I think for a race car folks keep it simple. These discussions on other sites go on for 10s of pages. While folks understand all the individual piece, not sure folks have thought through all the integrated elements. Some times vacuum adv advances for motor needs, other times it retards timing.

Most know generically what vacuum does, but I doubt many have mapped vacuum under driving conditions.

For HP motors with dual points in the day, before emissions you would have around 8 to 12 degrees initial, and get another 16-22 out of the mechanical, with a light and heavy (gapped) spring. Vacuum added or takes away another 18-24 degrees. Stock motor, stock calibrated carb designed for the motor and the distributor. Once CAP hit in 66-67 initial was between -2 to +2 usually around 0 for BB certainly hemi. Mechanical gave 30 degrees, some unit 34. Vacuum still around 22.

So as you go down this path, what changes have you made. What is engine vacuum look like intake/ported. When does ported come in, or is it already on at idle because of cam, carb change and other things tuning changes you made for idle.

Seems a little dangerous to have a high initial, and vacuum on intake, especially if you made changes to mechanical adv springs to get all in earlier. As stated earlier, even when vacuum goes toward 0, the vacuum timing retards slower, and with mechanical advancing faster, your probably pretty close to detonation.
If your doing this in your drive way in neutral your really not testing a loaded motor.

None of my vacuum advance units have ever retarded the timing. They drop out their added advance when vacuum drops, but the timing only returns to the initial or mechanical advance. I haven't had one retard the timing beyond that.

Determining what the engine likes is not that difficult with a tach, timing light, and a hand held vacuum pump.

Some of the Mopar ECU will retard the ignition timing a little (1 to 2 degrees per 1000 RPM above 5000 RPM :scope) above 5000 RPM
when I first started paying attention to drag race tuning a lot of the faster racers had a switch to turn off one set of points to retard the ignition timing in high gear
It seems like some of the street and strip MSD boxes would also, maybe not as i never used them

Is this what you were referring to?

https://www.cbperformance.com/CB-s-Black-Box-Programmable-Timing-Control-Module-p/2013.htm

For all of you talking about having to shorten the advance slots, or make 'em longer, etc. etc...there is a far easier solution...this setup has been out for some time now and if you haven't picked this type of a distributor up yet, well, if the right price comes up it's well worth it. You simply use a plastic gauge to measure out how far to slide the plates in the distributor around (there is a gap provided already).



It is my understanding Mallory has been making this setup for MP for a few years now. The bonus is that you have a pile of adjustability (curve wise) with their spring kit...fast, slow, mid, etc., all the variations. That's kit# 29014.

That's what I have setup in my coupe right now.

The kit you list appears to be for Mallory distributors. Not a lot of guys have those. The reviews for the kit are not stellar. And the kits do not seem to be available....anywhere.

And finally, this thread is about vacuum advance but the distributor you suggest doesn't even have vacuum advance.

ALL ignition boxes retard with RPM. All of them. The best box I’ve tested for that is a early 1980’s DC Gold Box. It retards 1 degree in 10,000 RPM. I think it took 8,000 RPM to get that.

I just tested a well know box for street stuff and it retards 6 or 7 degrees (I can’t remember if that box was 6 or 7 because I tested 5 boxes that day and that one was one of the boxes that had the most retard) and IIRC started retarding at about 4,000 or so and kept retarding to 8,000.

I am talking total timing. Dropping out is retarding. Your cruising on the highway at part throttle. What is vacuum? How much of your total timing is mech and how much is vacuum? What happens when you punch it to pass? Vacuum goes away and total timing retards. The motor does not want all that timing under load and starting to accelerate. As RPM climbs mechanical runs up and vacuum timing is 0 and your at your total around 32-34. Prior you where probably around 44-46 total timing maybe more. The vacuum can adjust time up and down based on load. Mechanical adjust up and down based on RPM of motor. Both are factors in determining what timing is needed for peak efficiency.

Go read the Chrysler master training material on this. That will explain what the Chrysler engineers were doing, and why things changed for emissions. It really explains what the motor needs at various operating conditions.

So I have read were some race guys use a low adv vacuum can to help their car idle after start. The high initial keeps car from starting hot. So they dial back initial to start hot, but vacuum can on manifold gets total back to were it is needed for idle. Once they get on it, the vacuum drops out and they are on the mechanical. So you use it as you thing your combo needs it. Manifold versus ported is basically an on off switch for the vac can. Ported is off at idle an on at a specific point of throttle blade opening. Manifold is on all the time.

George, do you have a link for this?

http://www.imperialclub.com/Repair/Lit/Master/index.htm

These are great reads and a wealth of information.

Wow. Thanks. I have a number of Master Technician booklets from the day and they are a wealth of information.

DaveRS23,


Well, as I stated in my post, it was my understanding that Mallory became the distributor supplier to MP some years ago. The pic shows the guts, the focus being the adjustability given the design of the sliding plates. This setup has the VAC can provisions, it being a regular MP distributor replacement.

I went that route some years ago, have the kit here, the VAC can hooked up, all that. Working great for me, for what it's worth!

Bummer if the kit is out of production b/c it is pretty handy.

What does your training materials say about tuning 500 inches, big cams, open plenum intakes, Dominator carbs, straight through exhaust, etc, etc? Good info to know, but not necessarily pertinent. Chrysler engineers had to make many, many compromises to accommodate every idiot with a payment book. Not necessarily what was best for the engine or even for peak performance at the time. Let alone half a century later.

This is a discussion of semantics. Retarding vs dropping out. Maybe the same thing, but to say that the vacuum advance retards timing could be confusing for those that are not as familiar with the function. To describe the vacuum can's function as advancing and dropping out that advance is just more explanatory. Saying that the vacuum can advances and retards the timing could lead someone to believe that it could take the timing both above and below the mechanical timing. It may be technically correct, but is not as clear.

With all the mods going on with our cars, it is possible that some unique combos, particularly drag cars, could benefit from manifold vacuum. But for most street combos, dialing in the distributor and adding a tuned vacuum advance with ported vacuum is the way to go.

No, Manifold vacuum drops to nothing as soon as the throttle opens. See post #2888120

Are we talking MSD boxes etc. or Mopar ECUs / Ignition Modules that like came from the factory 1972 and later ?



I literally have tested a handful on my 71 Superbee 383/432 Stroker , and none of these Chinese knockoffs , fake transistor ECUs , work well over 5,000 RPMs - Like mentioned a few start failing at 4,000 RPMs

Really had me digging deep over a few summers

My 1990s Mopar Performance Chrome ECU was the best unit , until that failed a few years back

Now I have some brand new never mounted Standard LX101s from 1986 and 1987 that where made in the USA I need to try out


I remember reading this some years back

https://www.hotrod.com/articles/mopp-1211-3-2-1-ignition-box/

I have never seen a situation where the manifold would be the best source for vacuum advance. But I did not want to make the statement that it was never the best place.

So far so good with the Jegs / Firecore Distributor
Has the adjustable mechanical slots / numerous springs / and adjustable vacuum advance

I run ported vacuum

I can adjust the mechanical advance with the distributor in the engine using different size drill bits , very easy

https://www.jegs.com/i/JEGS/555/40504/10002/-1#

Ok, Chrysler engineers just did things for idiots??? What does that even mean? It is not my semantics, it is the words the Chrysler engineer used. Yes the vacuum can reduce timing certainly below the mechanical. You are in the Racers forum, I think they can understand what was said.

My only point was this is a total integrated product. Too many variables can be changed and folks don't always recognize how they have changed the SLOPE of the timing curve when you add a vacuum can. You seem to be trying to simplify the solution.

In general, springs are linear. With an inflection point as the heavy spring is applied when the loop closes up. You can control when they apply, and rate of advance with a spring change and total amount of mech adv by cam stop plate. Without a vac can you move up and down that timing line based purely on RPM.

With vacuum can it is not just moving further up the line, you now have a variable slope depending on acceleration or deacceleration and load. Your RPM timing is now different under different conditions because vacuum adv may be going lower or higher depending on load.

For a street car you need a device that can advance timing more then the car can handle when under WOT and full load. You don't want 55 total timing at WOT and your not efficient at cruise with 34. And yes and engine is an engine. The principles and engineering laws have not changed because displacement increased and a cam was added. Principles are the same.

It is more pertinent to discuss the use of the car rather then the size of the motor. Street, Strip or both. Yes, no (unless solving a problem), yes (maybe). There is a ton of cars out there running no vacuum or running plugged. The car will still run and perform.

I gave one...hot starts on a race car that needs a lot of advance to idle. But cannot start with that much initial.

Above statement is not correct. You still have vacuum and now you have it at the ported position and they both are about equal. Vacuum lowers though.

Now under WOT and load vacuum goes to near zero, but guess what, so does ported. It is exposed to manifold vacuum because throttle blades are open.

Ported is a switch. Normal stock cars designed on ported (Chrysler). There is no vacuum to the cannister until the throttle blades open enough to expose the carburetor port to manifold vacuum. Turn your idle up to about 2K and you will probably see ported vacuum turn on and start advancing timing. That is why you remove the hose when setting initial timing.

Well, you are welcome to your opinion.

Chrysler often had to engineer things just because of idiots. Manufacturers still do today. To deny that is to deny reality. Although they have never achieved true 'idiot proof' status, they do try.

And I do not agree that our typical Mopar vacuum advance cans retard timing below the mechanical. You accuse me of oversimplifying this, but you are really making this much more complicated and difficult than it needs to be. In fact, you are down right confusing.

It is vacuum advance. Been around since the 50s. A tach, timing light, hand held vacuum pump and a little knowledge and you are good to go. Shouldn't be above most guy's pay grade. Unless they make it way more complicated than it really is.