356 Spark plug gap and compression ratio

[Al Zim]

Fred: I know you are correct about the voltage across the points. I have the unit that was installed in the early 911's (before 1969) on the shelf in my office. Finding this unit in the shop manual and parts books is an impossibility (you had to be working on those cars, by accident we were) I was powered off the starter solenoid and provided 12 volts to the coil in starting. Once the engine stopped cranking the voltage returned to 6 volts at the coil. I will be glad to send you a picture of this unit and the wiring. Over the years we have sold hundreds of 6 volt Blue coils which will give you a hotter spark. Also over the years the spark plug wires have been MESSED with possibly increasing their resistance. This is especially true on the plug ends that had a loose resistor held in by the brass end that screwed into the plug end of the connector.

[Greg Bryan]

Here's the system from my early 911 wiring diagram that Al is referring to.

• 12v comes into the WECO connector from the ignition switch #15 where it splits and powers the fuel pump and the #15 side of the ignition resistor
  Note that the output side of the resistor is #16 and is about 8 volts - during normal running, the wire goes directly to the coil - also note that it is #16 on the input of the coil.

• During engine starting, the ignition needs more that 8 volts to start because the battery voltage drops during cranking and the output side of the resistor drops proportionally, therefore the Resistor Relay comes into play - the relay has in input from the starter #50 which energizes the relay coil during cranking

• With the relay contacts closes, full battery voltage (picked up at the Alternator B+ terminal) is applied to the coil to give it an extra boost. As soon as the key is released from the start position, presumably as the engine starts, the relay opens and the lower 8v is supplied to the coil.

Sorry for the diversion, but it is an interesting system.

The domestics had these systems as well as was pointed out earlier, but usually dispensed with the expensive relay and just ran a wire directly from the starter #50 terminal to the coil + side.

[Neil Bardsley]

Can someone tell me about my how my ignition is now affected by dwell angle? I had to change back to the STD setting on Fred's CDI unit to set it but it occurred to me that because it's now only a switch I wouldn't be so sensitive to the dwell?

[David Jones]

4 cylinder engines are not very sensitive to dwell because there is such a relatively long interval between point operation. It is much more of a concern for V8 engines.

[Neil Bardsley]

Makes sense David. Thank you

[Jacques Lefriant]

Hi Neil
the nuanced dwell requirement is to achieve the proper rotor phasing.
j

[David Jones]

Jacques, you may want to put that in less nuanced language. Do you mean that the rotor has to be in a position to transmit the spark to the plug within the limits of its travel and width of rotor tip? Is that not a relatively fixed dimension and not subject to change?

[Jacques Lefriant]

Hi David
thanks. the point i was making is the event occurs when the switch breaks contact and the angular position is dependent on the gap/dwell if points are used.
j