Limit total advance???

[azfordman]

I've spent the last couple of weeks trying to tweak the timing on my car to get optimal performance. Could never find the "sweet spot" where I was getting great power in every situation. Finally got under the car and put a tape on the balancer with marks at BTDC 0, 10, 20, 30, 40 degrees. Seems I am getting best idle and low end performance at around 18-20 BTDC initial, but the total advance shoots up past 45 degrees at higher RPM's, 2500 or so and I start to get pinging. If I retard the timing down so my total advance is say 35 to 38, the low end suffers. The other night I cranked in the allen screw in the vacuum advance all the way, and it brings the total advance down to a more reasonable 40 degrees BTDC at 2500 RPM's. My question is there any way to limit the total mechanical/vacuum advance on Ford distributors? I think I could put a screw or a pin in the advance arm or the base of the distributor to mechanically stop the advance from moving, but is there a better way? FWIW, the vacuum on the engine off the manifold at idle is 17" Hg and it goes up to 20" Hg at 2500 RPM's, the ported vacuum off the side of the carb where the vacuum advance is hooked up is 0" Hg at idle and goes up to 23" Hg at 2500 RPM's.

[PaulProe]

There are a couple different lower plates in the distributors that establish the upper limit. They also have a little fitting that clips over a tab that limits the movement.

Suggest you contact Tim Connor in Indiana. He is the guy all the FE people go to for distributor setup / recurve. You can contact him thru this website: http://428cobrajetcars.homestead.com/

Paul

[dcdoug]

Tim is the man on Ford distributors.

[Excaliber]

I think the vacuum advance and your HG readings at various speeds done when the engine is NOT UNDER LOAD, are a red herring. A snipe hunt. A,,,, well you get the point.

WHY do we even have vacuum advance? What purpose does it serve for PERFORMANCE? It doesn't have anything to do with performance, in that regard, take it out, block it off, throw it away. It's PRIMARY PURPOSE in life is to maximize mpg by enhancing cruising speed/rpm efficiency. That's it, thats all it does, all it was ever intended to do.

You will ever see a vacuum advance unit on a serious race engine, or even a serious street engine. Step 1. Maximize your timing on the base and mechanical side. I have found that most modified engines do indeed run better at low to mid range rpm with 18 to 20 degrees advance. That alone solves SO many problems, like stumbling, hesitation, flat spots, etc. But there are a couple of problems with running this much BASE timing. One is it may be hard for the starter motor to crank the engine, especially when hot. Two is it may be difficult to limit the MAX mechanical advance to maintain the desired total amount. Anywhere from 32 to 38, not generally more than that. Now ADD to that 32-38 the vacuum advance of say, 10 to 12 degrees. The new total may be (38+12) 50 degrees! That is reasonable!!!! Most motors could handle that, like it in fact, maximize your mpg. All though, 50 is pushing the outter limits in some cases. IF, your engine is under light load, like steady state cruise rpm with light throttle, it's amazing how much timing you can throw at it.

The reason is simple: As the HG (vacuum) drops, due to increased throttle opening, climbing a hill, accellerating, heavier load on the engine, the vacuum also drops. The radically advanced timing goes away, drops back to mechanical advance only!

It's unlikely your motor will be pulling anywhere near the HG at 2,500 your reading in the garage as it will at 2,500 rpm under load! Try this, get a really long vacuum hose and mount the vacuum gauge on the window or in the cockpit so you can see what the REAL HG is under various conditions, rpm, load, cruise, WOT, etc.

I run both mechanical and vacuum advance on my side oiler, with an MSD "ready to run" (does not require an MSD box) and comes with a vacuum advance canister. I run so much base timing that I HAD to find a way to limit the mechanical advance. MSD does not sell a stop bushing that offered enough "stop". I had to buy a special, bigger, stop bushing made by a speciallty racing shop just for this purpose. The fact that they are available is indicative of how many people are having the same problem, your problem! To much mechanical advance and no way to limit it enough.

The other thing you need to know is: When does the vacuum advance begin to advance? At what HG? Second is: How MUCH advance does it offer at what HG?

MSD starts at about 15" and offers about 10 to 15 degrees of advance (I forget the exact amounts).

For now, disconnect the vacuum advance, work on the mechanical advance, then go back to the vacuum part. Good luck.

[Rick Parker]

AZFORDMAN:
The overall limit can be changed. The distributor must basically be disassembled. The round part that the rotor slips onto has the mechanical advance mechanism attached to it under the base plate. It has 2 slots, one that may permit 16 crank degrees and another that will allow for instance, 24 crank degrees, the number of available degrees is limited by the length of the chosen slot and the pin that limits its travel. That number plus the intial (18??) is the total advance you will have available. The distance the slots allow can be altered by reducing the length of the slot. (Heliarc works nicely.)
To disassemble it, carefully remove the small wire clip in the center under the rotor, remove the c clips on the vacuume advance mechanism, remove the reluctor without breaking any of the tangs, then remove the baseplate. At that point the mechanical advance will be visable. The distributor bodies are pretty much the same between an electronic as pictured and a point type ignition.

The calibration is best done by someone with access to a Distributor machine.

[azfordman]

Originally Posted by Excaliber

I think the vacuum advance and your HG readings at various speeds done when the engine is NOT UNDER LOAD, are a red herring. A snipe hunt. A,,,, well you get the point.


The reason is simple: As the HG (vacuum) drops, due to increased throttle opening, climbing a hill, accellerating, heavier load on the engine, the vacuum also drops. The radically advanced timing goes away, drops back to mechanical advance only!

It's unlikely your motor will be pulling anywhere near the HG at 2,500 your reading in the garage as it will at 2,500 rpm under load! Try this, get a really long vacuum hose and mount the vacuum gauge on the window or in the cockpit so you can see what the REAL HG is under various conditions, rpm, load, cruise, WOT, etc.


The other thing you need to know is: When does the vacuum advance begin to advance? At what HG? Second is: How MUCH advance does it offer at what HG?

I appreciate the informative post! I had always thought that the intake vacuum decreases considerably when under acceleration. Unfortunately with this car that is just not the case!!! I took the car out after reading your post and here are some results from a drive with vacuum gauge in cockpit:

In 1st gear up to 3000 RPMs the vacuum reading is 20" Hg
In 2nd gear up to 3000 RPMs the vacuum reading is 19" Hg
In 3rd gear up to 3000 RPMs the vacuum reading is 17" Hg
At this point I am already at 55 mph and speding.....

As far as putting a load on the motor, from a rolling start of 25-30 mph

In 3rd gear vacuum drops to 15" Hg
In 4th gear vacuum drops to 10" Hg
In 5th gear vacuum drops to 7" Hg
this is under very light throttle.

Under heavy (WOT) acceleration the vacuum will drop to around 10" Hg initially, but as I am ready to shift, around 5500-6000 Rpms the vacuum is back up around 20" Hg. The vacuum reading rises steadily as the RPMs increase. When shifting, between gears (clutch in and throttle off) the vacuum will jump up to 35" Hg (35" is incorrect...it is actually 25" Hg) then drops down to the 10" Hg as the next gear is engaged and throttle is applied again.

Normal cruising in 4th gear, 2500 RPMs, 60 mph the vacuum is a steady 20" Hg.
My question is, do I have an issue since this motor seems to pull so much vacuum???

As far as the advance goes, with the vacuum line disconnected and plugged, the mechanical advance only goes up to about 28 degrees BTDC, it begins to advance almost initially and is at full advance around 2000 rpm (a guestimate since I do not have a hand held tachometer.) With the vacuum advance hooked up, the timing advances the same way, but goes up to 45 degrees BTDC. So it seems with the vacuum advance disconnected I don't get enough advance, but with it connected I get too much.

[azfordman]

Originally Posted by Rick Parker

AZFORDMAN:
The distance the slots allow can be altered by reducing the length of the slot. (Heliarc works nicely.)
To disassemble it, carefully remove the small wire clip in the center under the rotor, remove the c clips on the vacuume advance mechanism, remove the reluctor without breaking any of the tangs, then remove the baseplate. At that point the mechanical advance will be visable. The distributor bodies are pretty much the same between an electronic as pictured and a point type ignition.

The calibration is best done by someone with access to a Distributor machine.

Rick, I take it the base plate is in my picture with the 8 and 6 stamped into it, correct? Where is this wire clip that is supposed to be under the rotor? Someone else had mentioned welding the slots.....are these slots on the underside of the base plate? I will get out and try to pull this apart tomorrow. I appreciate the input! Mark

[Excaliber]

Generally speaking, the vacuum signal is directly related to the cam profile. And generally speaking the higher the vacuum, the better the engine's overall condition.

At any rate, I bet your mechanical advance is frozen, stuck, sticking, needs cleaning. Your not getting nearly enough mechanical advance added to your base of 20. Maybe 8 degrees mechanical? Pfffft, thats nothing... Follow Rick Parkers excellent post, but instead of worrying about how to LIMIT the advance, look for MORE advance down in there where the sun don't shine. Not welding, get the grinder!!

There are a number of ways to limit what seems to be a very fast and high vacuum signal to the dist. It also appears that you have a very aggressive vacuum advance unit, advances a lot and it doesn't take much vacuum to do it!

For now, stay focused on the mechanical side of things. Putting the advance mechanism UNDER the dist plate was not one of Ford's better ideas. The GM and MSD type dist's with the advance mechanism on top is far easier to service, modify and actually "see".

[vector1]

so what do you get when you disconnect the vacuum advance and set the total at 3000 rpm to 32-34 and then reconnect everything. if your idle timing is then acceptable without the vacuum, hook the vacuum up and see what happens, if it is pinging somewhere it could also be a lean rpm in the carb. engine specs would help for generalization. dual plane intake from your vacuum readings?

[dcdoug]

Originally Posted by azfordman

Rick, I take it the base plate is in my picture with the 8 and 6 stamped into it, correct? Where is this wire clip that is supposed to be under the rotor? Someone else had mentioned welding the slots.....are these slots on the underside of the base plate? I will get out and try to pull this apart tomorrow. I appreciate the input! Mark

The wire clip is down in the rotor housing hole and is a bit of a pain to get out. I had to mess around with it with needle nose pliers for a while to get it out. Remove it, unscrew everything, and then you will see that the bottom plate that everything is bolted to is removable and beneath that is the advance mechanism.

Here is an article that might be useful. It's for slightly older model Ford distributors, but most of the principles should be the same.

http://www.mustangmonthly.com/techar.../photo_12.html

[Tom Cimino]

AZ,
Chech out this link. Lots of info on mech and vac advance and how to set up Ford distributors.
http://www.carbdford.com/viewtopic.php?t=5543

[azfordman]

So...I've partially disassembled an extra distributor that I had sitting here. Pretty much tried to follow the instructions on the carbdford site. 1st off, the reluctor did not just pry off....I actually had to take a propane torch to it. 2nd, I did manage to get the clip for the rotor housing out...what a pain!!! I am now at a point where even with the clip out the the plate that has the mechanical advance will not budge....am I missing something? I don't mind screwing with and possibly busting an old distributor, but don't want to damage the one in the car. I will try to work on the one in the car a little later today. A question though, it looks like the mechanical and vacuum advance work independently from each other....is this correct? So even if I get the mechanical works going properly, the vacuum advance is still going to advance the timing farther than I want it to go. Am I looking at running without the vacuum advance? And back to my original question, since the vacuum on my engine increases as the RPMs increase and the whole point is to increase advance as engine speed goes up, couldn't I just put a mechanical stop on the vacuum advance arm to limit the amount of advance??? (Note....I am not asking for advice then not following it, I just enjoy learning about how things like this work!)

[Excaliber]

Vac and mech advance units, totally separate, one has no influence on the other. As engine rpm increases it NEEDS and can handle more advance. If your existing vac advance is to sensitive, either comes into play to quickly or advances to much, yes, you might have to disconnect it entirely and run only mech advance. Adjusting the vac advance (beyond the allen screw limit which SOME have, others do not), or controlling it precisely is very difficult and often unpredictable because of flucuating vacuum signals. Which is one reason many simply choose to disconnect it.

More advance is required with higher rpm because the burn rate of the fuel/air needs a "head start" to fully burn before the combustion cycle is completed. At low to mid rpm the mechanical advance may not come into play, very much or at all. In such a case (low rpm, light load, cruise speed) the engine vacuum signal is strong enough to compensate for the inaction of mechanical advance by utilizing the vacuum advance instead.

So low rpm, light load, you get vacuum advance. High rpm, the mechanical advance comes into play. Generally speaking, you need one or the other, not usually BOTH working at the same time. How soon the advance (from either source) comes into and how much advance it offers is called the "advance curve" in the tuning process. While the mechanical advance can absolutely be perfectly dialed in based strictly on engine rpm, the vacuum advance will always remain a bit of a "wild card". That wild card could enhance the overall performance, or hinder it if it's not setup correctly for YOUR cam profile, combustion chamber shape, quench or no quench, rich/lean conditions, engine rpm and load. It can get nasty trying to tune the vac advance.

[mr0077]

AZ, what you just described ("The vacuum reading rises steadily as the RPMs increase. When shifting, between gears (clutch in and throttle off) the vacuum will jump up to 35" Hg then drops down to the 10" Hg as the next gear is engaged and throttle is applied again.") is not possible; an absolute vacuum is 29.92" Hg, so you can't read 35" Hg. If the gauge is reading high by 10" Hg, you would get the readings you recorded. When you get around to working on the vacuum side of things, I would get a second gauge to check your readings.

The advice above is very good and complete, you should concentrate on the mechanical side first; once set, then fiddle with (or eliminate) the vacuum advance. It can improve mileage, but does not improve power.

[vector1]

mechanical and vacuum are two independent type advance mechanisms. the mechanical will always be there, from 10*-32* for example as the rpms increase, usually all in around 2500-3000 depending on engine, and can be considered base advance. the vacuum will be on top of this. when you light cruise down the highway the high vacuum is going to advance the "base" timing which helps mpg, my crate engine sees close to 50* advance. but when you open the throttle the vacuum drops and vacuum advance will drop also so you don't ping on acceleration, but never less then the mechanical. i have read some don't like the vacuum advance for racing as it will not react quick enough to retard the timing on acceleration. vacuum advance can be tuned also as you have noticed along with different tension springs i believe. some vacuum setups will also retard the timing, ala oem.

[azfordman]

Originally Posted by mr0077

AZ, what you just described ("The vacuum reading rises steadily as the RPMs increase. When shifting, between gears (clutch in and throttle off) the vacuum will jump up to 35" Hg then drops down to the 10" Hg as the next gear is engaged and throttle is applied again.") is not possible; an absolute vacuum is 29.92" Hg, so you can't read 35" Hg. If the gauge is reading high by 10" Hg, you would get the readings you recorded. When you get around to working on the vacuum side of things, I would get a second gauge to check your readings.

My bad....that's what I get for taking notes at night while trying to drive the car!!! The gauge only goes to 30....so I should have said 25.

[Excaliber]

mr0077, he could be seeing the vac gauge "bounce" that might indicate a really high vac reading just for a moment as it settles back to actual HG. He will never see a steady state on the gauge while driving around, shifting gears, on/off throttle pressure. That needle is going to be all over the place, low, high, medium. But I concur, it's possible the gauge is not calibrated correctly, which would only exasperate any tuning procedures.

I mostly use my vacuum gauge for two things I want to know. What is my idle vac reading? I can use that number to determine what size power valve I may want to start with for tuning my carb. It is also a good "base line" for the overall condition of the motor and a general health indicator. I also want to know my cruise speed at normal rpm vacuum reading? That will give me an idea of how much vacuum advance is adding to mechanical advance.

Reading and interpreting a vacuum signal with a gauge is more of an art than a science. To get really good at it you need to see, perhaps over a period of years, different gauges, different engine combo's and different conditions. It's tricky stuff that can easily mislead you to wrong conclusions.

[lnfletcher]

A little more information.
http://dnr.louisiana.gov/sec/execdiv...p/auto/h/h.htm
http://www.sweptline.com/tech/engine2.html
I run a vacuum advanced to help with my idle. When my base and mech are set correctly, my idle is pretty bad because of the cam I'm running and the vac advance helps smooth it a little.

[azfordman]

Originally Posted by lnfletcher

A little more information.
http://dnr.louisiana.gov/sec/execdiv...p/auto/h/h.htm
http://www.sweptline.com/tech/engine2.html
I run a vacuum advanced to help with my idle. When my base and mech are set correctly, my idle is pretty bad because of the cam I'm running and the vac advance helps smooth it a little.

Interesting reading. Although the thing that is still confusing me is the following...

"Vacuum advance also adds timing to the engine, but it is based on engine load and is controlled by intake manifold vacuum. When the throttle is partially open (highway cruising), manifold vacuum is high. This vacuum signal is routed to the distributor and "pulls" on the vacuum advance canister to create more ignition timing. As the throttle opening increases, load increases and manifold vacuum decreases. At wide open throttle (WOT), manifold vacuum is at or near zero and the vacuum advance produces no additional ignition timing."

Almost everything I read says the vacuum in the manifold should be dropping to almost zero with the throttle wide open....so the vacuum advance does not come into play. For mild mannered street driving I am not having any issues, but on the occasion I decide to stand on it and go all out, is when this vacuum advance issue is kicking in. Am I the only one who has this issue? Again, when everything I read contradicts what is happening with this car it just makes me wonder what is going on. Ahhhh....the joys of a light weight car with a big engine!!!

[lnfletcher]

You may be confusing a timing problem with a carb tuning problem which is being masked by the timing changes.
I would plug the vacuum adv and tune without it and if it causes a problem and you are not worried about idle or fuel mileage, then leave it plugged.
I went rounds with mine and it turned out to be a carb jetting problem instead of timing!