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Old 09-13-2010, 07:07 PM
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Setting vacuum advance on SBC HEI

Hello guys I was checking my timing today on my 350 sbc and have done a lot of reading on vacuum advance but there is a few things maybe I am not getting right and want to ask your advice.

I have my 350 with a 260/270 212/218 @50 510/510 lift hydraulic roller cam and I have my timing set at 16 degrees initial and 32 degrees all in by 3500 rpm.

Then after hooking up my vacuum advance to full manifold vacuum I had way close to 40 degrees plus at idle so I knew I had too much vacuum advance for my setup. So I got out my 3/32 allen wrench and changed it to the lowest setting it would go to and the lowest it would go was about 14 to 16 degrees of vacuum advance. Balancer numbers are very hard to read but I get very close within 1 degree.

So at idle with vacuum advance hooked up I have about 28-30 degrees of timing. So if I am reading correctly about vacuum advance and rate etc When I have it all hooked up and rev it steady I get about 48 degrees total. So I take it I have 16 initial with 16 degrees mechanical for a total of 32 degrees and with vacuum advance of about 14 to 16 degrees I have around 46 to 48 degrees total timing when cruising correct?

I was looking at a chart with specs the amount of turns it takes to get said amount of degrees of advance. I had to back mine to its lowest point so I would not have too much vacuum advance. Am I setting it correctly then?
Thanks guys your help is appreciated.
Eric
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Old 09-13-2010, 08:11 PM
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Yep seems as if your math is bang on and those values are actually pretty good,

I am curious as to what type of HEI this is, is there a vacuum limiter plate in there? when you say that you are adjusting the vacuum adv I assume that you mean the set screw inside the can? what is the full engine vacuum at idle with the can connected? the set screw will only load up the internal spring so it effectively shifts the range up, but when you come off the gas lets say and pull more vacuum than at idle or at mid unloaded rev it will advance more than you are seeing at idle cause your pulling more vacuum. I have always use a limiter plate inside the HEI to limit the allowable travel of the vacuum adv push rod, this way you are sure it will not go higher cause it lessen's the travel.

Example; the total travel of the rod is say 1/2 inch and 1/2 inch adds 20 deg advance when a vacuum pressure is applied from 6 to 14 inch hg" thats an 8 hg" range to get the 20 deg adv the motor pulls say 14 hg" at idle.

now tighten the spring as you are doing, this raises the starting point of the movement to say 10" but it does not effect the total amount of travel available, it will still want to move 1/2 inch when the vac goes up but this time it will have to get to 18 hg" to give the 20 deg adv. and since it only see's 14 hg" it only adds 16 deg adv lets say.

So without a limiter plate in there you are only changing the the starting point of you vacuum adv, the only limit being effected is that your idle and mid rev vacuum is probably not enough to pull it in all the way, but if the engine vacuum goes up due to deceleration lets say and reaches 18" vac the can will still add 20 deg at that point.

This is the way that my particular HEI works and it will apply to most but not all HEI distributors. Hows that for a disclaimer
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Old 09-14-2010, 08:10 AM
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Well I don't know how to check the thing while I am driving it even if I hooked up my vacuum gauge. I don't know how much travel it has. The only way I know of checking is the numbers at idle and reeving it that's about it.

My vacuum at idle is about 18-19 inches. The gauge was steady but I have a stupid misfire that keeps happening and makes it drop just .5 of an inch of vacuum. I have checked plugs wires cap, coil and ground and everything else before and just can't figure out what is causing it.

I guess there is more then what I have already learned. I don't know about limiting travel of the vacuum canister. The hei I have is a summit hei that I got cause of the melonized gear it has.
Eric
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Old 09-14-2010, 09:41 AM
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Quote:
I guess there is more then what I have already learned. I don't know about limiting travel of the vacuum canister. The hei I have is a summit hei that I got cause of the melonized gear it has.
Eric
You are still OK here, my previous post was to describe the possibility that without a limiter plate on the vac adv it (the vacuum advance canister assembly) may add more deg advance under certain conditions than what you see at idle. My advice would be to add the crane limiter plate in there to pin down the absolute maximum vacuum advance it can provide and to make sure that you are getting all the rod travel at idle, this will help steady the idle quality as well.

http://www.summitracing.com/parts/CRN-99619-1/

The miss fire is most likely not related to this senario but you are on the right track by going through the ignition checks to try and rule out that part of it as the cause. Does the miss fire happen at all times including idle and actually driving?
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Old 09-14-2010, 02:21 PM
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Well I had a previous post about the miss fire and could not come up with anything. I have had different plug wires, complete distributor change along with the coil, cap, rotor, and control module and no change. I have also checked all my wires and ground wires and added extra ground and am getting proper voltage to my distributor as well.

I originally thought it was part of the overlap of my old cam but after the cam swap and everything else the dam miss is still there. It does not really seem to be there while cruising but at idle you can't miss it. The idle will be nice and smooth and then it will cut out and start to get rough then smooth out again. I have checked for vacuum leaks but as far as can tell there are none.

I have plugged my vacuum hoses and went through everything and still its there. The worst time you hear it is when the engine is cold and on first start up it will be smooth miss then smooth again and you can tell when it does a cylinder drops and picks back up. I even just replaced the plugs again. I also tried the idle mixture screws and tried to adjust them and it really did not change anything.

I am so sick of this engine I just want to through it out to the scrap yard. I have done everything to it and always just have to have something to not work good on it.

I even took my plug wires and checked them for fire as the motor was running and they all fire just fine so go figure. But on the limiter plate I might get one but I still have not had any issues with pinging or anything so would I really need it? Thanks again for help.
Eric
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Old 09-14-2010, 04:07 PM
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But on the limiter plate I might get one but I still have not had any issues with pinging or anything so would I really need it? Thanks again for help.
Eric
Yes get one in there its cheap and it is an easy install, once set up correctly it will ensure that you have all of your vacuum advance is in at idle and that you are not floating around in the middle of the vacuum advance travel at idle, the set screw you are adjusting is there to set the starting point of advance and not to set the amount of advance. Its just one thng you can do to help pin down your issue which is causing you obvious grief, procees of ellimination my good man.

If you can get a vacuum pump and connect to the vacuum can with the engine running and the hose plugged. Vary the pressure using the pump and note at what pressure you reach maximum advance. I bet the pressure needed to reach full advance is higher than the vacuum your engine pulls. This is because you have the adjustable screw in the can all the way out raising the starting pressure of the movement. A rule of thumb is to have the vacuum advance all in at about 2 hg" below your nominal max engine vacuum level or in your case all in by about 15-16"hg
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Old 09-14-2010, 06:05 PM
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Well I did buy a accell 31035 vacuum advance canister that says you can limit the amount of advance you get and the travel amount . I have not installed it caused I guess I am a little confused on the instructions on the chart they give. Maybe you can help me out if you wish.
http://www.accel-ignition.com/pdf/31035.pdf

Eric
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Old 09-14-2010, 06:10 PM
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If looking at that chart if I turn the allen wrench about 4 or 5 turns from the stopped position then it would give me around 12 to 13 degrees of advance and no more then that correct? I guess I don't know why there vacuum advance canister would be any different then all the other ones you can get from jegs and summit. I don't have a problem getting the plate heck its only 4 bucks
Eric
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Old 09-14-2010, 06:40 PM
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The problem is you hooked it up to "Manifold Vacuum", the vacuum advance is suppose to be hooked to a "Ported Vacuum" port on the carb that doesn't have vacuum at idle.

What happened is when you hooked the vacuum advance to "manifold vacuum" it pulled the 10 degrees in that the vacuum advance advances, which raised your idle and thus kicked in the mechanical advance and that is why you had 40 degrees of advance at idle.

Just hook up the vacuum advance to the ported vacuum port on the carb. And that should fix the problem.
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Old 09-14-2010, 07:41 PM
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Quote:
Originally Posted by eric32
If looking at that chart if I turn the allen wrench about 4 or 5 turns from the stopped position then it would give me around 12 to 13 degrees of advance and no more then that correct? I guess I don't know why there vacuum advance canister would be any different then all the other ones you can get from jegs and summit. I don't have a problem getting the plate heck its only 4 bucks
Eric
I had typed in a responce but it got lost, so I will sum it up

Yes I think the accel unit is advertised as something different than what you have in there already, according to the graph it is a limiter and the set screw is used to set the limit and not the start point as in a typical adjustable can. Test it with a gage and see if changing the set screw changes the rod travel or if it just adjusts the start and stop "hg. curious

Quote:
Just hook up the vacuum advance to the ported vacuum port on the carb. And that should fix the problem.
welcome to the board tubbed,,, What problem are you talking about? although your statements are valid we are discusing ways to set up the vacuum limits on his HEI, not ported vs manifold vac.
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Old 09-14-2010, 08:01 PM
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Quote:
Originally Posted by TubbedCamaro
The problem is you hooked it up to "Manifold Vacuum", the vacuum advance is suppose to be hooked to a "Ported Vacuum" port on the carb that doesn't have vacuum at idle.

What happened is when you hooked the vacuum advance to "manifold vacuum" it pulled the 10 degrees in that the vacuum advance advances, which raised your idle and thus kicked in the mechanical advance and that is why you had 40 degrees of advance at idle.

Just hook up the vacuum advance to the ported vacuum port on the carb. And that should fix the problem.
You maybe should begin a post like this with "In my opinion..."
Ported vacuum is not the answer to every ignition advance problem, so please don't come on here with an empirical answer like you gave.
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Old 09-14-2010, 09:32 PM
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Quote:
Originally Posted by techinspector1
You maybe should begin a post like this with "In my opinion..."
Ported vacuum is not the answer to every ignition advance problem, so please don't come on here with an empirical answer like you gave.
If you would have read what he said, you would know that the engine ran at 16 degrees advanced before he hooked the vacuum advance to "manifold vacuum". Then once hooking it to manifold vacuum the timing advanced to 40 degrees, this is because the high vacuum at idle is causing the vacuum advance to pull ALL it's timing in, which is normally about 10 degrees. So with his base timing being 16 degrees that would bring total timing to 26 degrees, but he had 40 degrees, which makes me believe that the mechanical advance is kicking in as well. Remember with all his advance coming in by 3500 rpm, he's running the "light" springs on the advance weights.

Anyway his first action in this case would be to hook the vacuum advance to the ported vacuum port on the carb. That way he will stop the vacuum advance from pulling in all that timing at idle and make it work like it was designed to.

Put it this way I was a ASE master certified mechanic for 10 years and have been building and tuning engines and hot rods for the last 25 years. I think I know a little about the subject at hand.
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Old 09-14-2010, 09:52 PM
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Originally Posted by Custom10
I had typed in a responce but it got lost, so I will sum it up

welcome to the board tubbed,,, What problem are you talking about? although your statements are valid we are discusing ways to set up the vacuum limits on his HEI, not ported vs manifold vac.
Thanks for the welcome.

As far as I know most vacuum advances are good for about 10 degrees of advance. So if your trying to limit it to even less advance than that, then why even run one??

Then there is the fact that once you nail the throttle to WOT, the vacuum advance will stop pulling in advance(timing) since vacuum at WOT is very low. So at WOT the only advance you'll have is your mechanical advance.

Vacuum advance is for part throttle driving..........
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Old 09-14-2010, 10:39 PM
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As far as I know most vacuum advances are good for about 10 degrees of advance. So if your trying to limit it to even less advance than that, then why even run one??
First off he is getting 14-16 deg, I never suggested he go below that. Also most HEI cans will add around 20+ deg advance un-altered, thats way too much and is the very reason for setting the limit.

I was looking at the big picture and taking it step by step, my point thoughout was that the limiter is needed to ensure that he can set up the dist to have it all in at idle and that it won't add any more if the engine vac increases, also we don't want the vacuum can rod to be floating around, we want it to be pinned at its max travel at idle. His 30ish deg advance at idle is a good place to be with his setup, he could even use a bit more perhaps and yeah we don't want any mech adv at idle but i doubt that is going on cause he's got 16 base + 14-16 vacuum adv that accounts for his idle timing at 30-32.

Quote:
Then there is the fact that once you nail the throttle to WOT, the vacuum advance will stop pulling in advance(timing) since vacuum at WOT is very low. So at WOT the only advance you'll have is your mechanical advance.
Yeah so? that is just a reminder I take it,,, thanks

Quote:
Vacuum advance is for part throttle driving..........
Yeah so? that is just a reminder I take it,,, thanks
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Old 09-15-2010, 01:05 AM
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Sorry I got a little confused here, thought we were talking stock HEI and didn't realize we were talking about an Accel HEI. Ya depending on how far you turn the screw in the vacuum canister, will determine how much advance you'll get out of it. 7 turns will give you full advance, where 3-4 will give you about 10-12 degrees. This is starting with the adjustment screw turned all the way out counterclockwise and then turning it in clockwise the number of turns stated.

But I don't understand what any of this has to do with you having to much advance at idle. I guess before I opened my big mouth, I should have asked a couple of questions.

One, what do you what the timing to be set at Idle with the vacuum advance connected??

Two, how much advance do you want from your vacuum advance??

Third, have you tried setting the base timing at 16 degrees and taking the car for a drive with the vacuum advance disconnected??



You see the problem is your referencing the vacuum signal from the wrong side of the throttle blades. On the manifold side vacuum is highest when the the throttle blades are closed, but as soon as you crack the throttle the vacuum in the intake manifold drops and so does the amount of advance the vacuum advance is adding. Do instead of advancing your timing as the throttle in opened it actually retards your timing as the throttle is opened. That is why your suppose to hook the vacuum advance to a ported vacuum port on the carb, it is a port that doesn't see vacuum until the throttle is cracked. It is the way a vacuum advance was designed to work.
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