General SAFC Timing map discussion

[Larry L]

I've read in many articles how the S-AFC, when leaned too far can alter your map signal thus changing your timing maps as well. Well at what point has it been seen? Say with
680cc injectors. And what is a common baseline setting for 680cc injectors. Thanks guys.

 

[bfdahl]

any time you use the SAFC to trim for larger injectors you have this as a byproduct.

the larger the injector the more the timming is effected.

on a 2G the best way to go is a DSM link.

Start with -17 or -18 and start logging from there.

 

[Gromit]

I've read in many articles how the S-AFC, when leaned too far can alter your map signal thus changing your timing maps as well. Well at what point has it been seen? Say with
680cc injectors. And what is a common baseline setting for 680cc injectors. Thanks guys.

Here's an equation that let's you calculate approx. how much more flow you get with larger injectors:

[1 - (stock injector size/new injector size)] * 100 = % more flow

As an example, the 680's you ask about will flow approx. 34% more than stock 450's:

[1 -(450/680)] * 100 = 33.8%

So -34% would be the baseline for those injectors, however, it's safer to start out a little rich & work your way lean.

Keep in mind that the SAFC can only be adjusted up to 50% in either direction. As you get closer & closer to the max the SAFC can adjust, the drastically altered airflow signal pushes you further & further off the ECU's stock timing maps. Most people suggest staying at or below 660's if you're only using a SAFC to tune with. DSMLink or a custom chip lets you run whatever you want.

 

[andymoraitis]

Nicely said Neal.

As an addendum for the original poster, beware of ND 660's as these can flow much more than 660cc's. Many of them are closer to 700-720cc and while it's nice to have the extra fuel on board, tuning will become that much more difficult considering Neal's statements above regarding airflow subtraction and the resultant changes in timing advance.

 

[CanadianTSi]

At -33 on the S-AFC for my 650's I am getting 15-16 degree's of timing at the Top of 3rd, and that is with 5 degree's of knock retard, so the ECU is really trying to give me 20+ degree's of timing which is way to much for high boost and pump gas.

 

[Kevin Jewer]

Another point that I never see mentioned is that one way around the airflow signal and timing problem is to simply flow a shitload of air. The ECU will eventually get back on the highest load map. This is why some poeple can run 10s on an AFC while somone running 12s or 13s struggles with knock.

There is some math that can be done to determine how much air you have to move. Let's use a 2g and 550s as an example. The 2g ECU needs to see over 2.1 g/rev to get on the highest load map. With a -18% correction, that becomes 2.56 g/rev. Now, you'll want to maintain this airflow/rev until redline, and it will taper downward from peak boost to redline. Most loggers don't read in g/rev, but do show lbs/min. Some simple conversion get you the rest of the way there. 2.56 g/rev is equal to 0.005643776 lb/rev. At 7500 rpm that's 42 lbs/min. To put this into perspective, you'll need to be maxing out an EVO 16g turbo to achieve this level of airflow. With 650s it moves up to 50 lbs/min. Now, also note that this is not taking into account the extra negative correction to lean out the AFR to 11 or 12 to 1, that will make the actual airflow requirement even higher.

To find out what you are actually flowing, take lb/min from the logger, add your correction factor back in (logged airflow/(1-correction factor), then convert to g/rev (convert lbs to grams, divide by RPM). You can also plot your airflow/rev (VE) curve from a datalog this way. DSMlink makes life so much easier

 

[donmagicjuan]

This is definitely an interesting point. I think another way to sum up what Kevin is saying is "if you're using a piggyback management system, use the smallest injectors that will net you <85% IDC." That way you should be as close to the highest map for your given setup as possible. Although, there is the consequence that with enough airflow, even the highest map could possibly call for too much timing.

 

[CanadianTSi]

Another point that I never see mentioned is that one way around the airflow signal and timing problem is to simply flow a shitload of air.

I was actually just discussing this with a friend of mine as we were logging my car with DSMLink but still letting the S-AFC control fuel.

I was able to peak just over 2.1 g/rev around 5500ish but as the rev's climbed that number dropped. If I could have just turned up the boost some more I could have maybe held it and got a close to stock timing curve. Only thing about that is that I'm around 95% Duty cycle on my 650's and turning up the boost would mean going to a bigger Injector and farther into the Negative on the S-AFC...Its a viscious cycle.

Another thing I noticed was timing was sky high at spoolup, now i'm not quite sure if this would help or hurt but I can remember it was way up there.

 

[tom04841]

beware of ND 660's as these can flow much more than 660cc's. Many of them are closer to 700-720cc

I can relate to that. With my Evo 8 560's, i had to set my global to 610.

 

[donmagicjuan]

I was able to peak just over 2.1 g/rev around 5500ish but as the rev's climbed that number dropped. If I could have just turned up the boost some more I could have maybe held it and got a close to stock timing curve. Only thing about that is that I'm around 95% Duty cycle on my 650's and turning up the boost would mean going to a bigger Injector and farther into the Negative on the S-AFC...Its a viscious cycle.

I think this is just the nature of the beast when tuning with a piggyback. The g/rev curve will take the same shape as the VE curve as Kevin points out. Unless you're able to achieve a constant VE for a given boost setting all the way to redline and defy physics, your g/rev will always peak at the RPM where max torque occurs. For getting the best results when tuning with an AFC, there will always be some degree of compromise regarding injector sizing and boost setting.