Gotta share some good news. I've been fighting pinging on my 351 ever since I put it in. Not a lot, but more than it should and I've been running 93 through it. Well, just had time to analyze and with some help from someone on the board who explained ported vs. manifold vacuum on the distributor advance. Well, he said that running directly off the manifold vacuum would tend to lean out the carb. Bingo, I think that nailed it.

Also, I did a timing check. I set my timing at 8 BTDC with the vacuum disconnected. Popped on the vacuum from the manifold and WOW, jumped to 28. I swapped to the ported vacuum and rechecked, stayed at 8. So...then I check total advance and found that I was getting a total of 40 that peaked at around 2000 rpm (8 32degree advance = 40). I didn't check the total advance off the manifold vacuum and I doubt I advanced much over the 40 degrees total, but what a difference on the bottom end. Feels like a different truck!!!

I'll let y'all know how it affects my mileage.

'82 F150 w/351W (orig 300 I6)
Slik

 

I thought all vacuum advance units were supposed to be connected to a ported vacuum source anyway. It doesnt make since to have it on the manifold.The manifold has higher vacuum at idle so it would already advance the timing at idle and when you give it gas the manifold vacuum decreases. Just wanted to make sure im not missing something.

 

I have been trying to sort out this one myself. From what I can gather, both sources of vacuum have been used for the vacuum advance, it depends on other items as to which is the correct one. Before the advent of emission controls, when performance was the name of the game, manifold vacuum was used. As emission controls tightened and manufacturers had to add lots of gizmos and miles of vacuum hose under the hood, ported vacuum was used in conjunction with these gizmos to reduce emissions.
My personal transportation vehicle is an 89 F-150 with a 351W with the Edelbrock performer package including heads. I am running around 10 to 1 compression and experienced pinging under part throttle conditions until I recurved the centrifugal advance on my Duraspark distributor. All the cent. advance (25 degrees) was in by 1500 rpm, which is way too much when combined with the initial 10 degrees I had dialed in. I ordered Crane Cams adjustable Vacuum advance canister which also comes with springs to modify the centrifugal advance curve. I am going to tinker with it a little more, but right now with 12 degrees initial timing, I have a total advance of 37 dgrees at 2500 rpm. I am running vacuum from the manifold and are no longer experiencing pinging even under hard lugging conditions. I am running 93 octane, but it could be I got hold of a real good tankful because I have noticed my pinging will vary slightly from tankful to tankful. I am getting around 12 mpg. It's a 4x4 with 31x10.5-15's and 4 sp tranny so I think that is fairly decent for the combo I have. I have looked for articles on this subject dealing strictly with High Performance Fords, but haven't had any luck. Since principles of internal combustion engines are the same regardless of Manufacturer, I relied on info gleaned from tech articles for the 350 Chevy. Come on now, it's not a dirty word we can learn alot from the other camp sometimes. One can never have too much knowledge. Well thats about it. This is a subject that I am very interested in because timing has a huge impact on economy and performance. I eagerly await further posts.
Danny

 

I read a tech article that talked about the manifold vacuum vs ported and basically was told that the Ford engines like a lot of vacuum at idle and should be run off the manifold vacuum. Other Ford folks on this board told me I should run on the ported, but couldn't really tell me why. Until I read the following posted by Gary Peters:

"You can actually use either one but manifold is simpler in many cases and some after market carbs simply don't have a port for this. The distinction is the fuel mixture. You have to re-tune the idle circuit both mixture and speed when you change from one to the other.

A ported vac setup will allow a richer initial idle mixture so the transition will be richer also which may, in some cases, eliminate that off idle stumble we are so familiar with. The throttle plate will also be slighly more open due to the richer mixture and retarded timing conditions so there will be less "transition" when you step on it.

Ported:
richer idle (due to timing)
more idle speed screw (throttle plate more open) but same idle speed
less advance (initial only, about 10 degrees)

Manifold:
very lean idle mixture (again, due to timing)
much less idle speed screw (throttle plate nearly closed) at same speed
much more advance (full vac which makes about 35-40 degrees total at idle)"

'82 F150 w/351W (orig 300 I6)
Slik

 

Hey, I guess I'll jump in on this one,

I have a 95 302HO with a eldebrook "RPM Performer" manifold and a Carter AFB 625cfm carb. I tried 3 different vacuum sources, one off the manifold, off manifold vac. on carb., and port on carb.

Port on carb.: I got the best idle on the port, but when at a stoplight the A/C was pullin the idle down below 400rpm, the idle compensator sel. is not strong enough to raise idle speed by itself. I also had a slight hesatation from a stop.

Manifold on carb.: Didn't have any vacuum at idle, it was basiclly the same as the port side.

Manifold off intake: Hard to get a really smooth idle, but when the A/C kicks on it seems to have more power to overcome the load. No hesatation from a stop and down shifts are better.

Base Timing is at 12deg. no vacuum.

I still have a fuel delivery problem with the fuel pump and gas tanks I have not figgered out yet. It's causing my carb. problems. I'm lookin into installing "in tank" low pressure fuel pumps, I just can't understand why the front tank runs empty with 6gals. left and the rear runs dry with 11 gals. still in there. I've pulled both tanks, and removed the "socks", cleaned the tanks, with a Carter 4070 fuel pump the truck won't move from a stop, with a cheap Puralator I can drive the truck but the noise drives me nuts.

I'll shut up now,

Moe
Houston

 

It just doesnt make sense to me to use manifold vac on the distributor. That would have the timing being advanced at idle and retarding when youre on the gas which is the complete oppisite of what you want. Is there something im missing?

 

>It just doesnt make sense to me
>to use manifold vac on the
>distributor. That would have the timing
>being advanced at idle and retarding
>when youre on the gas which
>is the complete oppisite of what
>you want. Is there something im
>missing?

I think when I had a gauge hooked up to the manifold the vacuum was at 14lbs. at idle and on acceleration it droped for a split second and then went higher than 14lbs.

I did this months ago, so I could be wrong, on my Porsche it has vacuum retard dashpot that must be hooked up when setting timing, and when it loses vacuum it goes into advance with springs.

Moe
Wondering if my Ford dist. could be a hair off?

 

I do not have a How to Hot Rod the Small Block Ford book, but I do have the Chevy version and this is what it says about the vacuum advance.
"Vacuum advance mechanisms provide advance in relation to load and should really be called load compensation advance."
"Over simplified explanations merely state that burning the fuel charge takes a certain amount of time and that burning time remains much the same regardless of RPM. Because the compression stroke requires less time as RPM increases, ignition must start earlier to allow burning to be completed at an ideal time in the stroke. T'aint necessarily so! Such simple explanations completely ignore combustion chamber turbulence which hastens burning. Turbulance moves unburned mixture into the area where ignition is occurring-instead of relying on slow travel of the combustion flame front. Moving the unburned mixture into the combustion area speeds up burning so that there's really no necessity for the advance mechanism to continue operating beyond 3000 RPM or so."
"Charge density furhter complicates the picture. A light charge-as created by part-throttle operation-is a "slow" burner...needing earlier ignition."
"At light throttle, high manifold vacuum advances the spark a considerable amount. As the throttle is opened further for acceleration or to climb a hill, etc., the manifold vacuum drops off and the vacuum advance diaphragm relaxes so that the distributor assumes a more retarded position
(to combat pinging). The vacuum diaphragm mechanism supplements the centrifugal advance mechanism-which provides RPM compensated spark advance."
This is only a small portion of what this chapter contains on ignitions but you get the picture.
As you can see, this book recommends manifold vacuum. Sure it is a Chevy book, but the principle is the same. I hope you all can appreciate the effort it took me to type all this as I am a lousy typist LOL. I am sure ported vacuum has its place on certain applications, but for now, I am sticking with the manifold vacuum.
Better go now cause I have to work tomorrow.
Hope to hear from ya'll.

Danny

 

I'm just guessing here, but I would think the difference is in how you adjust your carb to the vacuum advance. I think Moe is running larger jets than I am which gives him a richer setup anyway. I know I should probably have re-jetted my carb with the headers, but I haven't done carb work yet, so...running of the ported vacuum gives me richer intake, which works well with my set up. Any ideas on combos that work together, and what is ideal?

'82 F150 w/351W (orig 300 I6)
Slik

 

I believe that changing your source of vacuum advance doesn't really change your carb jetting per se, but instead as my previous post quoting the book suggests, the fuel is more efficiently consumed at this lower density with a greater advance such as that provided by the manifold vacuum. Have you all noticed an increase in idle rpms when you connect to the manifold vacuum as compared to the ported? I believe an increase in RPMS due to this simple switch should be indicative of the engine running more efficiently with the increased advance. Seems logical to me anyway, but I have been wrong before.
Danny

 

the reason the idle would increase is because the manifold vac is advancing the timing at idle from the manifold. you dont need more advance at idle. I bet if all you guys that hooked it up to manifold vac. would change it to ported vac youd get increased gas milage as well as preformance. If you want a better idle then adjust the whole distributor or play with that carb. Im not trying to be a smart a$# but it just isnt logical to hook it up to manifold vacuum. Just try hooking it up to the ported vacuum source and see if your lower rpm power picks up and check out that better MPGs. All this is just from my experience. Just my 2 cents.

 

My rpms increased, but my advance jumped to 30 degrees and my full advance was only 40. I believe this probably caused a low torque situation which in turned caused a ping on acceleration or long pulls.

I know that the ported doesn't change the jetting, but both advance and jetting will affect the fuel mixture (rich or lean), so you could probably work between the two to really tune in the engine to your driving habits, as well as other factors. It's just like when you put headers on, you will want to re-jet to dump more fuel or you'll end up in a lean condition. Everything works together and there are many combinations that will run "right".

'82 F150 w/351W (orig 300 I6)
Slik