Does the Prestolite dist have the external oiling port for the bushings?

Cecil

I haven't checked the total timing the MP distributor has, but my experience tells me it's too much, too fast for the street. Those super-lightweight centrifugal bobweight springs...! I noticed the mileage decrease with the installation of that distributor, too. I suspect it's because the fast centrifugal advance curve makes a street engine so ping-prone that you have to retard the initial timing too much, so part-throttle economy suffers.

I'm sure those who've checked and measured can give a more thorough response here.

Great post Cecil! (Note to Doc - I nominate this for placement in the Articles section).

I did this same mod to an LA distributor a while back. I didn't have sense enough to figure out the math though, and wound up without quite enough advance and without quite enough inclination to pull it back out. Wound up running something like 20* initial

I've long wondered why these distributors have so much mechanical travel in them. Must be that whole smog thing I guess.

Now, with your figures, I'm energized to pull that dist back out and fix it. I am a little corn-fused by your numbers, though.
I'm not too sharp, so bear with me. I see you get 15* advance for .235 movement. So, that's .0157 per degree, isn't it? (.235/15, rounded up)

So, .0157 times 9 degrees desired, gives me .141, plus your .240 gives right at .381. Ah, I get it. So if I want 20* mechanical, I use 10*.0157, plus the stud size, for a slot size of .397.

Sweet!

_________________
It's a Slant thing. Even I don't understand.

1974 Duster, EFI /6 soon to be turbo...

Get that Monkay! Get that nasty thing!!

GunPilot, you seen to have the right idea. Some distbutors may vary, so measure.
I don't know about an article, it would need more text and pics, this thread created a lot of questions.
There are quite a few of different advance rotors. The first dist I did already had a 9* (or 18* mech) in it. All it needed was a spring change.
This used to be a service almost all speed shops and some repair shops did back in the day of the points dist and the Sun dist machines. When the electronic dist came along, seems like no one bought the new equipment to do them, I don't think Sun even had a machine for electronic dists available for a long time. The only place I know that can do a dist for you is Don Gould in OR. He puts in new bushings, recurves and tests with your ECU or MSD box if you send it to him.

Cecil

A SL6 Distributor Recurving article would be a great addition to our articles section.
Chuck and I are here to help but we do need someone to "champion" the project. All that means is someone should start an article (just the text)and start collecting some images. Starting article text can be as simple as doing a "cut & paste" from this message thread, then do some editing work. Save your draft file in HTML format.

Once you have something in draft, send me a copy and I will help "tune" it. When we think it is good to go, we add-in the images and post it, done.
All the work is getting some nice text "content", I have enough "file photos" to cover the images we would need.
(FYI: Distributor Recurving and Valve Adjustment are two articles I have been collecting information and images on for a while so we can pull something together pretty easy)
Anyone interested should contact me or better yet, just dig-in and send me a draft of an article and we will go from there.
DD

Doc, what about the one you wrote for SSN #31?

That is a good article, too bad I don't have it in electronic format.
I do have a lot of the photos from that one.
DD

bttt.

hi, i bumped bttt to see dist slot pics, but they don't seem to be opening. will check later. ron

They where very helpful pics, I hope they can be re-posted.

Since this topic has been revived, I just want to chime in a correction to a common error in advance calculation. In regard to vacuum advance, it can't truely be added to total timing.

At wide open throttle, the vacuum advance should fall back out, leaving your actual "total" advance closer to only initial + mechanical. During light thottle cruise, the vacuum advance keeps the timing advance high for fuel economy reasons. How high? is a question I've been pondering recently. I've yet to see a good reference as to how much advance at what rpm and at what throttle position/ load the engine is under.

My "best guess" runs o.k., but I'm not getting any noticable fuel economy, (don't ask, it's really bad) and I feel a ping when the vac advance is connected during hard acceleration. Let's not get hung up on my exact problems (the list is too long)

Even if there was a reference for this, you'd need different charts for different weights, and different gear ratios, different compression, different cams, different fuel grades, etc, etc, etc. It would be a thick book to write and would only apply to a slant six. Not to mention you'd need a tach, a timing light, and a dyno to dial into your chart.

Anybody with trusted baseline info on this topic, please chime in.

_________________
1980 Aspen 225 super six

I've studied this stuff for both the / and my 383 and I still don't understand all of it, even though I know what your talking about.
I do know that whatever mine is set up at it runs great. But when I get going any faster that about 75, I start getting some pinging.

Doc, You say the slant likes a lot of initial advance. Would you explain if or why the engine knows if the timing at any given RPM is the result of initial, or advance. If the car is cruising down the road at 40 MPH at 1500 RPM and the timing is at 7* initial, and 15* centrifigal, plus 20* vacuum, how will this be different than 10* initial, 10* centrifigal (at 1500RPM) and 12* vacuum? At that particular moment, both sets ups give the engine the same timing. Is one superior to the other in what happens when you step on the gas and vaccum drops? If you have 2.5 initial, and very early centrifigal, why is 10 initial and a later centrifigal advance inherently better. Is it stability of idle timing, or inital throttle response?
At this point my turbo engine gives preignition under boost with initial timing greater than TDC. I have 22* in the vacuum, so it cruises at the same timing I used to get with 8* advance, and 12 vacuum. What am i losing in this formula? This is a genuine question, and not a retorical argument.

This is the spark table from a 1989 Mustang EEC:

http://www.mustangworks.com/cgi-bin/Pho ... s/fig6.gif

Instead of using a distributor with centrifugal advance plus a vacuum signal from the carb, spark timing is governed by engine load. Load is determined by how much air is entering the engine devided by the total amount of air the engine can injest. It can be thought of as volumetric efficency.

At low engine loads, you can see it takes a lot of spark to ignite the mixture. Closed and part throttle gives high manifold vacuum and low charge densities that burn slowly, so you have to start combustion early. As throttle setting and load increases, cylinder filling increases - charge density increases and your flame front or combustion speed increases, so you need less spark. Remember, the goal is to generate maximum cylinder pressure when the engine reaches maximum mechanical advantage. This is when the engine rotates a specific amount past TDC to where the connecting rod has reached a specific angle to the crankshaft. In most engines - depending on the rod ratio or length of the connecting rod with respect to the crankshaft stroke, this will be somewhere in the neighborhood of 20 degrees past TDC. So you need to vary the spark timing depending on the engine load to give max cyl pressure when you reach max mech advantage. The lighter the charge density (load), the more spark timing you need. The higher the charge density, the less spark you need.

With EFI, you can create a spark map similar to the one in the diagram. However, each engine will be different and it will take a certain amount of tuning to get your spark map to match your particular engine's requirements. With a distributor using vacuum/mechanical advance, this is not so easy. What you need to do is determine what baseline of initial spark you need and then dial-in how much more you can add with mechanical. At WOT where you have essentially zero intake manifold vacuum, your spark will be your initial + mechanical. How much mechanical advance you'll need and how fast it comes in is tuneable. With this as a baseline, you can now add vacuum advance. This works only under part throttle. Again, this is tuneable and needs to be determined for any particular engine.

Generally speaking, you will get better engine performance in terms of throttle response and part throttle power if you run more initial spark rather than less. Let's take one example: In some years, slant's used close to zero BTDC initial plus as much as 30 degrees of mechanical. When you accelerate at low speeds, you loose much of your vacuum advance and are left with your baseline initial plus mechanical. Because you cannot ramp the mechanical spark in too fast, under these conditions, you will have too little spark. If instead, you run a higher initial - something like 15 degrees - with less mechanical, you will have more spark at part throttle and, as a result, better performance. Most slants can take at least 15 degrees initial plus another 13 to 15 degrees mechanical.

The vacuum advance should be used to run as much spark as possible at light throttle (load) and tuned so it will remove enough spark under load to avoid detonation. The idea is to end up with something that will resemble the spark table where you loose spark as you go to the higher load areas. Again - this is much easier when you can use a lap top to change a spark map rather than changing spring weights and adjusting vacuum advance spring preloads. But the idea in both cases is the same. You want as much initial spark + mechanical with the modifier being the vacuum advance. The vacuum unit should add spark in under low load and take it away as load increases.

In the case of forced induction - either supercharging or turbocharging - you have a real challenge if you're using a distributor. Particularly in the case of turbocharging, you need to run a close to normal spark curve, but you need to pull timing in the midrange - and then add it back in at higher RPM. If you had a mass air meter on the car and you could look at the volumetric efficiency of the engine, you need to pull timing several hundred RPM before, during and after max volumetric efficiency to avoid detonation. Another way to look at this is to pull timing at the torque peak. Max torque output occurs at max volumetric efficiency. At higher RPM where volumetric efficiency drops off - even under forced induction, you can add spark back in. How you are going to do this with a distributor and carb is beyond me.

Mitch

Whoa. Thanks Mitch! Great post!

I agree, good info here. (I am collecting this info for an article so contact me if you have other inputs)

Mitch does a good job explaining what needs to happen. Basically we want to run the engine just before mixture detonation, at all times, that is where you get the max. cylinder pressure and the most power.

As noted, this is really hard to do with a distributor but you can get things much better then the factory settings, these were way conservative, especially with the later distributors.

As far as what the engine "sees" and how much advance it runs on at off idle RPM, it does not really matter where the advance comes from, the engine will simply develop a given amount of cylinder pressure (power) when the mixture is set-off. Again, the goal is to set-off the mixture with the spark, right before the heat of compression detonates it spontaineously.

A computer controlled system with sensors does a much better job of controlling the spark for a number of reasons, that is why all of today's new car engines have computer controlled ignition systems.

For our "old" SL6 running a distributor, we can adjust the "curves" to try to better match our car / engine combination, that is what this topic is all about. As MitchB and the timing plots point-out, a Slant can take a lot more initial timing at idle and low RPM then the factory gave it, usually 12 to 16 degrees of initial before you start to detect a mis-fire and manifold vacuum falls off. I tend to find that good initial timing amount as a first step, then build the mechanical advance curve from there. The last step is to add-in the vacuum advance.

Note: with the initial advance, you just have the amount of initial timing to work with, a vacuum gauge is the best tool to help you find that "sweet-spot".
With the mechanical and vacuum advance elements you have the amount of advance and the rate at which the advance comes in to adjust. A timing light with a retard feature or a distributor machine are the tools to use, along with good way to measure "seat-of -the-pants" driving performance changes. (a G-Tech or a Dragstrip time clock)
DD