Hello, all
I have just purchased a beautiful 66 Dynamic 88, 425 engine with a 2GC carburetor and am tickled with it. 61K original miles. It idled horribly when I got it with what I believe were original wires, plugs, coil, points, etc. I changed to an HEI distributor when I replaced all the other ignition components to improve reliability and ease of maintenance. The vacuum advance on the original distributor was connected to full vacuum on the intake manifold and I cannot find a ported vacuum outlet on the 2GC. Am I just missing it somehow? Any ideas where I can pull ported vacuum from for the new distributor? Will I cause problems running it without the vacuum advance? It seems to idle and runup just fine.

I'dl like to enjoy it for a while before tearing into it and installing a Performer 455 manifold and carburetor that is on the way.

Thanks and looking forward to being part of ClassicOldsmobile

 

Welcome to the site and congrats on your new to you project car! If you plan to do any work on it at all I'd strongly encourage you to purchase a 1966 Chassis Service Manual. They're on ebay and you can often get a used one for $25-40 or pick up one from a dealer for $75-90. I'm sure that would have a picture or diagram showing where the vacuum port is on the carb. Maybe someone here who has one will help you through this task. But I use the Chassis Service Manual like a bible when working on these cars. It's got pictures and diagrams, wiring schematic and step by step instructions on how to take things apart. John

Here's an example of one on ebay right now:

http://cgi.ebay.com/ebaymotors/1966-...item3efe6ca24d

 

Thanks, John 2blu442. I need the Chassis Service manual for sure. I bought it on CD as soon as I got the car and it's just not the same as a book. There is no detailed diagram for distributor advance vacuum on the CD, but the paper manual may have one.

 

if i recall correctly, the vac. advance can should be connected to manifold vacuum, not ported vacuum. that being said, i hope you selected an HEI distributor that somewhat matches the mechanical and vacuum advance curves for the '66 425. if you supply me with the original distributor number, i can supply the specs. for both advance curves.


bill

 

Thanks, Bill. The mechanical advance works like a charm. My vacuum line fed directly from the manifold to the distributor, which didnt look right to me.

 

I have a nice article about manifold vs ported vacuum,from a guy that worked for GM that i think is quite intresting and makes also sense,i mean the most of the guys i asked they all say "ported" but at idle you almost have zero "ported vacuum" why is it necessery to pull out the hose and plug it when adjusting?

"As many of you are aware, timing and vacuum advance is one of my favorite subjects, as I was involved in the development of some of those systems in my GM days and I understand it. Many people don't, as there has been very little written about it anywhere that makes sense, and as a result, a lot of folks are under the misunderstanding that vacuum advance somehow compromises performance. Nothing could be further from the truth. I finally sat down the other day and wrote up a primer on the subject, with the objective of helping more folks to understand vacuum advance and how it works together with initial timing and centrifugal advance to optimize all-around operation and performance. I have this as a Word document if anyone wants it sent to them - I've cut-and-pasted it here; it's long, but hopefully it's also informative."

TIMING AND VACUUM ADVANCE 101

The most important concept to understand is that lean mixtures, such as at idle and steady highway cruise, take longer to burn than rich mixtures; idle in particular, as idle mixture is affected by exhaust gas dilution. This requires that lean mixtures have "the fire lit" earlier in the compression cycle (spark timing advanced), allowing more burn time so that peak cylinder pressure is reached just after TDC for peak efficiency and reduced exhaust gas temperature (wasted combustion energy). Rich mixtures, on the other hand, burn faster than lean mixtures, so they need to have "the fire lit" later in the compression cycle (spark timing retarded slightly) so maximum cylinder pressure is still achieved at the same point after TDC as with the lean mixture, for maximum efficiency.

The centrifugal advance system in a distributor advances spark timing purely as a function of engine rpm (irrespective of engine load or operating conditions), with the amount of advance and the rate at which it comes in determined by the weights and springs on top of the autocam mechanism. The amount of advance added by the distributor, combined with initial static timing, is "total timing" (i.e., the 34-36 degrees at high rpm that most SBC's like). Vacuum advance has absolutely nothing to do with total timing or performance, as when the throttle is opened, manifold vacuum drops essentially to zero, and the vacuum advance drops out entirely; it has no part in the "total timing" equation.

At idle, the engine needs additional spark advance in order to fire that lean, diluted mixture earlier in order to develop maximum cylinder pressure at the proper point, so the vacuum advance can (connected to manifold vacuum, not "ported" vacuum - more on that aberration later) is activated by the high manifold vacuum, and adds about 15 degrees of spark advance, on top of the initial static timing setting (i.e., if your static timing is at 10 degrees, at idle it's actually around 25 degrees with the vacuum advance connected). The same thing occurs at steady-state highway cruise; the mixture is lean, takes longer to burn, the load on the engine is low, the manifold vacuum is high, so the vacuum advance is again deployed, and if you had a timing light set up so you could see the balancer as you were going down the highway, you'd see about 50 degrees advance (10 degrees initial, 20-25 degrees from the centrifugal advance, and 15 degrees from the vacuum advance) at steady-state cruise (it only takes about 40 horsepower to cruise at 50mph).

When you accelerate, the mixture is instantly enriched (by the accelerator pump, power valve, etc.), burns faster, doesn't need the additional spark advance, and when the throttle plates open, manifold vacuum drops, and the vacuum advance can returns to zero, retarding the spark timing back to what is provided by the initial static timing plus the centrifugal advance provided by the distributor at that engine rpm; the vacuum advance doesn't come back into play until you back off the gas and manifold vacuum increases again as you return to steady-state cruise, when the mixture again becomes lean.

The key difference is that centrifugal advance (in the distributor autocam via weights and springs) is purely rpm-sensitive; nothing changes it except changes in rpm. Vacuum advance, on the other hand, responds to engine load and rapidly-changing operating conditions, providing the correct degree of spark advance at any point in time based on engine load, to deal with both lean and rich mixture conditions. By today's terms, this was a relatively crude mechanical system, but it did a good job of optimizing engine efficiency, throttle response, fuel economy, and idle cooling, with absolutely ZERO effect on wide-open throttle performance, as vacuum advance is inoperative under wide-open throttle conditions. In modern cars with computerized engine controllers, all those sensors and the controller change both mixture and spark timing 50 to 100 times per second, and we don't even HAVE a distributor any more - it's all electronic.

Now, to the widely-misunderstood manifold-vs.-ported vacuum aberration. After 30-40 years of controlling vacuum advance with full manifold vacuum, along came emissions requirements, years before catalytic converter technology had been developed, and all manner of crude band-aid systems were developed to try and reduce hydrocarbons and oxides of nitrogen in the exhaust stream. One of these band-aids was "ported spark", which moved the vacuum pickup orifice in the carburetor venturi from below the throttle plate (where it was exposed to full manifold vacuum at idle) to above the throttle plate, where it saw no manifold vacuum at all at idle. This meant the vacuum advance was inoperative at idle (retarding spark timing from its optimum value), and these applications also had VERY low initial static timing (usually 4 degrees or less, and some actually were set at 2 degrees AFTER TDC). This was done in order to increase exhaust gas temperature (due to "lighting the fire late") to improve the effectiveness of the "afterburning" of hydrocarbons by the air injected into the exhaust manifolds by the A.I.R. system; as a result, these engines ran like crap, and an enormous amount of wasted heat energy was transferred through the exhaust port walls into the coolant, causing them to run hot at idle - cylinder pressure fell off, engine temperatures went up, combustion efficiency went down the drain, and fuel economy went down with it.

If you look at the centrifugal advance calibrations for these "ported spark, late-timed" engines, you'll see that instead of having 20 degrees of advance, they had up to 34 degrees of advance in the distributor, in order to get back to the 34-36 degrees "total timing" at high rpm wide-open throttle to get some of the performance back. The vacuum advance still worked at steady-state highway cruise (lean mixture = low emissions), but it was inoperative at idle, which caused all manner of problems - "ported vacuum" was strictly an early, pre-converter crude emissions strategy, and nothing more.

What about the Harry high-school non-vacuum advance polished billet "whizbang" distributors you see in the Summit and Jeg's catalogs? They're JUNK on a street-driven car, but some people keep buying them because they're "race car" parts, so they must be "good for my car" - they're NOT. "Race cars" run at wide-open throttle, rich mixture, full load, and high rpm all the time, so they don't need a system (vacuum advance) to deal with the full range of driving conditions encountered in street operation. Anyone driving a street-driven car without manifold-connected vacuum advance is sacrificing idle cooling, throttle response, engine efficiency, and fuel economy, probably because they don't understand what vacuum advance is, how it works, and what it's for - there are lots of long-time experienced "mechanics" who don't understand the principles and operation of vacuum advance either, so they're not alone.

Vacuum advance calibrations are different between stock engines and modified engines, especially if you have a lot of cam and have relatively low manifold vacuum at idle. Most stock vacuum advance cans aren’t fully-deployed until they see about 15” Hg. Manifold vacuum, so those cans don’t work very well on a modified engine; with less than 15” Hg. at a rough idle, the stock can will “dither” in and out in response to the rapidly-changing manifold vacuum, constantly varying the amount of vacuum advance, which creates an unstable idle. Modified engines with more cam that generate less than 15” Hg. of vacuum at idle need a vacuum advance can that’s fully-deployed at least 1”, preferably 2” of vacuum less than idle vacuum level so idle advance is solid and stable; the Echlin #VC-1810 advance can (about $10 at NAPA) provides the same amount of advance as the stock can (15 degrees), but is fully-deployed at only 8” of vacuum, so there is no variation in idle timing even with a stout cam.

For peak engine performance, driveability, idle cooling and efficiency in a street-driven car, you need vacuum advance, connected to full manifold vacuum. Absolutely. Positively. Don't ask Summit or Jeg's about it – they don’t understand it, they're on commission, and they want to sell "race car" parts.

 

THANK YOU, PANOS. That is a terrific education for me. I hope others will read as well. I'm going to hook up full manifold vacuum on my 425 and enjoy the ride!!

 

lol

 

So what your telling me is that the way I've been timing these old motors is asu for all these years!

Normally I disregard curb idle timing and set my timing between 32 and 36 deg @ 3000 rpm. Then I hook up the vacuum advance and set the vacuum canister to somewhere beween 50 and 60 deg decreasing the amount for pinging or surge.

Always and I repeat always I connect the vacuum advance to timed or ported vacuum!!!!

If I connect it to full manifold vacuum at this point it will bump timing up another 12 to 15 degrees at idle which introduces pinging.

Please note, if you look at all the carb and dist install instructions, they recommend the same setup.

 

I see a couple of problems with your process. First, while total timing is important, the advance curve is equally important. You can't assume the curve is correct if you only set total timing. In fact, most distributor advance kits include a bushing to limit total mechanical advance so that you can use more initial advance and still keep total in the 32-34 deg range.

As for pinging if you use manifold vacuum, as soon as the throttle plates uncover the port, you ARE at manifold vacuum. Usually there's more load on the engine at that point as opposed to at idle, and thus more pinging.

 

I understand that Joe, What your doing is throwing in 12-15 deg of additional timing at idle when you connect to manifold vacuum and then it raises your idle speed, which then in turn you have to reduce. All I've seen happen when people do as you guys suggest is an erratic idle and the engine runs rich.

Again my question is why, no matter which manufacterer you look at for carb and dist install directions, they all recommend hooking your dist to ported vacuum.

 

My 1962 Chassis Service Manual shows manifold vacuum for the distributor. Aftermarket parts use ported because they are primarily being used to replace parts on cars that came from the factory with ported vacuum due to emissions requirements.

 

Not trying to be argumentative, just trying to understand. I don't believe my 63 plymouth (68-440 magnum with purple cam, edelbrock 750 with vacuum secondaries, electronic ignition) or stock 67 olds 330 HC, had emmission requirements (excluding Ca).
I remember back in the late 60's through 70's, and possibly 80's, there was a temp switch that would change vacuum source from manifold to ported after warmup.

(Granted back then and more now my personal car appreciation ended at model year 72.)

So where am I to set my curb idle timing to compensate for the 12-15 deg increase as you suggest, because setting my timing as stated above gives me a 10-12 degree initial timing with all in at 3000 - 3500 rpm. And what about the rpm increase, do I decrease it back down to stock levels after connecting the vacuum line to manifold?

I was always led to believe that at the higher rpms you needed the additional timing under less load as stated in article above, not the lower rpms.

I'm an old dog but still able to learn new tricks!!!

 

I'm not sure I understand why the increased idle RPM with the canister connected bothers you so much. There won't be any pinging at idle - there's no load. As soon as you move the throttle off idle, ported and manifold vacuum are exactly the same thing, so any issues with manifold vacuum are already there with ported vacuum. The accepted procedure for setting the timing and idle speed is to disconnect and plug the vac advance (presumably because the factory thinks it will make a difference), set the initial timing to spec, connect the vac advance, and set the idle.

Note that even when the factory used ported vacuum for the advance, there was a thermal vacuum switch to bypass the ported vac and run full manifold vac if the engine started to overheat at idle. The fact remains that the factory knew that additional advance at idle (to a point, obviously) would help the car run cooler.

Of course, you can add an adjustable vacuum advance canister if you feel the need to do that.

 

Again, I'm not trying to question anyones integrity or be argumentative. If I set my timing at 3500 rpm, with no vacuum advance connected, to 32-34 degrees, and then I plug my adjustable vacuum canister to ported vacuum and adjust to approx 54 (it surges under light load with it set any higher). Then I set my curb idle at 700 rpm. This usually equates to a initial timing around 10-12 degrees.

What I'm trying to say is if I go from ported vacuum to full manifold the idle is going to increase and my timing will advance. In the past this required me having to reduce the rpm and led to erratic idle quality.

Wasn't the thermal switch there to provide manifold vacuum to assist in warmup and then drop off to ported???

 

Let me say that if you have something that works for you, far be it from me to tell you to change it. Not knowing your carb or cam specs, I can only guess that reducing the RPM and getting an erratic idle could be caused by a more radical than stock cam, a non-stock carb, or the fact that you need to readjust the idle mixture screws after setting the idle speed. That last one is an iterative process.


Nope. The TVS only opened when coolant temps exceeded 220-230 F.

 

I've always been taught that distributor vacuum goes to ported. So now I'm confused. I understand the article but now I'm wondering which is correct.

 

What's "correct" is what your car is set up to use. The mechanical and vacuum advance curves may be different depending on the intended use of ported or manifold vacuum. Couple to that any modifications, and this rapidly becomes trial and error.

 

It seems to me that there is no difference between ported or manifold advance at anything above idle. They both equal manifold vacuum as soon as throttle plates open. As Joe says, the only thing ported does for you is load the engine at idle which helped in emissions and also a faster warm up. I have always used manifold vacuum as I kind of like the engine running "fee and easy" at idle. I guess since all is the same after idle it is kinda what works for you type of thing!

 

I agree with the above 2 posts. Mine is not stock. Like Joe said, through trial and error, I ended up with manifold vacume at idle. I couldn't get her to idle worth a crap with ported. Although still a little rough at idle (Lunati Voodoo 60803), it runs freely.