Best advance curve for SC 180 HP ROW full stock?

[Alex_92]

Hi all,

I've just bought a CDI+ from Classic Retrofit for my Porsche 3.0 SC EURO 180 HP from 1979 with low compression (8.5).



For your information, I use only RON 98 gas (equivalent to 93 in US).
Now I would like to lock the distributor and run a full ignition map with CDI+, and so that optimize the ignition of my engine.

I've got on Internet, especially on Pelican forum, some personalized advance curves for 3.0 SC that people set up in their CDI+ but there are none applicable to my setup: Porsche 3.0 SC EURO 180 HP from 1979 full stock.
Indeed advance curves I found are for SC 204 HP or 188 HP, or upgrades with SSI, 964 camshaft and PMO.

So could you tell me please what will be an optimized advance curve for my SC EURO 180 HP in regards to the stock curve displayed below.



My idea was the following with a distributor timed at 5 degrees BTDC at 900 rpm:

2 degrees below 800 rpm
800 to 1200 rpm = 0 degrees
then rising to max advance of 26 degrees at 3,000 rpm
At the end I would have 31 degrees as total max advance.
The workshop manual indicates it must be between 24 and 31 degrees at 6,000 rpm.

Thank you in advance for your help and advise.
Have a nice day.

Alexandre (from Paris)

[mysocal911]

Originally Posted by Alex_92

Hi all,

I've just bought a CDI+ from Classic Retrofit for my Porsche 3.0 SC EURO 180 HP from 1979 with low compression (8.5).



For your information, I use only RON 98 gas (equivalent to 93 in US).
Now I would like to lock the distributor and run a full ignition map with CDI+, and so that optimize the ignition of my engine.

I've got on Internet, especially on Pelican forum, some personalized advance curves for 3.0 SC that people set up in their CDI+ but there are none applicable to my setup: Porsche 3.0 SC EURO 180 HP from 1979 full stock.
Indeed advance curves I found are for SC 204 HP or 188 HP, or upgrades with SSI, 964 camshaft and PMO.

So could you tell me please what will be an optimized advance curve for my SC EURO 180 HP in regards to the stock curve displayed below.



My idea was the following with a distributor timed at 5 degrees BTDC at 900 rpm:

2 degrees below 800 rpm
800 to 1200 rpm = 0 degrees
then rising to max advance of 26 degrees at 3,000 rpm
At the end I would have 31 degrees as total max advance.
The workshop manual indicates it must be between 24 and 31 degrees at 6,000 rpm.

Thank you in advance for your help and advise.
Have a nice day.

Alexandre (from Paris)

What do you mean by the expression, optimize?

[Jonny042]

You could actually use the programming capabilities of the CDI+ to better effect, it is possible to program curves that are physically impossible using weights and springs. Here is my curve:



I haven't really pushed the limits on this - but the theory is to have lower timing during cranking to avoid kickback, a flat area to keep the idle advance (and the idle speed) stable, extra advance in the mid-range where cylinder pressures are somewhat lower (also coincidentally coincides with cruise RPM, but I'd like to steepen the approach a bit) then backs off at high rpm for safety.

I am using 31 degrees as total advance like you are planning.

[Alex_92]

Originally Posted by mysocal911

What do you mean by the expression, optimize?

I wanted to say maximize the output of the engine with the best advance curve as possible.

[Alex_92]

Originally Posted by Jonny042

You could actually use the programming capabilities of the CDI+ to better effect, it is possible to program curves that are physically impossible using weights and springs. Here is my curve:



I haven't really pushed the limits on this - but the theory is to have lower timing during cranking to avoid kickback, a flat area to keep the idle advance (and the idle speed) stable, extra advance in the mid-range where cylinder pressures are somewhat lower (also coincidentally coincides with cruise RPM, but I'd like to steepen the approach a bit) then backs off at high rpm for safety.

I am using 31 degrees as total advance like you are planning.

Hi Jonny,

Do you think your curve could be set up in my SC EURO 180 HP low compression (8.5) which is full stock, unlike yours with a very nice engine preparation (3.2 cylinders, camshafts…). Both engines are configured differently.
What is your idle advance? I plan to keep the 5 degrees as recommended by Porsche.

Otherwise, I’m surprised to see that you have lower timing during cranking. For my car, it is a flat area to the idle speed but the SC after 1980, there is an higher timing during cranking (below 800 rpm).
Jonny Hart from Classic Retro fit configured his curve like that for his SC 204 HP (see picture below).
Could you tell me why do you think it is better please. What do you mean with “to avoid kickback”?

Thank you in advance for your help.
Alex

[trond]

with programmable ignition and stock distributor; how are you guys managing the rotor tip phasing ? To avoid arching issues between rotor tip and cap terminals. If timing does not coincide with position of rotor tip vs terminal inside distributor cap there may/will be issues. Curious as I am struggling with the same and have found unable to set ignition as I want because of limitations in distributor. Especially with a locked advance mechanism..

[GaryR]

Originally Posted by trond

with programmable ignition and stock distributor; how are you guys managing the rotor tip phasing ? To avoid arching issues between rotor tip and cap terminals. If timing does not coincide with position of rotor tip vs terminal inside distributor cap there may/will be issues. Curious as I am struggling with the same and have found unable to set ignition as I want because of limitations in distributor. Especially with a locked advance mechanism..

Possibly an answer - https://www.classicretrofit.com/collections/cdi-ignition/products/sc-turbo-distributor-lock

I had my dizzy rebuilt/recurved with vacuum plugged way back when... before programmable CDI, which I think is the bomb!

[Jonny042]

Originally Posted by trond

with programmable ignition and stock distributor; how are you guys managing the rotor tip phasing ? To avoid arching issues between rotor tip and cap terminals. If timing does not coincide with position of rotor tip vs terminal inside distributor cap there may/will be issues. Curious as I am struggling with the same and have found unable to set ignition as I want because of limitations in distributor. Especially with a locked advance mechanism..

I covered this in my build thread way back (link is to page 52), there should be some useful info there:

http://forums.pelicanparts.com/porsche-911-technical-forum/979976-project-heavy-metal-all-steel-classic-remastered-52.html

Short version - use a Carrera 3.2 Rotr and adjust the rotor position vs. trigger vs. "base" setting to get it firing on either end. Should be no issue to get it dialled so can fire at either end of the rotor depending on if you are at minimum or maximum advance.

[Jonny042]

Originally Posted by Alex_92

Hi Jonny,

Do you think your curve could be set up in my SC EURO 180 HP low compression (8.5) which is full stock, unlike yours with a very nice engine preparation (3.2 cylinders, camshafts…). Both engines are configured differently.
What is your idle advance? I plan to keep the 5 degrees as recommended by Porsche.

Otherwise, I’m surprised to see that you have lower timing during cranking. For my car, it is a flat area to the idle speed but the SC after 1980, there is an higher timing during cranking (below 800 rpm).
Jonny Hart from Classic Retro fit configured his curve like that for his SC 204 HP (see picture below).
Could you tell me why do you think it is better please. What do you mean with “to avoid kickback”?

Thank you in advance for your help.
Alex

I'm running quite a bit higher idle advance (the curve I posted uses a base timing of 11 degrees), so zero is 11 degrees.

I previously had even more timing at idle (12 degrees) and during cranking I would get an occasional "pop/kickback" that I couldn't really explain but once I reduced the timing at cranking it went away.

[Alex_92]

OK thank you

[mysocal911]

Originally Posted by Jonny042

Short version - use a Carrera 3.2 Rotr and adjust the rotor position vs. trigger vs. "base" setting to get it firing on either end. Should be no issue to get it dialled so can fire at either end of the rotor depending on if you are at minimum or maximum advance.

Besides that, there still needs to be some centrifugal movement of rotor to avoid misfires.
This becomes more problematic with advance curves that exceed 30 degrees.

[mysocal911]

Originally Posted by Alex_92

OK thank you

Here's an abstract from a research paper which evaluates ignitions that produce multiple sparks with the conclusion that the second pulse has little to no value:








Hopefully, you are aware that the second pulse from CDI+ is about 100us, right?

[Glenfield]

We’re talking about mappable ignition timing here not benefits (or lack thereof) of multi-spark. Nor are we debating the cost / benefit of CDI+. For those that have a CDI+ or an ability to customize ignition in an SC, this is potentially a very useful thread to keep on track.

[Jonny042]

The little scraps of white paper seem to argue the point that dual sparks are better than 1 and stretching that to 3 has no effect, so I'd say the CDI+ has it nailed. I'm too enedumacated to even understand the second part but the traces seem to start at 10,000rpm and go up from there. I know my 3.2 MFI idles and runs way better than either of my motronic cars do but that's just me I guess.

[Jonny042]

Originally Posted by mysocal911

Besides that, there still needs to be some centrifugal movement of rotor to avoid misfires.
This becomes more problematic with advance curves that exceed 30 degrees.

The 15mm wide tip on the 3.2 Carrera (which fits the SC distributor perfectly BTW) covers an arc of approximately 54 degrees of crankshaft rotation. There does not need to be a centrifugal mechanism in place if the rotor phasing is correct.

[Jonny H]

Originally Posted by Jonny042

The 15mm wide tip on the 3.2 Carrera (which fits the SC distributor perfectly BTW) covers an arc of approximately 54 degrees of crankshaft rotation. There does not need to be a centrifugal mechanism in place if the rotor phasing is correct.

It is actually very easy and quite an interesting thing to validate. We made a distributor cap with a hole drilled either side of number 1 post so you can see the rotor and the post. You then use your strobe light to observe the rotor position.

What you see is that the rotor tip is offset on the the post when the engine is idling. E.g. One end of the rotor tip is adjacent to the post but there is still good overlap. When you rev the engine, the tip moves to a more central position. In other words, the positioning is optimal (centred) when at high RPM which is what you want.

[Jonny H]

Originally Posted by Alex_92

OK thank you

Hi Alex, for a stock motor, I think your most likely improvements will simply be from multi spark. You can usually richen the mixture a little to produce more power (but remember it may already be rich).

Generally, it is fairly safe to add a little bit more advance (say 3 degrees) when the engine comes on cam around 4000 RPM but make sure to take it out again by 5500 to avoid detonation.

The kick back Jonny042 is referring too is when you have too much advance for starting. Generally speaking, engines start better with less advance.

[Jonny042]

Originally Posted by Jonny H

It is actually very easy and quite an interesting thing to validate. We made a distributor cap with a hole drilled either side of number 1 post so you can see the rotor and the post. You then use your strobe light to observe the rotor position.

What you see is that the rotor tip is offset on the the post when the engine is idling. E.g. One end of the rotor tip is adjacent to the post but there is still good overlap. When you rev the engine, the tip moves to a more central position. In other words, the positioning is optimal (centred) when at high RPM which is what you want.

I did this, exactly.

[mysocal911]

Originally Posted by Jonny042

The little scraps of white paper seem to argue the point that dual sparks are better than 1 and stretching that to 3 has no effect, so I'd say the CDI+ has it nailed. I'm too enedumacated to even understand the second part but the traces seem to start at 10,000rpm and go up from there.

You missed the basic point that a second spark of about a duration of 100us has basically no benefit.
Furthermore, no automotive OEMs use CDIs for production vehicles or race vehicles.
The inductive discharge ignitions provide better spark energy and pulse duration,
e.g. 1-2ms (3-4 times) a CDI spark. Use a scope and measure the pulse width of the second CDI+ pulse.

Originally Posted by Jonny042

I know my 3.2 MFI idles and runs way better than either of my motronic cars do but that's just me I guess.

You're joking, right?

[mysocal911]

Originally Posted by Jonny042

The 15mm wide tip on the 3.2 Carrera (which fits the SC distributor perfectly BTW) covers an arc of approximately 54 degrees of crankshaft rotation. There does not need to be a centrifugal mechanism in place if the rotor phasing is correct.

Then your ignition isn't providing the max advance you're expecting or your spark is weak at the higher RPM.
Porsche engineers purposely added some centrifugal advance for a margin of safety.