I'm trying to convert my old school knowledge into ECM knowledge...

TIMING: On an old school distributor, you get the motor warmed up at idle, say 700 rpm, throw a timing light on the thing and set the timing for ... say... 12 deg advanced. Now even though the motor is already pulling vaccum and advancing the distributor a little, you SET it at 12 deg so that is exactly what you have at idle.

In the ECM world, pull the wire going to the ecm and basically go through the same process but probably set the timing to... 6 to 10 degrees. I've seen and tweaked on all the tables that modify the timing but something I still don't understand is how can we be runnning (idle) in the neighborhood of 20 deg SA on an ECM when basically the same motor would idle at 12 deg with an old distributor.

If the ECM (727/730) spark table ($8d) shows 20 deg SA @ idle and you throw a timing light at it the what does the crank (dampener) show??

The bottom line to my problem is that I'm putting a 727 and $8D on a unique 4.1L stroked Buick V6 race engine. It had a carb on it and I understand most of what the engine wants for most things. It had a Mallory mechanical only distributor that maxed at 28 deg. So when I set it for 14 deg SA at idle, I know I was getting (verified with timing light) 42 deg SA at about 3200 rpm. All this is very similar to the old school Chevy 350 or 305.

So what is your recommendation for setting up my timing tables in 8D??

 

If your old distributor had no vaccuum advance, you can transpose your old timing settings directly into the timing table (it would be the same timing at all load points so that it only varies with rpm). Set your base timing to whatever works well for getting the engine started (just make sure the ecm knows what it is set to so that the numbers you put into your timing table end up being absolute)

 

The base timing is only used during cranking.
Once the engine is over ~400 RPM the ecm takes control of the timing.
8d is about optimum, IMO. Too much gets ya a slow crank when hot, and the engine just windmills with much less then that in cold weather.

The timing in the chip should match what you see with a timing light. BUT, there are numberous adders, and misc tables. If you're not sure about where you are, one by one *0* out the various timing corrections to make sure you do understand where you are.

Timing is timing, there's no rocket science involved. It's tuning, and keeping the engine happy that makes the TUNE work.

What sets ecm spark timing above a distributor is what you can do with the curve, you just can't move the timing around with a dissy like you can with a spark timing, much less, having coolant temp., or air temp corrections.

Getting the corrections CORRECT, is just as important as the main timing table....

 

This is my first project converting to EFI and it also happens to be a unique motor in that there isn't a stock program that comes close. It is very important to me (and my investment $$$) to have the intitial program very close and very safe for the engine.

Here is what I have so far... Please let me know if I missed anything.

Highway mode-disabled
EGR-disabled
Spark advace, Initial-12 deg SA (This is what I will have on the distributor as well)
Low Octane SA-disabled
PE SA-0 (not using this)
Temp correction bias-20.04

SA temp correction vs load vs temp- I dumped this into excel, made another table minus the temp correction bias so I could see the actual SA being added or subtracted from the SA Main... Take a look.


We will ignore the startup SA and Knock Retard for the moment. As far as I can tell and as long as I am at a normal warmed up temp (180 to 200 for this motor). The only thing left for timing is the SA Main and SA Main Extended, RIGHT?

So what I did for my first fire-up with the new EFI system (727, 8D) is took the spark table from AXXF and toned it down a bit. What I mean is that I flattened out the curves a little. I'll set the Max SA to something like 34 deg at first to make sure I don't have a melt down.

Take a look at the SA Main and please let me know what you think.

 

It appears you are on the right track.

Note: the idle control software will make your idle SA jump around, incase you put a timing light on it and wonder why it's doing that... search the board posts to find more about that and how to tame it down if it causes you grief.

If you don't run a knock sensor, I'd also zero that table out.

 

You may want to go higher in the low 20-30 kpa cells so your engine braking is not as hard when decelling. More timing there will make it glide better rather than putting your head in the glass. A short drive with a log will help you gauge the feel of it.

 

Good!! Thanks BTW my idle is around th 50 MAP area

 

You can setup TP so that it will subtract the bias from the SA vs Temp table so it will read how much is added or sub'd.
Goto the conversion tab for the table item. And choose the edit button.
add a "-Y" w/o the ".
Select the new argument that will be added automacally.
Change the argument type to other xdf item output.
Then select the bias.
Now your stock table should read all values less than 10 and show + or - values at certain temps. And lots of zeros.

This is in the TP help section too IIRC.

Don't forget to tweak the Idle SA table too.

 

COOL!!!

Now I have a whole lot more to play with.

Thanks

 

not to hijack here, but I have to set the inital timing by rotating the distributor with the EST connector unplugged to 10d. It feels alot stronger this way. in the software, I have tried setting the intial advance to 10 and it runs just like as if the initial timing is set at 0d.

It seems that the two negate each other. And it makes sense. These engines don't have a crank sensor, it only goes by the ref pulses generated by the distributor. The ecm really has no idea where true TDC is. It seems like when i set mechanical to 10d, the whole curve shifts 10d. But if I enter 10 in the software, the ECM adjusts it back down 10d. I can see it with a timing light.

Am i missing something here???

 

Exactly right.
By setting the mechanical to 10* and setting the bin to 10*, the table values will be reality.
Having them different is just adding confusion for no reason. You'd always have to remember that when viewing logs and making changes.
You can experiment if you are running into high knock situations with cheap gas by increasing the bin setting to reduce the overall SA by that amount. That is only a patch so to speak to solving the actual problem.

 

The short version-
SA = (various SA tables values) - base setting in bin + mech base setting of dist.
That's why the tables all summed together are what you should see with a light.
Adding 5 deg. at the dist and leaving the bin at the old setting is the same as adding 5 deg everywhere in the main SA & extended tables.

 

ok, that makes sense. So now I have to ask, why would you ever want to add any mechanical advance by rotating the distributor from 0d?

I checked my timing last night with a timing light. With the EST disconnected, my base mechanical timing is 12d. I don't have any knock problems. Should I set the mechanical to 0 and add 12d to my main SA table?

 

I actually had that in my reply but removed it because it was off the subject a bit and am not 100% that my belief is truth.
By having the distributor tuned closer to the actual point of fireing, you reduce the amount of gap that has to be jumped by the spark.
Keeps a bit more power potential that way, or so I believe.
Seems only logical that if you alway have to jump a gap of 35 degrees before the terminals are directly lined up that power would be wasted into heat from the longer jump.
Seems like the terminals in the dizzy should be advanced to actually use all of the available power (rather than have extensions of a couple degrees on either side of the center as they are)
BTW, I'm still looking for a vacuum Vs advance rate from "old school" type distibutors to help with my gameplan on my spark tables.
right now its pretty much trial and error work. No defined rate of removal for load. Other bins provide a little info but I haven't seen enough to come to a conclusion on that aspect.

 

yeah, that seems to make sense, I thought about how far the spark would have to jump if it was all done by the computer.

But I wonder if you took the mechanical advance out of the equation (by setting the distributor to mechanical 0d) and let the ECM take over totally how things would work. THere are alot of different spark tables that are used, so instead of just offsetting the whole result of SA (by the mechanical fixed amount) the ratio of spark advacne would be affected.

Problem is if I add 10 degrees to my main spark table the numbers are pretty large. Some values are up there, like 40 - 50 d! Do you think it would work?

 

FWIW -

There's a discussion of vac-advance distrib cans here:
2005 C6 Corvette Information - 2005 Corvette C6 Information - Corvettes for Sale

Alas when it was posted in HTML it seems to have lost all its line-breaks, so reading
through it is a chore.


In essence the author calls out where GM spec'ed the vac adv to start, and limits
on total movement for various cans. Since the cans are spring-controlled a good
working theory would be that the vac advance probably decays in linear fashion.

Only way to know for sure how an individual can-spring was loaded would probably
be to hook one to a vac pump (brake bleeder) and measure linear travel with
applied vac... For a typical can that would verify the beginning and end points, and
you could read the total spark advance off the model number.

HTH

 

Greg, for a good primer on ECM SA hit the stuning guide book sticky, then under '8063/'8746/'7747 there is a SA Logic write up. Explains how and where the ECM controls the SA. It isn't exacly the same for every GM ECM, but gives good info.

As for the distributor base timing that same value needs to be properly set in the BIN. This way it is a wash. What is added by the distributor is subtracted from the SA, so it has no affect on what the main SA tables are set to.

RBob.

 

The posts in a distributor cap and rotor should have several degrees of sweep in them such that it makes no "jump" under normal spark advances (6-45degrees). Should be about 35-40 degrees of CRANK angle (17-20 or more, degrees at the rotor/cap interface).

Base timing is used to get optimal starting at the most situations. It usually should match the cam, i.e. a small cam will need less crank advance, and less total advance, where a big cam may like a lot of cranking advance, and a lot of total advance. Trying to associate a jump/spark energy benefit is mostly placebo, IMO.