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Old 10-10-2010, 09:37 AM
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Ignition Timing Questions

Here's some basic info on the engine

4 bolt 355 (350 0.030 over)
Comp Cams XE268 Cam
Operating Range: 1600-5800 RPM
Duration Advertised: 268 Intake / 280 Exhaust
Valve Lift: .477'' Intake / .480'' Exhaust
Lobe Separation Angle: 110
670 CFM Holley Street Avenger (electric choke, vacuum secondaries)
Vortec L31 heads
Stock (but not original) AC Delco HEI distributor with Accel Super Coil and MSD 6A ignition
Fairly healthy bottom-end
12" vacuum at 750 RPM
3.55 gears with Powertrax No-Slip

So, on to the questions.

I've been reading a lot on timing and wanted to get my total timing set (bought a re-curve kit as well). The first thing I did was work on setting the initial timing (@750 RPM), and I noticed that it was staying exactly the same with the vacuum advance hooked up and disconnected. So I hooked up a vacuum gauge to the port on the carburetor that went to the vacuum advance on the distributor - the gauge read 0". What the hell. Looked at the carburetor diagram, and that port is labeled "timed spark vacuum (connects to distributor advance system)". Then, I note the location of the port - it's in the metering block right above one of the idle mixture screws; it's not going to have any vacuum until the throttle blades open, which they are not at idle. What good is it to not have any vacuum at idle from this port, if you're hooking it up to the advance on the distributor? At idle, my initial is only going to be what I set it at, say 12, instead of 12 + 14 (as an example of advance provided by the vacuum advance on the distributor) = 26.

From what I understand, at idle you have a very lean mixture, and therefore need the spark timing advanced more, which is one of the reasons for vacuum advance on the distributor, right? If that port isn't opening until the throttle blades do, then vacuum is going to zero, and the amount of vacuum given to the vacuum can is therefore going to zero. Maybe I've got things mixed up, but I don't see why anyone would ever hook the vacuum advance up to that "timed spark" port on their carburetor.

From an article I was reading the other day, it sounds like this "timed spark" port was a weak attempt at emissions control a while back. So, why is Holley still putting this port on their carburetors, and telling you to hook up to it for distributor vacuum advance?

I should be hooking the distributor vacuum advance up to the full manifold vacuum port on the carburetor, right?

After more reading, I decided to do this, and noticed that the distributor was now advancing the timing above what I'd set it to with the vacuum disconnected. But that brings me to another question. I set the initial timing to 12 with the vacuum advance disconnected. With it hooked back up, I looked at the timing again, and it was only 15.5. Is that all a stock GM vacuum advance can gives at 12" vacuum? Shouldn't it be providing at least 10? It is not adjustable.

At this time I stopped adjusting timing and pulled it back into the garage. I'm thinking it's about time to get a decent aftermarket distributor with adjustable advance.

Last edited by skizot; 10-10-2010 at 09:55 AM.
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Old 10-10-2010, 11:21 AM
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Quote:
I should be hooking the distributor vacuum advance up to the full manifold vacuum port on the carburetor, right?
IMHO yes this will add the advance at idle and is how I run with a very similar engine config as yours

Quote:
Is that all a stock GM vacuum advance can gives at 12" vacuum? Shouldn't it be providing at least 10? It is not adjustable.
There are different types/mod numbers of these. A typical GM vacuum adv can will add at least 15 more like 20 deg. You need to check on the operation of the pickup as compared to the vacuum can rod travel, it could be binding and/or partly siezed up or there could be a limiter in there. The rod travel should be about 1/4 of an inch. My advice would be to take out the dist and check it out.

I would also suggest a higher base timing of say 18-20 deg, then another 12-14 via mechanical all in by 2800 for a total of about 34, then add in about 12 with the vacuum adv for a total around 46. Tell us some about the trans, car, and rear end gears to help determine whats right.

There are some very inexpensive ways to accomplish these setting with an HEI, maybe take a pic or post the dist details who makes it and part#. Not sure how the MSD will fit into the equation but the standard mods I am talking about have worked time and time again.
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Old 10-10-2010, 01:31 PM
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You have all your facts mixed up. The vacuum advance gets hooked up to ported vacuum source, not manifold. If you hook it up to manifold vacuum the timing will be advanced at idle instead of advancing as the rpms increase.
Your timing is supposed to be less advanced at idle and advances more as the engine turns faster.
Your only seeing the timing increase 3 degrees because the mechanical advance doesn't happen until a certain engine speed. That is why you hook the vacuum advance to ported vacuum. because it works with the mechanical advance to give you the correct timing curve.
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Old 10-10-2010, 01:57 PM
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[QUOTE=bigdog7373]
Quote:
You have all your facts mixed up.
who me? I beg to differ if your going to state that.

Quote:
The vacuum advance gets hooked up to ported vacuum source, not manifold.
That is a matter of opinion and can work both ways depending on the application, lets not get into another debate on manifold vs ported vacuum adv, gwitarJoe is on vacation.

Quote:
If you hook it up to manifold vacuum the timing will be advanced at idle instead of advancing as the rpms increase.
We are adding vac adv at idle thats the whole idea, we are not talking about mech adv yet, his question had to do with manifold vs ported...here we go again lol

Quote:
Your timing is supposed to be less advanced at idle and advances more as the engine turns faster.
Yes mech advance here too is creaping into the equation but thanks for pointing out how mech works. The vacuum adv at idle is OK with cause it provides a cleaner burn and can help the carb stay in the idle circuit as designed, that is one big advantage with vac adv at idle and it is the main reason I use it, you can do it your way all day long if you wish.

Quote:
Your only seeing the timing increase 3 degrees because the mechanical advance doesn't happen until a certain engine speed. That is why you hook the vacuum advance to ported vacuum. because it works with the mechanical advance to give you the correct timing curve.
If his vacuum can was working and he had no vac leaks he would see more than 3 deg adv on manifold vac, thats why I suggested he check into the dist workings, pretty simple, mech adv the way i set it up anyhow is all out at idle....I think this subject is cursed.
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Old 10-10-2010, 02:34 PM
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Quote:
Originally Posted by bigdog7373
You have all your facts mixed up. The vacuum advance gets hooked up to ported vacuum source, not manifold. If you hook it up to manifold vacuum the timing will be advanced at idle instead of advancing as the rpms increase.
Your timing is supposed to be less advanced at idle and advances more as the engine turns faster.
Your only seeing the timing increase 3 degrees because the mechanical advance doesn't happen until a certain engine speed. That is why you hook the vacuum advance to ported vacuum. because it works with the mechanical advance to give you the correct timing curve.
LOL. 516 posts and you've got your facts mixed up. You're talking about mechanical advance, not vacuum. With vacuum advance, as the the throttle increases, vacuum drops, decreasing the advance you're getting from the vacuum advance on your distributor. At the same time, the mechanical advance on the distributor is increasing (advancing) the timing, as your RPMs are increasing.

I don't have things mixed up, I just don't understand why someone wouldn't want the additional spark timing at idle, when your mixture is going to be at its leanest. With the vacuum advance can hooked to the timed spark port on the carb, you don't get any advance until the throttle blades open - giving you no extra advance at idle. I would continue to elaborate, but Custom10 has already done so. Maybe you should take a look at the link I posted in the thread's first post.


Quote:
Originally Posted by Custom10
There are different types/mod numbers of these. A typical GM vacuum adv can will add at least 15 more like 20 deg. You need to check on the operation of the pickup as compared to the vacuum can rod travel, it could be binding and/or partly siezed up or there could be a limiter in there. The rod travel should be about 1/4 of an inch. My advice would be to take out the dist and check it out.
I will do that. I have also been thinking of replacing the distributor altogether, and getting one that has an adjustable vacuum advance.

Quote:
Originally Posted by Custom10
I would also suggest a higher base timing of say 18-20 deg, then another 12-14 via mechanical all in by 2800 for a total of about 34, then add in about 12 with the vacuum adv for a total around 46. Tell us some about the trans, car, and rear end gears to help determine whats right.

There are some very inexpensive ways to accomplish these setting with an HEI, maybe take a pic or post the dist details who makes it and part#. Not sure how the MSD will fit into the equation but the standard mods I am talking about have worked time and time again.
The transmission is a TH350, and the rear end is a 12-bolt Chevy with 3.55 gears and a Powertrax No-slip third member (it's a spider gear replacement).
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Old 10-10-2010, 03:18 PM
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OK well dependant on the car/weight I don't think my suggested numbers are too far off, the heavier the car the more lag on the mech adv adder and all that,,,

With the MSD, I would like to digg into this some more, I am not savvy with the features/benefits and how they apply to the HEI curve/adj we are getting into. I understand that the MSD has its own module installed in the HEI correct? If not then never mind

Can it (the 6A) be used to "monkey" with the dist characteristics in any way during the operation of the motor?, I understand that it can retard timing for start ups, what else? anybody, thanks (hope I don't hijack u here skizot).
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Old 10-10-2010, 03:25 PM
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First of all, custom 10, i was not talking to you. You gave your opinion and i was giving mine. Second, Skizot, you asked a question and in your question and i gave you an answer, simple as that.
Third, my number of posts has nothing to do with my knowledge, i would have more but i haven't been spending a lot time on the computer because i'm finishing up a build of my own.
Fourth: You vacuum advance is hooked up to a ported source because although you vacuum decreases as rpm increases, the vacuum increases at the ported source. You don't try to get as much timing as posible at idle, you try to get it to where the engine isles the smoothest, has the best throttle response, and uses the least amount of fuel doing so.
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Old 10-10-2010, 04:28 PM
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OK big dog its all good no biggy, I just disagree with your take on it when it comes to the idle with or without vac adv thats all, your explanation of ported effects on advance are true as well but not my cup of Joe.
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Old 10-10-2010, 05:40 PM
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Quote:
Originally Posted by bigdog7373
Fourth: You vacuum advance is hooked up to a ported source because although you vacuum decreases as rpm increases, the vacuum increases at the ported source. You don't try to get as much timing as posible at idle, you try to get it to where the engine isles the smoothest, has the best throttle response, and uses the least amount of fuel doing so.
I'm not sure I follow you. You're talking like this ported vacuum source is some magical port that has vacuum separate from intake manifold vacuum; it is not. As load increases, vacuum decreases. The same thing will be seen at the full manifold vacuum port, or at the timed spark (ported) port, once the throttle blades are opened slightly.

To keep it simple, at idle the timed spark (ported) vacuum port sees no vacuum as the throttle blades are closed. At idle with a full manifold vacuum port, the port sees full vacuum (obviously). Once the throttle blades open, the ports act the same.

I really think you are confusing timed spark (ported) vacuum with venturi vacuum. Venturi vacuum is completely independent of manifold vacuum, and is not what we're talking about here.

I don't think you should be telling others that they have their facts mixed up, when you don't know what's going on.

Last edited by skizot; 10-10-2010 at 05:46 PM. Reason: typo
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Old 10-10-2010, 08:29 PM
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i know from experience that my way works. instead of arguing, take 5 minutes and give it a try.
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Old 10-10-2010, 09:20 PM
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how to listen about advance.

BigDog7373 is totally on the button perfectly. Just listen to him. I build engines a lot of them and do a lot of mech and vac timing and why when you stop to think would want full advance(manifold vac) all the time. Set your timing as basic usually 10/12 on a car and 8/10 on a truck because a truck weighs a lot more and will ping to easy with the additional load. Most vehicles I set with a vacumn gauge due to having a diff cam than stock, I set my timing basic for the car/truck and put my gauge on manifold vacumn adjust to full vacumn it will pull running usually about 18(w aftermarket cam) then back off timing about 2.5 lbs on gauge and this usually works with every Gm carb motor, then if needed change my distrubitor springs as needed for best performance, which can be effected by type of trans used or gear ratio,lower gears needs less advance because of less load on car to excelerate.
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Old 10-10-2010, 09:45 PM
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Quote:
Originally Posted by mud.man.rj
BigDog7373 is totally on the button perfectly. Just listen to him. I build engines a lot of them and do a lot of mech and vac timing and why when you stop to think would want full advance(manifold vac) all the time. Set your timing as basic usually 10/12 on a car and 8/10 on a truck because a truck weighs a lot more and will ping to easy with the additional load. Most vehicles I set with a vacumn gauge due to having a diff cam than stock, I set my timing basic for the car/truck and put my gauge on manifold vacumn adjust to full vacumn it will pull running usually about 18(w aftermarket cam) then back off timing about 2.5 lbs on gauge and this usually works with every Gm carb motor, then if needed change my distrubitor springs as needed for best performance, which can be effected by type of trans used or gear ratio,lower gears needs less advance because of less load on car to excelerate.
You don't have full advance all the time when running off the full manifold vacuum. I think that's what you and bigdog are missing. The ported vacuum port behaves the same as the full manifold port, except at idle. I'm not sure how else to explain it, and this thread is getting off topic trying to explain it to some.

The question is, why would you not want the additional advance the vacuum can gives when you're at idle?
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Old 10-11-2010, 01:02 AM
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vacumn advance

I am not sure I want to further pursue this, you don't seem to believe anyone and thats fine, but with manifold vacumn(suck on the hose to distrib when running, RPM will increase) at distrib at idle your timing will advance throwing your initial setting of 10 or whatever off, it is to be used to produce a bit of advance to help the car not lag durring initial takeoff only then the mech advance takes over, otherwise it would be a bit of a dog if it was at full vacumn advance and retimed to 10 degrees then it wouldn't really be of use at all would it. It is really inly jyst for the first part of the cars launch, similar to your accelerator pump on your carb, back it off and it will lag at takeoff. Hope this helps.
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Old 10-11-2010, 07:55 AM
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My goal was not to turn this into a "ported" vs. full manifold vacuum thread (I've already read the 9+ page one from a while ago). My goal was to understand the actual reasons why you would, or would not, want the additional advance at idle. I was hoping to get some answers from the posters on this board that would explain it, but I'm still waiting.
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Old 10-11-2010, 08:11 AM
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I am not sure that I want to pursue this either but I am going to try and pin down were myself and skizot are coming from and then maybe we can ALL be content with our own take on the subject.

There is nothing written in stone or penciled on shi% paper that says ported vacuum is the only way to run vac advance. It seems that the ported crowd here is basing there view point upon a more or less factory or stock engine config. It was GMs pre catalytic convertor idea to start shipping cars with ported adv because of pollution control strategies, the late timing increased the combustion temperatures (among other things) which meant that some of the mixture could continue to burn off in the exhaust reducing emissions. A complete waste of the potential energy in the AF mixture that in my opinion should be fired off to its full potential in the comb chamber.

We are running high duration cams here and at idle they need the added advance that the manifold/vac adv gives to increase the combustion efficiencies and lower operating temps just to name a couple reasons. So stop coming across with your 8 or 10 deg BTDC base timing engine comparissons please, they do not apply to my/his setup.

I don't care if the ported crowd wants to run there way, we (the manifold proponents) have never said that you guys can't but on the other hand we are being told there is only one right way to do it aka ported, we just chose different methods, let it go.
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