Ignition 101 - Page 2 - Chevelle Tech
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post #16 of 102 (permalink) Old Aug 18th, 07, 12:37 AM
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Re: Ignition 101

Quote:
Originally Posted by kirkwoodken View Post
Again: Re-read entire post. If you have a car, any car, with any engine, and you drive that car at cruise speeds at part throttle, you need vacuum advance. How much simpler can this be stated. ALL engines running at part throttle under light load need vacuum advance. It's not about compression. It's not about the camshaft. It's not about the carb size. It's not about the weight of your car. It's about running at part throttle when mixtures are leaner than at wide open throttle.

Let's say it another way. If you do not drive your car with your foot to the floor 100% of the time, you need vacuum advance.

Does your car have power brakes? Is there a pipe plug anywhere on your intake manifold?
Yes, power brakes as well as a pipe plug under carb tapped into manifold (Vic jr).

By the way, I re-read it a few times and it all makes sense to me. Just trying to get over the brainwashing that we need race parts on street cars!
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post #17 of 102 (permalink) Old Aug 18th, 07, 3:09 AM
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Re: Ignition 101

Steve, thanks for the great post. That was alot of work for sure. Well done.


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post #18 of 102 (permalink) Old Aug 18th, 07, 10:15 AM
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Re: Ignition 101

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Originally Posted by 69-CHVL View Post
Steve, why would one not chose a vac can that fully deploys at say 1-3" of vacuum vs. getting one that deploat at 2" BELOW your lowest vacuum?

For the record, my GM can deploys at 1-3", and adds 20*, but I trimmed it back to 14*.
Vince if you had such a vacuum can the additional timing added would never drop out. You would be running the engine almost constantly with too much timing. When you step into the throttle you need the vacuum advance to drop out as it is no longer needed. If you are able to use full vacuum advance at all times you either have an extremely low initial or a slow and limited mechanical advance or very low cylinder pressure.
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post #19 of 102 (permalink) Old Aug 18th, 07, 10:30 AM
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Re: Ignition 101

Thing is, I dont *hear* any pinging at all, even under high-load, low rpm situations, even when I had it adding the full 20* of advance.

Will have to keep an eye (or ear) on it.

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post #20 of 102 (permalink) Old Aug 18th, 07, 10:34 AM
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Re: Ignition 101

Vince how did you find an advance can that would pull in at such a low vacuum level?
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post #21 of 102 (permalink) Old Aug 18th, 07, 10:49 AM
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Re: Ignition 101

Quote:
Originally Posted by vrooom3440 View Post

ed: There is a similiar list for point-type distributors which I have left out as I see little point running points with their increased maintenance requirement on a performance engine.
These are the cans that fit the small body distributors, like MSD, Accel, etc., not specifically a 'points' setup. I see that they should be included so the small-body guys might see what numbers they might choose:

You can find them here:

http://65corvette.nonethewiser.net/t...um_advance.pdf
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post #22 of 102 (permalink) Old Aug 18th, 07, 11:20 AM
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Re: Ignition 101

Quote:
Originally Posted by M.Maner View Post
Vince how did you find an advance can that would pull in at such a low vacuum level?
Mike
This actually comes with the ZZ crate motors. Evidently, they tell you to change this out too for a 10* unit. Another GM special

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post #23 of 102 (permalink) Old Aug 19th, 07, 1:49 AM Thread Starter
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Re: Ignition 101

Vince,

You actually answer your own question here somewhat.

First for the record if you follow my postings on ignition you will find that I often suggest using a hand vacuum pump to tune your vacuum advance. You really cannot beat the utility of a vacuum pump to tell you when (what vacuum level) your vacuum advance starts to move and when it is all in (again in terms of vacuum level). Doing the actual measurements is even better than using the manufacturers specification of a range. If you measure it you know what you have without any ambiguity or manufacturing tolerances.

Quote:
Originally Posted by 69-CHVL View Post
AR35. Looks like it starts at 2-4 and is pegged 6-9
Very similar to mine, mine starts at 3" and is all in at 9" and provided 20*.

Quote:
Originally Posted by 69-CHVL View Post
Steve, why would one not chose a vac can that fully deploys at say 1-3" of vacuum vs. getting one that deploat at 2" BELOW your lowest vacuum?

For the record, my GM can deploys at 1-3", and adds 20*, but I trimmed it back to 14*.
I believe you later elaborate that 1-3" is only the start and not full advance. I have not heard what your idle vacuum is but starting to deploy at a rather low vacuum would be appropriate with an engine combination that idled with low vacuum.

But that aside, this is a very common situation, needing to trim or reduce the amount of vacuum advance provided. Many of the vacuum cans provide too much advance and need to be limited to a lesser value.

Note also that your later response says that you really have full advance at 6-9". This would be appropriate for an engine combination that idles at 8-11" of vacuum.

Steve
1968 El Camino, 402 with TKO
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post #24 of 102 (permalink) Old Aug 19th, 07, 8:38 AM
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Re: Ignition 101

Steve, the post where I stated 1-3" was off, the later response is correct of (start) 2-4" and (fully) 6-9". I'm pulling 11-12" of vac at 750 rpm.

Doesnt dither in/out so were good. The only problem I could envision is at WOT if there was any vacuum, I guess the advance could potentionally begin to come back it, but I'm pulling any vacuum as I did check before (taped a vac guage to my windsheild).

Seems to me that it would be nice to find a can that would fully deploy around 3-4", that way the most radical combos could use vac advance.

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post #25 of 102 (permalink) Old Aug 19th, 07, 11:32 AM
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Re: Ignition 101

Any reasoning in swapping my Holley 950HP baseplate to a non HP baseplte so that I can have a manifold vacuum port to hook up a vacuum can?

Also, anyone know if an MSD p.n. 85551 will accept a vacuum can as an add on?

My combo only shows 6in of vacuum at idle (255/263 cam at .050). I run 18 initial and distributor is set up for 20 degrees for a total of 38 all in by approx 2500.
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post #26 of 102 (permalink) Old Aug 19th, 07, 11:46 AM
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Re: Ignition 101

Before I spent that kind of money I think I would tee off from the feed to your brake booster.
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post #27 of 102 (permalink) Old Aug 19th, 07, 2:03 PM
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Re: Ignition 101

Chris_69_SS: Almost every manifold has a pipe plug in it for a vacuum source. Even if you don't have power brakes, there should be a manifold plug. Are you running a spacer plate? You could tap a hole in that.

Beautiful car!!!!!

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post #28 of 102 (permalink) Old Aug 19th, 07, 11:01 PM
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Re: Ignition 101

Quote:
Originally Posted by kirkwoodken View Post
Chris_69_SS: Almost every manifold has a pipe plug in it for a vacuum source. Even if you don't have power brakes, there should be a manifold plug. Are you running a spacer plate? You could tap a hole in that.

Beautiful car!!!!!
No port in a Vic Jr. I did tap one in rear of intake (under carb) for Power Brakes and pcv. makes no difference anyway as my dizzy has no vacuum can.

Got the plugs looking a lot better with idle screws 1 turn out on all 4 corners. Gas mileage is awful but what ya gonna do with a 9'' stall, 4.10s and 575+ hp? I also jetted down to 76s in the front with the stock 78s out back.

Thanks for the compliment on the car. It was a lot of work to get the body/paint done.
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post #29 of 102 (permalink) Old Aug 22nd, 07, 1:43 PM
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Re: Ignition 101

I have embarked upon a quest to comprehend the limits of the GM 4 pin HEI Ignition system. Here is a compellation of data I have collected from many sources across the web and from some of my old GM training manuals as I understand it.
The Stock GM HEI System Is a well engineered system for most street hod-rodding applications. The Stock GM HEI works well for the lower to mild end of the compression and RPM spectrum. All hype aside if you combo is well under 12:1 compression ratio and or rarely sees continuous heavy load 5000 RPM duty you have nothing to gain from other styles of ignition unless you are going to mask other problems. The meat of it is this. The HEI Coil needs time and current to charge. If you limit either one of these you reduce the spark available to the cylinder. How much spark do you need? That is the Million dollar question that an entire aftermarket ignition industry is built upon. In reality, for a spark to initiate a jump of a spark plug gap it takes 10,000 to 11,000 volts. For it to maintain and complete the coil discharge process takes much less voltage, more along the lines of 3000-5000 volts. An inductive ignition system maintains the coil discharge time for a longer time than a CD System. The reason for the “multiple firing” at lower rpm of CD is not because it’s better than a single long spark but because the duration of the CD spark is so short it needs repetitive firing to ensure there is a spark in the cylinder at the opportune time. A typical OEM 4 pin module is limited to 5 amps output so as to avoid overheating the coil and module itself. This is more than enough amperage IF you have enough time. If you do not allow sufficient charge time (High RPM) then you must push additional amperage through the module and into the coil to assure the coil is charged up enough to provide the level of spark you need. Most high performance aftermarket HEI modules are available in a 7.5 amp output version. Now it is important to realize this additional current may cause problems (excess heat) in the coil and there for it is wise to upgrade to a matched coil also. With these two upgrades there should be adequate spark energy through the13:1 compression ratio and 6000 RPM range.
There is a timing retard that occurs at a rate of approximately 1* to 1.5* per 1000 RPM after full advance is reached. This is due to the Magnetic pick up trigger and not the module. I do not interpret this as a problem for most of us and I believe that high speed retard devices are becoming popular for a little more MPH. Hmm We have a built in device….The reason the HEI system retards the total timing the way it does is this: as the pole piece spins inside the magnet (pick up coil) it creates an AC voltage signal (looks like a wave).The module uses this signal to trigger the coil. The module looks for the back side of the wave to cross the zero point to fire. As the pole piece spins faster it creates a larger AC wave and that larger wave crosses zero a little later and we have a later firing time (retarded timing).
I am not suggesting there is no need for other types of ignition. HEI does have its limits. I am saying that the Stock HEI will take you a lot farther than most people give it credit for. Now if you need shiny things to go fast then spend away…….

Take Care
Milan Atanaskovic

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post #30 of 102 (permalink) Old Aug 22nd, 07, 2:06 PM
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Re: Ignition 101

I see Lars and John Z are getting around!

Good stuff and both folks understand and communicate excellent.



JIM

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