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Old 04-22-2004, 12:36 PM
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Timed Vac Port or Manifold Vac Port

Here we go again!!!! It sounds dumb............but if you don't know you don't know.

Got a new 350/250h.p. crate motor with a new HEI Distributor. No emisson controls on this engine. Edlelbrock Carb instructions say to use Timed Vac Port for emission-controlled engines and Manifold Vac Port for non-emission controlled engines.

I've tried it both ways. Most of my buddies agree that I should use the Timed Vac Port on the passenger side. No Vac while idling. Does anyone really know the correct answer? I am using Timed Vac Port as of now. If Timed Vac Port is correct did Edelbrock make a mistake in the instructions?

All help would be appreciated. I also put in a spring kit in the distributor with the heaviest springs after my buddy lost the stock springs. I didn't change the weights. I used the stock weights with the heavy springs. Is this OK???? The weights that came with the kit wouldn't even fit without filing the holes egg shaped. The timing is set about 16 degrees. It seems to run and idle fine. I'm not looking for peak performance, I'm looking for fuel economy and a fine running/good starting engine. This 34' is a boulevard cruiser.

Would I be better off getting a stock set of springs for my new distributor from the company that made it and setting the timing at 8 degrees stock as the motor manual states? I purchased it from Queens Automotive in PA.

Thanks to everyone for your patience and help.

Sweet-34

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Old 04-22-2004, 12:49 PM
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Running the vac advance on manifold vacuum works the same way as advancing the timing by hand.

Using the timed vac port advances the timing when you need it most, at acceleration.

I know of no benefit in using manifold vacuum on the vacuum advance, but I would like to know if there is.


Different springs/weights are ok, but you need timing tape to make sure you don't go past 34 deg total timing with different springs and weights.
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Old 04-22-2004, 12:53 PM
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Have you read these?

http://hotrodders.com/showthread.php...or+vacuum+port

http://hotrodders.com/showthread.php...or+vacuum+port
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Old 04-22-2004, 01:54 PM
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So manifold vac is good, but you have to change the vac advance pot/can to make it work right.

Still, if you use lighter springs the timing will advance at idle, but not while starting. The only problem I could see with that is the total timing would be too far advanced.

The way the HEI works on my truck is when I mash the throttle the timing is advanced quickly, but at WOT there is no vacuum going to the dist, the dist weights are doing all the work.

I've always herd that 34 deg total was the max, but that one post said 52. Who do I believe?

And why would I want 8 deg initial crank timing + 8 deg dist timing (which = 16 deg crank timing) to make 24 deg advance at idle?
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Old 04-22-2004, 03:28 PM
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As you can tell from my comments on the previous threads, manifold vacuum is the only thing that makes sense to me for anything but a smog engine. You want

- medium large advance @ idle for max vacuum and economy,

- little advance @ low speed WOT acceleration (reduced detonation),

- medium advance @ WOT acceleration at speed (reduced detonation,

- but more advance than low speed case), & big advance @ part throttle cruising speed.

That scenario is exactly what manifold vacuum/ centrifugal weights give. Doc Jacobs also makes a case for that in his book on ignition theory, with some smart guy arguments. Ported vacuum function in the above scenarios doesn't quite make sense to me but OEM has been doing it for decades (I suspect for reasons other than performance, i.e., emissions control) so take your pick!
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Old 04-22-2004, 08:43 PM
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I still think hooking the dist through ported vac is the fast and easy way to do it, but I like the idea of manifold vac now that I've thought about it more because...

1. I can start the engine easy at 8 deg initial timing.

2. The vac advance will make up for the low initial timing. (8 deg +-)

3. If the engine pings all I have to worry about changing is the weight springs.

Now all I need to know is the max degrees of advance I can have at idle.
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Old 04-22-2004, 11:57 PM
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The best way to set initial advance is with a vacuum gauge. Disconnect the vacuum advance on the distributor and connect a vacuum gauge to manifold vacuum. Now set the idle on the carb to factory spec and start advancing the distributor until vacuum stabilizes at it's lowest value. Retard it a degree or two and it should be perfect. Readjust idle speed if necessary. Now by using an adjustable vacuum pot on the distributor and playing with the weights and springs, you should be able to zero in on the perfect combo. Again, Doc Jacob's book tells how to make all of these settings scientifically. If you are serious about the topic, I suggest getting a copy of the book.

The Doctor's Step by Step Guide to Optimizing Your Ignition
by Dr. Christopher Jacobs, Jacobs Technical Publications, 500 N. Baird St., Midland, TX 79701


Amazon listing
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Old 04-23-2004, 01:44 AM
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JEEEZ $50?! I can buy another junk car for that!

Tell me if this is a bad idea or not. If I use the stock HEI setup, run the vac advance to manifold vac, and use lighter springs to get the total timing I want.

If that works, all I would need to know is what the max advance at idle should be. (I run cheap gas)

And I got a Jeg's cat, and they sell those damper covers that I need, so I won't have any trouble using a timing light.
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Old 04-23-2004, 06:14 AM
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Quote:
Originally posted by FASTCHEVY

2. The vac advance will make up for the low initial timing. (8 deg +-) [/B]
First off you need to understand a few things. Changing from manifold to ported vacuum does not really affect your total timing under WOT acceleration. You don't just change your base timing to 8* without screwing up your total timing under WOT. The weights also change the speed that your mechanical timing come in but not your actual amount. Most distributors use some form of stop to control amount. So you need 34* total mechanical timing for max performance. If your distributor has 22* of mechanical advance you would need to set your base timing at 12*(34 - 22) of base timing. Once you establish this then you need to decide whether or not you need or want that additional timing at idle from your vacuum advance. Setting it to ported vacuum does everything willys said except increase timing at idle. This was all discussed in depth in other threads like willys linked as well as others. The 52* Fastchevy mentioned is because under mild acceleration you are allowed extra timing, this is what the vacuum advance is for. It can provide additional advance beyond mechanical for various driving conditions. My base timing is set at 14*, my distributor provides an additional 24* of mechanical, which gives me 38* of total mechanical advance. My vacuum advance is hooked to ported vacuum since 14* at idle is more than enough and as soon as I tap the gas to take off the vacuum advance immediately supplies the extra advance to accelerate smoothly.


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Old 04-23-2004, 12:11 PM
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I tow cars sometimes up to 4000 lbs with my truck, so do I really need all that advance on acceleration?

It works fine the way it is, as long as I don't give it too much initial advance, or it will start pinging.
That's why I would set it at 8 deg, then have the vac advance kick up the advance at idle.

I remember on my El Camino the computer would advance the timing at idle to about 20 deg. it ran really good that way to.

I guess that's why computers have taken over.

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Old 04-23-2004, 05:47 PM
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Quote:
Originally posted by FASTCHEVY
It works fine the way it is, as long as I don't give it too much initial advance, or it will start pinging.
That's why I would set it at 8 deg, then have the vac advance kick up the advance at idle.
By turning your base down to 8* you are reducing your total timing which in your case may be good since you tow. In your case 8* base and manifold vacuum to the advance may be you're best bet. You are not looking for max performance. Without knowing how much mechanical advance your distributor supplies it is hard to say what total timing you are getting. Both ways have advantages and disadvantages, use what works for you. Ford small blocks generally can handle between 38-42 total timing. This requires my base timing to be at 14-16* (with 24* of mechanical advance) and I don't want any more than that at idle so I adjusted my advance can to work well hooked to ported vacuum. Every car is different, every driver different. Use what works best for you.


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Old 04-23-2004, 08:52 PM
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Thanks Willys, dmorris.
Sweet-34 we havn't herd a word out of you, I hope that helped you out.
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Old 04-23-2004, 10:31 PM
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I have never had any luck using manifold vacuum, I always revert back to ported and dial in the distrib to be all in by 3000 rpm or under if the engine/fuel can handle it. I find using manifold vacuum to vary timing slow reacting compared to ported, there is always is that sharp ping when going into full load. I have tried both and gave them even shots but in the end for a daily street driven car that has to perform under varied conditions I find ported superior. Doesn't matter at full load anyway, it's all centrifugal at zero vacuum (WOT).

Try it both ways and see what works for you? I would put the light springs in and dial that distrib in before playing with the vacuum advance timing, you need the centrifgal dialed in first. If you don't know how take it in and have it done.

Here's a good discussion.

Another good article.
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Old 04-24-2004, 07:07 AM
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Good links Chuck. Do I even want to know where the Captain Underpants came from?


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Old 04-27-2004, 05:35 AM
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Problem Solved

Pulled distributor and installed buddies new Pro/Form. Worked fine and timed fine. My new Pro/Form would bind the cam and weight plate when installed and tightened down. Works free when out of engine.

Called factory and they are going to fix it for free. Thanks for all your help!!!

Sweet-34
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