New distributor nerd out!

[daniel911T]

Hello Pelicans!

I'm still slowly grinding on my 2.2T-2.4E engine swap, but I took advantage of a quiet morning to look deeper into my new ignition solution. I'm trying to grok how the stock vacuum input impacts timing. What's its purpose? Emissions? Efficiency? Idle control?

I'm aware that the original distributor works great without vacuum input, and so will the one I'm trying to set up, but I'm a curious sort and I want to understand.

I got a shiny new 123ignition programmable distributor (https://123ignitionusa.com/porsche/) and am working on building ignition maps that match the Bosch original specs for 0 231 169 004. I was able to google the original data thanks to some very old Pelican threads and websites like Early 911 Distributors - Carbs-Fuel-Ignition (CFI) - Repair & Sales



The centrifugal curve was very easy to reproduce. I know the advance is supposed to be anywhere between the lines, but for accuracy and performance I plotted the top line. If it's a bit too much, I can physically turn the distributor a degree or two to shift the whole curve down.



Here's the result. I'm very happy with it.



This is the original vacuum curve:


Using Google I was able to convert Torr to kPa, although I'm not at all confident with the numbers.




I don't like the result at all. When I try to use negative timing values the program gives an error. Also, in spite of the Torr to kPa conversion being rather straightforward, the values seem very low. For example, 200 Torr, the far end of the Porsche chart, is only about 35 kPa. The distributor recognizes inputs as high as 200kPa. I'm wondering if I'm missing a decimal point somewhere?


I'm also finding it confounding that 123ignition set an immovable point on the graph. The 100kPa, 0 deg is fixed and immovable. I don't think it's an issue because the vacuum range from Bosch is all far below that value, but I still don't understand why it's there.

So, vacuum system on the 2.4/2.7L engines... what's it supposed to do? Given the freedom to make the vacuum and centrifugal curves anything you want, where is there room for improvement? Anybody else fiddled with this system?

Thanks all!

[Trackrash]

I think you are over thinking it. But here are my thoughts/experience.

First the original purpose of a vacuum advance was to aide in better idle and light cruise conditions. I other words when the throttle is closed or only slightly open there is a higher vacuum in the manifold. Most motors, at least before emissions and electronics, benefited from more advance when at idle. Also during a light throttle cruise more advance helped due to less fuel being burned and perhaps a leaner condition could be tolerated. Once more throttle was added and more load on the motor less advance was wanted or needed so the vacuum advance was reduced and the mechanical advance in the distributor took over.

However the older 911 motors did not have manifolds that could be used for a manifold vacuum. In addition whether or not it would have made much difference on the older carbed and MFI motors is debatable.

I your case, I would look at what CR your motor will have and what cams you will be running. Then look at what 911 motors from the carb/MFI years were similar, then select a timing graph to suit.

Then of course that could/should be fine tuned when driving and/or on the dyno.

[RSTarga]

On original 2.4 and 2.7 MFI Carrera engines, the vacuum pot retards the timing about 10 degrees at idle.
This brought 5 degrees ATDC on 2.4's and 0 on the Carrera at idle.

[Trackrash]

To clarify what I said and to add to what RST said, you, IMO, should look at the older ignition curves, not the 2,4- 2,7 curve, since you are not running a vacuum advance on your distributor. The 911s up to and including the 2,2s did not use a vacuum advance.

[RSTarga]

set it up to give your total advance of 32 at 6000. Then see what that gives you at idle. The important part is your total advance.

[daniel911T]

Over thinking it is the point!

Trackrash,
Thanks a lot for your explanation. I asked the question because I had seen two different ways to use vacuum discussed, and I wasn't sure which scheme Porsche was using.

While reading about the magic SVDA distributor for VW it clicked that MAP is a pretty good approximation of engine load. High manifold vacuum, low load. Low manifold vacuum, high load. A vacuum system that is intended to advance timing at high vacuum conditions gives more mileage and maybe lower emissions, probably runs a bit nicer too... I would be interested making this kind of system.

The other kind I've read about was part of the emissions malaise awfulness that infected engineering in the mid '70s to early '80s. In these systems the vacuum is used to reduce advance at idle and I think the system tends to reduce overall performance. Looking at the vacuum curve from Bosch, I read it as increasing vacuum reduces timing by up to 10 degrees. This may help with idle, but my distributor can be electronically set to give 5 degrees ATDC without having any impact on total advance. It also means that in lightly loaded conditions, highway cruising, its actually removing advance. (This assumes I'm reading the chart right.) If this is the Bosch design on the stock 2.4, then I'd plug the port and never think about it again.

The engine I'm working on is a stock '72 2.4E that formerly had MFI, type 911/52. It's a 8:1 engine - nicer than my old T, but still not in great danger of detonation. I'm going to run it with Zeniths.

I think I have the vacuum port issue worked out. The Zeniths had an idle enrichment system that the majority of people immediately disable. (I'm no exception) This leaves me with a set of ports significantly below the butterflies that are unused and should provide MAP. My plan is to run 3 into 1 and take the reference from only the left side. If that's not good, then it's just a matter of plumbing to get the other 3 involved.

To RS Targa,
This distributor I'm working with is a nifty device. All I need to do is give it an accurate TDC and it figures the rest out from there. If I feel so inclined I can program it to give 45 at 6K, and still have -5 at idle. (That would likely break something...) The point is that you can change any point along curve without impacting the others; I'm completely free to program any curve I want, and that I can use or not use vacuum to modify it.

It's almost too much freedom.

So, if you could run any curve on a relatively low compression motor, what curve would you run? Would you use vacuum?

[ed mayo]

The vacuum retard on the 2.4/ 2.7 engines was to reduce H/C at idle. It did not cause loss of performance since it immediately went away once you opened the throttle. I would not bother with the retard unless you want to use it to bring the idle down. Start with no retard and aim for about 0 to 5 * at idle and see if that allows for a nice consistent 900 or so RPM idle speed.

[manbridge 74]

Does this distributor still use the CDI box or is it an all in one unit?

[daniel911T]

Originally Posted by ed mayo

The vacuum retard on the 2.4/ 2.7 engines was to reduce H/C at idle. It did not cause loss of performance since it immediately went away once you opened the throttle. I would not bother with the retard unless you want to use it to bring the idle down. Start with no retard and aim for about 0 to 5 * at idle and see if that allows for a nice consistent 900 or so RPM idle speed.

Got it. So the stock distributor used vacuum for idle control only. I don't feel much desire to implement that. I can, and will, do the same thing electronically.

I think I'll rig the vacuum lines, then take some notes while driving and observing the MAP levels. There may be some performance or efficiency to be gained in fine tuning and providing some mid RPM vacuum advance.

I'm going to start with the reproduced stock curve. In the future, I really like the higher ultimate advance of the 2.2T curve, and I may try that one too.

Thanks for the thoughts and discussion.

I'll post about how experimenting goes.

[daniel911T]

Originally Posted by manbridge 74

Does this distributor still use the CDI box or is it an all in one unit?

I was going to run it without a CDI on the 2.2T, but the 2.4E unambiguously requires the better spark. I'm going to hook up an MSD Streetfire with a Bosch red coil.

I expect great things.

[Jonny H]

Be very careful when using a 123 with a CDI system. Do NOT connect the 123 to the coil or you will destroy it. You probably know this already but other people may not.

Btw, You will need to run your 69 tacho from the coil A terminal. It won’t run from the distributor like the 72 engine harness would be wired for.

[daniel911T]

This is my plan:


I'm really hoping I can get away without the tach adapter.

[Trackrash]

As far as the tach goes, does your car have the tach intermediate unit? If you are using the '69 tach and your car originally had the CDI and the intermediate unit you can probably run your tach off the coil.

This is what I did when I upgraded to a 6 pin CDI on my '71. My tach runs fine from the coil, as it was originally wired, and I am still using the intermediate unit.

[Jonny H]

Hmm, MSD. When driven from the coil, you might find that the slow multi spark output from MSD unit causes your tacho to over read below 3400 RPM. Above that RPM, the MSD units aren’t multi spark.

Best bet is Bob Ashlock’s ‘TachAdapt’.

[Trackrash]

Originally Posted by Jonny H

Hmm, MSD. When driven from the coil, you might find that the slow multi spark output from MSD unit causes your tacho to over read below 3400 RPM. Above that RPM, the MSD units aren’t multi spark.

Best bet is Bob Ashlock’s ‘TachAdapt’.

You are probably correct about the multiple spark unit not working with the old tach.

Maybe someone has tried it?

[daniel911T]

My plan was to initially try the grey wire from the MSD. If that gives a problem, then I suppose I'll try the coil, and if that fails, then I'll go to the tach adapter. I just like simplicity, and really don't want the extra device or wires if I can help it.

Would the tach work if it was hooked up to the signal output of the distributor? 123ignition black wire, MSD white wire. Would that work?

That's how my pertronix unit is driving my tach right now. Seems to work great...

[Jonny H]

Originally Posted by daniel911T

My plan was to initially try the grey wire from the MSD. If that gives a problem, then I suppose I'll try the coil, and if that fails, then I'll go to the tach adapter. I just like simplicity, and really don't want the extra device or wires if I can help it.

Would the tach work if it was hooked up to the signal output of the distributor? 123ignition black wire, MSD white wire. Would that work?

That's how my pertronix unit is driving my tach right now. Seems to work great...

Your tach works now because either:

A) You are running the Pertronix direct to the coil (without CDI / MSD), or

B) Your tach is ‘71 or later.

Pre ‘71 tachs need at least a 150V ’spike’. This is present on the coil primary but not on the signal output of the distributor when running CDI/MSD. Later tachs do work with the distributor signal so that could be an option for you.

[daniel911T]

Very short history:
Car came with no CDI. Installed MSD 6AL in 2005. Drove it for a long time.

In 2016 the car sat for a year after a mechanical problem. When I restarted it, it drove great, then suddenly started backfiring, then died. I found an enormous scorch mark on the rotor and melted metal. Installed a spare, resistorless rotor I kept in the car, and it met the same fate within minutes. In a desperate bid to get it home, I wired around the MSD - connecting the Pertronix directly to the coil. It worked.

I got braver and tried to drive it more. It would start and run great, but as it warmed up it would start to misfire and lose power. That problem was solved by replacing the coil. That was as good as it ever got. I'm pretty sure the failed MSD damaged the coil.

Now I'm changing the engine, and I want to get the ignition sorted the first time.

I'm pretty sure my tach is a '69 original.

Jonny H ,
If I was using just the 123ignition and wiring straight to the coil, would my tach work?

[Jonny H]

^ Yes, tach will work direct from the 123 without CDI. Just connect tach to coil+.

You will be down on power though. CDI makes a noticeable difference on these cars.

[daniel911T]

Originally Posted by Jonny H

^ Yes, tach will work direct from the 123 without CDI. Just connect tach to coil+.

You will be down on power though. CDI makes a noticeable difference on these cars.

Sorry for any misunderstanding! I am using a CDI no matter what. I've already purchased a Street Fire by MSD and it's sitting on the desk next to me right now. The only open question is how to drive the tach. It seems the black wire on the 123 would have the correct type of signal - although I take your point about the voltage being low.

I'll probably just have to experiment a bit. I'll try the gray wire off the MSD first, then the coil, then the black wire on the 123, and finally, if none of that works right, I'll install the MSD tach adapter.

Somehow the tach will work.

Then the fun part! I'm looking forward to seeing how different advance curves change the personality of the engine.