Low manifold vacuum?

[hunt2elk]

Some very good posts here guys. Bare in mind I am only a HVAC guy and play with these old cars as a hobby. So my next question is what does Don do to the distributors that they won't work hooked to a ported vacuum, or can they just by unscrewing the 3/32 allen head in the can?
Seems like there is no grey shady area on this discussion. 1/2 the guys are adamant that you should run ported, and the other 1/2 feel strongly about manifold vac. Meeps explanation of how your timing will go retarded as stepping on the gas makes perfect sense. I wonder why I don't notice that in my other car?

 

[superfragl]

Pretty much every distributor advance mechanism is set up to advance the timing as RPM increases. At idle you set your initial and the idea is you should have no mechanical advance action at this point. The vacuum advance provides some amount of advance at some level of vacuum and only works when vacuum is present. Manifold vacuum at idle is usually enough to pull the diaphragm to it's fully rated advance setting but what happens when you open the throttle quickly? The manifold vacuum drops suddenly and that will almost always cause the timing to RETARD right off idle and that gives the impression of a flat spot causing people to blame the accelerator pump and whatnot. So, some might suggest very light advance weight springs to avoid this timing retard during the drop in vacuum. The weights fly out quickly to try and counter the effect of sudden timing retard, which may or may not work. Or just lock the distributor to full advance less ~10 degrees and use the vacuum can to pull the rest in once you generate manifold vacuum. This might be the best application of using manifold vacuum on the advance can but at this point you have no curve and your total timing is subject to a vacuum advance can working off a manifold vacuum signal against a spring. I call this a band aid and subject to problems. In my opinion if I wanted to retard the timing on a locked distributor to start the car (without killing the starter) I'd just do it electronically and MSD makes just the thing to do this with electronic precision.

My take for a street driven car is this: Select the components that produce proper cylinder pressure with a static compression ratio that is suitable for the available fuel - usually pump premium 91 or 92. For the street you are 9.5-10.5:1 depending on the engine and heads. Set the mechanical curve for performance, usually all in by 2500 and never cheat the timing! Meaning, run the maximum advance for that engine (MoPars are 36 - 38 deg, again, depending on heads, etc..) with a curve that works. Now connect the vacuum advance to the ported source and this is the brilliant part so pay attention. I call this a mechanical computer. Why? Because it gives you added advance during the times that you need it most and is dictated by engine load and throttle position.

During light loads like flat freeway cruising when your throttle plates are cracked open slightly you want more advance than what the mechanical curve can provide at that RPM. Remember, the mech curve is RPM dependent so adding about 10 degrees of advance during that point in the curve you get a pretty crisp running engine. 45 degrees may seem high at 2800 RPM, and it is under load, but this is the brilliant part. As you roll the throttle and the vacuum signal starts to fall off, the advance can defaults to the mech curve at whatever point it is in the RPM range!!! At WOT you have no vacuum advance so no danger of over advancing your engine and causing damage. The vacuum advance is a "smart switch" and poses no threat to your basic performance inspired timing curve because your engine does not rely on the vacuum advance to run properly. It's just an enhancement for that sustained light load cruising to the next car show. If you race you will likely leave with your foot to the floor or darn close to it, right? Well, the advance can won't even come into play because the throttle blade will pass the ported port so fast no significant vacuum signal/time will be generated. Hook it to the ported port, set timing @ in/Hg, forget it and reap the benefits.

That is a very good explanation, thank you.
I did some manual reading last night and have one correction. Factory 68 manual indicates that mechanical advance will not kick in at starter cranking speed. When the engine starts, mech advance will give roughly 5-8* of advance (depending on application). As I understand, this could help starter crank easier.

I tweaked ignition last night. Ended up with 22* advance at idle and reading 14" of vacuum at 750RPM. The vacuum will drop to 11" when in gear. Starts fine with no kickback. If I bring RPM up by 150 I gain another 1" of vacuum and if I advance ignition to 26* I gain one more inch, but for now I will leave it the way it is.
I tried to achieve same advance with vacuum can hooked up to manifold vacuum and the timing would float between 18 and 24 which makes idle unstable.
Now I have to weld the distributor slots to limit the total advance. Anyone has an idea how much to weld? I have to remove 10* from the slots.

Oh, and factory manual suggests connecting vacuum can to ported vacuum source.

 

[747mopar]

Some very good posts here guys. Bare in mind I am only a HVAC guy and play with these old cars as a hobby. So my next question is what does Don do to the distributors that they won't work hooked to a ported vacuum, or can they just by unscrewing the 3/32 allen head in the can?
Seems like there is no grey shady area on this discussion. 1/2 the guys are adamant that you should run ported, and the other 1/2 feel strongly about manifold vac. Meeps explanation of how your timing will go retarded as stepping on the gas makes perfect sense. I wonder why I don't notice that in my other car?

I'm not to different, Industrial Maint - Machinist that loves tinkering with these old turds haha..... my piece and quite time I guess.
I see it like this, nobodies motors are the same and nobody has the same approach but we all eventually find our happy spot even if not by the same route. Like you I have never noticed a flat spot running manifold vacuum? Some have said it's a bandaid fix for worn parts or a "snake oil" sales? My distributor is a brand new billet Mallory that I've spend a fair amount of time tearing apart and re curving not to mention it wasn't sold to me by a salesmen so those are both out not to mention I've done the compression test, degreed the cam, tested for vacuum leaks, etc.... it's just likes it! I like these threads because they're educational, people like Meeps have been there done that but ignition timing is one thing I've found you'll NEVER get a lot of agreement on.
Can't answer your question on the can though, like I said mines a Mallory. Good luck.

 

[mentor70]

Running manifold vacuum at Idle like an FBO ignition is NOT a band aide. it is actually just a method of changing the properties in the combustion chamber to give lean fuel mixture additional time for a more complete burn....its just science...science not understood back in the 50s and 60s.

some of us, like me, have NO ported provision on the carb (Holley HP Ultra) so using this method makes sense.

 

[Banzaiii67]

this thread has already gone slightly off topic so I'll add my :2cents. I have a early FBO ignition kit with the ballast restitor and a curved distributor he did for me back in 2005. So it isn't the new stuff he sells now but I have had zero problem with running off a ported vacuum source. To add to that I haven't (knock on wood) had to replace anything with his kit. I don't quite understand this nonsense that you can't run ported vacuum with FBO distributor. One of Don's Canadian distributors even doesn't know why Don recommends manifold vacuum and suggested to run on ported vaccum (see below).

Per Dave from Dave's Mopar from the other mopar site.

I curve distributors on my machine with a secondary system built on, for many different combo's. Decent results so take this for what it's worth.

Question
why would it ever hurt, to run vac advance?

Answers

- on a more radical build, the mechanical advance curve may not respond well to more advance at cruising speed. A surge at cruise is a common result.

-On a 4 gear car that is driven hard, the resulting vacuum applied on the shifts, throws an unwanted variable into the mix.

Q Why would you simply not bother with vacuum advance?

-vacuum advance's purpose, is to make the lean cruise mixture burn more completely, giving better economy.

On cams with say 235 @ 50 duration or more, it simply doesn't help. That much cam makes the economy poor, and adding the vac advance makes no measurable difference.

- Some feel that there are performance benefits to running vacuum advance. The truth is, if your car runs or performs better with the vacuum advance hooked up, there is something wrong with the set up. Vac advance is for economy. That said, I do understand that fuel is expensive, and these cars use a lot of it, so trying to get some more with vac advance makes sense for most mild street builds.

Running manifold vacuum over ported

-It was only done by the factorys in the late 70's. By then the things going on with way the cars ran, and what they were trying to get emmisions wise from carburetad vehicles made it necessary. Makes no sense on one of these, tuned for performance (and even economy) and not emmisions

Why shouldn't you run manifold vacuum?

-I know Don at FBO, and learned many things from him directly in the area of distributor curving. That all said, I have no idea why he recommends guys run manifold vacuum.
By Don's own school of thinking, we curve the distributor to run the MOST initial timing possible, and keep the total mechanical around 34. It improves throttle response and idle quality DRAMATICALLY to do so. SO after doing that, I don't see how adding even more timing at idle is the right thing to do. Using ported, ads it only at light cruise, and THAT is what vacuum advance is for. To improve economy at light cruise. To give the lean cruise mixture, more time to burn.

Don is VERY knowledgeable, and does a great job on his distributors, BUT most of the ones he curves are for more radical builds, and he is a big supporter of not using vac advance at all.

Ported will always do what vac advance was designed for, while I don't see how manifold will

 

[cdr]

I already posted what to do, & it will run great.

 

[hunt2elk]

this thread has already gone slightly off topic so I'll add my :2cents.

Slightly off topic is fine with me. I am learning of new things to try and also other peoples opinion on something I am interested in.

 

[hunt2elk]

I already posted what to do, & it will run great.

I will try that also Charlie. Maybe I was mistaken when I thought the starter was kicking back with more timing and the engine was hot.

 

[dla4567]

My limit for good starting is 22 degrees. It does idle and respond better with more advance so this topic interests me as well. It's nice to see a civilized and intelligent discussion on the subject. (I wonder how long that will last).

 

[cdr]

I will try that also Charlie. Maybe I was mistaken when I thought the starter was kicking back with more timing and the engine was hot.

mine is 512 ci, & it has never kicked back, even when i had it at 26 initial, but it likes 24, most alum closed chamber heads like 33 to 36 total mech advance & 4 to 8 ported vac advance depending on the cam & comp ratio, mine is 10.1 & cam is a solid FT 248 @.050 it is a small cam.

 

[Meep-Meep]

That is a very good explanation, thank you.
I did some manual reading last night and have one correction. Factory 68 manual indicates that mechanical advance will not kick in at starter cranking speed. When the engine starts, mech advance will give roughly 5-8* of advance (depending on application). As I understand, this could help starter crank easier.

I tweaked ignition last night. Ended up with 22* advance at idle and reading 14" of vacuum at 750RPM. The vacuum will drop to 11" when in gear. Starts fine with no kickback. If I bring RPM up by 150 I gain another 1" of vacuum and if I advance ignition to 26* I gain one more inch, but for now I will leave it the way it is.
I tried to achieve same advance with vacuum can hooked up to manifold vacuum and the timing would float between 18 and 24 which makes idle unstable.
Now I have to weld the distributor slots to limit the total advance. Anyone has an idea how much to weld? I have to remove 10* from the slots.

Oh, and factory manual suggests connecting vacuum can to ported vacuum source.

Thanks! Somewhere there is a chart that gives the target timing vs slot length. I think lewtot posted it a while back - of course after I spent a bunch of time figuring it out! Just a suggestion from someone who done it. Tack weld with a TIG (or spot weld) 1/16" welding rod as a bridge across the slot. Minimal heat during welding will not affect the heat treatment of that part (too much) plus the 1/16" wire can be bent for minor adjustment to make sure both weights hit the stops at the same time. Yeah, I may be obsessive compulsive...

 

[Meep-Meep]

if he is running an FBO kit, he HAS to run the vacuum off the manifold, the distributors Don sells are modified to run vacuum off the manifold.

Excuse my ignorance but I have no idea who Don or what an FBO kit is. I just use the factory electronic distributor and usually end up putting Ford springs in it or use the older Direct Connection stuff. As a side note I like to keep the one heavy spring but have it more as a stop to provide the last 2 deg or so. This gives the weights some cushion so they don't have a tendency to bounce off a hard stop.

 

[moper]

Discussion and respectful debate is always good. That's why I tend to preface or include "I feel" or "IMO"...

While I agree it is science I see little reason to aim to change something that factory engineers much smarter than I, FBO, and probably most posters involved in this thread sought to keep in service from the 50s until fuel injection came into production at which point they added the timing at idle constantly varying it based on the sensor inputs but also kept the load based timing changes. Manifold vacuum as an advance source is not a better mousetrap. On a Mopar it's a different mousetrap that compensates for a lack of idle quality by taking away the load-based timing control. An idle quality that is the result of mismatched parts, poor tuning, poor assembly, poor machining, or wear. That is also just basic scientific fact. It was also used for years on Fords and some GMs. But these packages were engineered in their entirety for that. If you take one of those and change it to ported you can expect there will be some changes to the overall performance.

I'm talking about a loss in economy more than anything else. For some perspective... If one runs no vacuum advance, or ported with limited centrifugal and high initial, there is a very high likelihood that their mpg is 5-10% lower than it could be. Potentially much more if the tuning abilities of the tuner are marginal or poor. A customer's 496" B wedge in an E body that makes 480hp at the tires and gets 13mpg mixed, almost 15 on the highway with 3.23s and that makes close to 600hp at the crank.
Another example - a 422" small block gets over 17mpg mixed in a Challenger. That one has 15K miles on it now. That's 3,000 fewer gallons of gas used than one that gets 12mpg. At current prices by me that's almost $7500 saved over 8 years.

Both these engines have cams reasonably mild given the strokes, mild gearing, tight convertors, and ported vacuum advances that function properly.

To me it all comes back to awareness. If the owner does not sense anything negative (I won't say "something wrong") but if the owner doesn't notice anything while in the midst of enjoying the drive - does that mean it's not there? I'm a firm believer in ignorance is bliss in some facets of my life and I've repaired hundreds of cars whose owners apply that to their antique cars.

 

[mentor70]

all points taken

but i have yet to see and evidence of negative aspects of this tuning method for street machine application.

when you mention mismatching of parts, dose this include a 1975 440 with 95cc open combustion chambers with flattop pistons that are 1/4 inch down in the bore?

and as i concur that a 1960s chrysler engineer was probably smarter that i am now, those guys didnt know jack about cylinder quench as it showed in their designs.

 

[moper]

See, I'd submit you have yet to accept evidence presented. Not that there's anything wrong with that as it's ultimately your (or any buyer's) choice.

In terms of your example - yes - when taken in the context of what it was designed to run on vs. whats available to for fuel it's a mismatch. Based on that example the FBO approach is one of several that would improve the performance of that particular deal - ALL of which are crutches to get past the mismatch which in that case is stock original design and changes in the fuels that are available.

The engineers actually did know some. At least the beginnings of the dynamics of the piston & chamber interplay and efficiency. The reason the baby hemi was built in the 50s was because of the basic beginnings of how a cylinder's chamber works done by aviation engine developers. Following those efforts in the 50s it took a couple decades for the development to get past Pro Stock and NASCAR engine programs. I think what they honestly lacked was the computer power to crunch the data that the development required. But that's a guess on my part. When my dad started as an engineer in the 60s the guys took days to crunch numbers to work on designing jet airplane turbine blades. By the time he retired in the early 2000s he was troubleshooting computerized process modeling for developing turbine blades. The processes that were computerized was staggering to him.

 

[Meep-Meep]

all points taken

but i have yet to see and evidence of negative aspects of this tuning method for street machine application.

when you mention mismatching of parts, dose this include a 1975 440 with 95cc open combustion chambers with flattop pistons that are 1/4 inch down in the bore?

and as i concur that a 1960s chrysler engineer was probably smarter that i am now, those guys didnt know jack about cylinder quench as it showed in their designs.

Actually, understanding the effects of quench is not new (goes back to early 1900's) and I bet Chrysler engineers knew all about it. The "open chamber" head such as the 906 is still a quench head because it has that area away from the plug that is close to the piston. The quench area is not as tight to the piston as the "closed chamber" head but the swirl effect is still present. I'm reasonably certain the open chamber head was designed to burn that bit of fuel mixture that is otherwise shrouded in the closed chamber head. It's about reducing hydrocarbon emissions, which is not necessarily a bad thing. If they are in there we might as well burn them, right? The hemi is NOT a quench head and the main reason for its existence is port flow at high RPM, which means HP. How else would you stuff 2.25" x 1.94" valves in a 4.25" bore?

 

[mentor70]

well, i would challenge that idea. IF they understood it fully, they would have implemented design changes. but it also could be blamed on Chryslers financial issues in the late 1970s. the only other thing that i could support that with is the fact that it wasn't until the purchase of Lamborghini in 1987 that there was a radical change in cylinder head development across all Chrysler engine platforms.

 

[Meep-Meep]

well, i would challenge that idea. IF they understood it fully, they would have implemented design changes. but it also could be blamed on Chryslers financial issues in the late 1970s. the only other thing that i could support that with is the fact that it wasn't until the purchase of Lamborghini in 1987 that there was a radical change in cylinder head development across all Chrysler engine platforms.

A pretty good rule of thumb (for business in general) is car manufacturers do things because they have to not because they want to.

 

[ChargerST]

What about using a CAP/CAS distributor vacuum advance control valve?

Apparently it gives you the best of both worlds or vacuum sources (ported vs. manifold):
Taken from Allpar:

This valve is a pneumatic logic gate – a very simple mechanical computer. It advances the timing under closed-throttle, high-vacuum situations: deceleration or coasting with your foot off the accelerator. As long as the manifold vacuum is equal or less than that from the spark advance port, the distributor vacuum advance receives an ordinary ported vacuum signal as though the valve weren’t there at all.

When the throttle is closed, the spark advance port is above the closed throttle plate, so it produces no vacuum signal. But manifold vacuum is very strong under the closed throttle plate above idle speed, so it overcomes the valve’s spring tension, and manifold vacuum is sent to the distributor vacuum advance, advancing the timing.

Full text: http://www.allpar.com/fix/engines/cap-valve.html

 

[Darthomas]

I see your cam timing @ 50, but cannot compare to "stock" 383 cams.
Is this a manual or auto?
If you can crank the engine at 20 degrees advance, I'd say there's something amiss. An engine with good compression should be having a HARD time cranking with that much initial timing, unless the cam is too much, or installed in a slightly retarded position. You're quite sure the cam gear marks lined up correctly?
I put a slightly hotter cam in my 66 383-4bbl, the DC '68 Road Runner cam.
Since I did not want to lose bottom end response, I put in a DC 5 degree offset cam key, and advanced the cam (and ignition) all at once, to compensate for the change in timing.
The best way to have lightning quick throttle response off idle is to have high manifold vacuum, and the hotter the cam, the more it degrades low rpm breathing efficiency.