Timing Curve

[midnight340]

Finally got warm enough that I got around to graphing my timing curve. 340 with MSD Ready-to-Run distributor, 2500 PTC converter, 4:10 gears.

Right now timing is at 18/34, (may bump it a degree two.) I’m thinking maybe I should bring the curve in a little sooner???

 

[Mattax]

Here's my 2 cents.
N to D drop is good. Leave the initial at 18 at 700 to 800 rpm.
It might be a little more chipper coming off idle if the advance started before 1200 rpm.
> On a Chrysler advance, that would be done by reducing the primary spring's tension. If the MSD does not havea primary and secondary spring, might have to take a little tension off of both.
> This suggestion assumes ported source for vacuum advance. Not sure sooner mechanical start will be helpful if using manifold source.

The touchy area for too much part throttle timing is 1800 to 2800 rpm. With the steep gearing in the car it may not cause audible gurgle. Or it may not show up until the engine is heat soaked. Heat soaked being something like 2 - 3 hours into a road trip.
Bottom line is that I'd stick with Chrysler's recommendation on that total for timing at cruise rpm, 50*. Take a little vacuum advance out of the curve.
>Should be able to do that by increasing the spring pressure on the diaphram - and that should delay the start. I haven't messed with MSD distributor but I'm assuming that option is there.

The AFR caught my eye.
Part throttle AFR should go leaner than cruise AFR.
If the car cruising at 60 mph, 17"Hg, 3000 rpm and the AFR is 14.2:1
Then as throttle is applied for a hill or increase of speed, AFR should go leaner until the throttle is so far open that vacuum drops to 8 to 11" Hg. Only then (when the power valve or step up rod opens) should the AFR get richer.

Its best to make the timing change or make the carb changes and retest. The AFR interpretation by the WBO2 often will change with a timing change. So its best to have a new baseline AFR or we will be unsure whether changes in the AFR readout were caused by carb adjustments or timing change.

Chrysler recomendation reprint in the last illustration How To Limit and Adjust Chrysler Vacuum Advance Cans

 

[midnight340]

Thanks, Mattax! I’ll ponder this a bit then respond more. I appreciate the helpful drawn curve lines.

 

[Mattax]

Maybe the part throttle AFR is not going upside down. That's more of a modern holley thing. With the T-quad, it might just be going rich at a relatively high vacuum or your part throttle is more like 3/4 throttle rather than something like 1/3 to 1/2 throttle.

 

[midnight340]

1) Yes, Mattax, there is the issue of what "part throttle" means, huh?
Just took a half turn out of the rod tree setting, went for a drive, and at about 1/3 throttle it is now going 1/2 to 1 point leaner on the A/F till about 2/3 throttle, then it's getting richer... the 13.5 or so. So we're probably good there.

I opened the air door just a wee bit more. Is that the right way to go on the TQ??? Seems like more air with bit wider opening would lean it, but still not sure how the door works next to the spray bars. Didn't change much. Still pretty rich at WOT. The motor does seem to like it, but if I can lean it a bit without having to change the jets for now I'd try that.

2) As to the curve, right now the MSD has one heavy and one light silver spring in it. Their next quicker curve uses two blue light springs the same weight, but doesn't show it coming in earlier, just steeper. The step after that is to use one silver light and a blue light spring, so I may try that, see if it helps the bottom end.

(I forgot to mention it is a 9:1 motor (actual, cc’d) so (likely) can use more total with vac. adv. than for higher compression motors, correct? And with lower compr. I've read that going higher than 50 is OK???)

 

[midnight340]

I've got a few out of the way areas to test drive, but it's kind of hard to accurately test WOT conditions with winter streets/roads since they tend to have a bit of sand on them!!!!

 

[Mattax]

1) Yes, Mattax, there is the issue of what "part throttle" means, huh?

Cruise too! For some guys think it means tooling along at 25 mph...like cruisin for girls in town.

Just took a half turn out of the rod tree setting, went for a drive, and at about 1/3 throttle it is now going 1/2 to 1 point leaner on the A/F till about 2/3 throttle, then it's getting richer... the 13.5 or so. So we're probably good there.

Definately in the ballpark now.

I opened the air door just a wee bit more. Is that the right way to go on the TQ??? Seems like more air with bit wider opening would lean it, but still not sure how the door works next to the spray bars. Didn't change much. Still pretty rich at WOT. The motor does seem to like it, but if I can lean it a bit without having to change the jets for now I'd try that.

The air valve controls when and how much additional air-fuel mixture to provide. The secondary jets control how rich or lean that additional air will be.

2) As to the curve, right now the MSD has one heavy and one light silver spring in it. Their next quicker curve uses two blue light springs the same weight, but doesn't show it coming in earlier, just steeper. The step after that is to use one silver light and a blue light spring, so I may try that, see if it helps the bottom end.

There should be a way to keep the same springs but change the initial tension. The Chrysler built advances have ecentric mounted spring perches which can be rotated. Prestolite and Mallory advances require bending the spring perch slightly inward to reduce tension.

(I forgot to mention it is a 9:1 motor (actual, cc’d) so (likely) can use more total with vac. adv. than for higher compression motors, correct? And with lower compr. I've read that going higher than 50 is OK???)

At idle speeds the low compression and high overlap will typically produce an exhaust diluted slower burning mixture. So at 800 rpm it makes sense that the engine likes 18* BTDC. Once the rpm starts getting up into a range its working then I wouldn't assume its any slower or less efficient of a flame front than a stock version of the engine. A lot of factors come into play, heads, compression, squish, cam, fuel...

That's why I figure if you're looking for starting points, go with the ones published by Direct Connection/Mopar Performance. They probably had more than a gut feeling that those numbers work.

I thought it was pretty interesting that the factory timing curves I looked at add up to roughly the same 50* BTC in the highway crusing rpms (3000 ish).

 

[Mattax]

MSD is a GM Delco type advance, right?
You might have to bend one of the spring loops a little bit.

There are two variables with every spring:
Spring rate.
Length.

The force restraining the weights from moving depends on how much the springs have stretch.

L i is the initial length of the springs when stretched over the perches.
The amount the springs stretched or extended is the variable MSD doesn't mention.
But that's the one that lets us change the starting rpm without changing the slope.

You could try to so some measurements with calipers and math first, or just dive in and bend the loop of one and see what happens.
Force = spring rate x distance stretched.

 

[Mattax]

I've got a few out of the way areas to test drive, but it's kind of hard to accurately test WOT conditions with winter streets/roads since they tend to have a bit of sand on them!!!!

If those AFRs are at the begining of WOT, I wouldn't worry about it. It takes a bit to stablize. Just read a post where a guy said he couldn't fix it with a key pad. So its not just carbs. LOL

Not the best example but shows the idea. After the shift into higher gear and the load was higher the AFR stabilized.

 

[midnight340]

So simply lengthening one of the loops on the lighter spring would start the curve sooner but keep all else pretty much the same? Do I have this right?

 

[Mattax]

So simply lengthening one of the loops on the lighter spring would start the curve sooner but keep all else pretty much the same? Do I have this right?

Yes. First part Yes.

With the MSD I don't know which spring is the more likely to effect the start.
I'd measure the length of both springs (inside of loops), and the perch distance.
If they are the same length, then I'd probably make the heavier spring longer.

If they're not the same length, then its math time or gut feeling.
Could take one off, feel how much resistance there is when you try to move the advance. Then swap them and see if there is more initial with the other.

In Chrysler and Motorcraft distributors the heavier spring has slack when installed. Some mallories too. So with those the heavier spring does nothing at idle and low rpm. That may not be true at all with the MSD.

 

[midnight340]

OK, thanks! On second reading I think I understand re: spring length. Same length, then obviously the heavier spring is mostly holding back the starting point. Different lengths, maybe not.Will play with the springs.

And thanks for the tip to pay more attention to the later part of WOT.

 

[512Stroker]

Mattax - Great write ups
This is what a Tech Forum is suppose to be all about, one member helping another.
Keep posting I love it.

 

[midnight340]

I haven't had time to graph another ignition curve, but I did decide to test lighter springs and stretched the ends about .015"-.020" from factory MSD length so it would come in earlier. Every day is different with changing temps and air pressure, etc. but I can tell the bottom end (off idle) is quite a bit more responsive. It will be interesting to see at what rpm gives full advance. Next warm day!!

 

[Mattax]

One change at a time!

 

[midnight340]

OK, OK, hard not to start having "more" ideas when trying to do one first thing! So, yes, I wanted to start the curve earlier as you suggested, with some slight lengthening of spring. But thinking it needed to get to total advance sooner as well (to match what I thought I saw in your drawn curve.) I didn't think starting earlier would affect that, but wasn't sure.

These comments are based on my assuming that you were advising to not change spring length AND stiffness at the same time? Correct?

I'll get another graphed curve in a few days and see where it's at. I do really appreciate the help in understanding all this!!

 

[midnight340]

Too curious, had to see where the new curve is. I took it to a remote parking lot so as to not disturb neighbors. Hooked up the tach and light. Right away I could see it was coming in too soon, too fast, so pulled the springs from the MSD, and with needle nose pliers did some "field adjustments" shortening a loop a very small amount on each spring. (one silver light, one blue light)
Fired it up and this is what I now have:

(Red line is the new curve after previous changes!)

 

[mbaird]

How did you plot the vacuum advance ?
You should have another Horizontal scale below the RPMs for inches of Hg.

 

[midnight340]

Well, I'm sure the vacuum advance is not meaningful as there was no load on the engine. I'm still learning a lot about the interrelationships here. A/F #'s are better than they've ever been. (details on that in my Thermoquad threads.)

Vacuum I've been watching. (Ran a hose to inside and have a large vacuum gauge hanging on the mirror.) So far it does seem like the vacuum is creeping a bit higher as I get things tweaked better. Does that make sense?

This cam is 235/243 at .050, more than I've had before, but torque is good, and the power brakes have worked great.

 

[RustyRatRod]

Now that looks ok to me! I don't even include vacuum advance at all. I just get initial and total right then plug in the vacuum can and go. .........unless sometimes if it's adjustable, I "might" tailor it a little bit.

 

[midnight340]

Thanks, RRR, I'd feel like I was doing something really wrong if you didn't have some kind of opinion about this!!

 

[Mattax]

Well, I'm sure the vacuum advance is not meaningful as there was no load on the engine.

Agree but you can make use of it. **

Vacuum I've been watching. (Ran a hose to inside and have a large vacuum gauge hanging on the mirror.) So far it does seem like the vacuum is creeping a bit higher as I get things tweaked better. Does that make sense?

Yes.

This cam is 235/243 at .050, more than I've had before, but torque is good, and the power brakes have worked great.

I'm impressed too. Mine is around 11"Hg at idle (N) with 230 @ 50. I'm sure this is due to the tighter LSA shifting the intake closing and maybe a little bit to do with the lobes Hughes used at the time.

** If you have the vacuum for each of those measurements timing with vacuum advance, then subtract mechanical from the timing measured. That will reveal the vacuum advance contribution..

Using your first graph:
900 rpm, 33 - 18 = 15* at what vacuum ?
1250 rpm, 40 - 18 = 22* at what vacuum ?
and so forth.

You already know that the engine crusing vacuum is around 17"Hg. With the vacuum advance graphed then the amount of timing added during cruise will be known.

 

[mbaird]

Mapping the vacuum adv is not real hard. Get a Mighty Vac .. they are pretty cheap.
Once you have done that you will be able to visualize whats happening a little more clearly.
Its actually kind of fun !

 

[Mattax]

(Red line is the new curve after previous changes!)

All I can say is try it.
I think you know my opinion, but in case not, I'll repeat. LOL
The new timing from 1800 rpm to 2500 may result in pinging during part throttle acceleration and uphills.

You now know how to change it, so you can experiment. No harm, no foul. Just back off the throttle if it starts to detonate under heavy load - that's when there can be serious harm.

 

[midnight340]

I do have a Mighty Vac so will put this on my “to do” list.