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Discussion Starter · #1 ·
Just got the 67 GTO back from the shop after an engine rebuild and transmission swap. The car runs really well but I'd like to get my vacuum advance into the mix. The engine is a 461 with a HR cam, Eheads and about 10.4 compression. The car only sees street use so I would like to gain the efficiency of the extra timing advance under partial load and idle. I dusted off my timing light and with the vacuum pot disconnected the initial advance is 23 degrees. I didn't dial it back far enough or rev it far enough to see what the total advance was but it was at least another 12 degrees if not more. Is there any possible way the vacuum pot can be used without introducing pinging under light load?
 

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Engines rarely if ever ping under light load, but considering that you're already running quite a bit of initial lead I'd recommend starting out with the can connected to a ported vacuum source as opposed to a non-ported source. It's not going to hurt anything to hook it up and give it a try to see if your engine like it.

Bear
 

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Discussion Starter · #3 ·
Thank you for the quick reply! I'll plug the vacuum line into the ported vacuum on the Holly carb and see how it does.
 

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Just don’t have much more than 12 on top of 23…that is 35 already so not much up room.

if you knocked that centrifugal up to about 20….ran 16 base + 10 from the vac cam at idle, you would have a nice smooth runner. Have the springs bring in all of the 20 by 3000 Rpm or so.
 

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Discussion Starter · #5 ·
Hooked up the canister with no adjustments to the timing. No audible ping under light load/partial throttle driving. Even low rpm up a hill. Mission accomplished?
 

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Don't forget your Eheads are a "fast burn" design and usually run less timing. Most people with the fast burn heads run 32-34 total from what I've seen. From the googling I've done it seem like KRE heads don't like more than 32 and Edelbrock don't like more than 34, but it varies of course.

Check around your "all in" rpm maintaining speed up a hill. That's where I first start to get some detonation with my Edelbrock heads. I'm all in at 3000 and if I run too much timing, the vac advance plus max timing will induce some ping/miss holding speed up a hill around 3000rpm.

Any issues starting with 23 initial? I run 18 initial but I've always been reluctant to run more with the aluminum fast burn heads.
 

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With 23 initial and then adding vac advance of what 20 or more?…..that is 43? Maybe? But somewhere too high….just as you start moving even using ported vacumn. Many views on it, but seems way too much timing to be any good.

Bear is right that detonation under light throttle is not a problem, but detonation can occur under acceleration with too much timing, plus it is completely inefficient for anything.

you don’t get any tremendous benefit from just adding timing. It has to be the right mix and amount for each of the delivery systems, Centrifigal, Vacumn and base. It is like an orchestra.
you just can’t bang the drum louder and think it is better.

How much timing does the vac can bring in? And when? Is your cam so radical in duration that it needs tremendous base timing? Some do….

you are on it, so you will get it. You first have to figure out where you want to go with it. And yes 32 or 34 are safer total amounts. You can start there and see. But with 23 base that means 9 or 10 Centrifigal…….you think it is that low. Maybe, but rarely see that. Not saying a race shop couldn’t have set it that way.
 

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Discussion Starter · #8 ·
The engine fires up with less than one revolution it seems like. No hard starting. I was very surprised to see that much initial timing as well. I'll get some time in the next few days to get all these numbers down and figure things out. I'll let you know what it looks like.
 

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Discussion Starter · #10 ·
Checked the vacuum advance and it is 23 degrees. Is there any scenario where this would not be too much timing? I thought this was the adjustable vacuum pot
but I guess it's not. It's an MSD "ready to run" unit with the vacuum advance. I set initial to 18 degrees with 16 degrees mechanical by about 2800.
 

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Just have to listen to your motor. 23 is a lot though. My motor doesn't like more than about 8 degrees advance. It will start to miss when cruising around 2500rpm+ up hills and such (usually at highway speeds). But every motor is different.
 

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One common mistake on setting timing is assuming that the “all in” is all of it at 3000 RPM or so. There may be more in there but if you are sure then you know that you have 34 total timing.

Most vac can, including MSD ready to run cans have 20 or more degrees of timing and yes that is way too much. The adjustability of those cans is for RATE OF VACUMN only, not for timing. For that you need to get a fixed stop. MSD has a prototype in development.

Or you can get one from Lars @ [email protected]. Either one will work.

once the corrector is on then set your vac can rate to 2HG below your idle vac and hook it to full manifold vacumn. I always toss those adjustable cans and use a fixed can with the corrector, much better keeps it all solid. But you can make it work.

once set up you will have 18 base and 10 from vac for 28 degrees at idle. These settings I usually use for the most radical cams. But since these electronic modules now retard crank timing by 4 degrees, so you don’t get starter kickback, it is more forgiving. And can be used.

I would probably set yours up more in the mid twenties at idle, with something like 14 Base + 10 vac for 24 degrees of idle timing…then 20 for Centrifigal timing for the same 34 total.

The reason the racer or builder sets that idle at 18 is because he is eliminating vacumn advance and therefore has to run the initial up high to get a good idle. The engines really were not sold or designed for the street that way, they had vacumn advance to compensate for load, that is your foot or driver demand.

Sounds like you are getting to a better place with it either way….
 

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"The reason the racer or builder sets that idle at 18 is because he is eliminating vacumn advance and therefore has to run the initial up high to get a good idle. The engines really were not sold or designed for the street that way, they had vacumn advance to compensate for load, that is your foot or driver demand."

This also applies to some EFI systems. My timing is controlled by my ECU, I run vacuum advance but the computer doesn't apply advance at idle. So, I run 18-20 initial, 33 @ 3000 with advance starting at 1200. I only run 8 degrees of advance because going up a hill at highway speeds will see around 38 degrees of advance. I'd like to run more to try and squeeze a few more mpg, but it's not worth the risk of inducing detonation. Especially on a car that isn't driven often.
 

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Discussion Starter · #14 ·
Checked everything again and ran the rpms up past 3k. It turns out that "all in" the curve is actually making 23 degrees mechanical. I pushed the initial back to 15 degrees and drove the car and I didn't have any audible signs of pinging. That's 38 total and I remember the car running pretty well on that. Now there's there's the issue of the vacuum advance. It wasn't plugged in on this test drive but I'd sure like to put it into the equation. I measured it at 23 degrees total advance. That seems like a ton of advance I'd get at partial throttle with my 15 initial plus 23 mechanical along with the vacuum pot.
 

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Yes I suspected that you were under counting your centrifugal advance, RPM’s have nothing to do with determining that. It is a total mechanical fixed point.

you now believe it is 23 not 16. And set your base at 15 for 38 total timing.

you do not want more than 8 to 10 degrees of Vacumn advance. With today’s gasoline that is the perfect number. I described how to achieve that above. With a 10 degree vac can hooked to full manifold vac you will have 26 degrees of idle timing.

it will idle perfect there. 38 is aggressive, but some engines can take it.

to be sure of your total centrifugal, you can remove one spring from the distributor weights put the cap back on hook up the timing light and retest until the timing advances no more. That way you will know that it really has reached it’s mechanical limit. And you won’t have to rev it too high to determine that.
 

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Discussion Starter · #18 ·
It pulls around 16" at idle. Better than it did before the rebuild oddly enough. The engine seems to run very well with the initial and mechanical I have dialed in right now. I'd sure love to get this vacuum working though. Can you tell me how I can limit this to 10 degrees? Its the MSD 8528 ready to run model.
 

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It pulls around 16" at idle. Better than it did before the rebuild oddly enough. The engine seems to run very well with the initial and mechanical I have dialed in right now. I'd sure love to get this vacuum working though. Can you tell me how I can limit this to 10 degrees? Its the MSD 8528 ready to run model.
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Make a stop? Look at my pic #1. Cut a strip of metal that will fit inside/between the allen screw and what appears to be a raised dimple (?). This will keep the strip from moving side-to-side.

Then down at the base of the strip, first drill a small hole through the strip and through the vacuum pull. You are going to use this as an attachment point. Slot the hole you drilled in the strip,above/below the hole, so you can slide the strip in and out up against the vacuum arm attached to the distributor plate.

Tap the hole you drilled in the vacuum pull for a small/short machine screw, as you will want to tighten/loosen it to adjust the length of the strip in/out that will now be used as a stop to limit the amount the vacuum advance arm can pull the distributor plate in, and then tailor the number of degrees of vacuum advance.

Now if you think the screw might interfere with the pull rod, cut the strip with a dog leg as seen in pic #2 and add your screw/slot for adjustability using the dog log section.

I have never done this and am throwing this out there just looking at it and not having the distributor in hand. So you may have another idea or way..........so no laughing. ;)

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