1978-1980 Cam Specs - Page 2 - JeepForum.com
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post #16 of 42 Old 05-29-2009, 11:46 AM
John Strenk
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Could be. Making sure it burns completly before it leaves.

So now that we all have this info.... What micro changes are we going to make in the ignition for an engine that rarely goes over 3000 RPM. ??

Wasn't that what the computer was for? Wouldn't it be better then to hook the computer back up? Heck the computer could detect which cylinder was pinging and retard just that cylinder. Now we would have to retard all the cylinders if one started to ping or knock.


Last edited by John Strenk; 05-29-2009 at 11:59 AM.
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post #17 of 42 Old 05-29-2009, 12:02 PM Thread Starter
chill254
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Well we are trying to figure out if when you remove the computer by the Nutter bypass if you should replace the mechanical advance limiter with one from a pre computer model.
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post #18 of 42 Old 05-29-2009, 12:17 PM
John Strenk
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So we should be getting these charts from different years:

This is a 1984 Distributor curve


I though someone had a cam timing vs ignition advance formula out there.

Last edited by John Strenk; 05-30-2009 at 12:03 AM.
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post #19 of 42 Old 05-29-2009, 12:22 PM Thread Starter
chill254
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I agree John, however, Mike brougth up the fact that he thought the cams were different. So we started a thread to see if this was true. The 79 timing chart is somewhere around here. If we could collect them all that would help. We were also trying to figure out the year of our rebuilt engine by looking at the engine stamp and casting codes.
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post #20 of 42 Old 05-29-2009, 12:31 PM
John Strenk
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Hmm I wonder if the fact I'm running an 84 Distributor in a 76 block is why I'm haveing such good luck with mpg and power even though I have a little 1 bbl.

Here is the 79 curve:

Note two curves in each graph show the difference between automatic and manual trannies. Big difference in mechanical advance limits. Maybe that's why we see the difference in two different advance limits available. Less load in the automatic

Here is my 84 curve:
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post #21 of 42 Old 05-29-2009, 12:46 PM Thread Starter
chill254
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That is the difference between the mechanical and vacuum advance. Both types of advance are on the same distributor. The mechanical has to do with rpm and the vacuum has to do with the load on the engine.
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post #22 of 42 Old 05-29-2009, 12:58 PM
Mike Romain
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Quote:
Originally Posted by chill254 View Post
Check out this web site.

http://www.amcrc.com/tech/engine.html
Thanks,

That site matches my engine date code up as a 258 2BBL built in 1977, Mar 19. Older than I thought.

Mike
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post #23 of 42 Old 05-29-2009, 01:03 PM
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The cam shafts did vary. Year build changes, auto, manual, elevation, equipment, head design, carburetor.

Notice the fine print on the charts John has posted, can't see the #20 script in the example, but the #18 post clearly indicates the distributer being used is the 3242409 and the Hg advance chart indicates the advance mechanism as being the 8134678.

IIRC the '81 TSM has the broadest examples, I suppose cause most of these changes occurred during that era.
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post #24 of 42 Old 05-29-2009, 01:04 PM
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Quote:
Originally Posted by John Strenk View Post
Hmm I wonder if the fact I'm running an 84 Distributor in a 76 block is why I'm haveing such good luck with mpg and power even though I have a little 1 bbl.

Note two curves in each graph show the difference between automatic and manual trannies. Big difference in mechanical advance limits. Maybe that's why we see the difference in two different advance limits available. Less load in the automatic
So you are thinking the other set of notches are for the automatic right?

Which set lines is which, it is hard to see?

Mike
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.
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post #25 of 42 Old 05-29-2009, 01:12 PM
mcmud
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I have always assumed that the two lines represent the + or -2* we see in the AMC/Jeep timing set instructions which are as you know what very, very few will follow.
As to the auto/manual timing setting each will vary within the instructions but those charts will defiantly describe "all applications" .
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post #26 of 42 Old 05-29-2009, 01:15 PM Thread Starter
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Originally Posted by mcmud View Post
I have always assumed that the two lines represent the + or -2* we see in the AMC/Jeep timing set instructions which are as you know what very, very few will follow.

I agree it would make more sense for it to be the tolerence.

Also sorry John I thought you were talking about something else when I said the mechanical and vacuum thing.
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post #27 of 42 Old 05-29-2009, 01:17 PM Thread Starter
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However the basic argument that started this thing between me and Mike is that we are wondering why the change in mechanical advance. If the computer did supply some of the timing should the mechanical advance limiter be replaced with a pre computer one once the computer is bypassed.
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post #28 of 42 Old 05-29-2009, 01:29 PM
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However the basic argument that started this thing between me and Mike is that we are wondering why the change in mechanical advance. If the computer did supply some of the timing should the mechanical advance limiter be replaced with a pre computer one once the computer is bypassed.
Yes, basically can the 1986 engine benefit from the older 78 distributor advance plates or does it's cam prefer the 86 distributor?.

Just because there is more advance available is it a good thing?

Mike
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.
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post #29 of 42 Old 05-29-2009, 01:29 PM Thread Starter
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It is also good to know that parts stores don't have different part numbers for the different year distributors. You could go in and ask for an 80 that would have the larger slots and end up getting the smaller ones from a computer model.
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post #30 of 42 Old 05-29-2009, 05:54 PM Thread Starter
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Quote:
Originally Posted by John Strenk View Post
Wasn't that what the computer was for? Wouldn't it be better then to hook the computer back up? Heck the computer could detect which cylinder was pinging and retard just that cylinder. Now we would have to retard all the cylinders if one started to ping or knock.

That is a great point John. Though I don't think the computer could retard individual cylinders. I think it would just retard the ignition as a whole.

The thing is that the computer could retard the ignition if it got a signal from the knock sensor and if it could retard it must have been able to advance it.

You see the computer would have no control over the mechanical advance in the distributor or the vacuum advance. Both of those are actually in the distributor. So they would have to take out some advance and give it to the computer. That is the reason for the smaller slot. The remainder of the advance would be given to the computer for knock control, or emissions, or whatever.

The computer only has about 6 deg of control but think about how much your idle would change if you moved it 6 deg.

Call me crazy but I think this is an excellent point.
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