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  #16 (permalink)  
Old 01-04-2005, 11:09 AM
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Thanx for posting that quote Triaged. That is what I have always understood and I recall reading Doc Jacobs has the same explanation in his ignition book too. Anyway, that is how I always set up my ignitions and it works great. Have tried the ported vacuum port a few times and couldn't get it to work. Doesn't make sense to me why it would work!

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  #17 (permalink)  
Old 01-04-2005, 11:36 AM
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I´m going cross eyed reading web sites on this topic.
Read this through http://www.gofastforless.com/ignition/advance.htm now I see why full manifold vacuum may be an advantage.
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  #18 (permalink)  
Old 01-04-2005, 11:42 AM
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Yep, that agrees with what I have always understood. And this guy give actual experimental data to back up his theory.
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Old 01-04-2005, 12:07 PM
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Quote:
TIMING AND VACUUM ADVANCE 101

The most important concept to understand is that lean mixtures, such as at idle and steady highway cruise, take longer to burn than rich mixtures;....
Rich mixtures, on the other hand, burn faster than lean mixtures, so they need to have "the fire lit" later in the compression cycle (spark timing retarded slightly
I totally disagree.
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  #20 (permalink)  
Old 01-04-2005, 12:30 PM
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I have tried both, AND FOR MY APPLICATION, the ported works best. Using full manifold vacuum, there is too much drop in RPM between idle and gear (auto). WHY? As many have explained, rpm is raised due to the advance from the full source. My car makes 10" of vacuum at idle/park, put in gear and it drops to 6.5. My vacuum can starts advancing at 4-5", and full advance is at 10-11". Due to the cam in my car, I have to have rpm at 750 in gear or it will die. In order to achieve that using full vacuum, then the rpm in park would jump to 1100 rpm. Due to lack of vacuum the timing is retarded in gear. Also, I have a mechanical advance that starts at 800 rpm, well, when using full time manifold, that advance is lost too because I have set my idle so high, 1100 rpm, and then in gear it drops to 750 rpm. Not good on the tranny, ujoints, or new rearend. I could get by with it after driving, by shutting down with the car in gear. But on startups, it is too hard on the drive train. Also, the engine took too long to warm up due to the added cooling benefits of running full manifold source combined with blocked off heat passages in the intake

I went BACK to ported, to set my rpm at 900-950 at idle, and it drops to 750 in gear. Purrs like a kitten. No jumping around going from park to gear, no clunking. If I use stiffer springs in the mechanical advance, I can get it to idle at 850 rpm, then drop to 750 rpm in gear, but then I lose my curve.

I wish I could take advantage of the benefits of using the full manifold vacuum, but I can't.

Just try it both ways, and see what works best for you.
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  #21 (permalink)  
Old 01-04-2005, 12:40 PM
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This is an interesting laboratory study on flame velocity in IC gasoline engines. Note the observations on the bottom of page 700 and top of page 702 discussing figure 8 on bottom of page 701. In summary the author found that highest flame travel occurs at optimal air/fuel ratio with slower travel at rich mixture and slowest flame travel at lean mixture. This study thus supports the posts above that prefer manifold vacuum - extra advance at lean conditions.
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Old 01-04-2005, 12:47 PM
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On Ford distributors

When manifold vacuum is high, as in idle or cruising, that is when you will have the most vacuum advance. When the manifold vacuum drops off, the vacuum advance drops the advance back to whatever the installed total of initial and centrifical advance are at that given rpm.
So if at say 2500 RPM, your total initial and centrifical advance is 32 degrees,and you are pulling a total of say, 36 with vacuum advance, when you start stepping down on the throttle, the vacuum advance will retard in relation to the loss of vacuum signal, to the point where, the 4 degrees of vacuum advance disappears, if enough vacuum signal is lost.

Flame Propagation:

That is an interesting thing about fuel air mixture. That is the true story of it, that it is harder for the flame to travel in lean conditions than in richer, up to a point, when over richness occurs, then you wind up with wet plugs, and no spark.
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  #23 (permalink)  
Old 01-04-2005, 01:36 PM
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Interesting blip from NASA but I still will richen a mixture if timing is correct and ping is heard. I will hold to my guns that with a rich mixture more advance can be used, less with a lean mixture. this isn't rocket science

I do not dispute that manifold vacuum is superior to ported in 90% of the applications we will come across.
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  #24 (permalink)  
Old 01-04-2005, 01:36 PM
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454c10

learned something today, thanks, no wonder it sucked on ported

underdog, use ported on your Chevy, I was wrong...

and now we all know why there are 2 ports
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Old 01-04-2005, 02:09 PM
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It cannot get more complicated than this, read on
http://www.jagweb.com/aj6eng/vacuum.html
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  #26 (permalink)  
Old 01-04-2005, 02:58 PM
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Quote:
Originally posted by NXS
Interesting blip from NASA but I still will richen a mixture if timing is correct and ping is heard. I will hold to my guns that with a rich mixture more advance can be used, less with a lean mixture. this isn't rocket science

I do not dispute that manifold vacuum is superior to ported in 90% of the applications we will come across.
What you're speaking of is a bit different than what's being discussed. A slightly richer mixture will burn cooler. That's why the pinging will sometimes disappear. Make it too rich and the pinging will get worse because of poor fuel atomization. This is the wrong way to eliminate detonation.

If the engine temp., fuel octane rating, air/fuel mixture and ignition timing are correct for the engine, it will not ping. The only unknown variable when tuning a new engine is what the optimum ignition timing will be. All other aspects are decided beforehand, timing must be tuned for the engine and it's operating environment.

If it pings off idle you have too much initial timing, if it pings under acceleration but not off idle the timing curve is too steep, if it pings when you are cruising under no load you're vacuum advance is coming in too soon. It's fairly simple.

If it pings when it gets a little hot, improve your cooling.

If it pings no matter what you do you have the wrong fuel for your cylinder pressure.

The first article posted was dead on the money and I agree totally with using ported vacuum in an engine with a huge camshaft.

Use a vacuum pump and timing light to determine the vacuum necessary to have vaccum advance function properly. With the engine idling and in gear, the pump connected to the distributor and your timing light aimed, apply vacuum to the vacuum canister and monitor the vacuum and timing readings. Apply vacuum slowly until no more timing advance registers. Check the vacuum pumps gauge, that will be the vacuum required to achieve the necessary advance at idle.

If youre engine does not provide at least 90% of that vacuum reading at idle and in gear, you need to use a ported vacuum source for your vacuum advance or switch to a vaccum pot that will function with a lower vacuum reading.

I hope this was clear enough.

Larry

Last edited by coldknock; 01-04-2005 at 03:05 PM.
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  #27 (permalink)  
Old 01-04-2005, 03:06 PM
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To make it easy, just get a mechanical distributor.

to underdog305
There are over 600 postings on this subject in this Forum, how about reading some, and tune it to your application not what is good for some one else.
You have a 50/50 chance of being correct on the first try.
It seems it has to be explained over and over, you will never understand, just do it. Almost troll time.
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  #28 (permalink)  
Old 01-04-2005, 05:02 PM
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Malc , The car reacts better than it ever did since I have started using this set up. When I pull away slowly its great, When I stomp on it from a standstill such as at the track it has instant throttle response. I'm pretty sure at W.O.T there is very little vacuum if any. At that point the mechanical advance takes over, which we have altered with different springs and weights. We are using a GM HEI with a remote coil also. I have heard lots of different opinions about the coils also. I just did it because my coil went bad and I had a good MSD blaster coil so I bought an adapter cap. But regardless the car idles great and very smooth almost like a "sleeper" you cant hear the cam at all. But if I hook back up to ported advance it has a rough idle. I am sure you can make either way work, but both take completely different timing-vac advance-initial settings. I would try them both and see what works best for you. ccnova
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  #29 (permalink)  
Old 01-04-2005, 06:53 PM
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1BAD80:
Sorry if I bothered you on this. I was simply trying to get a couple specific opinions on the subject. I have to say that there were a few that really helped alot. Thank you for the effort everyone!!!!

Especially Malc, you cleared up a lot. Thanks!!
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  #30 (permalink)  
Old 01-04-2005, 07:23 PM
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You did not bother me, you have to do some reading to understand it completely. I could have said Please read the Guidelines for posting and doing a site/KB search.
Plus I put quite a few sites in the Knowledge Base for reference on vacuum advancement.
The sites I referred to were taken from Malc's posting on the same subject.
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