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 Too much advance at light throttle? 
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Joined: Sun Jun 23, 2013 1:41 pm
Posts: 375
Location: Phila. Penn.
What happens if there is too much timing advance under cruise or engine braking?
Will there still be detonation? Will it simply misfire?

I ask because there are two adjustable controls for most vacuum advance modules. The first is the spring which controls the how much vacuum (load) is needed to move the diaphram. Setting this seems straightfoward: any time there is part throttle detonation advance is being added too early; while on the other hand, the spring resistance needs to be soft enough to allow advance under cruising conditions.

The second available adjustment is the maximum advance allowed by the notch on the arm. This one I'm asking about. Slightly backing off the throttle before a corner or engine braking will result in more vacuum. If the advance module's arm was not already maxed out, the the distributor will advance more under these conditions. What happens then?

I'm asking for street as well as road courses or rally where conditions or rules require a light pedal or engine braking. I realize that vacuum advance is generally not used for racing, but why not take advantage of a little more part throttle response as well as fuel savings in an enduro situation where you want to carry just enough fuel to finish the race ?

Matt


Sun Nov 23, 2014 9:54 am
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I have been using vacuum advance on race engine distributors since back in the days of points. If I had a dollar for every time someone told me “That’s’ the first time it cooled off on yellows” I could buy a tank of gas, even at today’s prices.

You have upper vacuum limit, lower vacuum limit and range of degrees. If you describe your engine and situation I will offer an opinion.

It is possible to have detonation at part-throttle. You used to hear it a lot back in the late 70’s when the Mopar and Ford cars using distributors with the adjustable spring tension vacuum advance units got some miles on them and the constant spring force with vibration and hot/cold cycles pushed the adjustment to weakest spring tension – therefore most advance, usually too much – and the vacuum had to drop to nearly WOT for the vacuum advance to retard at all. A long gradual uphill section of freeway I traveled regularly would almost always have a few cars, usually Mopar V8’s, pinging merrily along at a steady 60 MPH.

Naturally, how much additional timing you add at part-load depends on the mechanical curve. Most high compression circle track stuff I use 10-12 crank degrees between 10” and 15”. Pre emissions 10.5/1 CR 300 HP 327 Chevys used 15 crank degrees between 8-10” and 15-17”. Low compression SBC, 15 degrees between 6”-8” and 13”-15”. Those would both have mechanical curves with about 30 degrees at 3000 and 35 at 4500. An engine used for circle track or road race is operated in a fully thermally soaked condition, as is a highway car, and cannot stand, does not require, the “all in at xx??” curve, they want the best timing for best torque at the torque peak RPM and then continuous advance to more timing which gives best power at the power peak. The rate and amount depend on C/R, chamber design and manifold pressure, restricted carb or not, etc.


Sun Nov 23, 2014 11:02 am
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Joined: Sun Jun 23, 2013 1:41 pm
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Location: Phila. Penn.
Well thats good to know. Maybe we can convince some of my friends to use it on their road race v-8 Darts and the /6 powered beamer. But we'll start with me first :)

This is for a small block mopar. My current situation is lower compression 340 with a moderate cam (223/230 at 50, 0.505/.515 lift). I'll probably use it this way another year. This will include mountains and (if work allows) a track day (non-competitive) and autocross (competitive).

The vacuum gage indicates a solid 9" Hg in gear at idle and 11" in neutral.
Cruises with a little load around 17" Hg, a little less on secondary roads a little more on Interstates. Just a little less load and it will jump up to 18-19" and 21" plus when completely off throttle.

For the past several years I've been using a Mallory built Mopar Performance distributor. At first this seemed easier than tinkering with the Chrysler built distributors, but it has design issues. One is the mechanical advance increases exponentially (as shown in Mallory's instructions below).
Attachment:
MalloyYT-YH-curves.jpg
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The other is retard starting around 3500 - 4000 rpm when measured on engine. Never had this issue with the old DC tach drive dizzy.**

So now I'm setting up a Chysler built version to go back in. Plan is to use a secondary spring very similar to the one from the tach drive dizzy. The trick will be to get loop length just right to get a slight increase or at least steady advance from 3000 rpm up. The Mallory dizzy is current set for total mechanical at 36 degrees 3500 rpm. I was getting better performance on tach drive dizzy set around 31-32 at 2000 which would slowly climb to 34 at 3000, etc.

On the distributor I'm setting up now, the vacuum pod's arm is marked 8.5 so should add 17 crank degrees when pulled fully in. However I think, if needed, this can be limited with a carefully folded shim. There also is (or was) an Airtex/Wells adjustble pod with a 9.5 arm.

Bench testing the shows the following when the spring adjuster is:
Fully in clockwise, advance starts at 6"Hg, fully advanced at 11" Hg
3 turns out, advance starts at 9" Hg, fully advanced at 14.5" Hg.
6 turns out, advance starts at 12" Hg, fully advanced at 18"Hg.

The old MP instructions recommend LA open chamber engines use 50 degrees at cruise.*
But if I set the vac pod spring for 15 degrees at 17"Hg then under engine braking, its going to advance another 3 degrees. I'm thinking it could misfire or backfire into the carb.

*Scanned version of the Mopar Performance electronic swap instructions here:
http://www.dippy.org/upgrade/ignition.pdf

**part throttle pinging was eventually resolved by slowing mechanical and vac advance, but here is a graph of mechanical timing when I was addressing the problem. I think the solution is costing both power and economy. Still not really sure of main culprit: changing pump gas, TC or the dizzy. The "ideal" is based on Dynomation simulations. Take it FWIW.
Attachment:
Pinging.jpg
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Sun Nov 23, 2014 12:55 pm
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The Mallory guts DC units are easier to setup for a quickie job, but they aren't as reliable in the long run because the geometry and quality of the advance parts is less than ideal. The original Mopar distributors are the best, though they are a bit more difficult to setup at least they don't deteriorate as rapidly.

I limit the vacuum advance by making a shim of whatever is necessary size wire and crimping it around the stem in the notch where the limit occurs. Coat hanger is good for this because the wire is heavy and old hangers are good steel if you have any US made from the 50's and 60's, I have a stash.

If you have the DC tach drive distributor why aren't you using it? Is it the aluminum housing with old-school Mopar advance guts? A light spring and a very heavy spring?

The lower of the two green curves is about what I would do, except above 3000 the slope should be flatter and extend to 8000+ RPM. 18 initial until 1200-1400 RPM, then to 30-32 @3000, then 35°@5500, 38°@ 8000, near 1 degree per 1000 RPM.

The mechanism should not hit the mechanical limit until above the engine's RPM limit. This requires a humongus secondary spring, (like the one which comes original in the 4120701 DC tach drive distributor, which should be (as I recall) .056” or .058” WD x 4.5 coils, .312” or smaller diameter) to make the high RPM slope continue to above the maximum engine speed, approximate 32°@3000, 35°@5500, 38°@ 8000, near 1 degree per 1000 RPM.

That is close to the spring rate in the old Prestolite roller bearing distributors for NASCAR Race Hemi and produces the curve described in the Mopar engine books, which is an advancing slope of about 4-6 crank degrees from 1000 to 8000.

The mechanical advance compensates for the retard in the electronics to provide a flat static total or continue to advance all the way through the RPM range, however the engine likes it and you set it up.

This is a technique I have used for distributor curves since the 60's, though I didn't come to understand the reasoning behind it until the early 70's when I replaced a points ignition that did not retard as RPM climbed with an electronic ignition that, like all electronic ignitions, has slew-rate retard, and the car slowed down.

A-B-A-B it was faster with the GM dual-points distributor, even though the spark energy was higher with the electronic, a GM Magna-Pulse, and the electronic had the same “curve” except for the retard after the “total” was reached at 2500. Jenkins book “The Small Block Chevrolet Racing Engine” details the high-RPM curve and explains his reasoning, so whiz-bang and it seems like only yesterday. I'm glad I got to thank him for that a few years ago.

Instruction about advance curves like this can be found in all the manufacturers' “off road” instructions and shop manuals. Smokey Yunick and Bill Jenkins both advised such advance schemes in various print media. Odd that the magazines ignored this and the “all in at 2500” deal is all anybody knows or cares about. When you get it right where the engine likes it, it is almost an unfair advantage.


Sun Nov 23, 2014 5:45 pm
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Quote:
The Mallory guts DC units are easier to setup for a quickie job, but they aren't as reliable in the long run because the geometry and quality of the advance parts is less than ideal. The original Mopar distributors are the best, though they are a bit more difficult to setup at least they don't deteriorate as rapidly.
So I've learned, the hard way.
Quote:
I limit the vacuum advance by making a shim of whatever is necessary size wire and crimping it around the stem in the notch where the limit occurs.
Thats an option I didn't think of. I was going to drill two holes in some flat sheet and cut it like a finger bandage. But this begs the question - what happens if the maximum vacuum advance is not limited? Phrased another way, how should the limit be determined?
Quote:
If you have the DC tach drive distributor why aren't you using it? Is it the aluminum housing with old-school Mopar advance guts? A light spring and a very heavy spring?
Only reason I stopped using it was the lack of vacuum advance. I drive the car to events; I've driven it as far north as Vermont and as far south as Florida. I've driven it in the winter too, although if my Wagoneer is running, the 'cuda is going back into hibernation with the first threat of salt.

For a few years I had rigged up the MSD timing control pot to a switch mounted on a GM style vacuum pod. This was set to provide on/off additional advance at cruise. I was glad to ditch that setup and the retard box. The 6T itself has been reliable, so I'm still using it.

The tach drive distributor is exactly the one you describe, P4120701. I think I waited 4-5 months before they made a batch. This was around 1990.

I appreciate your advice on the curves. I wasn't sure how useful the Dynomation would on this, but I figured it was better than nothing. I'll take your experience over the program any day!

Your memory is pretty darn good! My notes are that the heavy spring measured:
0.053" dia wire, 0.293" coil OD, 5 coils, which works out to 4.5 active coils and a .24" coil diameter:
My calculation is this works out to 174 lbs/in.
The loop to loop length is 1.09"

After poking around for, I noticed some of the slant six distributors have a similar spring. It looked heavy and 4.5 coils. I'll have the details soon, as one is one the way to my door.
Quote:
... when I replaced a points ignition that did not retard as RPM climbed with an electronic ignition that, like all electronic ignitions, has slew-rate retard, and the car slowed down.
I did not know that! I always assumed the retard was in the engine - timing chain through dizzy, and that's someone at Direct Connection or earlier was compensating. You may have noticed some folks are producing ECUs that they claim will not retard. Not sure how good they are. I'd prefer to do it the old fashioned way for now...
Quote:
A-B-A-B it was faster with the GM dual-points distributor, even though the spark energy was higher with the electronic, a GM Magna-Pulse, and the electronic had the same “curve” except for the retard after the “total” was reached at 2500. Jenkins book “The Small Block Chevrolet Racing Engine” details the high-RPM curve and explains his reasoning, so whiz-bang and it seems like only yesterday. I'm glad I got to thank him for that a few years ago.
That's very interesting! Back when I wet behind the ears, a co-worker who had been into class racing (stock) told me to never remove that heavy spring. He had, and lost time no matter what springs and curve they tried; that is until they put the heavy spring back in. However, he didn't know why.
Quote:
Odd that the magazines ignored this and the “all in at 2500” deal is all anybody knows or cares about. When you get it right where the engine likes it, it is almost an unfair advantage.
Maybe its not the magazine's fault. Most only "report" on what the aftermarket is producing.... Grump Grump.


Sun Nov 23, 2014 7:36 pm
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Quote:
what happens if the maximum vacuum advance is not limited?

It feels like a lean miss or surge. It may be an occasional hard miss like a bad plug wire, or a gentle surging that sort of nods your head. A common euphemism for the sensation is “trailer hitching” because it feels sort of like a 2” hitch on a 1&7/8” ball.

In all circumstances, the object of ignition timing is to peak the cylinder pressure at or near 15-20 degrees ATDC.


Mon Nov 24, 2014 11:54 am
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Joined: Sun Jun 23, 2013 1:41 pm
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Location: Phila. Penn.
Thanks Tuner. I wish someone around here had a 5 gas. I'll have to do it by seat of pants and innovate datalogger.

Do you think the Mopar's vac advance needs would be fairly similar to the SBC numbers in your first post above? I assume most SBC are running more of a closed chamber head.


Mon Nov 24, 2014 1:00 pm
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Tested three distributors last weekend on the Sun DT404 with a Sun amplifier. Some interesting stuff.
The first was a MP tach drive dizzy, no vacuum advance. The other two were stock distributors, one from a junker and the other a rebuilt unit from the parts store.

On the MP tach drive distributor, advance starts between 200 - 250 rpm (500 rpm crank) and increases by 8-9 degrees at 400 rpm. Then the advance slowly rises to 11 degrees between 600 and 850 rpm. After that, it was steady at 11 all the way out to 3000 rpm. This was measured using a Sun amplifier. My experience with an old Mopar chrome ECU and the MSD 6T is this is about right. Perhaps a real ECU with less slew rate would show a slight rise?

Graph added below. 'Reverse' is the timing as rpm is reduced from max. I do not know why there is so much difference between the upper and lower scale or when going up in rpm vs going down. Primary advance spring is .72 long and I calculate to be 1.3 lb/in. The long looped secondary spring is 1.09" and about 174 Lb/in
Attachment:
File comment: MP/DC SB Alumin Housing tach drive Distributor
AdvanceTested-P4120701.jpg
AdvanceTested-P4120701.jpg [ 149.88 KiB | Viewed 1070 times ]

Slightly OT, but for those unfamiliar with these, there is no vacuum advance provisions. With this advance curve, the timing must be set at total - pick an rpm between 2500 and 3000 and make that the standard if, like me, you don't want to rev the engine to 6000 while standing by the fan with your timing light! :O The easiest way to explain how it works is to use an example. Lets set the timing at 33* at 3000 rpm. Then using the above plot, subtract the advance and find that the base timing will be 11* (33 minus 21-22 crank degrees). This makes easy starting. Once the engine fires, the idle rpm is going to be something above 400 rpm, so idle timing will be someplace along the advance curve. Lets say the engine idles at 800 rpm, then the timing will be 19*. But the same engine cold might idle at 650 rpm, and timing will be 15-17*. This sensitivity drives many people crazy. Its another variable to keep track of and try address while adjusting idle/off-idle mix and throttle position. Otherwise, my experience has been it works great. Since there is no vac advance, it never caused pinging even in street use.


Last edited by Mattax on Mon Aug 10, 2015 9:46 pm, edited 1 time in total.



Thu Jan 01, 2015 2:48 pm
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Next is the stock vacuum advance distributor. The one from the junk 318 unfortunately had timing jumping all over - I think the top of the shaft was worn and thats not fixable AFAIK. So no test results worth sharing. The other was a rebuilt unit and is in decent condition. Its a one size fits many replacement, so all bets are off as to whether this set of springs and curve is really correct for any particular stock application. Anyway, it works as generic spare and serves a baseline.

Primary spring: , 0.72" loop to loop, abt 8 lbs/in
Secondary Spring: 0.92" loop to loop, abt 45 lb/in
Gov. plate slots: 11R (on bottom) slots straight

Image

On the same "remanufactured" replacement dizzy, the vac canister added 22 degrees (crank). Wrapped the arm in with a piece of .031" piano wire (from carb experiments). Not ideal because its hard to bend, but its what we had available. That limited the advance to 13 - 14 degrees.
Attachment:
Limiting-Chysler-Vac-Advance-_03-r-c.jpg
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I believe the adjuster was nearly all the way out on the first test, and then all the way out on the second. Why did I do this? :ummm?: I wasn't thinking
Attachment:
VacuumAdvanceTests-A1rebuild.jpg
VacuumAdvanceTests-A1rebuild.jpg [ 150.71 KiB | Viewed 1064 times ]


Mon Aug 10, 2015 9:46 pm
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Here are results from testing the vacuum advance pod that came on mopar performance's distributor. The arm is stamped 8.5 distributor degrees. The test on the engine was using a timing light and subtracting the timing with vacuum and without. The advance was hooked up to a vacuum pump. The test on the Sun 404 was at 1000 rpm. Target is what I think my current might like based on Tuner's comments
Attachment:
VacuumAdvanceTests-P3690430.jpg
VacuumAdvanceTests-P3690430.jpg [ 102.8 KiB | Viewed 1040 times ]


Fri Aug 14, 2015 9:47 am
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