Hi all,

I know this has been addressed a few times with how to do it, but I'm curious what you all think is a good curve to recurve to.


I mapped out my distributor's curve a little while ago and will post it up for reference:

First is the base timing, which, when I time for best vacuum, generally averages around 10°.

Next is the mechanical timing:
My engine:

RPM - Advance
0750 - 0
1000 - 0
1250 - 0
1500 - 3
1750 - 9
2000 - 12
2250 - 16
2500 - 18
2750 - 18
3000 - 18

So, my mechanical advance has a range of about 18° and doesn't kick in until around 1400 - 1500 RPMs. How does that affect acceleration from idle to 1500? For a 300 I6 that can often live in this range, no mechanical advance is even taking place yet.

My mechanical advance is then fully in by around 2500 RPMs. This seems pretty good to me, but I could be mistaken.


Next is the vacuum advance:

Vacuum (hg):
12 - 24
11 - 24
10 - 24
9 - 22
8 - 20
7 - 18
6 - 15
5 - 13
4 - 12
3 - 10
2 - 3
1 - 1.5
0 - 0

In this case, any engine vacuum above 10hg is giving me FULL vacuum advance.
The big drop off in vacuum advance is when it drops below 3hg (rare). So that means most, if not all the time, I have an extra 10° advance from my vac advance canister. Seems kind of odd to have vacuum advance almost all the time. Maybe not though?


This is the curve that was on the distributor from the parts store. The one I had recurved myself a few years ago had some issues so I had to take it in for warranty. I haven't had a chance to recurve it again.
What are the positives and negatives of this advance curve? What would be recommended? Why?

Thought this may be good info for everyone, instead of just "following the recurve instructions" without knowing why.

 

The cool part about charting this out is that if I have a vac gauge in the cab, I can know my engine timing at any given moment.

For example, if I'm at 65mph in 3rd gear:
2150 RPMs
vac = 10hg

10 initial + 14 mechanical + 24 vac advance: = 48 total advance

If I'm at 65mph in OD
1550 RPMs
vac = 7hg

10 initial + 3 mechanical + 18 vac advance = 31 total advance

Shows me I have a lot more vacuum advance in 3rd at higher RPMs.


Another example would be lulling around town at 35mph in 3rd.
1150 RPMs
vac = 14

10 initial + 0 mechanical + 24 vacuum advance = 34 total advance.

That's more advance than I have on the interstate in Overdrive...
Good to know!




Now, how does that type of timing curve weigh in on low RPM acceleration?

 

I run with a gauge too.
Your mechanical advance would benefit by starting at around 1000rpm, really anything above idle.
Having it maxed at 2500 is good and ideally should be a straight line curve.
Try setting the timing at 32 or 34° at 2800rpm and see where your initial then falls. You may well be into 15 or 16°, but if it starts easily you might notice a lot better response.

Vacuum advance is a little more tricky. You should easily be able to handle around 50° at cruise. General rule it to go as far as you can without pinging. for best gas mileage. You might have to limit the amount of travel. Try and use manifold vacuum.

Remember that mechanical advance is strictly rpm related, nothing at all to do with vacuum. You might get similar readings, but vacuum is controlled by the load on the engine and will vary greatly even at the same rpm. Even though you can time an engine well using vacuum, that is only one specific running condition.

The only way to curve the dist properly is on a dyno, but you can use timed runs up to a certain rpm and get pretty close.
The goal is to always have maximum cylinder pressure at roughly 15 to 20° ATDC whether at idle of max rpm.

 

i like this thread already. very, very interesting stuff regarding the vac advance.

i use Microsoft Excel to graph my advance curves. unfortunately (for the sake of this thread, at least) all my digging has been done on my 390, not my 6. the graph makes it easy to visualize what is happening.

i agree with Tiap- seems like the mechanical should come in sooner, especially on such a low-revving motor.

the last vacuum advance can i bought had an instruction sheet. it was screwed all the way down (no movement), and then listed how many turns out you should adjust it, depending on which vehicle you were using it on. not that the spec sheet is the authority- but it was interesting to see how the different models were spec'ed for different levels of vac advance.

Is 18 mechanical enough? i know they say the FE engines should have 36-38 total, but i've not done enough reading on the 6 (yet).

 

Thanks for the responses. I've done a lot of research on how each of them work together, but no one really ever goes into the specifics of what a good curve is. Just how to recurve it, how static, mechanical, and vacuum advance all come in to play separately, or how to install a specific kit.


I agree that the mechanical advance isn't coming in soon enough. What noticeable effects would this have?
It's on manifold vacuum right now, so, honestly, I should have around 25° - 30° advance (10° static, 0° mechnical, 15° - 20° vacuum) off the line with gentle acceleration.


I've heard that high 40s, low 50s for cruising advance is just about right. 48° for me is pretty good, although higher may net me a little more mpg. Thoughts? I imagine I could both fix this, and the low RPM timing, by bringing in the full mechanical advance sooner. 18° total doesn't seem like a lot either. However, I have heard that a smaller range is better for performance, since you can just start it out higher.


I haven't opened up this distributor to see what the mechanical advance is limited to. I'm guessing it must have a 9L slot (since the slots are labeled by half the advance they give). Interestingly, my last distributor was MUCH more, I think a 14L, but didn't reach full advance until the 3000s. I'm curious how one would know what curve they're getting in a distributor from a parts store?

These distributor's are kind of a pain to get the reluctor off because there's nothing to brace a screwdriver against to pry on it. This is the picture I've seen shown several times:


(Borrowed from Reincarnation Automotive)

That one clearly has a raised edge around it that's great for prying against. The 300 DSII doesn't have that, so there's nothing to pry on. I've bent the cover plate before and it's a pain to straighten back out and can cause the magnet to hit the reluctor.



Now, as for increasing my static timing to, say, 14° to 16°, how does that work with setting timing by vacuum? If I time by vacuum and my results average around 10°, why would I change that?

 

Another question is, is the statement "the most advance you can give it without it pinging" applicable under all conditions?

As such, right now, if I accelerate gently, and never let the vacuum drop below 10hg, my timing advance is really up there.
1000 - 2500 RPMs is 0° to 18° advance. Anything above 10hg is 24° vac. + my static.

So that means while I'm accelerating moderately, I go from 34° to 52° advance. That seems like a lot.

If I give it some modest oomph, and let it get into the powervalve (but don't floor it and pin the vacuum to 0), it still goes from 25° to 43° advance.

Normal? Or can too much advance make it sluggish as much as too little?

 

i have always thought that setting the initial timing by vacuum is a poor idea...because it does not take into account the mechanical or vacuum advance.

its a great way to make the truck idle smoothly...but that's it.
who drives around town at idle?

for example- lets say setting your initial timing with a vacuum gauge puts you at 10BTDC.

but say (hypothetically) the dizzy is using the 15L slot, and you've got 2 light springs in.
BAM you've now got 40 degrees advance by 2500 rpm, and that's *without* your vacuum can pulling in the other 24, for a total of...64!!

timing by vacuum is dangerous.

(btw- i think 24 is a LOT of vac advance. the can on my FE maxes out at 8. you might be better off to trade some of that vac advance for mechanical.)

 

My assumption on this particular distributor is that the small amount of mechanical advance was meant to be offset by the large amount of vacuum advance.

As you can see, anything above 3hg engine vacuum (which is pretty much 99% of the time) is an entire extra 10° advance. It's almost as if my static timing is set at 20°.
I'm not saying that's ideal, just that that's how it's set up right now.

The problem in my particular setup comes in at WOT. From what I've read, a good 30° - 36° advance is pretty ideal.
As I have it now, at WOT, I'll never even come close.

10° static + 18° mechanical + 0° vacuum = 28° total
And that's only above 2500 RPMs.
From 1000 - 2000, it goes from 10° to 22° advance. That's not very much, and maybe explains why my engine falls flat on its face when I try it.


As for moving on, and designing a curve...

The distributor needs lighter springs, that much is for sure. From what I've heard, this kit is the one for the DSII distributor. I've heard differing stories as to whether or not one or both springs should be used.

Mr. Gasket 925D - Mr. Gasket Advance Curve Kits - Overview - SummitRacing.com

This'll allow the advance to come in sooner. Right off idle would, I assume, be good. If I left my mechanical advance range as is, it would look like this:

RPM - Advance
0750 - 0
1000 - 3
1250 - 9
1500 - 12
1750 - 16
2000 - 18
2250 - 18

The next step would be increasing my mechanical advance. This can be done by opening up the gap. Often, most Ford distributors come with two settings inside. When you open it up, you can remove and reinstall it to the other setting. Won't know what other setting this one comes with until I open it up.

But, lets say I open it up to 24° total readjust the springs to bring that all in by 2000 again.

RPM - Advance
0750 - 0
1000 - 4
1250 - 10
1500 - 16
1750 - 20
2000 - 24
2250 - 24

Then, bump my base timing up to 12°

Then, at WOT, I'd be at 12° + 24° = 36°. A little bit better! Plus, this is by 2000 RPMs, not 2500.
Then, from 1000 - 2000 RPMs, I'd be at 16° to 36°. Definitely a bit more aggressive than the 10° to 22° it is now.



From what I've read, vacuum advance comes last after everything else is sorted out.
If I do the changes above, I'll need to drastically reduce when the vacuum advance comes in. If it still is adding 10° by 3hg vacuum, I'm going to be at 25° to 50° advance under moderate to aggressive acceleration. I'm probably going to be pinging quite a bit.

I know adjusting the spring inside the canister will limit when it comes in, but any idea on how you can change the rate of speed it increases? That 7° jump with 1hg vacuum (between 2hg and 3hg in the chart above) is pretty severe.


But, in the end, something like this would be preferable:



Vacuum (hg):

11 - 18
10 - 18
9 - 18
8 - 14
7 - 10
6 - 6
5 - 4
4 - 2
3 - 1
2 - 0
1 - 0
0 - 0

That'd get rid of that advance spike, and help to eliminate ping under even moderate acceleration. Plus, when I'm cruising, I'd have an extra 18° advance.

In the end, that'd give me around 54° advance for cruising as opposed to the 48° I have now. Or is more even better?

Thoughts, changes to this recurve layout?

 

The basic reason for more advance from a vacuum advance if because the mix is much leaner and the flame burns much slower so it can use a lot more advance.
That is why at idle your initial + vac advance is fine around 25°.
Under acceleration you have a much, much richer mix so you can have that same 25° but only with inititial + mechanical, but no vacuum advance since the vacuum should be at 1in". The richer mix burns much faster, so less advance is needed.

Remember your always looking for peak cylinder pressure at around 15 to 20° ATDC and that gives you the most power, therefore the best economy at any rpm.
At cruise rpm, you can add a lot of advance only thru the vac unit, because the mixture is leaner and that is why you can advance by changing only the vac can till just before it pings.


That's why I mentioned above to try setting your timing at over 30° at 2500+ rpm and if it falls at anything under 18° initial, it is fine as long as it starts well.

 

Thanks Tiap, I'll give that a try.

Probably should clarify a bit on the point of this thread. There's seems to be a wealth of information out there about WHY to recurve, the theory about what you're trying to achieve, and walk-throughs about how to disassemble, adjust, and reassemble the distributor to do it, but there's very little information on what to shoot for.


Seems like there's a lot of this:

"Well, for optimal performance, you'll want to recurve your distributor."
"Okay, what's that?"
"Read this, it'll explain how an ignition curve works."
"Awesome. Okay, so, now, how do I do that?"
"Read this walkthough."
"Okay... What do I recurve it to?"
"....."
"Anyone?"
"....."


So, I'm trying to put a guideline together and using my "stock" ignition curve as an example. What's wrong with this curve. How could it be improved. What should I aim for?

Okay, so peak cylinder volume at any given RPM is the best. But how would one know if they've achieved it?
Most don't have a dyno accessible, and with as long as it takes to open up a distributor and make changes, it would be nice to have a good starting point. A timing map to aim for.

Is it more desirable to start with a higher static, and a quicker, yet smaller amount of mechanical?
(ie. 16° static + 20° mechanical)
Or is it more desirable to start with a lower static, and have a larger amount of mechanical?
(ie. 10° static + 26° mechanical)

For a 300, what's a good RPM to have all timing in by? What a good total timing for WOT? What's a good timing for cruise?

Stuff like that.



Also, now that I just put a tachometer in my other Bronco yesterday, I'll see if I can get the curve on that one too. It'd be interesting to compare.

 

"Typically a high performance application needs full centrifugal advance in by 2,500 to 3,000 rpm. Towing and heavily loaded combos typically need to have full advance delayed to 3,500 rpm or thereabouts. Each combo is unique and you will have to make adjustments based on expected use and Compression Ratio." Got that bit of info from here:

Duraspark_distributor_recurve_instructions_page-2

 

you're absolutely right. my take on it:
1. the race guys guard their secrets
2. the guys who offer recurve services can't make money if they tell you how to do it
3. and us regular guys...just don't know.

your thread here is the most informative one i've seen for the normal guys. thanks for posting it.
(sorry i don't have more concrete info to give...i'm working through the same questions with my rig.)

 

http://www.msdignition.com/instructi...df?terms=85551

MSD's tech sheet for their dizzy has some graphs of the different curves by their spring combinations that may help with visualizing how a curve might look.

Depending on the cam, I think you're better off with a lower static timing and more mechanical. Assuming the engine likes it. Ultimately it will be performance that dictates what the correct timing curve would be for any given engine.

Unfortunately the only way to nuke the site from orbit is to recurve it, graph the curve, drive, and repeat until it feels best.

 

i had trouble starting the motor at 16 initial. you have an advantage here, though- the Duraspark's ignition retard feature. i'm working with a points distributor.

 

After reading this post, I was reminded of something the FTF once wrote. At one time, he had a similar situation and wired a dash mounted switch which when pressed, cut the power to the ignition. That way, when he activated the starter, the engine would spin up to speed and then he would release the switch and the engine would fire [already having the inertia of the rotating crankshaft assembly]. Ingenious!