Vacuum advance tuning

[Anonymous]

My other thread was getting a bit long, and diluted. Can somebody please tell me the normal level of vacuum at the intake log on a stock 200? I am getting way too much vacuum advance with the new carb I put on, and from what others say it has nothing to do with fuel mixture (I know the carb is too lean)

With 12 deg. initial advance and the vacuum advance hooked up, Vac. Gauge reading at the intake log was about 19in or 48cm. all the way from 1,000 rpm up to 3000 rpm (this gauge has two series of numbers on the dial - cm. of mercury and inches of mercury, I’m not sure which one to go by so I'm posting both numbers). That was with the new YF carb- did not check it with the old 1100.

If that is a normal level, the only way I can run the new carb is if I somehow adjust the vacuum advance. Will an aftermarket adjustable advance unit intended for a small block ford V-8 fit on my 6cyl distributor? If not, is there another way to adjust it? With 12 deg. of intitial advance, total advance with the vacuum hooked up is 50 deg. by only 1500 rpm! For comparision, with the old autolite 1100, total advance was about 32 deg. at 3000 rpm.

At idle, it took 10in/26cm of air to advance total timing to 40 degrees with the vacuum pump. With the engine not running, looking at the distributor plate and mechanism with the cap off, it took 12cm/20in to advance it all the way until the plate stopped rotating. I don’t know if this matters- just thought I’d report my results. Thanks,

Thor

 

[Anonymous]

Seems right for idle...

I get about 18.5" at the log with the neginge warmed up and idling smoothly at about 750 rpm or so. 14 degrees advance...pertonix 1, hot coil, MSD-6A ignition...autolite 1100 from Pony Carbs.

sounds like you have weak springs in your vac advance...50 degrees by 1500 rpm is a lot overall.

Cheers,
Steve-O

 

[MustangSix]

18-21" is normal. You'll maintain that level of vacuum under no-load conditions, so simply revving the engine while in the driveway doesn't drop the readings.

At WOT under load, vacuum should be near zero. It will be somewhere around 16" at a steady 60 mph cruise.

On a carb with ported vacuum, the reading should be zero at idle. The vacuum slot is not exposed to the manifold. That allows the distributor to deliver only the initial advance required for idle speed. As the throttle blade opens, the slot is exposed and the vacuum increases, but drops as engine load increases. It will increase again as load decreases.

The centrifugal advance is speed dependent. It should max out at 3000 or so rpm.

You may well have 50 degrees of total advance at no-load, high rpm operation (initial+mechanical+vacuum) such as coasting, but the vacuum advance will retard under load.

 

[Anonymous]

You may well have 50 degrees of total advance at no-load, high rpm operation (initial+mechanical+vacuum) such as coasting, but the vacuum advance will retard under load.

Is this now happening because I have the right carb for the '68 distributor instead of the old 1100 with the scv? I'm assuming the vac. line on the YF is ported vacuum as on the 1100, although I was getting no vac. reading at all on the 1100 at any rpm.

If so how do I a proper check for total advance? The article I saw in Mustang and Fords recently said to rev the engine to about 3,000 rpm and total advance should be no more than about 35 deg.

As for the vac. advance canister is concerned, does the V-8 fit the I-6 distributor so I can get an adjustable one? I haven't seen any listed specifically for the six- no surprise

 

[goinbroke2]

Mustang6...now you got me thinking! The ported vac does get manifold vacuum when the throttle is first cracked, but as the rpm and load goes up, the port is situated so that it's operating off of air flow. In other words, the siphon effect of fast moving air going by the exposed port causes a draw which we see as vacuum. The intake may only have 2" of vac under load at 5000rpm, but the air flow by the port maintains a higher vac reading.
I've never hooked up a manifold vac guage and a ported vac guage at the same time on an engine on the dyno to prove this, but when I set total advance on a race engine to be total of 34 or 36 or whatever works best, I only use initial and mechanical for obvious reasons. But when building hot street engines and running them on the dyno, I use the vac advance as part of the equation, and if it detonates, I limit the vac advance. Because this works, I naturaly assumed this is how it worked. If you've got proof otherwise, please share. I'd be very interested in any tests you've done.
In my experience though, if you've got 50 deg total, at any time, you're going to have problems.(except hemi's on alchy or nitro where because of the slow burn rate sometimes require up to 65 deg advance!)
Seriously though, if his total is 32 or 35 or whatever without the vac hooked up, I'd run it with it unhooked.
If the timing goes to 50 as soon as the line is hooked up, you've got manifold vac and that just won't work.

Man, after all that...I'm still thinking...I do so much race engine stuff and limited street stuff you've got me going over and over stuff in my head. If I had something carbed that's drivable in the shop right now I'd go for a drive to prove it one way or the other. I'm not into ego where I have to be right, I just want to find out what is right, regardless of who it is. A buddy has a carbed truck (with a 300 of course) and maybe tomorrow I'll convince him to do an experiment with it.

 

[Lazy JW]

I've never hooked up a manifold vac guage and a ported vac guage at the same time on an engine on the dyno to prove this,

If the timing goes to 50 as soon as the line is hooked up, you've got manifold vac and that just won't work.

Ported vacuum and manifold vacuum are virtually the same on my Carter YF mounted on my 300. I just hadda know Some folks do use manifold vacuum and claim good results. I'm just gonna have to try it myself, I reckon. Ya know, in the interest of science and the betterment of mankind
Joe

 

[Lazy JW]

If so how do I a proper check for total advance? The article I saw in Mustang and Fords recently said to rev the engine to about 3,000 rpm and total advance should be no more than about 35 deg.

As for the vac. advance canister is concerned, does the V-8 fit the I-6 distributor so I can get an adjustable one? I haven't seen any listed specifically for the six- no surprise

Yup, 35 degrees WITHOUT vacuum should be about right. I dunno about the V-8/I-6 canister though.
Joe

 

[Anonymous]

Mustang6...now you got me thinking! The ported vac does get manifold vacuum when the throttle is first cracked, but as the rpm and load goes up, the port is situated so that it's operating off of air flow. In other words, the siphon effect of fast moving air going by the exposed port causes a draw which we see as vacuum. The intake may only have 2" of vac under load at 5000rpm, but the air flow by the port maintains a higher vac reading.

As the load increases, you step into the throttle openning the plates further. Granted, more air is now flowing thru the venturi, but the air has a larger cross-sectional area exposed because the plates are open more. Therefore, when the plates are open, the velocity of the air is less than when the air is trying to squeeze around the edge of the throttle plate that is almost closed at idle. Throttle plate position and the proximity of the port to the plates controls the amount of vacuum signal obtained. Under the closed plate, as the intake valve opens, the manifold volume is exposed to a lower pressure cylinder volume, and the manifold volume expands to fill the cylinder dropping the manifold pressure. Crack open the throttle plate slightly, and you are choking back the amount of air that can enter the manifold, so the pressure waves are relatively strong even at the immediate area above the throttle plate. Open it further and the air volume above the plates is allowed to expand and fill the manifold, and the pressure in the manifold increases.

Doug

 

[Anonymous]

Will an aftermarket adjustable advance unit intended for a small block ford V-8 fit on my 6cyl distributor? If not, is there another way to adjust it?

I don't think a SBF vac advance would work, the dizzy rotates counter-clockwise on a SBF, and our 6's turn clockwise... so the arm on the vac advance has a different curve (shape) to it...

I know when you get a new vac adv for a SBF from most parts houses, they're adjustable... with a 3/32" allen wrench inside the vacuum port... IIRC

I dunno if you could be a 6 cyl one like that, I'm curious myself

 

[Anonymous]

Manifold vacuum hooked to the distributor = 40 deg. total advance at idle on my 200.

It sound's from everyone's comments that I should leave off my vacuum line altogether. But doesen't that hurt low speed performance? I know 50 degress will do more harm than good, but the first time I drove it with the YF on and the vacum line hooked up- ya-hoo!!!! Off-line performance is not like that now- I hate to think my only option is to leave it unhooked....

The aforementioned article in Mustang and Fords (april '04 issue, p56) say's that adjusting vacuum advance in original style distributors without the adjustable allen key can can "d or remove shims that are against the return spring inside" Where is this- below the plate the points mount on? Would I remove or add shims to retard the advance? And where would I get more if I need to add them? I appreciate all the knowledge you guys have, but I don't seem to be coming up with a solution here :roll:

 

[Anonymous]

Running without the vac advance will hurt fuel economy and performance.

I don't recall where you have ever stated drivability problems due to the advanced timing. You previous thread stated that you could not tell if it was pinging. And you mention it had good pickup with the vac can connected. Do you have a problem that needs to be fixed? I agree that 40 deg at idle sounds high, but is it pinging under load? Has the timing mark with relation to piston TDC been verified to ensure the balancer has not slipped? Another distributor may fix your problem of too much vac advance. It is possible that it was rebuilt improperly.
Doug

 

[Anonymous]

I had another distributor on there with the 1100 and had the misfiring, but never had it on with the Carter YF. I know the 1100 was too lean as well, wich I have now rectified. To many variables to tell, but I know that I only have that much advance with the carter- I've never gotten 50 deg. with the 1100 and either distributor

 

[Lazy JW]

It sound's from everyone's comments that I should leave off my vacuum line altogether. But doesen't that hurt low speed performance? I know 50 degress will do more harm than good, but the first time I drove it with the YF on and the vacum line hooked up- ya-hoo!!!! Off-line performance is not like that now- I hate to think my only option is to leave it unhooked....

Correction; I only stated to leave the vacuum advance unhooked until the carburetor/mechanical advance problems are solved. Once the carb is dialed in then it is a simple matter to hook it back up. Remember, it is NOT WRONG to use manifold vacuum for the vacuum advance source as long as it doesn't cause problems. Some folks like it because it can help give quicker off-line throttle response, and it does help keep engine temps down. For anyone in hot climates it is something to consider. Ford used ported vacuum to keep engine temps HOTTER for emmisions reasons. As long as it doesn't advance so much that it causes pinging at part throttle it should be fine.
Joe

 

[goinbroke2]

"Therefore, when the plates are open, the velocity of the air is less than when the air is trying to squeeze around the edge of the throttle plate that is almost closed at idle."

Nope, that is wrong.
Your velocity of air (the volume of air entering the engine) picks up as you increase throttle angle. The only time this will decrease is when the engine reaches max rpm under load, and further cfm is not required. For example, an engine on a steady state dyno loaded to 3000rpm, you open throttle and it pulls hard to it's peak torque and cfm requirement and after that the engine "lays over" and torque drops off.(albeit only slightly) An extreme example would be a 1100 dominator carb on a stock 200, going up a hill, or on a dyno it would pull harder and velocity would increase as the throttle was opened until you hit a certain point. Then it would start that "moan" and rpm would drop as you opened the throttle further because vac is basically non existent and velocity is dropping.

Besides all this, if you look at the chart mustang6 put up on spark control systems, and read it carefully, what it says in simple terms is;
The engine is running with full vac advance calibrated by a counterspring.When you accelerate the manifold vac drops and instead of losing all advance, it uses venturi vacuum to limit the loss of advance.

So, driving down the road the engine has full manifold vac and 40 deg advance for example, you step on it and drop to 5" of man vac. Instead of dropping back to 15-25 deg (initial and mechanical) you instead have 32 for example(initial+mechanical+a couple degrees of vacuum)

This is obviously different then vac advance used on later vehicles which were designed to use ported vac only and did not have a "counter spring" to counter manifold vacuum but instead are sprung to react directly off of ported vac.

Lazy, I'm not quite sure how an engine would work so well with manifold vac hooked to a modern vac advance unit because the spring inside the canister is calibrated to operate under less vac. As soon as man vac is hooked up, it would go full advance, and any amount of throttle (or drop in manifold vac) would retard (or more correctly decrease) advance.
But,
1) Anything can be cobbled to work a particular way
and
2) I've also seen engines do stuff that "isn't supposed to work" but actually work great.

 

[goinbroke2]

Also read about load o matic tuning, mustangsix writes;

"You "can" install a later distributor with an 1100 carb, but the venturi advance will add vacuum advance at speed and it may be too much, since the later distributor was designed to use ported manifold vacuum only. Installing a newer carb with a Load-o-matic distributor should be avoided since the carb without a venturi advance provision will not be able to provide enough advance at high speed/load."

That kinda says it all eh?

 

[goinbroke2]

66 fastback 200,
Ever heard of the saying, measure twice but cut once? It should also be read twice and answer once!
I reread what you stated about air in close proximity to an almost closed throttle valve. You are correct, in that situation, the velocity of the air trying to get around the butterfly would be very high, and would in fact decrease as the plate was opened.(less restriction on the air entering).
I read it fast and was talking about the air velocity entering the engine in general.
Besides all that, this carb is operating with venturi vacuum not ported vacuum. Venturi vacuum will be almost zero at idle and increase with the volume of air entering the engine.
I think the two posts at the top of this page would to stop all this confusion if we read them first!

 

[Anonymous]

goinbroke2,
You are right on both counts. I was referring to the air flow over the ports near the throttle plate just as you are moving off idle and the rpm's are low. I should have been more clear. And as you correctly pointed out, air flow and velocity by those ports would increase as the engine rpm's increase. I was not thinking about the venturi ports because I thought you were thinking about manifold vacuum sources versus ported vacuum sources. I wasn't thinking about the Loadamatic, but rather the majority of distributors.

Your comment about hooking up a manifold vacuum to a more modern emissions era distributor that was designed to run on a ported source is spot on. When I bought my 66 Pontiac, someone had installed an HEI distributor of unknown origin. It advanced the timing way too much. I changed the vacuum cannister and springs and it got better, but it still pinged at times and was doggy. An automotive engineer on another website told me that the emissions era distributors were curved to use more total vacuum advance and less centrifugal advance than pre-emission distributors. This all tended to run the overall timing more retarded (less advanced) throughout the rpm range. This increased combustion temperatures and reduced hydrocarbon emissions. Once I replaced the HEI with a pre-emission distributor, the Pontiac ran much better.

I enjoy these type of discussions since I have to scratch my chin and think about it for a while. Thanks for the education.
Doug

 

[Anonymous]

Lazy-

it is my intention to leave it unhooked when I get the YF put back on, only until I get the problem of too much vacuum resolved. But it sound's as though I'm just plain getting too much vacuum from it and there's not much I can do, so I may be SOL.

goinbroke2-

I feel like I'm repeating myself over and over :roll: I know about the carb/dist. mismatch issue. The only reason I'm running an SCV 1100 with my later distributor is that the 200 engine I just bought had the wrong carb. I then got the proper carb Carter YF carb for it, and that's when all this trouble (too much vac. advance) started!

There's 2 or 3 threads going about this so I'm getting confused, but I have tried manifold vacuum and it's way too much- 40 deg. of timing at idle! Again, this is with 12 deg. of intitial timing. Manifold or or carb- either way I connect the dist. vacuum line I am getting way too much advance. It's not the distributor- it's the (YF) carb.

Since we are reading from the bottom of the thread :roll: (sorry, sacasam starts to take over after dealing with a problem for 2 weeks- it's nothing personal!), I'll quote myself regarding the main concern (how to deal with too much vacuum) from 3 posts back;


"The aforementioned article in Mustang and Fords (april '04 issue, p56) say's that adjusting vacuum advance in original style distributors without the adjustable allen key can can "add or remove shims that are against the return spring inside" Where is this- below the plate the points mount on? Would I remove or add shims to retard the advance? And where would I get more if I need to add them? I appreciate all the knowledge you guys have, but I don't seem to be coming up with a solution here"

 

[wsa111]

Sedanman, seems like everybody has taken a shot at your concern.
If you have 68 distributor with the original vacuum advance canister, take a wrench and remove the end where the vacuum hose fits. Unscrew the fitting & expose the spring & spacer. you can add washers to limit the vacuum advance travel. You will have to experment.
If the canister is sealed you can try to adjust the advance with an allen wrench. See which canister you have & go from there.
It is common to have anythere from 15-20 crankshaft degrees of advance from the vacuum unit. At idle with a timing light attached just hook up direct manifold vacuum to the canister & see how much advance you are getting. Adjust from there, your choice is direct manifold vacuum or use ported vacuum as explained by jack. Any questions just give a shout. William

 

[Lazy JW]

#1. The ported vacuum on my YF is virtually identical to manifold vacuum except at idle when the throttle plate is closed. It will therefore give identical advance characteristics except at idle.

#2. Ford deliberately used ported vacuum to retard the timing at idle in order to give hotter combustion temps at idle for a reduction in unburned hydrocarbons. They even had the infamous "dual-vacuum advance" distributors that pulled in even more retard for the same reason. Bleah!

#3. If a mechanical/vacuum advance distributor is used that is properly calibrated for about 15 degrees of vacuum advance and is set for an initial advance of 10 degrees, plugging in to manifold vacuum will give 25 degrees at idle. This will also speed up the idle somewhat. Adjust the curb idle down, voila! Cooler idle temps while burning less fuel. Open the throttle and presto! you are on the same advance curve as with ported vacuum (i.e. low vacuum at full throttle/high vac at part throttle) Doesn't seem too outrageous to me.

#4. I have chosen to use ported vacuum on my 300 for the same reason Ford did; to give quicker warmup. In a hotter climate I would try manifold vacuum, if I didn't like it I would switch back.

#5. Results may vary
Joe