Vacuum advance

[Bill Monk]

Just bought a distributor and ignition box from 4secondsflat. This is a quote from their tech instructions:

"Hot Rods, Muscle Cars, Classic Cars or nearly any performance application the Vacuum Can is connected to the CONSTANT Ma nifold Vacuum source. Doesn't matter what you read on the internet if you don't have any computer controls, Catalytic Converters or other emissions equipment it goes to CONSTANT. End of discussion."

This is contrary to everything I thought I knew. I have always plugged the vacuum advance to the ported hole at the top of the carb so that you get advance as vacuum increases. Throw some input at me!

 

[69hemibeep]

It depends on the Distributor advance pod. Non emissions in non ported or the low one on the carb, emission controlled is ported or the hight one on the carb

 

[Ranger]

:yesnod:

 

[dmartin]

I plugged mine vacuum line off some run good with it and some run good without it just have to try and see


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[Don Gould]

I've answered this question at least 816,457 Times, here' goes 816,458 times

Performance engines should always be Power Timed with a dial back timing light the total timing is the most critical. Set the light at 34* then bring the RPM up to just above the all in number your looking for and set the distributor timing to read "Zero" where it falls down to is what you have for Idle timing. If your using our plate it should be dead on, if not then you need to double check to be sure the RPM was high enough to fully advance the weights.

You need to understand how Ported Vacuum works....
As the airspeed or air velocity increases through the Venturi it creates high pressure in the center (which explodes the fuel into small particles) and the equal and opposite reaction is Negative wall pressure. This is the source of Vacuum for your vacuum advance so the higher you spin the motor (velocity) the more vacuum is produced and more timing is pulled into the motor. This is exactly what you don't want, so if you have a vac can that has say 20* of stroke in it you will have 34* mechanical and another 20* vac for a total of 54* and good bye crank shaft and pistons. When we curve a distributor we have ways of limiting the stroke of the Vac Can to get to where we want it when Connected to CONSTANT Vacuum.
You might be able to get way with this if your using an old antiquated transistor box like an Orange box or some Chinese stock type box as they all retard the timing under load by at least 8* ... strange way to try and get your tune-up right.

Ported vacuum is for OBD1 and newer vehicles with computer controlled timing retard dictated by a cluster of probably at least a dozen sensors ( I believe a Viper has 16, O2 sensors alone) Timing controllers can only retard timing they only pull it back from the total allowable timing set in the timing MAP.

So now you connect it to Constant vacuum..... At Idle and part throttle you have vacuum timing which is responding to the high HG's of manifold vacuum. So when the engine has no load or very light load (cruise) you have high vacuum which advances the timing and under load releases the Vac timing to match the load being applied and the springs take over. This is a very simple design that has worked since the late 50's I believe our friends over at GM figured this all out and all GM performance distributors were always connected to MANIFOLD vacuum, until the curse of emissions controls in the mid 80's.

Why so much Idle timing? At Idle or Part Throttle cruise you are trying to burn a very lean mixture. A lean mixture takes longer to burn than a rich mixture, the particles are further apart so it takes longed for the flame front to propagate and go into the expansion mode, you need the piston to be at about 13* ATDC to apply pressure to the crank turn it (tork)
New cars run around 40-50* of idle and cruise timing because they run so lean to achieve mileage numbers.

More initial at idle will give you a cooler running motor and the RPM'S.

And why is that???
If you don't have enough initial timing in the motor it won't have time to burn the fuel before it gets to that 13* magic number so it can turn the crank. So if your expansion mode in the combustion chamber is late then the pressure is just chasing the piston down in the hole and because your not keeping the fuel pressurized your not burning it all. So now the piston is coming back up and pushing that smoldering fuel out the exhaust port where it reignites and creates a blow torch in the head runner and cooling system just can't keep up. Also results in poor throttle response, stinking rich idle, stumble and hesitation. Also leaving serious carbon deposits on the back of exhaust valves, exhaust ports, on the piston choking up the rings and eating your cylinder walls up. So even if YOU think it's running fine with 10* of idle timing it's not happy and down the road it will leave a message in your wallet.

Where to set your initial, total and Vacuum timing? When you purchase our products we're always here to guide you, it's included in the price.
Don't do HALF the job if you want it to live and perform like it's capable of don't think that just the timing plate or just the Ignition Box will do it all, do the whole job and enjoy the power of your Mighty Mopar for years to come.

Even Edelbrock finally figured this one out............