Ignition map for 1600 engine with Megajolt

[clonebug]

If you have one barrel pulling more vacuum than the others you need to fix that first.
Get the carbs and barrels synced then adjust the carbs.
You will never get it running good with a bent throttle shaft.
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[tzepesh]

The AFRs are 12.7 and lower through all RPMs.
If the engine is talking to me, what does it say? This is why I opened this topic. For me, it is all cotradictory theory, behavior and measurement.
The higher vacuum on one barrel is not the major issue. It ran like this for 5 years now, no problems. The difference is very small. I will address it when I take the carbs down again. I got several replies before that such a small difference will not influence the settings and running very much, and it didn't bother me in this time.
All these troubles started AFTER switching to MAP. On TPS, with sensor disconnected (so only rpm based advance), it ran very well.
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'74 1303S, L98B Viperngrun (extra glitter), German Look
'64 1200, Sea Blue, under restoration

[Boolean]

I tried using PU tubing exactly like you have done. They did not like backfires but did fine when mapping was done.
I still think you must fix the wrongs first. Stuff a working distributor in there and adjust carbs. Then try Megajolt again.
Even if it ran good before, things happen...
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I strive for perfection. Excellence will not be tolerated!
Build thread here:
http://www.thesamba.com/vw/forum/viewtopic.php?t=529379

[Boolean]

[tzepesh]

I did not make a map for load compensation yet, after switching to MAP sensor. I was just trying to get it running in some way, and see what the MAP signal looks like vs. throttle opening and RPM. As I said, I will tune for load compensation on road.
I will try to do one more thing this week, to rule out the faulty MAP connections: I will remove all tubes from manifolds and plug the quick couplers, then start the engine again. It shall be at atmospheric pressure all the time and no connection between intake manifolds. I will be very surprised if it will not run better.

I still want to find a reason why, using absolutely the same parts, but installing the MAP and tubing, it runs so bad. I did not even change the positions of the screws on carbs. I just took everything down, drilled and tapped the intake manifolds, installed the MAP sensor and removed the TPS input parts, and installed everything back. The ignition advance was also the same. So? Why does it behave differently??? I can not see anything else past the MAP connections...
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'74 1303S, L98B Viperngrun (extra glitter), German Look
'64 1200, Sea Blue, under restoration

[tzepesh]

Today I removed all vacuum connections and replaced them with plugs. The car did not start well, but I believe I forgot to increase the cranking advance (from 4 or 5 degrees). Then it ran much smoother, and no popping through the carbs. So It was all in the vacuum connection between all intake manifolds. I don't believe the barrel that had more vacuum created all the issues, but just the connection between all. Maybe it would have worked better if I put some one-directional valves on each line and then go to the vacuum accumulator. But that would have reduced the MAP signal.
I guess the next plan is to get down the carb with the twisted shaft and solve that first, then I will install a new TPS and just tune my initial map some more. For me all the hassle with MAP and individual barrels makes me think that it would behave similarly with ITBs on fuel injection. So the plan for that will be to use a single throttle body with plenum, and take the MAP signal from the plenum, and also use a MAF. There can be a combination of MAF and MAP with Megasquirt.
I'm now thinking how I can use the vacuum cleaner or something similar to check the flow on all barrels off the car.
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'74 1303S, L98B Viperngrun (extra glitter), German Look
'64 1200, Sea Blue, under restoration

[clonebug]

How did you swap the Megajolt over to Map??

Is there a jumper or do you control it with software???

Does it accept the map sensor you used??

Did you calibrate it to the Megajolt .
There are different sensors that are 2.5 Bar or 3.0 Bar or higher.
Which one do you have and is it setup in the Megajolt???
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[tzepesh]

To switch from TPS to MAP, I soldered the MPX4250 sensor and related components and de-soldered the series resistor from TPS input, as described on the Megajolt site. Then I re-flashed the microcontroller to have the calibration for the MAP sensor. The sensor was reading good.
With Megajolt, if you choose MAP input, it uses the calibration for MPX4250. If you want to use another type of sensor, it shall be connected to the TPS input and the calibration is set manually.

My point was that only connecting the 4 intake manifolds created all the issues. I used an ignition map that increased advance only with RPM, no load taken into consideration. So it didn't matter if you had 30, 70, 90 or 102 KPa. I just removed all tubes and put in plugs, and let the tubes to open air. So now the ignition map is the same as before, the sensor is reading ~102KPa all the time, but the engine is running much better.
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'74 1303S, L98B Viperngrun (extra glitter), German Look
'64 1200, Sea Blue, under restoration

[tzepesh]

Warning, long post!

I started to tune the ignition table in Megajolt and I kept pushing up the ignition advance values, making the engine run better and better, and finally got close to the first map I posted, which I considered to be too advanced. And then I wanted to see again how a distributor curve with centrifugal and vacuum advance looks like. I used a tool/template to plug in the values and get final Megajolt maps from here https://forum.autosportlabs.com/viewtopic.php?t=2632.

Please educate me on the following: I read everywhere that manifold vacuum is totally different than ported vacuum. But from what I obtained in megajolt during runs and from the calculation tool, I get mostly the same. And I thought the following: both MAP and ported are vacuum, the difference being that the vacuum port for distributor is above the throttle plate. But as soon as you push the accelerator pedal, the vacuum port is exposed, and it seems to see the same vacuum as MAP. With MAP, you also get vacuum at idle, but as soon as you push the pedal, both MAP and ported vacuum shall be close to the same value. Am I interpreting this correctly?

Next, I will try to explain how I started from distributor curves and got to some Megajolt maps that frighten me when I see the advance results.

Let's start with bosch 021 905 205 G. Here is the advance map from the manual (sorry for the picture quality, this was the best I found):

a is vacuum advance, b is centrifugal advance. Advance is in distributor degrees, and rpm is in distributor shaft degrees, so both degrees and rpm shall be doubled to get crank degrees and rpm. I plugged in the tool, also computing for vacuum transformation from mmHg to MAP Kpa:

Considering 7.5 degrees initial static advance, I got the final Megajolt map:

Now what I see right: if no vacuum is considered, I get a maximum of 31.5 degrees advance, correct from 7.5 initial plus 24 max from dist. But it seems that as I add vacuum (and from what I can think of from my runs, that ported vacuum behaves exactly as MAP), the advance gets to be huge at low loads (lower KPa). I do not have a car with a distributor to measure exactly what happens, but I bet it looks exactly as in the map I got from the tool.

I'm reading on http://www.oldvolkshome.com/ignition.htm#A1974MFD that for 1.6 manual transmission, the initial advance is 7.5 degrees, vacuum is 8-12deg Adv, and centrifugal is 7-12deg @ 1600rpm, 20-25deg @ 3800rpm, so very close to the values above.

Just for fun, I pluuged in the values for an autostick distributor, with 0 degrees initial advance (for this one, a is centrifugal and b is vacuum advance):

Again, this one looks close to the first.

What I need is feedback: do these Megajolt maps look plausible with all that high advance at low loads? Are these the normal plots for the said distributors, so the transformation from the datasheet to Megajolt was done right?

Moreover, I was lookng over this whole topic again and I see that what I get from my runs and from the tool is close to what Jonboylaw posted on the first page. Now this map which I considered extremely aggressive seems that could accept even a bit more advance in some bins:

[Juanito84]

Good job!

And yes you are correct. Manifold vacuum and port vacuum are exactly the same with the following exceptions: 1. When the throttle is closed it cuts of vacuum advance. 2. DVDA carbs many times had a two step vacuum port that caused the vacuum signal to transition less abruptly between closed throttle and part throttle. 3. Way back when, on single vacuum single advanced setups (most single port engines) the ported vacuum signal is very different from manifold vacuum because it's combined with venturi vacuum (which takes the place of mechanical advance).
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If a water cooled engine cools its water with air, isn't it just an overcomplicated air cooled engine?

[tzepesh]

Thank you Juanito for your opinion.
At idle I kept the static advance without vacuum, simulating the ditributor. As I increase the advance, the rpm also rise, for example at 10 degrees I got around 1000rpm, and the throttle plates are fully closed.
The distributor graph for vacuum also explains why I never got to see lower than 55 or 60KPa at idle... 300mmHg corresponds to 60KPa, so I guess the original motor never got higher vacuum, even more now with the W100 which would decrease the vacuum a bit.
Next step is to get the fuel sorted. I kind of understand now that the spitting through carb could come from too low advance (usually the reason is lean mixture, but in my case it was always rich, from 10AFR with 52 idle jets to 14 with 50s and 18 with 48s). As I have wasted spark, I guess the extra spark occured right when the exhaust was closing and the intakes was starting to open, so more advance could help with that issue.
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'74 1303S, L98B Viperngrun (extra glitter), German Look
'64 1200, Sea Blue, under restoration

[Juanito84]

[tzepesh]

Something else crossed my mind today, as I'm still wondering about the high combined advance - centrifugal and vacuum - I obtained (let's say all above 36 degrees). Do the dual advance distributors have any way to stop the combined advance at a maximum value (36 degrees for example), maybe a stop tab or something else? If yes, what is the maximum combined advance that would be allowed? from the table, I get that I would get to 34 degrees very fast and stay around that value (2750rpm and 77KPa is something I get during driving).
I saw that may people are using only centrifugal advance distributors on hot motors, and limit the maximum advance to 28-30-32 degrees, so they never go above that value.
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'74 1303S, L98B Viperngrun (extra glitter), German Look
'64 1200, Sea Blue, under restoration

[Juanito84]

[tzepesh]

Thank you once again for your advice! I will update the ignition table this weekend, as I reduced everything above 36 to 36. I hope to have the time to jet the carbs and I get to the perfect tuning I am targeting.
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'74 1303S, L98B Viperngrun (extra glitter), German Look
'64 1200, Sea Blue, under restoration

[jonboylaw]

So, did you end up using my map as the baseline in the end? I have been running this map in my T1 1955cc engine since my original post and covered many miles with no issues and smooth running. It should be close enough as a starter map for most T1 engines. The only addition I have now made is to add a single anti pulse valve between the plenum and the Megajolt vac sensor to smooth the signal.

[tzepesh]

Please see my logs below:

As you can see, as soon as I accelerate, the vacuum goes low (~90KPa) and even during cruise it is still low. I expected to see ~80-85KPa at cruise. Now the ignition advance seems to be too low at cruise (25 degrees instead of 35-40), and the ignition advance seems to stay in just a few bins, so I don't take full advantage from the map resolution.

How can I improve the vacuum signal in the most efficient way?

Maybe add an anti-pulse valve between the vacuum canister and MAP sensor?
I could also add restrictors in the 4 lines going from the manifold to the vacuum canister (like 0.5 or 1mm drilled jets), but I have read some time ago that this is not a good solution because it delays the vacuum signal and also provides a not so accurate signal.
Other ideas?
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'74 1303S, L98B Viperngrun (extra glitter), German Look
'64 1200, Sea Blue, under restoration

[käfertom]

Hallo tzepesh
To get the best vac-signal you have to use all four manifolds and each fitted with an anti-pulse-valve. But I think the difference between one, two, three or four ports is not very big. I think of max 30%.
In some double Weber IDF fitted Alfa Romeos there are two cylinders/throats fitted with these ports. but there is only one hose fitted to the distributor, I think.
If you have got a ported vac positioned just over the shut throttle plate, you have to consider that is a switched vacuum port. At idle it signals the almost ambient (simple saying) air pressure. When you open the throttle plate it passes the vac-port and the sicnal becomes a manifold vacuum.
Sorry, if you know that already.
Is it possible to read the current vac-signal while driving with MJ?

[jonboylaw]

I used a plenum chamber made out of 1 1/2 inch plastic pipe and tapped the inlet manifolds for M3 barbs. These join at the plenum chamber and then route one further pipe to the MAP input on the MJ. This pipe can have an anti pulse fitted so you only need one. Never had an issue running this system. Avoid the carb vacuum take off. When tapping the inlet manifolds, ensure the bans are at an incline to allow fuel condensate to drain back.

[tzepesh]

I could try to have an anti-pulse valve on each line. Next week i'll go and buy some. That will be interesting to see, I was thinking when I built the vacuum canister that when the motor runs, only one cylinder pulls vacuum, the other ones not, and maybe the vacuum signal is affected by the connection between all manifolds through the canister. But many engines use equalization pipes between the manifolds, without directional valves for vacuum...
The four pipes take the vacuum signal from under the throttle plate, so I see high vacuum at idle, but this is not an issue with MJ mapping, I just have low iginition advance at idle (8deg).
I can read the vacuum signal while running. You can see that in the logs, the white trace is vacuum signal in KPa (Load).

The pipes are inclined to drain the fuel.

Maybe i'll try both ideas, with one valve between canister and sensor, and with 4 valves between manifolds and canister. It will take some time to test, next weeks I have lots of activities.
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'74 1303S, L98B Viperngrun (extra glitter), German Look
'64 1200, Sea Blue, under restoration