I'm running a stock vacuum advance distributor on my 401 nailhead. I'm also running a Weind Drag Star log manifold with 6- 97's. Where do I pick up vacuum for the distributor?
You can see the vacuum ports added to this 6 deuce intake in the balance tube. These were built for all out racing and had no provisions for a vacuum advance. The big one on this intake was used for power brakes. The small one is for the vacuum advance.
You need to find "ported" vacuum. That's a port that gets vacuum AFTER the throttle is opened. You want the advance to "tip in" when accelerating for best response. If you hook it to a port that has vacuum at idle, the advance will drop off when you crack the throttle, diminishing it's affect
Thanks for the responses. I'm guessing since I'm running progressive linkage I'll want to be under the throttle plate of one of the primary carbs.
So if you have a 67 283 Chevy with a 2bbl Rochester carb...where are you supposed to get this "ported vacuum"? There is no ported vacuum source on Rochesters which came with a vacuum advance. They were always hooked up to manifold vacuum from the factory. You had no choice. It is only the new universal carbs that give you an option.
Groucho is correct. You want vacuum from above the throttle. If you use full manifold vacuum the vacuum advance will act exactly opposite of what you want it to do.
We will try our hand here at the facts. Full manifold vacuum is needed, that's all. Ported vacuum came about in the " smog era " days, well before the Nailhead engineers designed the curve in that distributor. TR
A lot of confusion on this. Try reading this ported vacuum is an emission era invention. I'm not a tree hugger.
For those that dont believe me and tommy, there is a simple test you can do. If you have both ported and full vacuum ports, hook up your trusty gauge to each port and record the findings. Check each one at idle and again each one at say 1/4 throttle. What do you see? The results and the explanation why has been gone over on this forum many, many times. Trust me, FULL manifold vacuum. TR
A book can be written about ported vacuum vs. manifold vacuum. The short answer is, it depends... It's first of all important to understand what vacuum advance is used for: additional advance under LIGHT or MODERATE engine loads, typically steady-state cruise, to improve mileage (and engine response under light acceleration, like hills). It's normally not used under moderate acceleration or takeoff, and isn't intended to be. As a basic rule of thumb, an engine with a stock/mild cam may do a bit better with ported advance (it might not, either). An engine with a healthy or radical cam MAY idle better with manifold vacuum, because it generally needs more timing at idle rpm...and for various reasons you might not want to increase the initial timing that much. If you do this, you want to make sure that the vacuum advance unit is sized to open about 2" inches of vacuum below whatever the engine manifold vacuum is at idle...otherwise the vacuum advance will be only partially open, and the idle will fluctuate all over the place. Especially in this situation, you may need to limit the total amount of vacuum advance possible; for example, the GM HEI vac cans will do 20 deg., but you generally want to limit them to about 12. GM points distributors have many different cans, with different amounts. Some are stamped and some aren't. They are interchangeable (not with HEI, though.) Without writing a book- the last thing you need to consider is the overall mechanical advance. If the mechanical advance springs have been lightened (or other changes made) so that the mech. advance is "all in" at cruise rpm, you will almost certainly have to limit the vac. advance down to 12 or so, otherwise the engine will probably ping badly. My overall advice to the OP: first, look at recurving your mechanical advance. If you aren't familiar with this, find a local shop to put it on a distributor machine, or try GMC BUBBA here on the H.A.M.B. Then go to NAPA and start with a VC680 vac. advance unit, which works pretty well for most moderate street engines. It's a 8" can, which means 16 deg. at the crank. Try hooking it up both ways if possible, manifold and ported, and see which one you like more. As I mentioned it will depend somewhat on the cam you've chosen...
Ported vacuum is only "above the throttle" when the throttle is closed. As soon as you crack the throttle, it just supplies manifold vacuum. No advance at idle makes the exhaust hotter so the air injection and/or catalytic converter works better. The ported vacuum is the little hole to the left of the transfer slot. It's just manifold vacuum unless its covered by the plate.
Lots of good reading here! On my Holley Avenger, they have what the instructions call a timed port on the front metering block where they say to hook the vacuum advance, which is above the throttle blades-this is more for the emissions type install? I remember the 3310s having an vacuum port on the base plate I'd use for advance, but for the Avenger, I hooked it up via their instructions. Maybe I'd be better off going to the manifold on my 390 Ford.
Exactly, timed or ported vacuum is the same as full manifold vacuum, EXCEPT at idle. That's the only difference. The O/P made no mention of having a radical cam or even any type of performance cam. TR
If anyone is looking to try homespun's vacuum canister VC680, it should have a B1 stamped on the canister. It will begin to open from 8-11" vacuum, and provide 16-18 degrees advance at approximately 8" vacuum. TR
I think homespun's reply is correct. There is no single "IS" . And swapping the vac adv hose from port to port sure is easier than a cam change to try and see what works better. Probably even quicker than debating about it, too. I've read the widely quoted GM engineer's explanation that include the statement that ported vacuum is an "emissions only" thing. There is some pretty solid evidence that is historically not true, even for GM. Here is what the 1959 factory Edsel manual says. "The vacuum advance port in the throttle bore is located above the closed position of the throttle plate to prevent spark advance at specified idle speed." Not many emissions requirements in 1959. Even legislated PCV was a few years off. I guess we need some stock Y-block owner to swap ports and report back. Ignition manufacturers' recommendations vary a bit too. Many Accel distributors call for full manifold vacuum. Mallory Unilite instructions say this "Vacuum advance applications: Connect original vacuum hose to the UNILITE® Distributor vacuum chamber. Replace hose if cracked, damaged, or if it is not long enough. If the previous distributor was not equipped with vacuum advance, connect a vacuum hose to a ported vacuum outlet on the carburetor." http://prestoliteperformance.com/Po...tructions_unilite_distributor_37_38_45_47.pdf One possible advantage of ported vacuum I can see is that a steadier idle speed could result, since the timing at idle would not change so much from the twin effects of higher vacuum as oil thinned and revs crept up during warm-up. But, with stock type vac advance curves being fully advanced at 14-16 inches of mercury, manifold vacuum would likely just provide full, steady 16 or so degrees advance at idle, at most idle conditions on stock-ish engines.
I learned in school that you not should use Full manifold vacuum. It messes the hole ignition up ! If you puts the timing at 12 degrees. And then put the vacuum hose back ! On manifold vacuum. You aint getting 12 degrees at idle ! well its a good way to destroy an engine.
So lets see if we have this correct, now we are bringing a 1959 Edsel into the story, a Accel Dual Point, a Mallory Dual Point, why dont also just compare a 63 VW Bug w/ a Bosch 009 distributor, why not? How about my MSD Digital 7 box? I can set each cylinder with its own curve for petes sake. Lets compare this to a 401 Nailhead from the early 60's. No one has yet to explain what any of this has to do with the original question, by the original poster about his BUICK 401 Nailhead??????????????????. Instead of quoting engineers about random ignition systems that have absolutely nothing to do with the O/P's question, why not stay on topic, and just answer the question instead of trying to outsmart each other with variables that dont even apply. I'm a engineer, I have 30 plus years of tuning on the dyno, I have the definative solution, put a T in a vacuum line, and hook the damn thing up to a Hollywood wolf whistle, and be done with it already. Now you can drive to achieve full volumetric efficiency with the whistle. When your under load the whistle will blow and you can back off the throttle until it stops blowing to maximize your fuel mileage. Good Golly Miss Molly.
This article helped me when I was recently having problems with my vacuum advance. Lots of good information http://www.460ford.com/forum/showthread.php?t=117504
Whether you use ported vacuum or manifold vacuum, the vacuum unit should be be calibrated for the vacuum source being used.
Running too much advance is a good way to loose power and hurt an engine. The mechanical advance needs to be curved and adjusted for what the engine wants at wide open throttle. You can just run with that curve and no vacuum advance, your part throttle and cruising fuel economy just won't be as good. Otherwise, no harm done. With radical cams and some intake designs the vacuum advance gets "confused". In those cases it's better not to use the vacuum advance. For that reason some performance cars were manufactured with no vacuum advance. No matter who says it, anyone who tells you they can time an engine by ear....... no they can't. Unless the setting is WAY off, here is no way you can tell by ear whether an engine has the correct amount of advance at high loads and RPMs
For those guys that keep saying ported vacuum...please explain to me and the OP exactly where some one might find ported vacuum on this set up??? You won't because there ain't none. Are you smart enough to drill holes in a perfectly good 97 carb to try and make your own? I'm not. The 401 Buick distributor vac advance was designed to work off of manifold vacuum. Yes after you set the initial timing with it disconnected The mark will jump way up off of the timing tag with the vacuum reconnected just like it is supposed to and exactly like it did in the 60s when it was new. I always left the light hooked up after the vacuum was reconnected just to make sure that the advance was in fact working. The car will idle cooler in bumper to bumper traffic with the timing advanced at idle. It instantly retards at the slit second that the ****erfly cracks open. That's the way GM made it and it worked great for years until emissions requirements changed things on later model smog engines confusing many hotrodders.
tommy, I agree with you. I'm going to drill a hole below one of the primary carbs in the intake. I've talked to a couple of Buick guys and they told me this will provide plenty of vacuum with good throttle response.
The location is not critical. The vacuum just moves a diaphragm a short distance so there is no air flowing into the intake. Where ever it's easiest and less obvious.
I'm with you traditions racing, You make the most sense. I like manifold vacuum advance. Everyone has an opinion. Ago
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