Turbo Distributor question

[Zmanco]

I've been running fuel-only MS on my NA L28 and I'm thinking I'd like to control spark as well. It seems like the easiest approach might be to find an L28 turbo distributor which includes the CAS. I understand how the CAS tells MS the position of the crank so MS knows when to fire the spark (MSD box in this case). But what adjusts the position of the distributor cap relative to the distributor so it can deliver the spark to the correct plug?

 

I can imagine that as long as the rotor is reasonably near the electrode in the cap when the MSD fires, that most of the spark energy will be delivered to the plug. Is that how this works? I know that the technology available in the early 80's wasn't all that advanced, so I figure it's got to be something pretty simple.

[speeder]

The ZXT distributor installed in the stock position will trigger the MS about 60 degrees ahead of where the rotor begins to contact the corresponding plug wire tower. Perfect as-is. Use the Optical input on the MS for tach in, and use ignition out to fire the MSD. There are good writeups on the MS sites on using this particular distributor as well as MSD ingnition amplifiers.

[Zmanco]

I follow what you're saying, but am still a little unclear on how the rotor distributes the spark to the plug. Are the electrodes inside the cap wider than on the N/A cap so that the rotor will be aligned with them for a longer range of rotation?

 

I'm guessing that if I had a turbo distributor I could just look at it and answer my own questions. TIA.

[naviathan]

In respect to rotor-to-post alignment it's just like the N/A distributor. All standard round distributors use this setup which is why you have a basic trigger advance setting in degrees. The degrees refer to the relationship of the crank position to the coil trigger, what position the crank is in when it fires. In the most basic form, the CAS pulses the trigger to the MS telling it it's time to send the signal to the coil. MS knows once it received this trigger it checks the advance table for current RPM and MAP readings, gets the needed advance and calculates when from the trigger event to fire the coil. Of course MS does this over a period of milliseconds.

[Zmanco]

I have a feeling I'm not asking this question clearly, so let me try a different tack. I'll create an exaggerated scenario to make the point. Let's assume that sometimes we want the spark to fire 30 degrees BTDC and other times at TDC. I understand that the CAS sends the crank (actually cam in this case) position approximately 60 degrees in advance of the TDC event. It seems to me that in the first case the cap would have to be rotated about 15 degrees so that the electrode in the cap is aligned with the rotor. But then for the second case where we want to fire at TDC, the cap would be 15 degrees away from the rotor when the spark fired.

 

My questions are:

 

1) is this indeed how it operates?

2) does this mis-alignment between rotor and cap electrode matter? (I know I exagerated to make this point - maybe using more realistic values means that the error is too small to matter?)

 

Am I making this too complicated?

[naviathan]

When prepping your engine to run you must set the cranking timing with the timing marks on the pulley which gives MS it's reference point or alignment point to run it's timing advance curves. Basicly you set this yourself. I had a hard time getting this together myself.

[Zmanco]

When prepping your engine to run you must set the cranking timing with the timing marks on the pulley which gives MS it's reference point or alignment point to run it's timing advance curves. Basicly you set this yourself. I had a hard time getting this together myself.

Ok, as I think about this further, it dawns on me that same issue occurs with a conventional distributor using vacuum advance. As the plate in the distributor is rotated by the vacuum solenoid, there's the same issue that the rotor and electrode are no longer "perfectly" aligned when the spark is fired. I guess the misalignment isn't enough to matter and that the spark just jumps across the slightly longer path?

 

Have I beaten the horse to death?

[mobythevan]

Are the electrodes inside the cap wider than on the N/A cap

 

ha ha ha, I know what you are trying to ask. Yes you are correct, the end of the rotor is much wider than an N/A rotor arm. So that it can account for the entire range of rotation across your advance curve. Go to autozone and ask for a 280zx turbo rotor and look at it, then tell them its not the right part and leave. Then you will smile and say "I knew it"

[Zmanco]

BTW, does the turbo distributor use the same base (part that bolts to the block) as the non-turbo?

[Zmanco]

Yes you are correct, the end of the rotor is much wider than an N/A rotor arm. So that it can account for the entire range of rotation across your advance curve.

Thanks mobythevan, that makes sense. Now to find a turbo dizzy that's in good shape

[naviathan]

Not sure, I can look. I have turbo in the yard (not mine, but close enough).

[mobythevan]

I have done the NA to Turbo dizzy swap with Prox's car. There is an aluminum spacer that sets the NA dizzy up about 2 inches, you need to remove that. Then you need the turbo dizzy, turbo dizzy oil driveshaft. Loosen the sway bar, drop the oil pump, put in the new turbo dizzy driveshaft, keep timing correct and you are set. Helps to put the engine at TDC before starting.

[Zmanco]

So the NA oil driveshaft won't work?

 

Too bad - I see the turbo dizzy for sale from time to time, but never the drive shaft.

[proxlamus©]

LoL.. yeh Bryan helped me with that!

 

I was freaking out because the dizzy wouldnt fit! I thought I wouldnt be able to do the swap!

 

then Bryan noticed there was a spacer.. whew.

 

 

=)

[Zmanco]

Any secret to finding the shaft? I see the dizzy for sale now and then, but never the shaft.

[naviathan]

Junkyards... Occassionally you'll find them on eBay with the oil shaft included because some people actually know it's needed.