gentlemen, you are mistaken. In Alba's, case, the hookup should go to full manifold vacuum. If the engine knocks on ported, but not on manifold, why would anyone recommend hooking it up in a fashion that we already know is causing knock ? I think some folks need a little history lesson here. Ported advance and those thermo jobs for vacuum control are the auto manufacturers early attempts to get their engines to pass the toughening standards of the late 60s and early seventies. Where is it that some of you live that you have to pass smog on a truck old enough to run an FE ? Manifold vacuum is the better way to go, and anyone who thinks otherwise doesn't understand what they are they are talking about. If you look, you will find trucks from Ford, chevy and dodge all came, at one time or another, with manifold vacuum. All 3 companies also used ported. Alot will depend not only on when, but where your truck was manufactured. Differant plants did things differantly. The Kalifornia trucks are all over the map as far as what was 'standard'. A truck or a car can be made to function quite well with either setup. The weights and springs inside your dizzy can be changed to speed up or slow down the rate of centrifugal advance. There are vacuum cans that pull in 10, 12 or 14 degrees of advance. Some of these are even adjustable, so they can respond earlier or later to the same amount of vacuum. It can take a considerable amount of time to sort out the best state of tune with either system, but it is worth the effort. It is the differance between the guys who get 8 or 9 mpg or the guys who get 14 or more. If you don't know how to change the curve in the dizzy, get someone else who can. It might cost you a few bucks, but it IS worth the effort to get it done. Now there are some who believe that just because your truck cam from the factory with ported vaccum that somehow makes it the correct way to go. Incorrect thinbking, I believe. Lets suppose we have a '75 390 powered pickup, built in kalifornia, and we just removed it from a time capsule. To meet the Kali emission standards of the day, that truck will have 7.8 to one compression and thermally disabled advance and some God-awful logs for exhaust manifolds. Just because that stock setup was the best Ford was able to do at the time, under those regulations, is that any reason for us to leave it that way ? Think about how lots of 390 powered trucks struggle to get more than 10 mpg, and earlier 352 powered trucks with more compression and manifold vacuum get 14 or 15 mpg easily. Which do you want ? My 71s both came with 2 barrel carbs and points ignition and cast exhaust. Both now have aluminum intakes, 4 barrel carbs, headers and capacitive discharge ignitions. Just because all three car companies tried some weird stuff to meet emissions, and it made our engines run less than their best, that is no reason to continue those past mistakes. Keep in mind that i love all of my FE brothers and sisters, but those of you who think ported is the way to go are wrong, it really is just that simple. Now if you'll excuse me, i am going to go and get some halon and put on my nomex suit in case there are any relpies. DinosaurFan, on the work computer

 

How the hell can an engine knock on ported and not on manifold? The ported vacuum will always, in all cases, be less than or at the most, equal to manifold vacuum. But never more. It's just not possible to knock on ported and not on manifold. I think someone is confused on which port is which.

 

Bear, it depends on where in the carb's venturis the opening to the ported vacuum are. If you took a sloppily made carb, like an unfortunately high number of Holleys, the opening for the vacuum ports is in throttle body. If the main venturi is just a bit smaller than the throttle bore, right where the two meet, the air sweeping past the vacuum port can sometimes pull a greater vacuum than there is in the intake. This would be most likely to occur in a situation where you had the throttles mostly open but the engine was working very hard. Like if you almost floored it when climing a long steep hill. The greater sensitivity of manifold vacuum is one of the reasons I prefer it. DinosaurFan, on the work computer

 

Dino, trust me, you're not going to win this one...

 

Gee, I guess I just don't understand Holley 4150/4160 carbs at all. But I don't believe I ever saw one work the way you say happens in over 45 years of tinkering with Holley carbs. Oh yeah, and when I worked with a Holley tech way back when in the late 60's and early 70's, he disagrees with you too. But then what did he know. Oh yeah, If you are at WOT and working the engine hard at lower RPMs, the vacuum would be very low, so low as to not be enough to not advance the ignition at all. Where did you get the idea Holleys were poorly made? Must be an Edelbrock/Carter or Q-jet guy.

 

Bear, soooo you admit to not understanding what is going on here ?! Yes, I know sarcasm, but you walked right into it didn't you ? Ported vacuum can and does exceed manifold vacuum in the case I described. Just because most folks, including lots of Holley employees, don't realize what is happening or how it is happening doesn't make it untrue. If you doubt, take your own advice and get a vacuum gauge. Better yet, make that two of them. Hook one up to manifold vacuum and 'T' the other into the ported line. With both of the gauges ready, have a copilot watch them as you accelerate hard. You'll see the manifold vacuum drop down and stay down as you accelerate, but the ported gauge will show little or no vacuum at idle and will rise with engine rpm, because of the air flowing past the opening in the carb throat. Go ahead and test me on this one. And yes, Holleys have been very sloppily made. For while in the late 70s early 80s, they were downright embarrassing. I believe they were resting on their laurels too much. I rather like the holley design, because they are easy to take apart and modify. But there was quite awhile where their quality was not what it should have been. That is why we now have Demons and C&S and Braswell and Proform all making new 'Holley style' carbs. If Holley had maintained their earlier quality, all of these new players would not have had to begin making their own parts. And yet they all have. Ported vacuum advance was and remains an aberation, a crutch, if you will, that the manufacturers used for a period of time while trying to meet increasingly strict emission standards. Power, mileage, and drivability were not yet a concern. 'Thats my story and I'm stickin' to it' DinosaurFan

 

The only thing I walked into is another self proclaimed uneducated expert who hasn't got a clue of the physic and mechanics involved or the history of ported vacuum advance. The only reason those other carbs exist is to make money for the guys that sell them. Strange that you think vacuum advance is an emissions thing, since it showed up in the early 1950s, long before any one had thought of emissions, as a gas mileage enhancer and nothing else. You can't even look up history right so how did you become a carb expert? Your story is the same old one of, "Don't confuse me with the facts, I already made up my mind!". Go away you are beginning to bore me, like the last several self proclaim experts like yourself.

 

If he's so wrong, why not just go do the test and prove him wrong?

 

First off manifold vacuum will always, in all cases, give you full vacuum advance at idle. Which means you will have to set your initial advance extremely low to compensate for this additional advance at idle. When you step on the throttle to accelerate the mechanical timing will be to low and cause poor acceleration along with poor gas mileage. Rusty, why should I do a test, that a guy that thinks vacuum advance is an emissions thing, recommends? He is clueless. The only vacuum thing with timing that had anything to do with emissions was the dual diaphram to retard timimg and that was a stop gap measure that lasted only a few years.

 

Bernoulli's Laws are a good read.

Air flow past a small hole (port) will induce vacuum. The higher the flow, the higher the vacuum in the port.

Otherwise, "ported" vacuum is useless.

I would assume that if the air flow is fast enough, the ported vacuum will exceed manifold vacuum. Dino is correct in this.

As to the finer points of what vacuum advance does to emissions, and what changes were made BECAUSE of emissions, well, unless we have the exact engineering documents from Ford, and the documents that explain the intent, it's all conjecture anyway.

Obviously, vacuum advance existed before very many emissions laws came into effect.

It was obviously twiddled with when emissions became more and more of a concern.

Now, who knows if it was twiddled to change emissions. Or because of changing emissions, HAD to be twiddled with to make it run right?



Which came first, the chicken or the egg?

 

This may not have been the way to do things, but here's what I noticed in my testing. Here's what I did. I tested it both ways, without changing anything other than vacuum routing and idle speed. The motor used was my 390 (with 343941 cam).

I had my timing set at 10 BTDC, with the vacuum advance disconnected.

I did have a vacuum gauge on the dash. This was connected to full manifold vacuum.

I noticed that when connected to ported vacuum, the manifold vacuum was down around 9-10. It idled rough, like it had a big cam in it.

When the vacuum advance was connected to manifold vacuum, the vacuum gauge read about 16-17. It idled smooth, like a stocker. I had to set the idle speed screw down to get it to idle at the 800rpm I wanted it to.

When comparing driving, other than the idle at a stoplight, I noticed no change. When driving on a flat, level road, the vacuum gauge read about 14-15 in both cases. WOT acceleration was unaffected.

These are my observations. Might I have noticed a difference on a dyno, that would have came out in favor of ported vacuum? Maybe. But for me, full manifold vacuum to the vacuum advance unit just seems to work better.

Edit: Krewat, you posted as I was typing! I can see where ported may have an advantage. At WOT, as the RPM's come up, it may cause the vacuum advance to add a little timing. At high rpm's, after the mechanical advance is done, this may provide some benefit.

 

Bear, you're slipping here my friend. Used to be you were able to discuss very articulately before the character impugning began. Where did I claim to be an 'expert' ? What makes you think I am any less educated than you are ? What if you found out I had a degree in physics ? Would that make either of us smarter or more knowledgeable than the other ? Would that make my arguement correct ? Since when is a degree or test any proof of thoughtfull ability anyway ? Thankyou Mr Krewat for correctly understanding the Bernoulli principle. And Bear, Just where did I state that vacuum advance is an emissions only thing ? If you read carefully, you'll note that I stated that PORTED vacuum advance was the abomination. I never made that claim against all vacuum advance. Cheers. DinosaurFan And yes, I am also old enough to be treacherous!

 

I believe this is a quote of your statement earlier in this tread. That's where.

 

Bear, look at the quote you pulled from my post, it is right there, it is PORTED advance I have no use for. We can't have a discussion with fine nuances if we don't pay attention to detail. DinosaurFan

 

Who hid my darn popcorn?