Buick 231 timing help

Buick 231 installed by PO.

Casting number missing from the 231. Suspect it is either a 1979 or 1980 even-fire...So I'm going with 15 BTDC for timing.

I have a yellow mark on the plate behind the crank pulley, and there is this tab that sticks out on the driver's side about the 2 o'clock position that doesn't seem to serve any purpose other than to mark the timing (it doesn't have any degree marks. It is just marks a point or a line over the pulley)

Is that the 15 BTDC mark, or the 0 BTDC mark?

Should I be using manifold vacuum, or ported vacuum?

I have seen one diagram of my Rochester Dualjet 210 showing the where the vacuum hooks up for the distributor is manifold vacuum. (attached picture) Is this right????

I know that the 225 runs off of ported vacuum, and I have read RRich's stuff on converting 225's to HEI distributors and vacuum issues.

I just don't have any info on the 231 and can't seem to find any.

I would like to time this thing correctly, but I can't figure out how...

Any help would be appreciated!!!!

 

It's probably the 15* mark but it's been years since I've worked on an even fire 231, got a picture?

 

Yeah, I figured it was supposed to be ported, but the diagram shows manifold vacuum...

I can't attach the image. Going to see if I can find the URL.

I've got a Hayne's Rochester manual on the way, as well as a Hayne's Buick manual. Maybe the answer will be in there.

I'm just trying to think of any reason why you would use manifold vacuum for the distributor - it would be max at idle, and decrease as RPM's go up. I don't know if the vacuum calibration lever inside the vacuum diaphragm on the distributor and the spring weights take that into account.

I have read from RRich's papers on HEI modification for the 225 that the Rochester 2GV has such a strong vacuum that you have to get an adjustable vacuum diaphragm, and adjust it to the least sensistivity.

But since this is the Rochester Dualjet that came with the engine, I'm figuring it was engineered for the vacuum port that is depicted.

The only reason I can figure is for EGR and smog reasons.

Just trying to get the thing tuned and figure out why it is the way it is...

Bottom line: trying to work on a 42 yr. old vehicle is hard enough, but when you try to figure in mods the PO made, it makes it infinitely more difficult...

 

Here is a 231 vacuum digragm from a 1978 Pontiac. Note how the vacuum line going to the HEI distributor is labeled "manifold vacuum." That's what has me cornfused... The point depicted is where my vacuum line attaches. It barely runs, set at 15 BTDC, until you attach the vacuum line - then the RPMs jump from about 800 to almost 1800...



http://members.dandy.net/~k0xp/Oldsmobile/1978/1978_231_Except_CA-a.jpg

 

Great information!!!!

Looks like these engines need full manifold vacuum, and where my timing is hooked up is correct. Ported vacuum signals are for later EGR controlled engines.

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.

 

PART 2:

Distributors & Vacuum Connections​

Most OEM distributors are a combination of vacuum and mechanical advance for ignition timing. The mechanical part is RPM dependant, while the vacuum advance is throttle "signal" dependant. I will cover both OEM and aftermarket distributors to give you a full scope of their intended use and proper setup.
It would be best to cover some functional logic first, in order for you to understand how distributor advances operate. I will start with the OEM Chevy distributor since they are popular in T Buckets. There are 2 basic functions which make up the TOTAL advance on the GM distributor. Vacuum and Mechanical. The vacuum part is controlled by a vacuum signal from the carb or intake manifold, while the mechanical is managed by engine RPM. The vacuum port on the carb is an active element, meaning it draws vacuum directly from the intake manifold. So, if you were to put a vacuum gauge on this port, you would see a vacuum signal. This vacuum signal actually "RETARDS" the timing on your distributor. Now when you open the throttle, the vacuum signal drops and the distributor "ADVANCES" until the vacuum signal rises again. Meanwhile, as the engine revs up the mechanical advance starts advancing proportional to the engine RPM. Under full throttle both the vacuum and mechanical advances are functioning. However while cruising, the vacuum advance will back off (RETARD) proportional to the vacuum signal and the mechanical advance will be advanced proportional to the RPM. This makes a good general use distributor especially for street driving.
One factor most overlooked when timing an engine is the TOTAL advance. While you could have your low side of the advance set correct, the total advance could be way off, due to wear or some other problem. It is imparative to check the total advance to be sure it is working within it's designed range. Too much total advance will cause serious detonation that will eventually damage your engine. For pump gas (the stuff you get at a gas station) won't handle lots of total advance too well. Try to keep your total advance under 38 degrees. On leaded racing fuel you can go over 40 degrees without worry.
Let's talk about vacuum line connections for a moment. Where your distributor vacuum line is connected to the intake manifold is important! First and foremost, it needs a continous vacuum signal. Second, it should have a stand alone connection to respond quickly and properly. Let's go into further disscussion here.First "continous signal" meaning there is a un-interrupted connection between the distributor and the vacuum within the intake manifold. Whatever your engine is doing is reflected in vacuum and your distributor needs to "see" this signal. Second, "stand alone" means the vacuum line has no other vacuum connections IE: Transmission modulator, power brakes, or PCV connections. The vacuum servo on your distributor requires a lot of vacuum signal to move the mechanical plate back and forth. If it has to share the vacuum signal with a transmission or other device, the distributor vacuum servo will not respond properly.
It's time to talk CONNECTIONS! All carbs which are designed specifically for street use have multiple vacuum line connections available on the baseplate. There will be one large nipple, and up to 3 small nipples. The large nipple is for your PCV connection. And, the others for Transmission modulator, distributor, and an optional timed port(Holley/Ford only). It is very important you connect these properly!
RULE 1: Do not "Tee" any vacuum connections except as a very last resort.
RULE 2: Never "Tee" anything to the PCV connection, I mean NEVER EVER. This line gulps huge amounts of vacuum to extract fumes from your block. Anything else connected here will virtually not function at various throttle positions.
RULE 3: check all vacuum connections with a vacuum gauge to be sure they have a continous vacuum signal, especially the distributor, transmission modulator, and PCV.
RULE 4: Wherever possible, direct connect the PCV and distributor to the intake manifold instead of the carb. The intake manifold is much faster at responding to vacuum signals than the carb.
RULE 5: (Holley Carbs) Never use the vacuum nipple located on the metering block for GM distributors! That port operates BACKWARDS to what your distributor needs. This is reserved for selected OEM Ford distributors ONLY.
RULE 6: Always check your total advance after setting the lowend timing. It is imparative that you do not exceed the limits of your engine/fuel type combination. Failure to check this could result in high speed detonation.

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Mechanical distributors are designed for 3000RPM and up launches off the line. While they work reasonably well on the street, that isn't their design intention. From a dead idle, mechanical advances can only do what the engine RPM tells it to do. Whereas a vacuum advance will go to full advance as soon as you open the throttle. This spontanous vacuum responce is very desirable for off-idle street hotrodding. A vacuum advance is much more forgiving while cruising as well. While cruising, the vacuum advance will retard to it's zero position leaving the advancing to the mechanical part. This makes for a slightly cooler running engine during cruise mode. You can think of a vacuum advance as a "power valve" for your ignition! Its function is IDENTICAL to that of a power valve in a carb. Both provide more "juice" when needed, and "conserve" when cruising. A full mechanical advance will tend to be more advanced than needed for cruising, and usually runs your engine a little hotter. This isn't a problem by any means, but it's not running as efficiently as the vacuum advance.
From a hotrodding perspective here's the difference; From a dead idle if you were to stomp on the accellerator pedal with a vacuum advance you would get instantanious power all the way up to where the engine vacuum rises and pulls the advance back. So, from an idle to near full power the vacuum advance wins hands down. The down side is, when you approach your power curve your manifold vacuum rises and pulls the vacuum timing back! From a dead idle if you were to stomp on the accellerator pedal using a mechanical advance, you would have a slower distributor advance thus taking longer to get in the power band. HOWEVER, once the mechanical advance reaches maximun, it stays there through out the entire RPM range. This is a major benefit, holding full advance. Since drag racers do not start from an off-idle, the slow RPM advance responce is of little interest. Drag racers much prefer to have 100% total advance when they launch off the line, which is something the vacuum advance cannot do under any circumstances.