mjb

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-

Dave_N

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

mjb

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-

articfriends

iTrader: (1)

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

800XCR

iTrader: (2)

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.

articfriends

iTrader: (1)

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

Dave_N

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

mjb

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

mats

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.

mjb

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-