Ported or manifold vacuum for vacuum advance

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82sbshortbed

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Watched some videos on this. All 3 said ported over manifold vacuum. This was the best diminstration. What yall think?

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SirRobyn0

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I think that you might just get a ton of opinions and this. I'm at work right now on a computer with no audio so I can't directly tell you what I think of what he has to say, but I'll tell you what I have learned as a mechanic. All engines ran manifold vacuum until early emissions starting coming into play. That's when they went to ported late 60s / early 70s. Because of that some people think that if they rip out their air pump and their EGR valve that they can switch it to manifold, but that's just not the case, because typically the camshaft is different. So pretty much the rule of thumb is to use whichever port the engine was designed to run, is the port that should be used, unless there have been modifications. Some modified engines like ported others manifold.

Setting all else aside, manifold is usually better for idle quality and off idle response. Ported there will be no advance at idle, but generally speaking you'll hold vacuum advance for longer creating more vacuum / mechanical advance overlap which maybe good or bad, you might get a little extra power, or a little extra knocking.
 

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Too many years ago,I was told ported and that's what I've done all my life

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hirschdalechevy

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All my cars and trucks, stock or modified over the year's I have always re-curved the distributor for faster advance and moved the vacuum advance to full manifold vacuum and they always seemed to me to pull a little harder and have a better idle. My engine guy that I have known since I was 17 year's old, (who taught me everything I know today) told me this trick when I was 18 year's old, (long time ago). He also told me back then nothing is set in stone and every engine is different so no matter what somebody tells you, the rule of thumb is, (give the engine what it want's), so you try different things.

My vortec engine I just put in my k5 likes a lot of initial timing, (I am at 5,500ft elevation), and it has a stout hydraulic roller cam in it, (357hp, 410tq). I have a vacuum can that gives only 6 degrees advance on full vacuum and my initial is set at 22 degrees and my h.e.i. is limited to only give 12 degrees all in by 2,800 rpm. So my total is 34 degrees when I am on it and at a cruise down the highway I have 38 degrees.

So I am 22 initial when I turn the key and when the engine starts the vacuum can adds 6 on full manifold so I am at 28 initial at idle, (what this engine likes). If I was to go ported vacuum I would not have the extra 6 degrees and have to turn the distributor to get 28 initial and then it start's to hard. So for this engine full manifold vacuum is the way to go, (which I have found to be true on most engine's).

Just my 2 cents.
 

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Watched some videos on this. All 3 said ported over manifold vacuum. This was the best diminstration. What yall think?

I saw that video a while ago and I agree with Uncle Tony using ported for the reason that if you run manifold you loosing advance the moment you put your foot on the gas and start to get into it.

Ultimately I think it's your truck and you should run it whichever way you prefer. There are inherent limitations to a mechanical distributor and, emissions aside, there are pros and cons to ported vs manifold and it comes down to how you like it to run. Some people are very insistent that it must be manifold because manifold was used since the beginning and ported was only created for emissions and that is true, but just because it was created for emissions doesn't mean it can't be useful in other ways.

They key to running ported is you have to give your engine more advance on the initial timing since you don't have vacuum to give you that extra advance at idle. You also have to be careful in doing so that your timing into too far advanced at high RPMs so you don't ping your engine to death.

ThunderHead289 on YouTube has a great video about the pros and cons and how to of running ported.

The best way around these pro and cons and the limitations of a mechanical distributor is to go with an electronic distributor. I control the timing from my iPhone. It works with any smart phone or tablet over bluetooth. It looks like a stock distributor. No boxes or extra wiring to hook up. I put one on the 454 in my '80 K25 and its fantastic. It set me back about $460, but the results are incredible. Having that kind of control over your timing vs having a hotter spark from a bigger coil or whatever is a huge deal. Compared to the amount of money I've seen people send on MSD and other aftermarket systems I think the juice is well worth the squeeze for the electronic distributor.

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82sbshortbed

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I saw that video a while ago and I agree with Uncle Tony using ported for the reason that if you run manifold you loosing advance the moment you put your foot on the gas and start to get into it.

Ultimately I think it's your truck and you should run it whichever way you prefer. There are inherent limitations to a mechanical distributor and, emissions aside, there are pros and cons to ported vs manifold and it comes down to how you like it to run. Some people are very insistent that it must be manifold because manifold was used since the beginning and ported was only created for emissions and that is true, but just because it was created for emissions doesn't mean it can't be useful in other ways.

They key to running ported is you have to give your engine more advance on the initial timing since you don't have vacuum to give you that extra advance at idle. You also have to be careful in doing so that your timing into too far advanced at high RPMs so you don't ping your engine to death.

ThunderHead289 on YouTube has a great video about the pros and cons and how to of running ported.

The best way around these pro and cons and the limitations of a mechanical distributor is to go with an electronic distributor. I control the timing from my iPhone. It works with any smart phone or tablet over bluetooth. It looks like a stock distributor. No boxes or extra wiring to hook up. I put one on the 454 in my '80 K25 and its fantastic. It set me back about $460, but the results are incredible. Having that kind of control over your timing vs having a hotter spark from a bigger coil or whatever is a huge deal. Compared to the amount of money I've seen people send on MSD and other aftermarket systems I think the juice is well worth the squeeze for the electronic distributor.

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Yes I agree the top video was one of the 3 I watched. Actually it was the first of the three a watched. You set your timing at idle with advanced disconnected. So why do you want to advance it at idle by hooking it up to manifold.

In the video I posted the guy Actually hooks the distributor to both so you can see it move. He explained that it's really only used at cruise speed to improve gas mileage. And that a lot of hot rodders and drag strip guys don't even use it.

When I switched mine back and forth it, the idle went up on manifold vacuum and stayed the same on ported. I left mine on ported.

I also have a 454 in mine and thought that distributor at the end of the first video was pretty cool.
 

hirschdalechevy

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When you have any type of sizeable cam with more overlap they need more initial timing to idle well and to get your carb. idle set up right, but you can only go so far with the initial, (mechanical advance), or your starter will hate you. So you put your vacuum can to manifold so you get the initial that you need for idle without hard starting. In turn when you are now started and running you now can turn your idle down to get your t-slots set right, (holley carb.) so they are not pulling fuel at an idle, (.020 to .040 open). Then you curve for a fast advance, (all in by 3,000 rpm or so) and that takes up for the loss of vacuum advance that drops out, (from vacuum loss), the minute you floor it from idle. Then you check your total advance, (32 to 34 vortec heads), (36 or so non vortec heads). Then you limit the total advance, ( in the distributor) to hit those number's and you are good to go. Then the vacuum comes back for a good cruise as well. Either way (give the engine what it wants) so you have to try all kinds of different things when it come's to performance.

On a side note, when I was young and trying to learn about engine's I had a few stocker's that was trying to make fast, ( with no little or no money), so I would pick off used performance parts etc... I had a 71 impala 2 door and put a eddie intake, header's, dual exhaust and a holley carb. on it and it pulled pretty good for the day and age. Took it to my engine guy and said, ( what can I do for more, cheap). He then showed me his trick.

Full manifold for the vacuum can, ( vacuum cans have different advance amounts and vacuum activation point's). Give the engine what it wants
Speed up the timing curve, ( all in by 2,800 to 3,000 rpm). Give the engine what it wants
Limit the total, ( 30 to 36 ). Give the engine what it wants.

So this is what I did, (he really did this and showed me how) and my stock inside impala with some used bolt on's pulled harder off of idle and stabbing it from a cruise as well.

I did this same set up on a few stocker chevy's threw my younger year's with the same gains. My dad bought a brand new 1978 k10, (when I was kid) and took it to our engine guy for the trick timing with full manifold vacuum set up with only 800 miles on it and he told me it pulled a bit better.

Latter on in life most of my drag car's I just locked the advance at the total I wanted and that was it but not for a street car.

If you have any type of stout cammed street car or truck you petty much have to need to run full manifold vacuum to get your idle correct, (I have to with my k5). I would do it for stocker's as well but thats just me.

My dad went to high school with our engine guy. He was a gm dealership mechanic his whole life and when he retired from gm he opened his own shop and built engine's for the outlaw sprint car circuit guy's. So I was at his shop trying to learn **** all the time.

I am not bashing ported vs. full vacuum, just throwing out what I have done. Back to , (give the engine what it wants), and every opinion is different so you have to try it for yourself, for me it's full vacuum when set up right.
 

hirschdalechevy

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I really do like the electronic , cellphone set up that, (dsteelejr) is running, that would be the cat's meow. That being said I would so do that but for my k5, I am always out in the deep desert, (100 miles from anything) and I am old school so I dont want any electronics that can fail going through water crossings and bashing water hole's at high speed etc... The only piece I carry with me is a extra h.e.i. module for the electronics department. But for a street machine I am all into that.
 

dsteelejr

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I really do like the electronic , cellphone set up that, (dsteelejr) is running, that would be the cat's meow. That being said I would so do that but for my k5, I am always out in the deep desert, (100 miles from anything) and I am old school so I dont want any electronics that can fail going through water crossings and bashing water hole's at high speed etc... The only piece I carry with me is a extra h.e.i. module for the electronics department. But for a street machine I am all into that.

You can always keep the old one in a box tucked away in your K5 for emergencies. I saved my old dizzy for that reason. I live in Wyoming and there are a lot of places in the middle of no where to get stranded. It's always good to be prepared.
 

Paladin

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In my current mental state ( only had an after lunch bowl ) I believe I have come upon a solution to this age old question once and for all. How about just teeing both ported and manifold vacuum together to the advance can and let it decide for itself. There,
problem solved! Your welcome!!!

:beatdeadhorse5:
 

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I know my truck is a lot less cold blooded on manifold vacuum.
 

Rusty Nail

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I think that you might just get a ton of opinions and this. I'm at work right now on a computer with no audio so I can't directly tell you what I think of what he has to say, but I'll tell you what I have learned as a mechanic. All engines ran manifold vacuum until early emissions starting coming into play. That's when they went to ported late 60s / early 70s. Because of that some people think that if they rip out their air pump and their EGR valve that they can switch it to manifold, but that's just not the case, because typically the camshaft is different. So pretty much the rule of thumb is to use whichever port the engine was designed to run, is the port that should be used, unless there have been modifications. Some modified engines like ported others manifold.

Setting all else aside, manifold is usually better for idle quality and off idle response. Ported there will be no advance at idle, but generally speaking you'll hold vacuum advance for longer creating more vacuum / mechanical advance overlap which maybe good or bad, you might get a little extra power, or a little extra knocking.

Ditto.

Most of the quadrajets and engines in this forum are probably built to run ported on their CCC but most people can't leave well enough alone.

You said it good Robyn. I no longer enter this debate because there is no correct answer.
 

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