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Unread 05-13-2002, 10:15 AM   #1
johny1i
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Hey Bob K & anyone else

It seems like I remember you going through something similar to this about a month ago, and I wanted to see what you came up with. I have finally completely de-emissioned my 258 by finishing the nutter bypass last week. I am still running the Carter, but the stepper motor needles are seated and it is unplugged. I also chose to keep the manifold vacuum for the vacuum advance because it is so much stronger than ported vacuum (about 14" vs. 1"--right off idle). Anyway, the jeep is running great and I'm getting about 15-16 mpg, but the amount of timing that I can now set it to is quite high relative to stock. I know stock is about 9 deg. at 1600, but when I put mine at 8-deg at idle (which is 16-deg at 1500 rpm), I still could not get it to ping even when flooring it, so I bumped it up to 10-deg at idle, which is about 17(!) at 1500 and it only pings when I really stomp it in 5th gear with the AC on (this is with 89 octane gas). Therefore, I'm inclined to leave it there because I seldomly ride it that hard, and just wondered if your results were this far from stock.

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Unread 05-13-2002, 12:40 PM   #2
OldJeepguy
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</font><blockquote><font size="1" face="Verdana, Arial">quote:</font><hr /><font size="2" face="Verdana, Arial">Originally posted by johny1i:
<strong>It seems like I remember you going through something similar to this about a month ago, and I wanted to see what you came up with. I have finally completely de-emissioned my 258 by finishing the nutter bypass last week. I am still running the Carter, but the stepper motor needles are seated and it is unplugged. I also chose to keep the manifold vacuum for the vacuum advance because it is so much stronger than ported vacuum (about 14" vs. 1"--right off idle). Anyway, the jeep is running great and I'm getting about 15-16 mpg, but the amount of timing that I can now set it to is quite high relative to stock. I know stock is about 9 deg. at 1600, but when I put mine at 8-deg at idle (which is 16-deg at 1500 rpm), I still could not get it to ping even when flooring it, so I bumped it up to 10-deg at idle, which is about 17(!) at 1500 and it only pings when I really stomp it in 5th gear with the AC on (this is with 89 octane gas). Therefore, I'm inclined to leave it there because I seldomly ride it that hard, and just wondered if your results were this far from stock.</strong></font><hr /></blockquote><font size="2" face="Verdana, Arial">Your manifold vacuum will decrease to virtually zero at W.O.T., whereas the ported vacuum will increase. Seems to me that the engine will be at full advance at idle and retard as engine speed increases and vacuum decreases. This explains why you can't get it to ping. Try hooking it back up to ported and time it again - I'll bet you'll get it to ping at anything past about 14-16 degrees initial advance.
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Unread 05-13-2002, 01:26 PM   #3
johny1i
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I appreciate the suggestion, OldJeep, but I'll admit that the ported vs. manifold issue has perplexed me since I first heard it suggested. Enough so that I hooked up a vacuum gauge inline with the vacuum advance to record the vacuum pressure at 500, 1100, and 1600 rpm. For manifold (stock), it was 14.5", 16", and 16", respectively for those three RPM settings. However, for ported vacuum, it was 0" (at idle of course), 1", and 2.5". Therefore, it seemed to me that it would almost be like not even having vacuum advance hooked up on a ported setup until you get the engine rpm's up, and even then 2.5" doesn't come close to advancing the timing as much as 14-16" of vacuum do. I'm not saying that either way is wrong, I just don't see how the little vacuum advance mechanism on the side of the distributor can function with such a small amount of vacuum. (as you can tell, I'm a tinkerer when it comes to engines <img border="0" title="" alt="[Wink]" src="wink.gif" /> )
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Unread 05-13-2002, 04:11 PM   #4
Bob K
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It doesn't take much vacuum to advance the distributor. I run ported vacuum and I like the results. I haven't done the Nutter Bypass. My Jeep runs so well I'm not sure I will try it. I had my timing set stock, then advanced it a few ti tune by ear. I don't really know what it is presently set at. And remember that 89 octane fuel is more resistant to ignition, thus you are getting less power and fuel mileage by running it. Oh, and one word, JUICEBOX!! Get it.

In many high-performance situation, drivers clamor for higher octane fuels, thinking this will give them additional horsepower and, thus, an advantage over the competition. But this is not the case--adding higher-octane race fuel to your vehicle may actually produce less horsepower. Here's why: Octane, an arbitrary number which is calculated as the average of the Research Octane Number (RON) and the Motor Octane Number (MON), and is only an indication of a fuel's sensitivity to knock, which is typically pressure-induced self-ignition. (Of these two ratings, MON is more applicable to racing fuels as it is measured under high load and high speed conditions.)

Octane, as you can see, is not a measure of how much power--or, more correctly, specific energy--is contained in a fuel. And remember that leaded high-octane race fuels burn slower than most unleaded fuels, and may reduce performance in stock or lightly modified engines. A high octane rating itself, however, does not mean that the fuel is slow burning. Hence, it has no direct bearing on the power characteristics of the fuel.
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Unread 05-13-2002, 06:43 PM   #5
OldJeepguy
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</font><blockquote><font size="1" face="Verdana, Arial">quote:</font><hr /><font size="2" face="Verdana, Arial">Originally posted by johny1i:
<strong>I appreciate the suggestion, OldJeep, but I'll admit that the ported vs. manifold issue has perplexed me since I first heard it suggested. Enough so that I hooked up a vacuum gauge inline with the vacuum advance to record the vacuum pressure at 500, 1100, and 1600 rpm. For manifold (stock), it was 14.5", 16", and 16", respectively for those three RPM settings. However, for ported vacuum, it was 0" (at idle of course), 1", and 2.5". Therefore, it seemed to me that it would almost be like not even having vacuum advance hooked up on a ported setup until you get the engine rpm's up, and even then 2.5" doesn't come close to advancing the timing as much as 14-16" of vacuum do. I'm not saying that either way is wrong, I just don't see how the little vacuum advance mechanism on the side of the distributor can function with such a small amount of vacuum. (as you can tell, I'm a tinkerer when it comes to engines <img border="0" title="" alt="[Wink]" src="wink.gif" /> )</strong></font><hr /></blockquote><font size="2" face="Verdana, Arial">Weird - my manifold vacuum is 18" at idle and goes to zilch at W.O.T. I used to have a gauge in the dash - in fact some old cars had them as an option - the idea was to keep your foot out of it and keep the manifold vacuum up and the fuel mileage up as well.

Think about this - you want the distributor to advance in time with the engine rpm - manifold is at it's highest at idle, thus giving you full advance at idle..

Oh well...
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Unread 05-13-2002, 07:05 PM   #6
johny1i
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Old Jeep, I know what you're talking about and that is absolutely correct. When I floor the accelerator, my vacuum drops off to nothing until the vehicle has enough time to "catch back up" and then the vacuum will slowly rise. This "erratic" manifold vacuum behavior is part of the reason that I may try ported because it is in fact much smoother. Anyway, I appreciate your responses and I'll keep playing with it.
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