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Unread 10-12-2011, 07:36 PM   #1
dylan0708
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Advance curve

I understand that timing on the 258 can be screwed up I put on a weber carb and I think my timing or advance curve is off so I was wondering what distributor to get and from where?

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Unread 10-12-2011, 08:09 PM   #2
gojeepin
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There's no reason to replace your distributor just because you installed a Weber carb.

What gives you the indication that your timing curve is off?
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Unread 10-12-2011, 09:46 PM   #3
BagusJeep
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Some posters on this Forum like to alter the ignition curve because they believe they get better performance, often combined with an ignition change for greater spark.

There is nothing wrong with a stock ignition curve, it was put together by automotive engineers who knew what they were doing. It can be a little fiddly to measure your setup to see if you are close to the stock curve.
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Unread 10-12-2011, 09:58 PM   #4
86cj74.2L
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+1 that's why I went back to the motorcraft dizzy. Added a MSD6a couldn't get the HEI setup to my liking.
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Unread 10-13-2011, 02:23 AM   #5
dylan0708
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I think I'm defiantly running enough fuel fast enough to need a greater spark then... I'm running rich but when I play with jets itll run rough. I have a weber and a tuned header. Is there any way to get a longer more consistent spark?
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Unread 10-13-2011, 02:33 AM   #6
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Quote:
Originally Posted by dylan0708 View Post
I think I'm defiantly running enough fuel fast enough to need a greater spark then... I'm running rich but when I play with jets itll run rough. I have a weber and a tuned header. Is there any way to get a longer more consistent spark?
MSD 6. Multi spark CD ignition system.



http://www.ignitioninfo.com/cdignition.html
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Unread 10-13-2011, 06:36 AM   #7
Matt1981CJ7
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Rather than "think" your curve is off, why don't you actually plot it? All you need is a timing light and a pencil and paper.

Start by disconnecting and plugging the vacuum advance. Then read the timing at idle and at every 200 RPM increment up to about 3200 RPM. Record and graph your numbers. Or, just post the numbers and I'll set up your graph.

Unlike the opinions of some, I think the stock advance curve is a joke. It was established by engineers who were more concerned about emissions than performance. Also, nearly every Motorcraft replacement will have a different curve, due to the variety of springs they put in them.

Let's see what you have.

Matt
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Unread 10-13-2011, 07:00 AM   #8
BioTex
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IMO, the factory timing curves are adequate at best for most people. After all, most people do have to pass emissions, and the six cylinder is only seeing low rpms. That said, there is definately room for improvement.

Here are the factory curves for reference.

I would like to hear others opinions on how to improve the curve if the OP doesn't mind the hijack...
6-cyl-timing-curve.jpg  
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Unread 10-13-2011, 08:38 AM   #9
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Here's mine:



http://www.jeepforum.com/forum/f8/wo...w-tune-973904/

To bad I don't have the money to put a stock engine on a dyno and try to get some proof out of the modified curves.
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Unread 10-13-2011, 08:48 AM   #10
swatson454
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Here's a really good thread about how to get a good curve out of a stock Motorcraft and how much better it runs when you do. Matt did a great job going the extra mile and he got the full benefit of the time he spent getting there.

http://www.jeepforum.com/forum/f8/he...-curve-1238634

IMO, a good curve is extremely important. People often say "we're not hot-rodding these things" but then they drop $300+ on a Weber, headers, CDI, etc. You'd be surprised how much better it'll run with just a little time spent on the centrifugal curve.


Shawn
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Unread 10-13-2011, 09:06 AM   #11
Matt1981CJ7
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John,

That would be interesting.

The performance difference in mine was considerable after re-curving. I have several hills near my house that I test drove and recorded top-end speeds at full throttle after each spring change. I improved my upper end by roughly 10% when I was done, based on my crude measurements.

I think these ol' 258s can take a lot more centrifugal advance than some think, but it can't happen too soon in the RPM range, or they start rattling.

Here are my before and after advance curves:

Matt
advance-curve-1.png   advance-curve-5.jpg  
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Unread 10-13-2011, 10:16 AM   #12
BioTex
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What causes the mechanical advance to drop off just before it climbs?
Doesn't make sense to me...
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Unread 10-13-2011, 10:46 AM   #13
John Strenk
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Quote:
Originally Posted by BioTex View Post
What causes the mechanical advance to drop off just before it climbs?
Doesn't make sense to me...
A great mystery even Jeephammer and I haven't figured out yet. But it's there in all the graphs. Both factory and empirically.
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Unread 10-13-2011, 10:55 AM   #14
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Quote:
Originally Posted by Matt1881CJ7 View Post
John,

That would be interesting.

The performance difference in mine was considerable after re-curving. I have several hills near my house that I test drove and recorded top-end speeds at full throttle after each spring change. I improved my upper end by roughly 10% when I was done, based on my crude measurements.

I think these ol' 258s can take a lot more centrifugal advance than some think, but it can't happen too soon in the RPM range, or they start rattling.

Here are my before and after advance curves:

Matt
I use to do that also but it seems to average all the resulting changes. Maybe your gaining a lot in the bottom but actually loosing something on the top. You overall would still show a great improvement but you can tune closer by restricting the range you test in. Wouldn't it be great to get gain in both top and bottom ranges.?

I like doing mine in timing RPM changes like from 1000 RPM to 1500 RPM for bottom end. Top end changes from 3000 to 3500 RPM. I'll pick a gear like 2nd were it won't ping or lug to badly and get a stop watch out. Even record it with your cell phone and play it back later.

This also works for carb tuning also.

Just don't try to do both at the same time. That will really mess you up.
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Unread 10-14-2011, 09:07 AM   #15
swatson454
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Quote:
Originally Posted by John Strenk View Post
A great mystery even Jeephammer and I haven't figured out yet. But it's there in all the graphs. Both factory and empirically.
Check this post out.

Quote:
Originally Posted by bill jones
-99 percent of every ignition I've dealt with has been distributors like GM HEI's, Fords DuraSpark, and MSD distributors with the Ford style magnetic triggers-----and about 90 percent have been oval track----and the ignition boxes were typically the MSD6's or 6AL's.
-I have had ignition testing machines since the early 1970's where I can test and modify complete ignition systems and crank triggers etc to about 10,000rpm.
--------------------------------------------------------------------
-One issue I have seriously dealt with when running distributors that contain the ignition trigger is rotational slop between the distributor gear and the cam gear---which is quite frequently around 3 degrees of uncontrollable timing scatter.
-This can be helped with oversize pitch distributor gears and attention to the exact height orientation of the distributor gear in relation to the cam center line.
-----------------------------------------------------------------------
-But---to fix ALL of the slop the only way I found to fix it requires offsetting the distributor shaft by installing a purposely offset lower bushing----to move the distributor gear several thousands closer to the cam.
-------------------------------------------------------
-Having not dealt with more than maybe a dozen crank trigger systems I know the original magnetic trigger systems that MSD used---that had metal reluctor tabs----have been obsoleted in favor of the flying magnet system----and there was timing retardation when using the original system.
-I really do NOT know how much if any the flying magnet system retards--and I have yet to test any of the MSD digital boxes----I've heard they do not retard
--------------------------------------------------------
-any ignition system that uses a distributor for the plug wires--where the timing trigger is remote from the distributor such as a crank trigger and/or any type of electronic ignition timing controller---the distributor always needs to be checked & rotated to get the rotor indexed to coincide with the timing event when the engine is running.


It could be that were seeing the effects of distributor shaft riding up on the gears as it begins to sping faster and the gear pitch is ever-so-slightly rotating a few degrees ccw while it happens.

Hell, I gues it could happen.


Shawn
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