Total timing for modified 502 MPI ???
 

I'm wrapping up my first season with my new modified 502 MPI's in my top gun. Engines are mefi 1's with 9:1 comp., 228/233-.600-114 cam, ported dart heads, AZ shortened/ honed intake, stainless marine exaust, MSD, AZ remapped ECU.
Based on Crockett dyno of similar modifications and my desktop dyno, I am estimating 500-525 CHP at 5300-5400. My rev limiter is set at 5500.
Using my old bravo-1-24's, I can only get about 5100 RPM's. My scan tool says my timing is about 33* between 3500 and 4000 but falls of to 30* at 5000. Base timing is set at 10*.
Everything I have ever learned about performance big blocks would make me think I need 34-36* at WOT for best performance.
Does 30* at 5000 sound right or will I get more RPM and power by having my ECM reprogrammed(again!) to bump up it up to 34-36*?? .
Thanks-

Re: Total timing for modified 502 MPI ???
 

If you have 10 degrees of base and 30 degrees of advance, you have total of 40 degrees advance. Are you seeing any activity on the knock retard circuit (under load)?

I have a lot of recorded data on a very similar 502MPI w/mods and AZSM reprogrammed MEFI 1 that shows similar computer advance (27 to 32 at WOT). I do see some (up to 5 degrees) of knock retard at times, so you may not want more advance. This was with 87 octane.

Dave

Re: Total timing for modified 502 MPI ???
 

Thanks Dave- you are just the one I was hoping would reply.
When I hooked up my scan tool, I was expecting that the advance it was showing me would have to be added to my base of 10* to get total advance.....but when I started seeing 33* at 3500 and 30* at 5000 I figured it must be showing me the actual total?? 43* would seem like way too much so I figured it must be reading total. I checked the knock retard and it was reading 0* at 3500, 4000 and WOT.

If it is only 30* total at WOT and the knock retard is not kicking in, then do you think more timing at the top end would help me? I would love to know how I can get any more data you may have on that similar 502 you reference.
Thanks again-

Re: Total timing for modified 502 MPI ???
 

Mjb,my mefi3 is set with 10 degrees of base timing(set w/light and timing tool on the marks) and ends up with around 30 degrees when not under boost. The program in mine pulls timing back as boost increases BUT as I give it more base timing it increases the total timing the same amount-example-8 degrees base/28 degrees total,10 degrees base/30 degrees total,12 degrees base/32 degrees total. I would think yours would be similar except the retard with boost part. It sounds like your motor is under-timed. I played with timing on a chassis dyno a few years ago with a small block on a circle track car that I had built for a local team I sponsered.The results were exciting,we found the ideal timing for max power on this particular application and tried varying it,with 2 more degrees from the optimum point the car made similar power but ran hotter,with 4 more degrees it started losing almost 20 hp.Retarding it was similar,with 2 degrees less than optimum it lost 12 hp,with 4 degrees less it lost 25plus,Im sure your motor is the same way,you need to experiment carefully with your total timing until you find the optimum spot,Smitty

Re: Total timing for modified 502 MPI ???
 

Arctic, your ecm may be programmed that way, but most are not, the efi timing table is not necessarily constant, in fact, its not and shouldn't be. You can have 36* at 3500 and only 30* at 5200 with 0* of retard. Not only that, it can be different under different load conditions.

Re: Total timing for modified 502 MPI ???
 

I didn't mean to imply the timing table is constant BUT without reprograming the computer you can add base timing which will increase your total timing acrossed the board that same amount. So as long as you weren't giving it too much somewhere else you could increase the base timing 2 or 4 degrees more to bump your top end timing to 32 or 34 degrees at 5000 if you originally had 30 degrees there,Smitty

Re: Total timing for modified 502 MPI ???
 

mjb,
I am not sure if more advance would help or not. Good input from Smitty and 800.

If your knock retard is working properly (you may want to test it), and you are not seeing any activity, You could try a few more degrees of base and see what happens.

As far as data, I can convert Diacom files to Excel files and you could view the data as a large data table. You could then generate graphs of the data (like the one below) for study. Let me know and I will email you a few files to mess with.

Dave

Re: Total timing for modified 502 MPI ???
 

Thanks Dave, Smitty &800xr- I will try a few more degrees of base timing and keep a good eye on my knock retard reading.
Dave, I would appreciate looking at those excel files. You can email them to me at [email protected]
Thanks again-MJB

Re: Total timing for modified 502 MPI ???
 

Just as a clarification, the scan-tool shows the total advance, assuming that the base timing is set to 8 degrees. If you bump the initial advance to 10 degrees, it will still show 30 degrees at higher rpms, but the real advance is 32 degrees.

Re: Total timing for modified 502 MPI ???
 

MATS- Thanks-Thats a very important clarification if I'm reading it right! Could you and others in the know confirm this?
I assumed the scan tool was reading total advance simply by the size of the numbers....but I assumed that somehow it could tell my base timing was set at 10* and therefore was adding 10 to the ecm advance to get the total.
Are you saying that all scan tools only assume 8* base and therefore only add 8 to the ecm advance??? Does the scan tool have even have the ability to know where base timing is set? If thats is really the case then I do have 2 more degrees of timing than I thought I did. Small details but VERY important!!
Thanks-

Re: Total timing for modified 502 MPI ???
 

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

Re: Total timing for modified 502 MPI ???
 

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.

Re: Total timing for modified 502 MPI ???
 

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.

Re: Total timing for modified 502 MPI ???
 

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

Re: Total timing for modified 502 MPI ???
 

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.

Re: Total timing for modified 502 MPI ???
 

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

Re: Total timing for modified 502 MPI ???
 

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

Re: Total timing for modified 502 MPI ???
 

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.

Re: Total timing for modified 502 MPI ???
 

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

Re: Total timing for modified 502 MPI ???
 

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?

Re: Total timing for modified 502 MPI ???
 

So it is a feedback loop at idle.

Re: Total timing for modified 502 MPI ???
 

Well, I was talking about the Delco EST distributor (since that is what this thread is about) and, call it "limp mode" if you like, but when running in Distributor Module Mode the engine is running on the Ignition Module's internal advance curve. The timing is not "stuck" anywhere.

What I was suggesting was that base timing is set to achieve proper total advance when running on the Ignition Module's advance curve. Otherwise it might make more sense to use 0 as base.

Dave

Re: Total timing for modified 502 MPI ???
 

Well, going back to the subject, I think that you have a point. But then again you can question why the ignition module has to assume an 8 degree basetiming. It would just be to throw in other numbers in the program. One thing I have thought about is that in the distributor, the gap that has to be jumped by the spark increases with the advance over (and below) the base timing. If the rotor is right in front of the , say, no 1 plug terminal at base timing, it would be 22 crankshaft degrees off at WOT, corresponding to 11 cam/distributor degrees off. If you went for zero base timing it would be 30/2=15 and it quickly becomes quite a gap to jump. If you have a base timing that is somewhere in the middle of the advance dynamic range the distance becomes smaller, otherwise you will need quite an ignition coil to make it. Just a thought

Re: Total timing for modified 502 MPI ???
 

Continuing ón the previous post; 15 cam/distributor degrees would correspond to almost half an inch gap to jump at 30 degrees advance, with zero base timing. If I remember it correctly from school that would take almost 35.000 Volts just to pass that gap, and you still want some Volts left for the plug. I read somewhere that the ignition coil on the 502 MPI delivers some 45.000 Volts, so you would definitely need a more powerful coil to have decent spark at WOT if you were to go for Zero degrees base timing.