Questions on my Distributor and Dwell

[High Life]

Im not familiar with this ignition system - I have never had an issue - but while learning about tuning and reading about Dwell and timing - I thought I better start understanding the ignition distributor

According to Bentley - this is a CDI inductive pulse generator. - So Im assuming this replaces the old points system..

My initial questions then are this:

Is there any maintenance I need to do or look for? - not seeing anything listed in the Bentley.

Timing measurement and adjustment is still done by rotation of distributor.. correct?

Looking through other posts and Wayne's book - I'm still unclear - can dwell still be measured?

Or does this system mean I bypass measuring dwell? I have K-jetronic w/Lambda -and I have the O2 disconnected - running a air/fuel gauge - so the Freq Valve is at default.

I know enough to be dangerous - so any insight will help. My main objective is that I am working through the system to help tune my car.

[cmcfaul]

CDI (Capacitor Discharge Ignition) does not replace the points, the points are a trigger that fires off the CDI. So you need both. Highly recommend you ditch the points for a breakerless ignition like Petronics. Best improvement you will ever make. Otherwise you need points and a dwell meter. Its a pain in the ass getting it perfect and it does not stay perfect very long. Petronics is perfect all the time. looking at the pic, I dont know what you have. Does the box in the left rear hum when the ignition is on. That would be the CDI.

Chris
73 E

[stlrj]

Timing is still done manually, the rest is all electronic.

Cheers,

Joe

[Trackrash]

Originally Posted by cmcfaul

CDI (Capacitor Discharge Ignition) does not replace the points, the points are a trigger that fires off the CDI. So you need both. Highly recommend you ditch the points for a breakerless ignition like Petronics. Best improvement you will ever make. Otherwise you need points and a dwell meter. Its a pain in the ass getting it perfect and it does not stay perfect very long. Petronics is perfect all the time. looking at the pic, I dont know what you have. Does the box in the left rear hum when the ignition is on. That would be the CDI.

Chris
73 E

NOT on the OP's distributor. What is shown is an SC unit. It does NOT have points.

However it does have a centrifugal advance which can be checked with a timing light. There is no adjustment or easy way to measure dwell. There is also a vacuum advance device on that distributor.

The distributor's magnetic trigger is what tells the CDI unit when to send the power pulse to the coil. Think of the CDI unit as a high power amplifier.

As far as maintenance goes. You should put a drop of oil in the felt dot in the center of the shaft under the rotor. You can check if the advance weights are free by turning the shaft by hand.

Timing adjustment is still done by moving the distributor.

[Bill Verburg]

Originally Posted by High Life

Im not familiar with this ignition system - I have never had an issue - but while learning about tuning and reading about Dwell and timing - I thought I better start understanding the ignition distributor

According to Bentley - this is a CDI inductive pulse generator. - So Im assuming this replaces the old points system..

My initial questions then are this:

Is there any maintenance I need to do or look for? - not seeing anything listed in the Bentley.

Timing measurement and adjustment is still done by rotation of distributor.. correct?

Looking through other posts and Wayne's book - I'm still unclear - can dwell still be measured?

Or does this system mean I bypass measuring dwell? I have K-jetronic w/Lambda -and I have the O2 disconnected - running a air/fuel gauge - so the Freq Valve is at default.

I know enough to be dangerous - so any insight will help. My main objective is that I am working through the system to help tune my car.

I'm guessing that you have a stock '81 SC

If so generally there's little to do in the distributor, I'd clean and lightly oil w/ wd40 the rust area

There is no adjustable dwell as there is in a points system, Spark advance is controlled by engine speed and vacuum, there are centrifugal weights that are controlled by springs, you can check the advance operation w/ a timing light
US cars should be 5° +/-2° @950 +/- 50 rpm w/ the vacuum hose connected and 26°+/-2° @6k rpm w/ the vacuum hose disconnected

sometimes the springs break or the pivots freeze up




The SC distributors also rotate counterclockwise unlike all other 911s







centrifugal advance curves


Vacuum advance curves, US only

[High Life]

Thank you all - that clears up those questions.. I'll step away from the distributor (besides the spot of oil and rust cleanup) and drop the dwell quest - this makes sense.

Thanks for the insight Bill and the visuals - that helps.