Programmable MSD 6AL-2 questions

[68Cuda]

Originally Posted by SSLance

Or just tell me what I need the MAP sensor to tell the box what to do...where and when do I want it to add advance and where and when do I not want it to add advance?

Overall adding vacuum advance will make the car run better under "normal" driving and have no effect on performance under full throttle.

So your MSD boost graph in the menu is in PSI. 14.7 PSI at sea level is 0 vacuum... every 1 psi = about 2" of vacuum. So, 10" vacuum is about 10 PSI and 20" vacuum would be about 5 PSI... now, if that does not confuse you, the graph adds retard, not advance. So, not only is the pressure graph kind of backwards to conventional thought, so is the advance.
EDIT: I think this is because this is usually used for BOOST retard... for example 10PSI boost (only airplanes use inches of boost) would be 24.7 PSI absolute. 14.7 PSI absolute pressure is no boost AKA 1 bar, AKA 0" vacuum.

Make sure you have the correct MAP sensor (1 bar) and the setting for that is correct on the MSD.

So, if I want 15 degrees advance at 20" of vacuum, I set 15 degrees more initial than previous, set my retard to 0 for anything up to 5psi, then slowly ramp in 15 degrees of retard all in by 14.5 PSI. This would give you a linear 15 degrees of "advance" from 0 to 20". What is the best curve - I don't know. I think the stock distributers can give you somewhere around 20 degrees at 20". I would start with, maybe 14, see how it likes it, progressively try more. This will cause you to have a higher advance while at idle, but that is OK, it will idle a little smoother, take less throttle screw and be less likely to diesel when you shut it off.

[SSLance]

Originally Posted by 68Cuda

now, if that does not confuse you, the graph adds retard, not advance. So, not only is the pressure graph kind of backwards to conventional thought, so is the advance.

Originally Posted by 68Cuda

So, if I want 15 degrees advance at 20" of vacuum, I set 15 degrees more initial than previous, set my retard to 0 for anything up to 5psi, then slowly ramp in 15 degrees of retard all in by 14.5 PSI. This would give you a linear 15 degrees of "advance" from 0 to 20". What is the best curve - I don't know. I think the stock distributers can give you somewhere around 20 degrees at 20". I would start with, maybe 14, see how it likes it, progressively try more. This will cause you to have a higher advance while at idle, but that is OK, it will idle a little smoother, take less throttle screw and be less likely to diesel when you shut it off.

I don't think I want any more advance at idle...do I? The throttle blades are completely closed and it's idling at 14.7:1 AFR at 850 RPM. Any advance at idle I add just increases the idle RPM and vacuum. I guess I could start fiddling with the idle air screws to dial them down, but am I just chasing my tail here? Heck I was trying to get the initial down to 6* to try to slow the idle down a bit more.

It's good like it is. It drives great, no issues anywhere. Maybe just a bit of a stumble when lugging it with the primary side about half open, but giving it just a bit more gas clears that right up.

I guess maybe that means it wants a bit more advance say at 1000-1500 RPM with high vacuum...right? and as soon as that vacuum drops (throttle opens) take that advance away so it doesn't detonate.

[68Cuda]

Originally Posted by SSLance

I don't think I want any more advance at idle...do I? The throttle blades are completely closed and it's idling at 14.7:1 AFR at 850 RPM. Any advance at idle I add just increases the idle RPM and vacuum. I guess I could start fiddling with the idle air screws to dial them down, but am I just chasing my tail here? Heck I was trying to get the initial down to 6* to try to slow the idle down a bit more.

It's good like it is. It drives great, no issues anywhere. Maybe just a bit of a stumble when lugging it with the primary side about half open, but giving it just a bit more gas clears that right up.

I guess maybe that means it wants a bit more advance say at 1000-1500 RPM with high vacuum...right? and as soon as that vacuum drops (throttle opens) take that advance away so it doesn't detonate.

I can't imagine you are going to surpass 20". I say try it out. It would only take a few key strokes to change it back.

[SSLance]

I'm not sure why I'm not getting this, I'm certain that the people that are helping me know what they are talking about, but it's like they are talking in different languages and it's not making any sense to me at all.

And I don't want to be the guy that is asking someone else to do the work for me, I really want to understand this myself.

The MSD Tech replied with this...

you will use the boost curve....and a 1 bar map sensor ( typical gm from 92 to 96 car or truck )

remember :...with key on ...a thin red line will appear...
left of the line is vacuum
right of the line is boost

and this graph...



I guess what I'll do is rig up a long piece of vacuum line and get the vacuum gauge and my laptop (hooked to the MSD) in the passenger area of my car and go for a little drive. I can't really see what the vacuum and advance are doing in their current state without putting a load on the engine, which I can't do in the garage.

Maybe once I see what all of the gauges and graphs are doing as I load the engine, I'll have a better idea of what to change to make it better.

I'll probably hook the GoPro up pointed at the gauges as well to record what I'm doing.

I took the AFR meter out as it was not hard wired yet and I didn't want the fab shop to have to worry about heating the O2 sensor up every time they moved it...maybe I'll throw it back in the car as well to watch it too since the fab shop won't be ready for me until this weekend now.

Wish me luck and thanks for all of the help...I really appreciate it.

[68Cuda]

Originally Posted by SSLance

and this graph...

OK... is this your boost retard graph, if so it is currently doing nothing!

This is an "example" curve to dial in 10 degrees of retard progressively with 10 PSI of boost. Your car is naturally aspirated - correct?

The 14.75 point you see on that chart where the red and blue meet is "0" boost -or- "0" vacuum. It is the 14.75" reference point of atmosphere, the same pressure as your outside air.

If you are not running boost, to get 14 degrees of "advance", do the following: From 0 to 5 PSI on that chart set the "degrees of retard" to "0" (same as it is now). Then, from 5 PSI to 14.5 PSI have the chart slope down from "0" to "14" degrees of retard. From 14.5 PSI to the right leave it at "14". After setting this curve, set your timing to your current 7 degrees initial with the vacuum unplugged from the MAP sensor. When you plug in the MAP if the vacuum is 20", it should jump to 21 degrees. If you have a hand vacuum pump you can test this through the range to make sure it does what you think. Anytime you want to disable the advance and go back to your base curve it can be as simple as unplugging the vacuum line from the MAP and capping the manifold port. This will dial in the full 14 degrees "boost retard" and effectively put you bask to your 7 degrees base timing.

I took the example they sent and superimposed the curve I described as a purple line, why MSD does not illustrate this is beyond me. If you like the outcome and want to dial in more, say, up to 20, then instead of going to 14, go down to 20 at the same point and reset your base timing.

[SSLance]

Okay, I think this is starting to sink in...thanks...

How my brain looks at this now is 14.75 PSI on the graph is zero vacuum or WOT and at that point there is no advance from the MAP sensor\boost retard curve. When the throttle is closed and vacuum increases, the graph moves to the left and the further it goes to the left the more advance is being added to the distributor. Furthest left is full advance at max vacuum at idle (based on your curve shown anyway).

Last night I drove the car with the laptop and vacuum gauge inside the car so I could watch. At idle and when at high vacuum when running the red line in the Boost Retard graph stays right around 7.00 PSI with the vacuum gauge at 20"

I never saw the red line go less than 7.00 PSI. So should I start the left side of the curve at 7.00 PSI or at 4.00 PSI as in your graph. And the bottom of the curve should level off at 14.75 PSI all of the time right?

The one thing I'm still fuzzy on, the relation of the 20" on vacuum gauge to 7.00 PSI on the graph.

[SSLance]

Sometimes I do some of my best thinking in the shower in the morning...

I did some playing around with the curves, please let me know your thoughts on these. I haven't put them in the box yet.

First, my engine likes 7* at idle and 32* total. Actually I think it would like 6* and 33* better, more on that in a minute.

Here is my current Run Retard curve



I don't think I want to add any vacuum advance in at idle, I think I want it to kick in just off idle...so here is my idea for a Boost Retard Curve...



It currently idles at 850 RPM and around 7 PSI

Now...and this is where my brain starts kicking in... Can I use the Boost Retard Curve to pull even more timing out at idle, say instead of the curve being at 14.0 from 6 PSI to 8 PSI, can I put that at 16.0 to pull another 2* of timing out at idle?

This would let me bump my full advance up to 33* and still let it idle at say 6*...

[68Cuda]

Originally Posted by SSLance

I don't think I want to add any vacuum advance in at idle, I think I want it to kick in just off idle...so here is my idea for a Boost Retard Curve...

I would not make sharp transitions like that, it could cause the engine to surge, oscillate between two points or do something funny. Sharp transitions in control systems are never a good idea.

Why the aversion to running so much advance at idle under no load? It may actually let the engine run cooler. As soon as you put a load into it the advance will drop out. People run locked out distributors at full advance on race cars, I don't think your idle speed is going to run away or anything funny. Just try it out and see how it behaves, you can always plug off the vacuum port and disconnect the MAP sensor until you can change it back if it does something you do not like. Disconnecting the MAP sensor will trick it into thinking you have no manifold vacuum and it will pull out all the timing. If you are worried try a smaller amount to begin with, like 7-10 degrees. Just match the amount of advance you put into the initial timing with the amount you take out with the "Boost Retard Curve".

It doesn't matter much if you start at 4" or 7", it just changes the amount of advance a little. Starting at 7" should be fine since that is where your MAP sensor is telling you the engine runs. You can also cheat the end point on the right a little. When you disconnect the vacuum from the MAP does it read 14.75"? I would want the full amount of spark advance pulled out before that point, maybe set it to 14" even to have some margin

The difference between 5" and 7" could be the MAP sensor calibration or your vacuum gauge, either or both could be a little off. 20 sounds like a lot to me for an engine that is not stock, maybe your gauge is a little generous or non-linear. It does not matter much, it is just a reference point, use the MAP sensor since it will be controlling the spark.

[SSLance]

Now that I'm getting a good handle on the timing and can make changes easily and monitor them immediately, I'll get back with my carb guy and discuss how to make changes to the carb as well.

As the carb sits now, with 10* of initial timing it was idling at 1100 RPM with 23" of vacuum with the throttle plates closed. If he says it's an easy enough adjustment to the idle\air screws (that a noob like me can handle) I'll make those changes today and report back.

I put the O2 sensor and air fuel meter back in the car last night and plan on driving it to work today and should have some time to tinker, drive, tinker, drive etc...

Thanks...

[SSLance]

Pretty interesting and educating trip to work this morning. I was mainly looking at vacuum and AFRs at cruise speeds.

In 5th gear at around 2200 RPM (70ish MPH) the engine was extremely happy. Pulling 20-22" of vacuum, barely touching the throttle, would pull any hill with no effort...and AFRs at around 15.7:1 and steady.

I didn't have the laptop hooked to the Ignition box so I don't know what the timing was or what the MAP sensor said the vacuum was...but with no Boost Retard Curve installed, it was just Advance Retard working which would have put the timing advance at around 20* advanced.

In 6th gear at around 1700 RPM (70ish MPH) the engine was not happy. Vacuum was bouncing between 10-15", way more throttle input to keep it moving, lugged down on hills...and AFRs got fat, down to mid 12s to 14:1. Timing would have been around 15* advanced.

Engine certainly needs vacuum advance if I want to use 6th gear on the highway.