Quote:
Originally Posted by THE NEW GUY
in the old times, the carburetor had a barb connector for a hose thar lead the the distributor to act on a diafragm to increase the ignition timing advance. This was the "LOAD" factor on the advance. The RPM factor was done with counterweights on the rotor.
We do not have distributors now. It's all on the chip. but I do not feel any increase of the ignition advance due to "load" on my SVX. May be the only way to confirm it, is with a camera and a timing gun, some tape and run the car....
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To see why the ignition timing is advanced over the engines running conditions, you have to look at why we time it, in the first place.
The whole objective is to develop the pressure on the piston at about 15/18* after top dead center, when the con rod is at an angle, that can exert the force on the crank.
Now while the rate that the combustion chamber full of air/fuel burns, is basically constant, the speed of the engine is not. When the engine is idling the spark starts the burn at 10*BTDC this takes 25* at this speed to build up max pressure by 15* ATDC. As the engine increases speed, we have to light the fuel earlier to allow for the pressure to build up. So as the engine increases speed, we increase the spark advance to allow for the time that it takes for the fuel burn, to develop maximum pressure.
At about 2200/3000 rpm another factor starts to speeds up the burn rate.
This is the turbulence created by the squish of gas squeezed out of the space between the edge of the piston and the cylinder head squish area. This turbulence has been increasing over the last 1500/2000 rpm or so, but now it has got to the point that the increase in the burn rate due to the increasing turbulence keeps pace with the increasing engine speed. The Ignition Advance then remains constant at about 36* BTDC, from about 3000 up. This means it takes 51* to get maximum pressure on the piston at 15* ATDC.
The fuel mixture, turbulence, temperature and pressure, all affect the rate of the burn. Any change in the rate of the burn requires a change in the Ignition advance to bring the maximum pressure back to 15*ATDC. So when we lift the foot, the cylinder pressure reduces, and the fuel mixture is leaned off. Both of these factors result in the burn taking longer, so the ignition timing is advanced, around 20* is added to the 36* to get the max pressure back to the 15/18* spot.
In the old distributor, the centrifugal weights advanced the timing with the rpm rise. The Vacuum advance worked to advance the timing, when the vacuum is high, that is when the cylinder pressure is low, and the mixture is lean, and the burn rate is slow.
Harvey.