A discussion on distributors, ignition, timing, etc for dummies

[skipdup]

This stuff has always been a mystery to me and I'm hoping someone will help me along, correct my mistakes, and teach me further...

(I think) I have a basic understanding of how spark plugs receive the electrical current to make their spark - the coil builds up a very high voltage charge and sends that to the distributor, the disty has a rotor that spins directly with the engine over 6 pins which then leads to each individual plug wire an then to the plug itself. As the rotor spins over each of the six pins, it allows the current to flow to each individual circuit/plug allowing for ignition of the air/fuel mixture in the individual cylinder.

Depending on when the spark/combustion take place in relation to the stroke of the piston (on it's way up, top (TDC) or down) affects the efficiency of the engine.

Distributors are directly driven off the engine and determine when the spark will be sent. They and can be adjusted/turned in relation the the engine/pistons to advance or retard the timing of the spark/combustion. Also, (at least in the case of my car), there is a mechanical and vacuum operated functionality of the distributor that adjusts the timing dynamically based on what the engine is doing at that moment (i.e. idling, cruising at low or high rpm, accelerating or decelerating).

All of this works in conjunction with fuel delivery, both which affect efficiency of the engine.

Is the above accurate? What else is there that am I missing? I'm guessing there's a lot???

What is an ignition curve? Is it simply how the mechanical and vacuum advances adjust at a given rpm and how fast they reach their maximum values?

I believe my car (US-spec) has a different ignition curve than Japan-spec which is different from RoW, which is different from Cali. What determines the values of this curve? Is it a function of gearing in the disty? Something else?

What part does the Capacitive Discharge Units (C/D Box) play in this?

What does an aftermarket ignition (like MSD) do? What parts does it replace?

Many thanks to anyone patient enough to enter this discussion with me,
-Skip

[IROC]

The June Excellence (I believe) has an excellent article by Jim Pasha that goes into alot of detail on Porsche ignition systems. I learned alot from it, for sure.

Mike

[emcon5]

Originally posted by skipdup
the coil builds up a very high voltage charge and sends that to the distributor, the disty has a rotor that spins directly with the engine over 6 pins which then leads to each individual plug wire an then to the plug itself. As the rotor spins over each of the six pins, it allows the current to flow to each individual circuit/plug allowing for ignition of the air/fuel mixture in the individual cylinder.

Pretty much, except the distributor spins at half the speed of the engine, so it only fires on the compression stroke.

Depending on when the spark/combustion take place in relation to the stroke of the piston (on it's way up, top (TDC) or down) affects the efficiency of the engine.

Yes, but there is a little more to it, size of the combustion chamber, location and number of the spark plug(s), rotation speed of the engine, Quality of available fuel, engine compression ratio, operating temp range, and probably some others I can't think of at the moment. All of these will effect the proper timing advance.

Distributors are directly driven off the engine and determine when the spark will be sent. They and can be adjusted/turned in relation the the engine/pistons to advance or retard the timing of the spark/combustion. Also, (at least in the case of my car), there is a mechanical and vacuum operated functionality of the distributor that adjusts the timing dynamically based on what the engine is doing at that moment (i.e. idling, cruising at low or high rpm, accelerating or decelerating).

Pretty much.

All of this works in conjunction with fuel delivery, both which affect efficiency of the engine.

Is the above accurate? What else is there that am I missing? I'm guessing there's a lot???

What is an ignition curve? Is it simply how the mechanical and vacuum advances adjust at a given rpm and how fast they reach their maximum values?

Advance curve reflects the fact that centrifical advance is RPM dependant. The higher the RPMs, the more advance. If you plot it on a graph, you get a curve:

I believe my car (US-spec) has a different ignition curve than Japan-spec which is different from RoW, which is different from Cali. What determines the values of this curve? Is it a function of gearing in the disty? Something else?

The mechanical advance is determined by weights in the distributor. The faster it spins, the more the weights move, the more advance you get. Changing weights, or the springs that hold them in place will change the advnce curve.

What part does the Capacitive Discharge Units (C/D Box) play in this?

What does an aftermarket ignition (like MSD) do? What parts does it replace?

The MSD is just another type of C/D Box. The primary difference between the MSD and the factroy Bosch is the MSD will send multiple sparks at lower RPMs. Whether this does any good seems to be a matter of much debate.

Hope this helps.

Tom

[skipdup]

Tom- Excellent information. Thanks!

Originally posted by emcon5
The mechanical advance is determined by weights in the distributor. The faster it spins, the more the weights move, the more advance you get. Changing weights, or the springs that hold them in place will change the advnce curve.

Does that mean that the difference between a US-spec and RoW-spec distributor is the weights and/or springs? Or, are there other differences?

The June Excellence (I believe) has an excellent article by Jim Pasha that goes into alot of detail on Porsche ignition systems...

Mike- I think I have that issue... I'll take a look and study up. Thanks for the lead!

- Skip

[911pcars]

Tom pretty much covers it pretty well.

My thoughts:

"Distributors are directly driven off the engine and determine when the spark will be sent."

Distributors are usually driven off of the camshaft which rotates at 1/2 engine (crank) speed. This allows the distributor to fire all the cylinders in its one rotation while the engine must make two revolutions (720º) to complete the 4-stroke cycle for all cylinders.

"All of this works in conjunction with fuel delivery, both which affect efficiency of the engine."

In a modern engine, yes. Without modern electronic control and engine status sensors, ignition is a standalone, independent operation.

What is an ignition curve? Is it simply how the mechanical and vacuum advances adjust at a given rpm and how fast they reach their maximum values?

Tom described the mech. advance curve well.

Vacuum advance is dependent on engine vacuum. Vacuum advance takes advantage of the slower-burning leaner mixtures at off-idle and cruising, thus increasing fuel mileage. The usual vacuum signal is just above the throttle plate so the ignition doesn't advance until off-idle. At higher engine speeds, vacuum diminishes and the mechanical adv. mech. in the distributor continues. The advance curves provided by the factory only show mechanical ign. advance.

In many cars (mid 70's 911s, SCs), there may be a double diaphragm, retard/advance vacuum mechanism, each with a separate vacuum hose. The retard hose senses idle vacuum (below the throttle vacuum); the other off-idle vacuum (venturi vacuum).

The retard chamber applies full vacuum to the distributor adv. mechanism to retard the timing at idle. Retarded ignition timing increases the combustion temperature to help reduce exhaust emissions. In this case, vac. retard only works at idle. The advance vac. chamber works as discussed above.

The difference betwee US and ROW dist. timing are the weights and springs and a mechanical stop that pegs the timing.

"What does an aftermarket ignition (like MSD) do? What parts does it replace?
"

Electronic and aftermarket ignition systems increase the available voltage to fire the spark plugs. Typically, factory ignition is capable of max. 20K volts. Aftermarket units provide around 50K. Note: Max. voltage is not usually needed. Nominal firing voltage is below 10K. Only when ignition loads increase (during acceleration, dirty plugs, wide gaps, high compression ratio, etc.) will voltage requirements increase. 50K provides more reserve capacity if the plug needs it to fire.

Most aftermarket ign. systems provide multiple sparks (up to about 3000 rpm). Some cars (carbureted and MFI) seem to benefit the most at low speed (smoother idling and part throttle).

Sherwood

[echrisconnor]

One thing to note about the distributor:

(I think) I have a basic understanding of how spark plugs receive the electrical current to make their spark - the coil builds up a very high voltage charge and sends that to the distributor, the disty has a rotor that spins directly with the engine over 6 pins which then leads to each individual plug wire an then to the plug itself. As the rotor spins over each of the six pins, it allows the current to flow to each individual circuit/plug allowing for ignition of the air/fuel mixture in the individual cylinder.

There are two components to the distributor. One does kind of what you've described with its routing the the spark to the appropriate cylinder/plug. The other is that it has a triggering mechanism for the coil in it. These are the points, hall-sender, magnetic or optical pickup. The coil charges up power when energized. The opening of the points, or other triggering causes the charge inside the coil to collapse and be discharged and routed throug the distributor to the appropriate place. This triggering is what ultimately affects the spark timing and is altered by the vacuum or mechanical advance systems.

[Bill Verburg]

Tom did a great job.

on a 911 the distributor is driven from a gear attached to the crankshaft, as stated at 1/2 crank speed.

[randywebb]

What part does the Capacitive Discharge Units (C/D Box) play in this?

You want a very high voltage to make a nice spark. The coil (an inductor) does this. A capacitor can help get an even higher voltage. That is what the CD box does. It is more than just a capacitor, but that is the basic idea.


Always useful to remember that all the 'stuff' is just there to do two jobs:
1. make a big spark
2. make it spark at the right time

[Bill Verburg]

You want a very high voltage to make a nice spark

You actually want more than just high voltage.

You want a big fat spark; high voltage, long duration and high amperage. All 3 are needed in measured proportions.

The stock CD box is very good at supplying whats needed.

[skipdup]

This is awesome! Thanks so much. I think I actually get it!

- Skip

[Brother]

Furthermore, when we all someday replace our lousy CIS systems with EFI, we will get the same thing without the distributor. Instead it will all be timed of the position of the crankshaft via either an optical or magnetic position sensor. Then the computer can time the spark based on that as well as several other things.

[Lorenfb]

For more info on ignition systems and how they function, check here:

www.systemsc.com/technical.htm

and here:

www.systemsc.com/waveforms.htm

[randywebb]

"You actually want more than just high voltage."

- I agree. I was trying to keep it simple for a primer, hence the sobriquet "nice" for the spark.


Also, the Bosch yellow booklets are pretty good for ignition, if they still make them.