Dave_N

mjb,
I believe that mats is mistaken on this point. It would be a good idea to verify with your scan tool and timing light (with advance). With 10 degrees of base, it should be easy to tell if your scan tool and timing light agree or are 10 degrees off.

I sent you a few files, did you receive?

Dave

mats

This is how I have understood it; Neither the scan tool nor the ECM can actually know what you have set the Base timing to. If they did you would not have to use a timing light, it could all have been solved in SW. Since the scan tool displays the total advance at any given moment, the scan tool (or the ECM) adds the initial advance while assuming that it has been set correctly. As far as I know the base timing is 8 degrees for all 502 MPIs. At higher rpms, the ECM advances the timing to what it believes is approximately 30 degrees (assuming that there is no knockretard or other anomalities), while assuming that the initial advance is 8 degrees. If you bump the initial advance to 10 degrees, and fool the ECM, the ECM will consequently advance the timing to 32 degrees but the scantool will still show 30 degrees. If you bump the initial timing futher, you will sooner or later get knock retard and possible engine damage.

mats

One more thought. As I said, either the ECM or the Scan tool adds the 8 degrees. The ECM obviously knows that it is sitting in a 502 MPI and that the initial timig should be 8 degrees so it would be most locical to add it there. The other alternative is that the scan tool adds the 8 degrees. In that case the scan tool has to be told that it is a 502 MPI, either by the ECM or by the one who is operating the scantool. Either way it doesn´t really matter for the result.

Dave_N

Well I guess I am wrong. I checked a MEFI3 system today, OEM spec is 8* base and thats exactly were it was set. Ran the engine, no load, and the spark advance that the scan tool displayed and the advance that my digital advance timing light indicated were exactly the same.

So if the base is set to OEM spec, the scan tool indicates TOTAL timing, including base, not in addition to base as I previously stated.

Dave

mats

Another comment. At higher rpms you can fool the ECM by bumping the initial advance. However, at lower rpms (If I recall it correctly up to some 3000 rpm) the ECM is actively regulating the advance in order to maximize performance. This means that if you bump the initial advance by two degrees, the ECM will in its search for max performance, deduct them and you are back to the same advance as before. The scan tool will however show an advance that is two degrees less than before.

mjb

I appreciate all of the good info on this thread. I called Jim at AZ Speed this morning....he remapped my ecm and originally told me to set my base timing at 10*. He told me that a stock factory ecm is programed to assume 8* base timing when it calculates total advance. If you were to bump the timing to 10* with a stock ecm, the total advance reading on the scan tool would be 2* less than the actual because the ecm is still assuming 8* base.
When AZ remapped my ecm, they reprogrammed the ecm to assume 10* base timing....therefore, the total advance shown on the scan tool IS correct as long as base timing remains at 10*. If I increase my base timing above 10*, the scan tool will be low by the amount I increased the base because the ecm still assumes 10*.
Thanks all for the help- MJB

articfriends

Are you sure the mefi3 and back do this on the 502'S? Without a cam sensor or a crank trigger and just a pickup coil in the distributer I don't know how the computer wouldn't be blind to cranksahft/camshaft position compared to the ignition firing if the ecm can't see it to compare it and make corrections,Smitty

mats

You are right. It is blind to the position of the crank/ cam. But since the ECM will try to regulate the advance to find the best performance (highest rpm in a given situation, throttle, load, etc) it will, if the algorithm is decent, find the best performance at the same total advance as before. If the initial advance was bumped say 2 degrees ,it will just result in that the ECM will find a sweet spot that is 2 degrees lower in ECM-added advance compared to before. Again, this is only at lower rpms and if you are fairly close to the correct initial advance.

Dave_N

Maybe the reason they use a base of , say 8, instead of using 0 as base is due to the fact that some base is needed for proper advance curve when the operating in "Distributor Module Mode" (during cranking and in the event of IC circuit failure).

Dave

mats

Well, I read it in a book somewhere. I may of course have misunderstood it. I understood it to be similar to the Mean Best Timing in the Thunderbolt V ignition system.
So if I understand you correctly, there are no feedback loops in the Mercruiser ECM controlled ignition systems. How does it work at idle?