Engine : 12a bridgeport

What are the benefits of locking the Dizzy VS having a stock dizzy which is timed over 4000 rpm ??????



Method of locking the dizzy was simply removing the spring which hold the weights in place, the vacuum pots are not used also

 

For racing there is no disadvantage. The engine will start and run fine up to 24 degrees of advance.

However, it is common not to understand the function of the vacuum pots vice the weights. The weights function only in response to engine RPM. Easy. The pots respond to various amounts of intake manifold vacuum. The pots actually hold in an amount of advance at idle and just off idle. Also do this at two rates, so as to vary the split. On sudden throttle opening the pots release the advance and the effect of the vacuum advance is lost, so a high load low RPM start does not detonate the engine.



There are some folks who will let the clutch out and go to full throttle at very low RPM. Remember that we are looking for maximum cylinder pressure at about 50 degrees after TDC. So ignition advance should go up and down as a function of RPM. So, the pots may hold in 8 degrees with max (idle) vacuum, and when lost drop back to 3 degrees of advance, Since the RPM is controlled by the load it may be just above idle, so nearly zero advance is required to get max pressure at 50 degreesa ATDC. So, although the vacuum system is called the Vacuum Advance System, it is in fact the vacuum retard system.



So now if we disconnect the pots and remove the springs and braze the upper and lower plates together, we have no system of retard in the high load low RPM situation. And that 24 degrees of advance is 20 degres too much, and the engine is operating very much over advanced, and after a few cycles begins to detonate.

So, for the street there is a major disadvantage to changing the advance system, for racing, a bit less complexity and a slight benifit in spark accuracy. Just make the driver slip the clutch a bit to get things rolling before letting the pedal all the way up. Been there done that.



All of this activity still goes on today in every engine, but it is done by the computer.



Lynn E. Hanover

 

Initially this engine will be for street use and weekend fun not a major racer may be some drifting. i am going with a 12abp and was told i should lock the dizzy so i just removed the springs from the weights (i was advised to do this rather than welding it ) .





What do u think i should do ?

I am unsure as to what i should do to make it run properly . I was told the vacuum pots need to be condemned and the dizzy locked but from research it seems i will have low power at lower rpm and i would have starting problems . Is there any way i could avoid this ?



lets say i condemn the vacuum pots as i have no where to connect them (48ida) and i leave the distributor unlocked and set the timing at 4000rpm will that work out well for me or will there be problems .

 

The pots stay on so that the points plate does no move around. I would leave the springs in, as the curve is all in by 2,200 RPM and has no effect at 4,000 RPM.



Lynn E. Hanover

 

Ok well i will put the springs back in and set the timing on the crank to 20-25 BTDC at about 3000 rpm with a split of about 8-10 deg (is that to much split?). the vacuum pots are still on the dizzy and holding the plate in place . then i will set the timing as follows .Rev the engine to about 3k and locate custom timing marks on the pulley( 20-25 btdc) with a timing light hooked to the leading circuit. rotate the dizzy housing until the desired timing mark lines up with the pin on the front cover.Swap the timing light to the trailing circuit and push or pull the trailing pickup plate untill the trailing mark is where you want it in relation to the pin on the front cover.That should cover it right ?





thank your for your help

i am 26 years old and new to this its the first time i am dealing with a bridge port i am learning as i go along . i spend most of my time on here reading posts for information but sometimes the information is unclear and i have no one ask so i rely on forums for alll my information so once again thank you very much

 

Your proposed timing method is correct. The smaller split is desired. The split is used to help idle and low RPM burn, and reduce HC for poluton tests. It has very little effect at high RPM.



Lynn E. Hanover

 

thank your for the advice my aim as of fight now is to get this thing fired up , i will look in to msd and direct fire later on , i am also going to be using a turbo2 gearbox so i am awaiting a act streetlight flywheel , fortunately the engine was a 12a automatic so i do have the automatic counter weight for the engine , i might be looking to step up to 51ida in the future also