Need help setting advance table

[pjo046]

Hi

I have a 1976 Carrera 3.0 engine (Engine code 930-02).
It has a CR of 8.5:1, and it originally has 200 bhp.

On my engine I have installed headers and a custom built 2 in 2 out muffler which is very non-restrictive.

This summer I ditched the CIS-setup and distributor, and installed Bitz EFI (Megasquirt I) for fueling and an Electromotive HPV-3b for ignition control.

The HPV-3b is the ancestor to the current XDI-2 ignition, so it is a programable crank triggered ignition with a 8x8 ignition map resolution.

Does anyone has inputs as to how I should set the advance table on this engine?
The initial map I have so far is this:


How does the WOT (102 Kpa) advance curve look? Can I go higher (Is stock curve a bit higher?)?

I know I can add roughly 3 degrees of advance per every 10 kpa I go down from WOT, but if someone has any good ideas for the table, please let me know!

[pjo046]

Reading through the Megasquirt manual I come across the following statements regarding ignition advance:

- "total advance at WOT: should be from ~24° to ~40° depending on your bore size and combustion chamber characteristics. Older design engines (i.e. push rods, domed pistons, etc.), and those with large bores (big blocks, etc.) need more advance, about 36 to 38°."

- "RPM based advance: This is the advance as read across a row (at a constant MAP kPa). Generally for a performance engine, you want the advance to be 'all-in' by 3000 rpm. So for a given MAP (say 100 kPa) the spark advance should rise from the idle value to the maximum by about 3000 rpm"

- "The basic principles are to determine a maximum advance for your engine and work backwards from there with heuristics ('rules of thumb'):

older engines (1960s up to 1990 or so) with two valves - max advance = 36°
newer two-valve engines - max advance = 30°
three or four valve engines - max advance = 26°
then adjust for bore size:
under 3.5" (89mm) - subtract 3°
between 3.5" and 4.000" (101.6mm) - no adjustment
over 4.001" (+101.6mm) - add 3°
then adjust for the fuel:
regular - subtract 2°
mid-grade - subtract 1°
premium - no adjustment
That gives us a maximum advance figure. It you have an aftermarket combination with a good squish area and optimized quench, subtract another 2°. If you have a flathead, add 3° or 4° or more.
We will use this to fill in the table at 100 kPa from 3000 rpm to the redline.

From idle to 3000 rpm, we want the advance (@100kPa) to increase fairly linearly from the idle advance to the maximum advance. idle advance is really a matter of tuning, but assume 8° to 16° in most cases, with stock engines being on the lower end, and 'hotter' engines being on the upper end. "

Do these guidelines sound reasonable also for old aircooled 911-engines?
Doesn't the stock advance curve continue to rise far beyond 3000 rpm?

Appreciate all input guys!

[jpnovak]

What is your MAP reading at steady cruise? Let's say 4th gear between 2500 and 4500 rpm and 5th gear in the same range?

What AFR are you running at >95 kPa?

The common advance curve for a 3.0SC will start at idle around 8-10 dBTDC. This will ramp to 22 dBTDC around 1300 rpm. The advance will then linearally ramp to 30 dBTDC at 3K rpm.

There are some things you can do to wake up the motor. More advance at 6K rpm (~35 BTDC depending on fuel, temperature, etc) and a lot more advance in your cruise load/rpm bins. This will help the motor be able to run leaner for better fuel mileage.

[pjo046]

Hi again

I haven't driven the car much after the engine started, only one very short trip around the block. I did however make a log during that run.

When checking the log I see that at idle (which is currently too high at 1100-1200 rpm I have an AFR of around 12.4 and a kpa of around 40.

When cruising around 2000-2100 rpm I have a kpa in the area 45-50 and an AFR in the area 12.3-12.8

When I have kpa >95 the AFR is mostly between 11.5 and 12.7.

Again, I don't have much log time yet. But, I can use an autotuning feature when driving, so if I set a target AFR map, the ECU will adjust the VE-table realtime while driving to achieve the pre set AFR values. So if anyone has a good AFR table for a 3.0 911-engine, you are more than welcome to share it. :-)

[pjo046]

How does this look? Given that I start rich on the VE-tables, and tune leaner gradually. This way I avoid beeing too lean when first applying the higher advance and risking detonation.

I have been told by a reliable tuner source that I can go aproximately 8 degrees more advanced on any given map-point compared to a 3.2 map because of my lower CR (8.5:1). I think that in the mid-partial load area I could go even higher. How does my attempt keeping a stable idle look (low kpa/rpm)? Do you think I can go higher in the mid PT? I am also not sure if my conversion from load to kpa seems ok.

100kpa__ 10_ 11__ 15__ 22__ 36__ 32__ 34__ 38
95 kpa__ 11_ 12__ 16__ 23__ 37__ 32__ 34__ 38
80 kpa__ 15_ 17__ 21__ 29__ 42__ 32__ 34__ 39
72 kpa__ 19_ 20__ 24__ 32__ 44__ 34__ 38__ 42
65 kpa__ _9_ 10__ 27__ 35__ 46__ 36__ 41__ 45
57 kpa__ _5__ 5__ 10__ 20__ 49__ 41__ 46__ 50
50 kpa__ _5__ 3__ 0___ 0___ 46__ 38__ 44__ 48
40 kpa__ _4__ 2__ 0___ 0___ 40__ 33__ 36__ 40
Rpm____ 900 950 1000 1300 3000 4200 5000 6000

[pjo046]

As you can see I have made the assumption that the Carrera 3.0 engine has it's torque peak at the same rpm as the SC, and thus I have lowered the advance in the 4000-5000rpm region. Is this a valid assumption?

[cgarr]

Subscribed because this stuff fascinates me! Can an old guy be taught this stuff?

[TimT]

Given that I start rich on the VE-tables, and tune leaner gradually.

Always get the mixture a bit fat, then adjust timing.... Adjust timing for peak power in all ranges, then back the timing off a bit. Then fine tune mixture...on the dyno we watch the power go up... then as power flattens and falls we take a few degrees out from that point..


I am not used to seeing so little advance at idle, I usually have about 10 deg at idle and even more at cranking rpm..


If the engine stumbles at idle having more advance can help to keep the engine running as more advance gives the engine more power.... helpful if you have lightened components etc..


Also emulating the timing curves that Porsche used in the distributor, is generally a safe place to start.... or find a cracked Motronic and copy that map...

fine tune from there ;-)

[pjo046]

It's just that lower advance at idle helps keeping the idle more stable, and also reduces emissions. The reason I set the ignition at zero for the the rpm's just above idle at the lowest kpas is to help keep the stable idle. Once the idle rises, the reduction in advance should help the idle settle back down in the 900-950 rpm area.

But I am very welcome to better solutions, this is my first attempt at setting an ignition advance table properly.

I have set initial advance at 14. That means from 0-899 rpm. So this helps cranking/starting. Also, should the idle drop below my target at 900-950, the advance rises rapidly from 4 to 14, helping to bring the idle back up.

My WOT-curve (100kpa) is set more or less like the original ignition curve for my engine, exept I use higher idle advance than stock, and I also have dialed in the reduction around peak torque. Is the reduction at peak torque necessary? I read somewhere that this was a wise move. Just to be safe.

[pjo046]

When I plotted my table into the software, I had to alter the kpa points a little. Also, I adjusted the table a bit.


100kpa__ 10_ 11__ 15__ 22__ 36__ 33__ 34__ 38
90 kpa__ 11_ 12__ 16__ 23__ 37__ 33__ 34__ 38
80 kpa__ 15_ 17__ 21__ 29__ 42__ 36__ 37__ 39
72 kpa__ 19_ 20__ 24__ 32__ 44__ 38__ 40__ 42
64 kpa__ _9_ 10__ 27__ 35__ 46__ 40__ 42__ 45
56 kpa__ _5__ 5__ 10__ 20__ 52__ 44__ 46__ 50
50 kpa__ _5__ 3__ 0___ 0___ 46__ 40__ 44__ 48
40 kpa__ _4__ 2__ 0___ 0___ 40__ 36__ 38__ 40
Rpm____ 900 950 1000 1300 3000 4200 5000 6000


This is what I ended up with (Put the table into excel and made a graph of it):



Again, if anyone has opinions or suggestions, please do share!

[pjo046]

Bump

It's an old thread, but I am about to convert to MS2-Extra with integrated ignition control so the topic is again in the wind. This time the table will be 12x12, which gives much better resolution than before. Not that it is very important as 8x8 was ok, but it makes it easier to fine tune the idle characteristics for example.

Are the different 911-heads of that era (911S, 3.0 carrera, 2.7 carrera, SC, Carrera 3.2) basically the same with regards to chamber design(shape, size, geometry, plug location)? I'm asking because if they are, then it shouldn't be a problem running 3.2 ignition maps on say a SC or a 3.0 Carrera. But if they differ greatly in design, then it is not possible to use ignition maps from the newer engines as a baseline for mapping older engines. I know the compression ratios vary, and so does valve size, but neither is affecting chamber design in a great amount. If anything, since my 3.0 Carrera has a lower CR than the 3.2, it should be possible to run even higher ignition advance.