I have a Buick Straight 8 engine with a 1942 compound manifold dual carb set up. I have a vacuum line that runs from my distributor directly into my intake manifold. I ran across a post over in a Buick forum and it states, "I would not tap the vacuum advance system into straight manifold vacuum. Some hot rodders do this because it helps the engine idle and generate some vacuum with a radical camshaft. The S-8 doesn't have problems with idle or poor vacuum. And it doesn't like too much advance at idle (it'll start popping in the exhaust)" So I'm seeking second opinions. Can or should I eliminate my vacuum line?
I don't have any personal experience with the str 8's but I'd just hook it up both ways and see what it likes.
I posted this before.......Lots of good info on why to use vacuum advance. Quotation thanks to JohnZ G.M engineer 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 arent fully-deployed until they see about 15 Hg. Manifold vacuum, so those cans dont 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 thats 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 dont understand it, they're on commission, and they want to sell "race car" parts. ____________
The Buick poster continued with this post, "I still stand behind my statement: if you are using manifold vacuum on your distributor's vacuum advance, it will advance your timing also at idle, which is not good for a S-8. Advancing the idle beyond 10 degrees or so, forces you to close the throttle blades further to bring idle rpm down. That, in turn, makes your fuel distribution even worse than it already is, manifesting as a "popping" sound in the exhaust. The sound is not loud, but more like "spitting". I have used manifold vacuum in all my V-8's, but for the Straight-8 with siamesed intake ports, it's not a good idea. It seems by his experience on Straight 8's it is not needed. Any thoughts out there
Brooklyn, it has been a long time since I worked on anything older. Having said that, my '50 vacuum advance line is connected to a port on the base of the carb, in front. I believe yours should be, also.Probably on the front carb. Per my '50 shop manual, the vacuum tube fro distributor is connected to an opening in the carb barrel. On the atmospheric side. So at idle, no vacuum advance. As throttle is opened, to about 18 mph, the vacuum starts to advance the timing. THEREFOR, hooked directly to manifold, at idle, where there should be no vacuum advance, one has full vacuum advance. Unhooked completely, one never has vacuum advance, which BUICK, at least IN 1950, THOUGHT ONE NEEDED. Ben
I would do it the way the factory had it. Years of messing around with motors taught me the engineers who designed the car usually know more about it than I do off the top of my head.
I think firstinsteel is correct. You need some kind of vacuum advance. Yours seems to want venturi vacuum. Don't just unhook the line.
Many(Most) factory calibrations in the 40's and 50s used timed or ported vacuum advance. They seemed to think, and this was way before Smog, they idled best that way. With your Buick set at 4 degrees you would have 4 with ported/timed advance and 18 with manifold vacuum. You can try it both ways, and choose which idles best. As conservative as those engines were it will probably like the ported, or work fine either way. Most of the info about ported vs manifold vacuum stems from higher performance engines that "want" 24 - 32 to idle. Sounds like the Buick guy knows his engines and certainly Buick's specs were for best idle quality, not smog related. The GM guy is referencing what was done in the 60's and later to increase the load at idle and decrease HC with elevated EGT. You will burn a very small percentage of total fuel at idle and the difference in ported/manifold would be too small to measure. One thing to do is plug the vacuum advance, hook up a vacuum gauge, and advance the timing slowly up to 18 and watch the vacuum. If it is higher at 18 and the engine idles smooth fine, manifold vacuum, if it idles no better (vacuum wise) or is rough, then run ported. Finally ported is NOT venturi vacuum. That is a special vacuum that is used to work the loadamatic type distributor which uses the vacuum signal from the airflow through the venturi. It is highest at WOT when manifold vacuum is near zero. This would cause massive knock if you had a carb with venturi vacuum hooked up to a normal centrifugal plus vacuum advance distributor. I think your Buick guy knows what he is talking about. A fellow Valve In head mechanic 52 263 specs, Automatic ====== Synchromesh
The factory vacuum set up for buick dual carbs goes like this. 1st vacuum line is Distributor to front carb. 2nd vacuum line is manifold to windshield wipers. Are you using factory buick carbs? You can eliminate the manifold to wipers only your wipers won't work. But you should keep the distributor to front carb vacuum. Sounds like it got switched around at some point
There are 3 kinds of vacuum ports in general use' Venturi Vacuum; Increases as airflow/power increase, maximum signal is lower than Manifold vacuum amd is used both for the old Loadamatic distributors and for Carburetors with Vacuum Operated secondaries. Note the strength of this signal varies by location (booster vs main venturi) and application, distributor or secondary control. Ported or Timed Vacuum: It is Manifold Vacuum but the port only "sees" the manifold vacuum after the throttle is opened past the port. Used for EGR, Vacuum Advance, and Canister Purge generally. Manifold Vacuum: Is the difference in pressure in the plenum of the intake vs the outside air. It is a useful load indicator and roughly approximates throttle opening. Often used to advance timing under light load conditions to compensate for lean, non-turbulent, low density charges which burn slowly. Not using this feature will increase EGT, Heat Rejection into Cooling System, and fuel consumption. Many carburetors connect a piston or diaphragm to manifold vacuum to enlean the mixture about 3 -4 numbers of A/F, say 12.5 - 16.1 for better fuel economy. If we have over 1.5 ' of mercury Vacuum at wide open throttle you could say we are under carbureted.
On my old 248 with duals I kept the distributor vacuum line hooked to the manifold. And only had that because I ran 97's with progressive linkage and had windshield wiper bullets so I never used my wipers. Here's my motor on YouTube. It was a long time ago so the quality sucks http://youtu.be/CKZbRNrPS7s
My car is a rat rod. I have dual Stromberg 97's for carbs, a Buick 1942 compound manifold, a freshly rebuilt Straight 8 engine with pistons bored on a 53 Buick Special. What brought this post on, I'm in the middle of installing my intake and exhaust manifold gaskets. In the process of installing my vacuum line on the intake manifold I sheared the vacuum line. Hence my curiosity to poke around the forums and do some research on vacuum or no vacuum. The previous response about Vacuum 101 paints broad strokes that all cars need vacuum; however, the post from the Buick forum that I referenced earlier, strongly disagrees specifically on Straight 8 engines. I've read and own my 53 Buick service manual... about 1/4 of it applies with modifications that are on my car. In this particular post, doing what the service manual states appears not to apply. I've found exactly 1 post on a Buick forum that strongly discouraged manifold vacuum on a Straight 8 with a stock distributor. Anyone else to second his notion?
I just got a response off another forum. Said to run vacuum off the carburetor not the manifold on Straight 8 engines. Any thoughts?
I'm with going with the factory connection; You can always experiment later, if the performance isn't satisactory. Isn't that what roddin' is all about ? Happy Roddin' 4TTRUK
I have some thoughts. I believe you've already received your answer concerning vacuum source location for the distributor. Generally speaking, the Buick straight eight engines are no different than most others. As the previous posters have stated in their "broadly painted responses"...you do need vacuum for initial timing, and the recommended source is ported vacuum at the carburetor, not the intake manifold. I wouldn't have a lot of faith in one poster's opinion on some other forum concerning special requirements for the Buick straight eights, which is contrary to the service manual and what has already been suggested to you here. Also, you must've heard that the Hamb is a good source for automotive information, hence your decision to "poke around". What you may have missed, is that its membership also appreciates a certain respect for the forum and the basic rules set down by the founder and moderators. And that includes (among other things) filling out your profile, doing an introduction, and not using the term "rat rod". Welcome to the Hamb.
brooklyn53- Reread what hoop98 said above. That makes the most sense to me. The stock Strombergs that came on the Buick are larger than Ford types. And had an air valve below the secondary carb. Really more like a four barrel than two twos. If I leave out the vacuum advance on a distributor, I don't just plug up the line. I rotate the vacuum plate to it's at rest position. Confirm that the points are opening as the roter is pointed at a terminal. And then braze it in that position. You don't want it just free to flop around altering your timing at will.
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