1) I no longer have the vacuum advance, so without the CDI+, I do not have the same curve.
2) I can now stick my 911 on a dyno and eek out a few more HP via tuning.
3) It's fun to tinker!

Well, I'm glad I checked. The numbers were not where I expected them to be.
https://i.imgur.com/IS3a0k9.png

Now to adjust the dizzy so I can set the trigger point so I can write a map so...

I rotated my distributor as far as I could. I then added to the Trigger Point until I found the magic number where my idle was at 7* BTDC. I double-checked all the other numbers and... They changed. (Why?) Anyway, here's my dizzy maxed out. It opened up 19* of room to play in.
https://i.imgur.com/4ehi3B1.png

Edit: Doh. That should be +1, not -1, at 1500RPM. Grr. And I've already disconnected the cable and bolted it back up!

I should figure out if I need to move the line below the red zone, or if the CDI+ automatically does it for me. eg, if I'm asking for 13* of advance, but it can only provide 10*, does it provide 10* or does it just not do anything?

The distributor vacuum system was only for exhaust emissions requirements.
It could easily be disabled/removed and the timing re-adjusted.

A simple use of a 13mm wrench and a slight turning of the distributor solved that in the past.

Yes, when it doesn't cost buying another ignition system.

Please explain. Indicate what your max timing is when you check it with a timing light.

With the Timing Point set to 0*, and a flat map assigned, I measured that I was getting 17* advance out of the mechanical advance mechanism. I rotated the distributor until it couldn't go any further, and then increased the Timing Point number while pointing the timing light at the crankshaft pulley. The timing light was set to either 5* or 7*, I can't recall which. I'll assume it was 7*. I increased the timing point number until the light showed TDC (So it was actually 7* BTDC) at 1000RPM. At this point, my ignition timing should have been the same as it was before I adjusted the distributor. It wasn't. Instead of measuring 17* advance at/above 2500RPM, I was getting 22*. My target was 35*. So, using the CDI+, I added the necessary 13* of timing to bring it up to the target.

As far as the "19* of room to play in", at 0 RPM, I could increase the timing by as much as 19*. The picture shows that red zone "adjust room" area, which increases with engine RPM. So by redline I don't have a full 19*, I'm down to about 11* I can add to the 22* measured. My target is 35* @ 7000RPM, but I'll only be able to reach 33*. This is still superior to the 22* the distributor gives me on its own, fully maxed out on adjustment with a 13mm wrench.

Now, if you were just using the stock Bosch CDI, you COULD just rotate the distributor and use your 13mm wrench, but you'd be destroying your timing at idle. If you wanted 35* BTDC @ redline using nothing but the dizzy-rotate method, you'd be at 18-20* BTDC at idle - When you're supposed to be at 5-7*. The CDI+ allows you to have the best of both worlds.

Doesn't sound good!

Sounds like you had a bad stock 911 distributor, i.e. the stock distributor should provide at least 33-35 degrees advance.

Hopefully, your engine doesn't get damaged with some Mickey Mouse setup, or your new ignition changes your timing in error and it becomes too advanced causing detonation.

I'm not certain you're understanding the function of the controllable ignition.

Yes, you can get 35* out of a distributor and the factory CDI unit. It's absolutely possible. It is NOT possible to get that number out of the CIS distributor I have (without the vacuum setup) and still have 5* at idle. The mechanical advance curve simply doesn't support it. Here's a chart that shows the mechanical advance curve with 7* idle (red), and the mechanical advance curve with 35* at the upper RPM range (yellow). That blue line in between is the curve you want. You can NOT get this curve with just a 13mm wrench. You would need to disassemble your distributor and recurve it, work some magic with a custom vaccuum system, or utilize an electronic solution (CDI+). That's all that happens when you adjust your distributor - the curve moves up or down the graph.
https://i.imgur.com/TRAN7K2.png

If your curve goes into the red zone, it is telling you that the curve you are asking for is not possible because the trigger point (in crank degrees) is not enough. In other words, the trigger isn't early enough for the CDI+ to fire at the advance you want.

Think about this example - say the box gets the distributor signal 20 degrees before TDC ( trigger point = 20) then how could it possibly give advance > 20 degrees (it would have to fire before it got the signal!). It's a clever piece of kit but it can't go back in time.

If the graph goes into the red zone, then the advance delivered 'tracks' the bottom of edge the zone. In your last screen shot, it looks like you need trigger point set to about 24 to get the advance under the red zone ( it may be in the red above 8000 RPM but you aren't going there with an SC engine so it doesn't matter). You will need to turn your distributor a but more to compensate.

You should always aim to have the red zone just 'miss' the top of your curve. That way you will minimise the amount of physical turn of the distributor.

Remember, always validate some point on your graph with a timing light after any adjustment. The advice is to dial it in using a straight portion of graph in a low RPM band (say 1500 - 2500) just to do the check. If this is correct, then the rest of the graph will be correct too!

The vacuum advance is - nomen est omen - a manifold vacuum based ported advance which simply cant be simluated by a system which actually calculates the advance out of the current rpm. The vacuum is only present at low and lower mid loads, means where the throttle butterlfy is just opened a few degrees where at this position it passes the port/hole in the manifold. It brings a significant benefit by advancing the timing by approx 5° at such low loads where you'll feel a smoother driving/crusing with a better throttle response/ torque and less fuel consumption.
Cause at such a low load the ported higher vauum within the manifold causes a leaner mixture which needs more time to ignite – and thats why the vacuum advance here in this state does not result in pining but in a perfect combustion and therefore noticable torque.

Very well stated Andrew! I am planning to port each one of my PMO manifolds into a single vacuum to my 123 distributor. I’m going to use 1/16” ID hose and fittings.

Teejay you should look into doing this if your system allows an actual MAP from vacuum.

So if I'm reading this right you can't turn the distributor any more and it's limiting your "adjust room". It might be time to move it a tooth so you aren't up against the end stop.

Well stated, but the vacuum signal is going to be very weak with the PMOs. The factory vacuum advance is likely to be useless when you're running triple downdrafts or ITBS. If the 123 dizzy can filter or smooth the signal it might work.

Plenty of cars running carbs without the advance pot.

As for idle advance, I run 10 degrees at idle, any RPM between 800-1200 since the MFI does tend to idle a bit low when cold. This way I can easily check the timing and the curve isn't going to be causing it moving around a bit while I'm checking the timing.

With the CDI+ you can also build a bit of idle stabilization - if your normal idle is, say, 900RPM, could could experiment with adding a few degrees timing if the idle drops to 850, and pull a few degrees if it goes up to 950. Something to play with.

Last but not least, if you get any sort of kickback during starting you can pull advance at cranking RPMs, could be especially useful with the carbs.

Oh and last thing teejay, sending you a PM....

Hello Mr. Hart!

Glad you popped in. Teejay, he's absolutely correct about the red zone, it explains why you weren't able to quite get the advance you programmed into the curve.

Any chance you are here to suggest he send his original CDI to YOUR COMPANY for repair?

https://www.systemsc.com/products.htm

If you really are that desperate for work, why don't you advertise in Panorama or something instead of sneaking around here wasting everyone's time by asking questions you should already know the answer to?

Poor teejay is just trying to have some fun with his car, he doesn't need you trolling him.

You are reading this correctly. I am RIGHT up against my limit. If I had about 2* more movement, I'd be perfect.

However, guys...

I drove it today.

It was glorious.

I went out to a movie with some friends. 45 minute drive to the theater, 3 hours of movie and hanging out chatting about it a bit. So the car had three hours to cool down before one of my buddies asked, "Why'd you go carbs? At this altitude? How do they run?"

I was able to say, "like this!", reach in through the window, and START THE CAR WITHOUT USING THE GAS PEDAL. It idled. (low, admittedly, but on its own!) My ignition timing was the last piece of the puzzle I needed, apparently. It started and idled in 57*F weather with no driver inputs. I don't expect that to happen every time, but man... That felt GOOD.

Fun and educational listening to y'all discuss this. Seems way more complicated in practice than I expected. Thanks so much for all the discussion.

TeeJay, glad to hear it's working so well!

Great to hear!!!

You are already enjoying the main benefit of the CDI+. The strong, powerful, double spark just blasts right through questionable mixtures. My car fires up on the key the same way, I love doing the same thing and amazing people at how well it runs.

I imagine there's a big change in the way the car drives, too.

You can just plug a CDI+ in, in place of the factory unit, it's a simple plug and play install.

However if you want to enjoy the benefit of being able to program a very precise, non-linear spark curve, you can easily do so.

I think this thread was a little longer and more complicated than it needed to be because teejay didn't read the detailed installation manual before diving in.....

The manual is a good read for anyone considering the upgrade:

https://www.classicretrofit.com/pages/manuals-and-software