questions about vacuum advance and load

Knebel

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I am not sure how to properly ask this but can someone explain to me, lets say I have a mechanical timing curve that is only rpm dependant, how do i know how many degrees the vacuum adance should add and when?

In Theory:
If i break this down, a regular distributor has so to speak different "steps" of advance. anything above idle, when the throtte opens is purely mechanical advance until it maxed out at "all in at 3000rpm". We reach this point and back off the throttle and the vacuum goes up and adds more advance to it.

no vacuum advance connected would that look like: idle 18° - 1500rpm 20° - 3000rpm 34°...or something to that affect

lets say I add vacuum adance to the picture, I know that I want to reach 40 or so degrees at cruise, which would be the all in at 3000 (34° plus 6° vacuum). But...how is the mechanical plus vacuum determined at rpms below "all in rpm". Lets say we are just above idle or cruising at 1500rpm and are having 20° mechanical timing, should the vacuum advance only be adding...2... 5° or whatever it takes to reach the 40+°? If in theory it would be adjustable that high?

What is the optimum timing at rpms below the 3000rpm threshold while no load is applied, which of course will have lower mechanical timing? Would you aim at 44° while coasting or cruising at any engine-speed below 3000?

would a cruise/coasting situation look like: idle 18° -off idle rpm cruise 44° - 3000rpm cruise 44°?

how do I determine how many degrees the vacuum advance SHOULD add between maxed out and not activated at certain rpms? Im trying to understand a little how a vacuum advance is normaly working and if (in building a timing curve) it can be improved with the abilities I have in my EFI. I am focusing on cruise timing values and am very unsure what to do about low load/low rpm situations. So think about it this way, if you could punch a number into the vacuum advance and it would add whatever number you put in, what would you do?
 
What's your engine? Stock, built and if so what specs?
 
Its a smog era 360 with an edelbrock intake and dual exhaust. I'm not sure about cam, I get about 8.5" of vaccum at idle in drive, so maybe a mild cam.
 
Back in the middle 1970s, I frequented a local Exxon service station. I came to know the man who ran it (he had a son I was in school with, at the time). I found some Exxon Service Manuals one day. They were like a Motor Manual of sorts, with all kinds of specs and information for different models of vehicles. All of which might be vehicles that might come into his service bays.

At this time, it was somewhat common for mechanics to seek to "alter" the emissions settings of newer vehicles of that time. One day, I was looking in the "Tune Up" section and noticed something I'd never seem before. "Total Advance @ 2500rpm". This would be an rpm where the mechanical advance would be working AND "no load" (for this spec) vacuum would be high enough to max out the vacuum advance capabilities.

I'd already known about the 30-40 degrees WOT advance that Chrysler B/RB and small block Chevy engines liked for best power, But this would be pure mechanical advance as there should be NO vacuum advance at WOT. The "2500rpm" specs listed were all in the 50-55 degrees BTDC range. MORE than I'd ever suspected! But then I realized that with the "partial charge" the cylinders were getting, with manifold vacuum in the range of "over 18" Hg", the weaker air/fuel charge took longer to get fired off, so the "more lead" was needed. So, after I thought about those dynamics, that way-higher part-throttle advance made more sense.

Look at the4 vac advance specs in the FSM for your car. You'll probably see that the advance starts at like 8" Hg (WOT should be less than 5" Hg) and maxes out at about 14-15" Hg. On a level-road-load situation, I'd suspect that the engine will have MORE vacuum than 15" Hg.

I might add that mechanical advance usually starts at about 900rpm, so that the slower fast idle speeds will not really affect base timing. Plus that it goes up in something of a "smooth curve" rather than "steps", per se.

The 38-40 degrees BTDC total timing spec is at WOT only, not part-throttle. Regardless of where the mechanical advance is maxed at a particular rpm (2000, 3000, 4200, etc.).

The amount of vacuum advance in a particular unit is usually stamped somewhere on the arm which extends to the breaker plate in the distributor, as assembled. A little more advance might be had by filing on the "stop" for the arm, but this is a cut/try situation. I suspect that most current replacement vacuum advance units, other than the stated amount of advance stamping, have an internal adjustment for when the advance starts.

Insert an Allen wrench into the vacuum port on the vac advance "can". See if it indexes with an internal Allen nut as you gently turn it in the port. If it does, then the unit is adjustable. Clockwise should tighten the internal spring, counter-clockwise should decrease spring pressure (allowing for quicker vacuum advance operation and reaching max advance quicker).

Use a "dial" timing light so you can check the advance at particular engine rpms. It's quite easy to do. Use an accurate tachometer to check engine rpm. Then you can see when the mechanical advance starts and how it progresses up to about 3000 engine rpm . . . CAREFULLY! Put the carb on the top fast idle cam location. Check the rpm and related mechanical timing advance (vac advance plugged). Then unplug the vac advance line and install it on the vac advance can. The engine rpm should increase with that ONE change! Then note the total timing in that situation to see what it is. That could well be the "Advance @ 2500 rpm" spec.

Tune the engine for WOT such that you get the desired mechanical advance at WOT with the FSM spec base timing, maybe add 2.5 degrees more base timing, so that it doesn't clatter at WOT. Then on a highway cruise, when you go up hills and maintain speed (as with a cruise control operating), there should be no clatter there either, although "trace rattle" might be allowable. IF there is clatter in that situation, adjust the vac advance spring to more tension to keep the vac advance a bit less than it was. IF you get a trace rattle, you might try using a higher octane fuel to see if the higher octane decreases or eliminates it.

The vacuum advance is necessary for the best cruise and general fuel economy AND drive-around throttle response. In the '60s, it was popular to put quick springs in the advance, even brazing the advance slots for a shorter total advance, deleting the vacuum advance all together, then using more initial advance to make that 40 degrees total advance number happen. Everybody raved about how much better it ran, but no mention of fuel economy loss. But then many of them had deeper rear axle gears (3.55, 3.73, 4.10), so what mattered most was acceleration from a dead stop than how much more fuel it took to do that.

Somewhere, in the Chrysler MasterTech Library, seems like there's a book on how this all works and interacts. Either www.mymopar.com or the www.onlineimperialclub.com website should have it.

Key thing is to get the max WOT advance calibration worked out first. Then tune the part-throttle/cruise advance with the adjustable vacuum advance unit. You might find that just a few more degrees of initial timing might be all you need, anyway.

Enjoy!
CBODY67
 
OK, sorry if I go over stuff that you already know, but I will try to break this down while adding some general comments (first) about your situation.

a. You have a vacuum leak...no doubt. In a smog-era, basically stock 360 you should have maybe 15-18"Hg vacuum....maybe even 20" if a fresh motor. I ran a barely-streetable 292/509 purple shaft cam in a big block and got 9" at idle.
b. If your motor needs 1500 rpm to idle, that is another indication of a bad vacuum leak. You would need a race-only solid lifter/roller cam to need an idle that high.
c. distributor basics: Initial timing set by distributor "clocking". Mechanical advance is created by centrifugal force (rpm). Mechanical advance is a balance of the weights vs. the springs, both of which can be changed. Also, the total mechanical advance can be limited by adjustment screws, limiter plates or the old "weld & file" method inside the distributor. With stock springs/weights, there usually is NO mechanical advance at idle, only initial advance (and vacuum advance). The vacuum advance is a "basically" a vacuum diaphragm & a spring (not unlike the vacuum secondaries on a 4 bbl. carburetor). The spring tension can be adjusted using a tiny allen wrench inside the distributor vacuum advance canister hose nipple. There aren't really "steps" in the mechanical/vacuum advance..."curves" is a better description.

Now to your questions:
1. Mechanical + vacuum at less than "all in" rpm? You have two competing "curves" you are trying to balance. First, the mechanical advance is increasing with rpm. Second, the vacuum is decreasing with "rpm" (really throttle position....floor it, vacuum drops & vacuum advance decreases). I "think" what you're asking is "How do I get 40 degrees total advance at 3000 rpm?"

Here's how (or at least one way)….Vacuum will be maximum when the throttle is almost closed, right? Vacuum will be highest at idle & "just a touch" lower at cruise. One way to get 40-degrees total advance at 3000 rpm would be to set your initial timing where the engine likes it. Then rev the engine & see what your initial + mechanical is "all in" & what rpm that happens at. Limit the mechanical advance using the screws/limiter plate/weld & file method so your "all in" is limited to 34-degrees initial + mechanical. Then play around with different weights & springs until that 34-degrees happens at 3000 rpm. Now, you hook up the vacuum advance (which has been blocked off up until this point) & read your timing. Use the little allen wrench to adjust the true total advance (initial + mechanical + vacuum) to 40-degrees at idle (maybe 41 degrees total at idle since cruise vacuum & vacuum advance will be slightly lower than idle rpm) and your done. You will be adding 6-7 degrees of vacuum advance at idle (& cruise). I have GROSSLY over-simplified this, but this should point you in the right direction.

2. Total advance at 1500 rpm cruise? In this case, you would still have 6-7 degrees of vacuum advance since the throttle is almost closed + initial advance (always there) + whatever mechanical advance is created by your selection of weights/springs..... the "mechanical curve" created by your selection.

3. Optimum timing below 3000rpm with no load? The timing that your particular engine performs the best at (sorry to sound "flip" about it). In general, you want the maximum advance, in all driving situations, that does not create detonation (knock/ping). For a track guy, it would be the combination that makes the car fastest.

4. Aim at 44* while coasting or at any rpm below 3000? See 1. & 2. above

5. would a cruise/coasting situation look like: idle 18° -off idle rpm cruise 44° - 3000rpm cruise 44°? No, more like 41* idle, maybe 40* off idle cruise & 40* 3000rpm cruise.

6. how do I determine how many degrees the vacuum advance SHOULD add between maxed out and not activated at certain rpms? The only time the vacuum advance does NOT add advance is when the engine is turned off or at wide-open-throttle when vacuum is near zero...it's basically "always activated" when you're not drag racing. I'm not sure how to answer this question better.

7. ".....how a vacuum advance is normaly working and if (in building a timing curve) it can be improved with the abilities I have in my EFI." If running EFI, you may want to consider a "lock-out" distributor (like a Lean Burn type) which doesn't have any mechanical or vacuum advance. All you have is initial timing + whatever timing curve is in your "computer". This is basically how modern fuel injected engines work....except most really new engines don't even have a distributor, so ALL timing (including initial) and the entire timing curve(s) is completely computer controlled. I've never run EFI on my Mopar, but I'm sure programmability for the timing curve is available. I see some guys on FBBO talking about this.

8. So think about it this way, if you could punch a number into the vacuum advance and it would add whatever number you put in, what would you do? I wouldn't do anything.... I'd use a computer to program the entire ignition curve... and fuel curve too for that matter.

HOPE THIS HELPS!
 
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The 38-40 degrees WOT "best power" advance I mentioned was for the B/RB engines. I believe the LA motors might be a few degrees less? More like 34-36 degrees total mechanical, at WOT?

As for the vacuum at idle in gear, after I got my '77 Camaro back together and was trying to ge the carb mixture better-adjusted, I happened to have it in "D" with the parking brake full on. The idle vacuum was 10.5" Hg. That was with a stock 305 V-8, 8.5 rated CR, Holley Z-Line 4bbl intake, emissions-spec Holley 4160 for a '76 Impala 350, stock exhaust, and the stock torque converter. Cam was a Cam Dynamics 266 (210 degrees @ .050" lift, .440" valve lift) The cam specs are "wilder" than a stock Chrysler 360 cam (which has similar numbers as the old Chrysler 383 2bbl 252 degree advertised duration cam).

The torque converter "tightness" WILL affect the load on the engine at idle, which will also affect related intake manifold vacuum levels. In my case, the 10.5" Hg at idle in gear was VERY close to the opening vacuum level of the Holley two-stage power valve in that carb, which opened at 10" Hg.

This was all taking place at about 1000ft above sea level, which ALSO affects intake manifold vacuum!

CBODY67
 
In the '60s, it was popular to put quick springs in the advance, even brazing the advance slots for a shorter total advance, deleting the vacuum advance all together, then using more initial advance to make that 40 degrees total advance number happen. Everybody raved about how much better it ran, but no mention of fuel economy loss. But then many of them had deeper rear axle gears (3.55, 3.73, 4.10), so what mattered most was acceleration from a dead stop than how much more fuel it took to do that.
You know, this is STILL popular. The new "rule of thumb" is about 34-36 degrees total advance with the vacuum advance disconnected...….the exact way I have my nitrous Roadrunner with 3.91 gears set up. If I was worried about mileage, I wouldn't be running a big block (haha). I get maybe 6mpg city & 8mpg highway.
 
okay thank you. couple of things, no vacuum leaks, and i never said I idle at 1500rpm lol and i think this goes deeper than what I originally just want to find out.

Anyway, lets try this: so I have two scenarios on hand that are adjustable. The first is "cruise". This is low load based and rpm based. The second is WOT. high load and rpm based. The WOT is essentially the same as " just mechanical +initial advance". I give the motor its 34°s at WOT at 3000rpm (thats the all in setting) and same at 6000rpm. and then i have a 1100rpm setting where I dont know whats a safe number to begin with.

The cruise is just the same except i am looking at low load situations where i would put in a number that resembles (initial+mechanical+vacuum advance). Again, higher rpms no problem but what is a safe starting point at 1100rpm? This is a situation when you just barely have the throttle open off idle or you would get off the gas at lower rpms.
 
At 1100rpm cruise, you'll still be basically on the base timing setting, little to no advance, depending upon model year (I believe that earlier model years started at 900rpm, with later years delaying that a bit) and probably full vacuum advance. The "safe" amount is the amount that keeps it from clattering when you accelerate to get up to faster speeds in traffic. But as soon as you open the throttle to accelerate, the manifold vacuum will decrease, as the engine is now seeing "more load".

Enjoy!
CBODY67
 
cbody67 put it very well. I would set the 1100rpm cruise just like base timing, then run some tests (same day, same load of fuel, etc.). Dial up the 1100rpm cruise advance until it starts to ping up a gradual hill (or something like that), then back it off maybe 2*. My 2-cents.
 
cbody67 put it very well. I would set the 1100rpm cruise just like base timing, then run some tests (same day, same load of fuel, etc.). Dial up the 1100rpm cruise advance until it starts to ping up a gradual hill (or something like that), then back it off maybe 2*. My 2-cents.
thank you, that answers a lot of my questions. i will try that!
 
I am always amazed, with Google and all how difficult it is to find fairly basic answers to questions like that!? ImI glad I have this forum with some very knowledgeable people who can explain this stuff to me!
 
I am always amazed, with Google and all how difficult it is to find fairly basic answers to questions like that!? ImI glad I have this forum with some very knowledgeable people who can explain this stuff to me!
Yeah, google is very useful BUT all you really get is a massive list of posts that takes hours to weed through. Then, you have to figure out who knows what they're talking about & who doesn't. There is a TON of wrong information on the internet.
 
I'm just thinking...how difficult can it be to find out "how much vacuum advance should be added below the cruise max". Nothing. like nobody has ever thought about that? Even with all the megasquirt tuning posts out there you would think someone would have addressed timing for economy?
 
Just an update, would something like this look okay?
SPARK.JPG


I run about 25° at idle. I've estabished that it produces max vacuum at 27°. So I backed out some degrees to have a slight vacuum drop. It idles very smooth and nice like this. 75kpa is just off idle for me (Idle produces 70kpa MAP). Cruising around using minimal throtte I am ust below that.


This is what the Low/medium/high load curves would look like.
Timing curves.JPG


The yellow line would be considered only Mechanical advance, the blue ine would be mechanical advance PLUS Vacuum advance! Red is just an additional curve for medium load or "normal driving". The Ignition is of course somewhat 3 dimensional but that is put the easiest possibe! Mash the Pedal to the floor and the yelow line is active, close the throttle and coast or have minimal throttle opening at steady state, it varies between red and blue. For example.
 
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