YOUR THOUGHTS ON THESE ADVANCE CURVES PLEASE

[CFD244]

Hi Folks

I picked up a distributor machine and have been fooling around with it a bit. The picture is the plotting of 4 different distributor advance curves. The associated tech info is from my 1971 Demon with 180lbs cranking compression. Let's hear your thoughts on the different plottings and how they would react in my combo. FWIW, the engine runs with about 17 HG manifold vacuum (idle), and the vac cans will give me another 18 degrees at that level.

Thanks FABO

Edit: The OEM distributor is of unknown origin.

 

[CFD244]

I thought for sure this post would spark some conversation.......Anybody?

 

[RustyRatRod]

No offense, but I caint really make heads nor tails out of your graph. Don't feel bad though. Mine would be even worse.

 

[4spdragtop]

I'm no help Troy but following. I know my initial is 18*. In my notes somewhere.

 

[CFD244]

No offense, but I caint really make heads nor tails out of your graph. Don't feel bad though. Mine would be even worse.

None taken.......Art is not my strong point

4 different distributors with their respective curves. 2 in pen, 1 in black marker, and the other in blue marker.

X axis is engine RPM starting at 0 and going to 3500 rpm.

Y axis is degrees of mechanical advance starting at 0 and going to 20 degrees.

**Example** with the blue marker line.......Advance starts at 1000 RPM and has 2 degrees by 1500rpm. By 2000 rpm 8 degrees; 2500 rpm 10 degrees; 3000 rpm 12 degrees, and by 3500 rpm 14 degrees where it will be limited.

The other 3 curves are read the same way......All curves will be limited at 14* since my initial is 20*

LOL...Does this remotely explain my atrocity, or am I still whacked

 

[RustyRatRod]

The only thing of substance I can add is this. Just try SOMETHING. That gives you a starting point. No one can say "this curve" or "that curve" is right for your engine, because they are all different. Just put a curve in it and see how it runs.

 

[CFD244]

Thanks for the reply. I guess I'm trying to understand the science. I thought this curve info and combo info. would be enough to get me started. I suppose that I'll have to figure it out for myself.

I appreciate you chiming in.

 

[RustyRatRod]

I think your "all in" RPM is a little low, IF I am reading it right.

 

[NCmtnDWELLER]

Contact our (own) distributor specialist (member) he can certainly assist if needed !!

 

[CFD244]

I think your "all in" RPM is a little low, IF I am reading it right.

Limiting the mech advance to 14* gives me an "all in" of 34* at 3500 rpm. (not including vac)

 

[SSG_Karg]

At first glance I like the looks of the black marker curve (MP dist.). But more info is needed on the combination. What rpm does it turn at cruise?, what is the general driving style?, what fuel are you usually running?, etc.
I would probably start with the MP curve and test, read plugs, and repeat.

 

[Newbomb Turk]

Those curves look pretty steep but it looks like you didn’t add the initial in at the start of the curve.

I’m not sure which advance mechanism they have. Some are easier to get the curve in shape than others.

 

[1MeanA]

I like the MP as timing is all in at 3000 rpm. Isn't 20 degrees a lot of initial timing?

 

[CFD244]

At first glance I like the looks of the black marker curve (MP dist.). But more info is needed on the combination. What rpm does it turn at cruise?, what is the general driving style?, what fuel are you usually running?, etc.
I would probably start with the MP curve and test, read plugs, and repeat.

Driving will be a combo of mostly cruising and city with stepping on it for passing. Car cruises at about 2500rpm @60mph......fuel octane would be limited to 191.

 

[CFD244]

Those curves look pretty steep but it looks like you didn’t add the initial in at the start of the curve.

I’m not sure which advance mechanism they have. Some are easier to get the curve in shape than others.

The curve plotted does not include the initial. Just the mechanical advance was plotted. The MP distributor is the gold shaft type with the light weights. The other 3 are regular weighted type mopar distributors.

 

[CFD244]

I like the MP as timing is all in at 3000 rpm. Isn't 20 degrees a lot of initial timing?

The engine actually seemed to like idling at 25* BTDC.......I dropped that back to 20* to get the T-slots and air mixture screws somewhat in check.

 

[AJ/FormS]

If I had to pick one of them for iron heads it would be the dark black one.
For alloy heads, the bent blue one.
Both for 20* Idle timing.
But honestly, with a 2800 TC it wouldn't much matter cuz look where the timing lands at 2800; namely, the three non-oems are 12>14, plus your initial.

With a manual trans, I'd like to run that OEM, but limited to 35* and with only 10>12 Idle-Timing. Oh wait, that's almost exactly what I an running lol, with alloy heads and up to 185psi, lol.

 

[RustyRatRod]

Limiting the mech advance to 14* gives me an "all in" of 34* at 3500 rpm. (not including vac)

Oh ok. I told you I couldn't read your graph. LOL

 

[mbaird]

What heads ? Open oor closed chamber ? Cam specs ? How much quench ?
All of this will change the curve .
My W2 race closed chamber and EQ magnums closed chamber don’t need more than 32 total . Where as stock open chamber heads need 36ish .
Big duration cams like more initial advance.
Single plane or dual plane intake ?

 

[CFD244]

If I had to pick one of them for iron heads it would be the dark black one.
For alloy heads, the bent blue one.
Both for 20* Idle timing.
But honestly, with a 2800 TC it wouldn't much matter cuz look where the timing lands at 2800; namely, the three non-oems are 12>14, plus your initial.

With a manual trans, I'd like to run that OEM, but limited to 35* and with only 10>12 Idle-Timing. Oh wait, that's almost exactly what I an running lol, with alloy heads and up to 185psi, lol.

I can slow the blue one down, would that make it better than the black one?

I was thinking about welding the inner edges of the slots to put more initial tension on the springs to slow it down.

 

[RustyRatRod]

I can slow the blue one down, would that make it better than the black one?

I was thinking about welding the inner edges of the slots to put more initial tension on the springs to slow it down.

You say it's all in by 3500, right? I don't think it needs to be slowed down. Maybe even sped up a little......but I'd try it as is.

 

[CFD244]

What heads ? Open oor closed chamber ? Cam specs ? How much quench ?
All of this will change the curve .
My W2 race closed chamber and EQ magnums closed chamber don’t need more than 32 total . Where as stock open chamber heads need 36ish .
Big duration cams like more initial advance.
Single plane or dual plane intake ?

This is pretty much a mild built stock 1971 340. XE 262 cam, 69cc chambers with 2.02's......Oem iron intake. Pistons are .018 above the deck then .053 gaskets. I think I figured about 9.5 :1.

 

[mbaird]

This is pretty much a mild built stock 1971 340. XE 262 cam, 69cc chambers with 2.02's......Oem iron intake. Pistons are .018 above the deck then .053 gaskets. I think I figured about 9.5 :1.

In that case I like the MP curve.
Lucky you ! I had a guy offer me hie Sun dizzy machine for free in the 90s but I got busy and didn’t move fast enough …. Gave it to another guy . Urghh….
What great fun !

 

[Mattax]

I thought for sure this post would spark some conversation.......Anybody?

Sorry. Had these conversations before and most of the time they go nowhere, and I've been too busy at work to even log on to FABO.
First thing you probably don't want to hear is that engine should not need or want 20 deg initial.
In fact discussed this with on a thread a few a months ago, it might have been with you, where the engine was on a test stand.
As stated above in this thread. No load on the engine doesn't tell us much about what an engine will need when loaded.
An engine load is based on the percentage of power being used versus the max it could make at that rpm.
If it takes 10 hp to turn the transmission pump and the engine with accessories is making 20 hp net at 600 rpm, then 10 hp is causing 50% load, which is fairly high.

As far as the curves.
The OEM curve will feel good coming off idle. Then it will ping at part throttle when hot (depending on the initial timing) and will lose timing and power by 4000 rpm if limited as shown.

The OEM recurve will be a little bit more sluggish off idle unless you've set idle too high to begin with. Otherwise it will be pretty good and probably won't retard until 5500 rpm or so.

The Mallory MP will be a slug off idle as will the aftermarket unless the idle timing is way too high. Additionally they will ping at part throttle when the engine is heat soaked. Finally they will retard around 4500 rpm.

 

[Mattax]

Yep. My memory is correct.

MP DISTRIBUTOR TIMING CURVE 340

Hi Folks Still playing around with my "mule" 318 getting Fuel and ignition straightened out before I break in the 340. Today, I ran my Mopar Performance distributor (bronze internals). Here's what I found......... Advancing began at approx 1500rpm. By 1600, I was at 5*, by 1800 at 10*, and...

www.forabodiesonly.com

This guy also has 340 and actually races it against a clock.

MP DISTRIBUTOR TIMING CURVE 340

What carb? The engine is no longer near stock, so stock timing curves go out the window. That is why the engine is going to need 20+* for best idle. If you want to dial in the timing curve with this modified engine, it has to be done on a dyno; anything else just becomes a good guess, which...

www.forabodiesonly.com

How does the VA adjustment factor in?

You asked this in the other thread but he didn't answer. H'm. That's a clue.
From the Chrysler Master Tech Conference Session 259

Igniting it sooner helps it burn in time to get maximim leverage when the fuel mix is less dense and combustion temps a little cooler. This happens at speed when loads are light to moderate.

Vacuum advance should go away when the load is high enough to call for enrichment.
I go into the weeds about that here

Get your initial and mechanical advance squared away first.
Idle is the foundation for everything. Mechanical advance is highest load/fastest accesleration under high load.
Then mess with the vacuum.
Without vacuum advance, can advance timing much quicker with no harm, and sometimes to advantage for drag racing.

Since you have a machine, run the distributors up to redline. Then you'll see the retard at hgh rpm from slew in the electronics.