Best advance curve?

[dipso]

What's the ideal advance curve? If there even is one.
I have a JB Racing distributor with an adjustable advance. At least it is semi adjustable.
There is a window that I can play with between the teens and 28 degrees.
The full advance comes on about 3500 rpms.

Right now I have it set with 28 degrees advance. I'm timed at 10 at idle and 38 at 4000 rpms. It seems to run fine. I was just curious since I have options.

This is on an internally stock 2.7 with 40mm PMOs, MSD, .028 gapped plugs and Magnecore wires.
The engine is old and has about 140,000 miles on it.

I had it set at 22 degrees before but I thought my initial timing was too high. I was at about 16, so I adjusted it.

Is there an advantage to having a higher initial timing zone, with a smaller curve. And can you go too high.?

Or is it better to keep the curve wide and have the initial timing lower?

Thanks.

[RoninLB]

i run 10 initial and 35 total with low compression 8.5:1. At 3,500 rpm i'm running much less than you. Be safe be conservative after 10 initial

i've found over the years that 10 or 12 initial with carbs on various engines helps starting especially in NE winters

[Steve@Rennsport]

Remember, the amount of ignition advance depends a lot on what octane fuel you use and the thresholds of detonation in an air-cooled engine are NOT static: they vary with cylinder head and outside air temperatures.

[dipso]

I use 91 octane fuel. So wouldn't I want a slightly higher advance?

[Steve@Rennsport]

Originally Posted by dipso

I use 91 octane fuel. So wouldn't I want a slightly higher advance?

Nossir,.......thats a relatively low-octane premium fuel so you should be conservative with your timing so you don't buy pistons & rings.

Higher octane fuels (92-93), permit more ignition advance.

[GaryR]

I would think even with 93 octane you wouldn't want to go past 35 degrees max advance.

[dipso]

It says in my Haynes manual, and in a few other places I read. That 35 to 38 is OK on a 2.7.
I'm mostly interested in the initial spot.
I could close the gap on the distributor down to 20 and have an initial of about 15 to18 if I wanted.
I could even go lower, but wouldn't that be too high of an initial advance?
Is there any advantage to a higher initial timing.

I'm just thinking that since the advance comes on so early, I would want a lower initial spot.
I figure at 1000 rpms I am at 10 degrees. With an early advance mechanism I am starting advance at lets say 2000 rpm and already advanced another 10, so I am at 20 at 2000.
Then 30 at 3000 and 38 at 3500.

Just a guess. I really don't know.
Just trying to understand this advance curve stuff.

[911st]

sub'd

[GaryR]

I believe you will have starting issues with an initial advance much over 12 but I could be wrong, and over 35 with just 93 octane? Love to hear from experts here.

[dipso]

I'm 91 octane out here. I keep reading things online about understanding advance curves but they are all for Chevy's and muscle cars.

I just keep wondering about when I had the CIS and the stock distributor my timing was different.
At first i had the vacuum retard and it was timed at 5 ATDC. Then I removed the retard line and retimed to 5 BTDC.
Now my timing is a 10 with the full carb set up and new distributor.

I am reading a lot that full advance is the most critical when timing and a lot of people just say let the initial fall where it may.
But since I have options on my advance band. I don't have to let the initial fall where it may.

I figure between 5 and 10 is pretty good. I never had starting issues when I was at 12 or above. I think i was even at 14 one time.

I wonder how far ahead the MSD fires with it's multiple sparks and maybe that is why the initial is high.
Or is high good. And as long as there are no starting issues, you should have it as high as possible.
Very confusing.

[red 2.7]

I have a similar motor and 34 at 3500 4000 is very good for me with 91 oct.

[dipso]

Originally Posted by red 2.7

I have a similar motor and 34 at 3500 4000 is very good for me with 91 oct.

Where does your initial fall? And have you ever moved it around and found that a lower or higher initial works better at low RPMs.
I think i could get both, if i just knew what it was.

[5:04]

Have a 9.5:1 2.7 with webers and I timed it to about 34 degrees of advance at 6k, and around 8-10 at idle

[Steve@Rennsport]

Originally Posted by dipso

I'm 91 octane out here. I keep reading things online about understanding advance curves but they are all for Chevy's and muscle cars.

I just keep wondering about when I had the CIS and the stock distributor my timing was different.
At first i had the vacuum retard and it was timed at 5 ATDC. Then I removed the retard line and retimed to 5 BTDC.
Now my timing is a 10 with the full carb set up and new distributor.

I am reading a lot that full advance is the most critical when timing and a lot of people just say let the initial fall where it may.
But since I have options on my advance band. I don't have to let the initial fall where it may.

I figure between 5 and 10 is pretty good. I never had starting issues when I was at 12 or above. I think i was even at 14 one time.

I wonder how far ahead the MSD fires with it's multiple sparks and maybe that is why the initial is high.
Or is high good. And as long as there are no starting issues, you should have it as high as possible.
Very confusing.

This is really a complex subject frought with a lot of variables.

If you call me, I'll try to straighten this all out for you.

I think we're now on the same page,...

[dipso]

Well, Steve just cleared that up.
With 91 octane and our head design we want about 10 12 initial and 32 max dynamic.

Thanks Steve.

[911pcars]

Originally Posted by dipso

Well, Steve just cleared that up.
With 91 octane and our head design we want about 10 12 initial and 32 max dynamic.

Thanks Steve.

That's fine for beginning and end. As for your question about the "curve", that depends on a whole lot of variables. The best curve produces maximum torque and HP at various steps in the rpm range, but with a healthy margin to avoid detonation. The most aggressive factory ignition timing curves are based on the engines they were designed for. If you don't have that engine, the curve is just a close approximation.

The best solution is to find a shop with a chassis dyno (rolling road) and a knowledgeable tech who can determine the optimum curve for your particular engine, then modify the distributor/ignition system to replicate that curve.

Sherwood

[RoninLB]

10-12 initial carbs sounds sharp and responds great

32 total is wise



gas has become crap all over
So crap + Calif gas regs crap = crap x2

[Steve@Rennsport]

Originally Posted by 911pcars

The best solution is to find a shop with a chassis dyno (rolling road) and a knowledgeable tech who can determine the optimum curve for your particular engine, then modify the distributor/ignition system to replicate that curve.

Sherwood

Hi Sherwood,

The problem with optimizing timing the advance curve that way with air-cooled motors is that the resulting values are only valid on that day's temperatures.

Given the dynamic cylinder head temps on air-cooled engines that mirror ambient temps, its very dangerous to do this unless the dyno operator/tuner is intimately familair with these cars. He must know precisely how much headroom to build into the advance curve so the owner doesn't wind up buying new pistons, rings, and cylinders when summer arrives.

This is where long-term experience really pays dividends.

[Grady Clay]

dipso,

The static and idle timing are going to be what they are for a given distributor.
It is the timing at 6000 rpm that you must set.

Porsche specifies “5°/ATDC at 900 rpm (warm)” for your ’75 engine.
This should give you 35° before TDC at 6000 rpm.

When starting from scratch, use static or idle timing to get in the ballpark.
Next set the timing at 6000 rpm. Then check the timing at idle.
If you reset the idle timing, you will also change the timing at 6000 rpm – you can
have too much advance or too little depending on your adjustment.

Whatever you do, the last things you do are check the distributor is tightly clamped (try rotating it)
and check the timing again at 6000 rpm.
Make absolutely sure it is not more than 35°BTDC (or whatever you decide is safe depending on fuel).

Too much total advance can put a hole in a piston.

A distributor advance curve is very much tied to the cam and ‘breathing characteristics’ of the engine.
That somewhat parallels the torque curve with rpm influence.
The ignition must advance with rpm due to the ‘slow burning’ of the air/fuel mixture.
However, it must not advance too soon as to have too much advance approaching torque peak.

With an engine dyno, you can find the best advance at each rpm level with wide open throttle (WOT).
Part throttle timing can be adjusted with a vacuum advance/retard device.
Heed Steve’s advice. He is one of the very few I would trust with my engine on his dyno measuring the ‘ideal’ timing.


Here is the advance curve for a ‘74S (911/93) distributor.


© Dr. ing. h.c. F. Porsche A.G.
(sorry, I cut off the Y-axis scale)

The 6000 rpm advance can be anywhere along the green line.
Once set, the idle timing should be along the violet line.
Every distributor is slightly different due to manufacturing tolerances and wear.
The actual timing at each rpm must be within the two lines on the graph.

For example, Porsche specifies for this ’74 distributor the following (vacuum disconnected):
1000 rpm – 3.5°-6.5° BTDC (violet line)
1500 rpm – 4.5°-14° BTDC
2000 rpm – 16°-24° BTDC
4000 rpm – 24°-31° BTDC
6000 rpm – 31°-38° BTDC (green line)

I can’t find similar specs for your ’75 distributor.
Perhaps someone can post.


I recommend you measure your actual advance curve and post the results.
Test static, idle (note rpm) and every 500 rpm interval to 6000 rpm.
Repeat a few times so you are sure of your measurements.

An adjustable timing light makes this easy but you can also paint-mark your pulley with 1° increments.
Make yourself a sharp pointer aligned with the case split.



As an interesting aside, the ’80 911SC distributor has a desirable ‘idle stabilizing’ feature.
As the rpm drop below idle, the timing advances causing the idle to come back up.
This has the added benefit that when you ease the clutch out at idle and the rpm drop, the car
will accelerate back up to idle. This makes starting from a stop better.

Note that this chart is advance and rpm at the distributor.
Double the numbers for engine advance and rpm.

Note that the advance is reduced as the SC engine 'comes on the cam' (2500 - 5000 rpm).


© Dr. ing. h.c. F. Porsche A.G.

This idle is a feature I’m going to include in my distributors.

Best,
Grady

[Tom F2]

Grady, how do the "idle advance" and "high rpm retard" features work. I have just been through a couple of SC distributors (1978 with vacuum advance and 1981 with vacuum advance/retard) to replace the fiber discs on the shaft and general crud removal, and I cannot see how the mechanical advance mechanism performs this feat. However, the advance charts definitely show the effect.

Also, do you know where to find the advance curves for Carrera 3.0 and a RoW SCs, both 78-80 and 81-83? These are not in the US factory shop manual. That manual only has US market distributors, at least for the SCs. I would like to see what the factory did, because I have a seat of the pants impression that these cars are zippier.

I can see that there are different springs in the US 78-79 SC distributor, compared with the 81, but I can't see any other differences in the mechanical advance governor.