so the advance is going to be small.
When the engine starts to get up in the RPM range a little, and starts making some horsepower and torque, the horse power to weight ratio comes up, and in effect, makes the load easier to move. As the RPM comes up, the engine comes up, but the intake vacuum is still way down, but the air volume through the venturies comes up, and the advance also comes up... and stays up until you close the throttle blades and cut the air flow off...
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*Basically, they vacuum advance ends up doing the same as the mechanical. Is this what we really want?*
It doesn't do the same thing as the mechanical advance. The centrifugal advance adds a set amount of advance at a set RPM, for about half of the total advance and engine gets.
The Ported vacuum advance system is load and engine RPM variable to an extent.
It is also very sensitive to atmospheric conditions and changes.
Cool, dense air, like on a rainy night, will cause the advance to come in sooner. The dense air creates more vacuum for any given CFM through the carburetor.
On hot, thin air days, the effect is reversed, and not as much vacuum is created, so the advance is smaller for the given CFM through the carburetor.
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It's an ingenious system really, and much too complicated for me to cover fully here.
It does work though, and has worked very well for many years...
I have only met a hand full of people (maybe 2 dozen) through the years that honestly and truly understand it, and know what they are talking about...
Lots of people think they do though, that's why so many people are misled about ported vacuum and how a vacuum advance works...
You can see what I mean by the opinions on this thread already...
Hope this helped answer your questions.... Aaron.
If Chris Columbus "Discovered" America (with 25 million already here), Can I Go "Discover" Florida?
