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Unread 05-29-2009, 11:20 PM   #31
John Strenk
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OK Here is what i'm thinking. It would have to take some type of Holistic computer program to predict firing a spark ahead of the time a signal is detected at the distributor. What if your RPM has changed in the mean time. How much simpler it would be to retard the timing or delay the signal to send out a spark. Particularly the computer technology in the mid 80’s. Were they not running 8088’s back then?

So say you take a distributor with a normal advance curve for an engine design like the 258 and set it up to run further advanced than normal. Say the static advance is 5* (withing the 6* of control chill254 mentions) and the advance at idle adds around 8* for a total of 13* at idle. Then the computer can detect the RPM and say “hey let’s subtract 5* or delay it for X number of milliseconds so I get 8* advance at idle. So as the RPM increases the computer keeps on taking out more or less advance listening to the knock sensor at all times to make corrections. If there is a knock, delay the spark next time. Computers can count and count fast so counting 1-2-3-4-5-6-1-2-3-4-5-6-1-2-etc. is no problem for a computer. So say the computer detects a knock just after (before?) #2 fires each time it can delay just #2 each time. So simple I could code it. IT doesn’t even care if it the #5 cylinder or not just when it occurs in the 6 count cycle. Interesting that the distributor in 84 is connected to Manifold vacuum which tends to keep the distributor further advance most of the time.

Modern engines get their timing off a crank sensor that is set up so you know exactly were the pistons are each time so a modern engine can easily take full control of the ignition with that set up but we didn’t have crank sensors in Jeeps at that time.

Now lets look at what happens when Mr. Nutter removes the computer from the picture. The distributor still has a advance curve built in. He has you adjust the mixture control to what I think Mike has said is a little bit rich so you are not leaning the mixture out. Why did he recommend using premium fuel? It’s not too lean but hey! Maybe the timing is to far advanced for the engine now and running premium fuel will stop the knocking. Make sense?

Some people recommend setting the distributor at 8* and others have problems with anything past 5* but this seems to reduce the power down a lot. Mine runs great at 8* but I’m running a lower compression engine with a cam with more overlap.

If I had an 84 engine I would run the 84 distributor with the timing set at 8* at idle at 650 RPM and re-connect the EGR valve to get the combustion temperature down so the tendency to ping and knock is reduced. Also connect it to the "S" port or ported vacuum signal. Maybe check the plug temperature range also. I really never looked for the proper advance curve for a higher compression, small overlap retarded cam before. Maybe JH can help us here.

The manual is pretty limited when it talks about the ignition part of the control on timing. It mainly mentions that it will retard the timing if it detects a knock and advance or retard the timing depending upon engine conditions. What we need is someone with a stock engine running the computer still and get us come timing curves to see if any other sensor has an effect on the timing.


Last edited by John Strenk; 05-29-2009 at 11:47 PM..
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Unread 05-30-2009, 04:32 AM   #32
Mike Romain
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I have been thinking the same as John which is why I started questioning the extra advance from the old style distributor.
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Unread 05-30-2009, 08:14 AM   #33
jfwireless
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I know with the GM TBI computer when it detects knock it removes 10 degrees of timing, of course with the GM TBI system the ECM has full control of spark. I would guess the concept to be the same with the Jeep computer. The GM TBI computer runs a knock test as soon as the coolant temperature comes up to temperature by advancing timing until it detects knock, if this test fails it removes 10 degrees as it assumes the ESC/Knock sensor are not working and throws a 43 code. This can really detune the engine. I do not know if the Jeep computer has a knock test.

I have never worked with the Jeep computer, but thought this might be similar.

I am just getting ready to play with the spark tables in my TBI 304, been running the 305 GM curves up to this point with base timing set at 10 degrees. I do have the ESC/Knock sensor installed, the 305 GM knock sensor will screw right into the coolant drain plug on the bottom of engine block.

Jim
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Unread 05-30-2009, 09:33 AM   #34
chill254
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You make a good point John and I had thought about the fact that it would be hard to send a spark before you actually get a signal. I think it would be much easier for the computer just to have the 6 deg permanantly and be able to retard from that point and advance back to it not beyond it. That is really what I meant when I said advance.

The main point was that the slot in the mechanical advance is smaller because that timing was given to the computer. You would have to take the timing out of the control of the distributor.

Also I know it would be easy to retard each cylinder on its own but you have to remember this was 1984. To add to that the knock sensor was located on the intake manifold. The computer is very small and very simple. It probably did not have much processing power and was not very fast. I think the simple fact that they even had a computer in the jeep controlling the ignition and carb is amazing.
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Unread 05-30-2009, 10:36 AM   #35
chill254
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Another thing I noticed is that the vacuum advance curves are completly different. The computer model does not reach maximum advance until 18 in of hg. The 79 mmodel reaches full vacuum advance at 11 in of hg.

Also it would make more sense that the full limiter be put into the computer model. That way the computer could retard the ignition back from full. If you only have the 6.75 limiter the computer could only retard from the 6.75. AS John said how would it advance beyond the current setting of the disributor. I wonder why they only put that 6.75 limiter in the computer models?

It would be great if we could get an answer from someone who knew the system. An answer like "we did it for the emissions" or "it was becaus of the increased compression".
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Unread 06-03-2009, 01:24 PM   #36
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I think this thread died out because of the title. The argument about the advance has nothing to do with the title. I will make a new thread with an appropiate title to reflect the current arguement. I will try to summerize the points this thread contains. I would like for you all to be involved in the other thread.

Thanks
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Unread 06-03-2009, 02:33 PM   #37
Mike Romain
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Please have at it, I am very interested in this thread and discussion. Not much of an argument really seeing as neither one of us knows the answer and we both want to know.
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Unread 01-13-2010, 10:53 AM   #38
swatson454
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I found this to be an interesting read and I'd like to learn more if you guys want to keep it going.

At this point, it seems to me like the computer isn't really capable of determining exact crankshaft position or speed because it lacks a crank trigger and since it isn't gathering more information about engine load from a TPS, or VSS that it's not able to anticipate the optimal timing for the next cycle.

If any of that is true, would it make sense then that the computer uses the signal from the distributors fairly steady input because of the timed advance that changes very little and a basically "locked down" centrifugal curve to then delay or retard as needed based on knock sensor and/or O2 input?

I ran a test several months ago to see how much fluctuation there was between manifold vacuum and ported vacuum when under driving conditions and was surprised at how little the timed vacuum signal varied with differing load conditions. After reading this thread, I'm wondering if that steady signal was used to give the computer something to retard from as needed.

Anyone interested?

Shawn

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Unread 01-13-2010, 11:18 AM   #39
John Strenk
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What carb and engine? I ran the same vacuum comparison test and the results were "Interesting". But on mine, both fluctuated a lot but at different times. That may be because the "S" port would shut off when I lifted on the throttle.

I don't think the O2 sensor had much to do with the timing. Some earlier Cal. version of the computer was not even connected to the ignition system. Knock sensor however was important and my studies at the time could detect one cylinder knocking and it would retard that cylinder on the next firing. Now if the Knock sensor was to fail. That would totally screw you up all the time.

What are you referring to on the "timed vacuum signal"? the "S" port?
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Unread 01-13-2010, 11:24 AM   #40
swatson454
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Hey John,

Yeah, the "S" port. Mine was a cali-smogged 83 with the 258. I was testing the point beyond activation of the s ports vacuum pull just to see how it changed in relation to throttle input. It remained pretty steady at around 15 inches (if I remember right) and didn't start to pull off until I was pretty deep in the throttle.

Shawn

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Unread 01-13-2010, 01:02 PM   #41
Cutlass327
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Was the new thread ever started? If so, got a link?
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Unread 01-13-2010, 01:29 PM   #42
Mike Romain
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Quote:
Originally Posted by swatson454 View Post
At this point, it seems to me like the computer isn't really capable of determining exact crankshaft position or speed because it lacks a crank trigger and since it isn't gathering more information about engine load from a TPS, or VSS that it's not able to anticipate the optimal timing for the next cycle.
Not so, it has a knock sensor that the FSM says is good enough to retard just individual cylinders.

Quote:
I ran a test several months ago to see how much fluctuation there was between manifold vacuum and ported vacuum when under driving conditions and was surprised at how little the timed vacuum signal varied with differing load conditions. After reading this thread, I'm wondering if that steady signal was used to give the computer something to retard from as needed.
Yes, that was a factor with the 10" and 4" vacuum switches that are involved. They acted like a TPS that the computer reads.

Jeep Adventures Under the Hood is a good read for component explanations.

The 84-86 uses a mixer valve and a CTO to give the distributor manifold vacuum on a cold engine and then ported vacuum on a warmed up engine.

The camshaft also takes a bigger volume gulp of air/fuel mix with the 86 cam which affects the compression pre-ignition factor or knock factor by changing the point the piston is physically sitting at when it reaches the pre-ignition fire or knock compression point so you can't have the timing advancing too far or it will ping out bad.

I think that is why the 84-86 distributor curve is best for that 84-86 cam. Hey I could be wrong, but..

One of the biggest complaints I see with new distributors on those engines is ping issues from too much or too fast advance.
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86/00 CJ7 Laredo, 33x9.5 BFG AT's, 'glass nose to tail in '00, 'New' frame,wires and plumbing in '09. Carter BBD Carbed 4.0 HO in '10.
89 YJ Renegade. BBD Carbed 4.0 HO. Locked front and rear with 33x9.5 BFG AT's
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Last edited by Mike Romain; 01-13-2010 at 01:52 PM..
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