some questions about tuning with SAFC

[daver36]

I've been reading up alot about the safc and got a pretty good handle on it (the tutorial by kpt4321 was awesome, plus everyone elses input) but I have a few questions left.

1) Intial low throttle settings. I want to set the low rpm setting to get a fuel trim of zero (2G) on the data logger. I'm wondering if this is a 0 on the STFT or LTFT, or and combination of the two where and -5 on the STFT and a +5 on the LTFT (for example) would net a perfect 0 fuel trim.

2) Advanced tuning. I just wanna make sure I'm understanding kpt4321 in his tutorial under the Timing vs. Airflow section. What I'm getting out of it is that is if I run bigger injectors (let's say 720's instead of 550's) on the same setup I could potentially see more power b/c there is more correction from the safc, meaning less air the ecu sees, meaning more timing advance? Obviously there's a limit, you don't want knock, high egt's or anything else bad, but am I understandng this correctly?

A second part to this question is how low of a duty-cycle can injectors run? The reason i'm wonderin is that I live in the snowy white north (canada) where we have good 'ol emmisions tests on our vehicles, which i'd like to be able to pass. I'm wondering for example if I was running in 400hp or so on a basic turbo/injector upgrade (what I mean by this is bigger turbo and injectors with all supporting mods and no other variables such as bigger cams or a stroker kit) could i turn the boost back down to stock and tune the larger injectors back down to run like 450's, and essentially make the motor run like stock to pass emmisions, or is there something i'm missing to make that impossible?

Any input would be great. Thanks in advance.

 

[romeen]

I've been reading up alot about the safc and got a pretty good handle on it (the tutorial by kpt4321 was awesome, plus everyone elses input) but I have a few questions left.

1) Intial low throttle settings. I want to set the low rpm setting to get a fuel trim of zero (2G) on the data logger. I'm wondering if this is a 0 on the STFT or LTFT, or and combination of the two where and -5 on the STFT and a +5 on the LTFT (for example) would net a perfect 0 fuel trim.

2) Advanced tuning. I just wanna make sure I'm understanding kpt4321 in his tutorial under the Timing vs. Airflow section. What I'm getting out of it is that is if I run bigger injectors (let's say 720's instead of 550's) on the same setup I could potentially see more power b/c there is more correction from the safc, meaning less air the ecu sees, meaning more timing advance? Obviously there's a limit, you don't want knock, high egt's or anything else bad, but am I understandng this correctly?

A second part to this question is how low of a duty-cycle can injectors run? The reason i'm wonderin is that I live in the snowy white north (canada) where we have good 'ol emmisions tests on our vehicles, which i'd like to be able to pass. I'm wondering for example if I was running in 400hp or so on a basic turbo/injector upgrade (what I mean by this is bigger turbo and injectors with all supporting mods and no other variables such as bigger cams or a stroker kit) could i turn the boost back down to stock and tune the larger injectors back down to run like 450's, and essentially make the motor run like stock to pass emmisions, or is there something i'm missing to make that impossible?

Any input would be great. Thanks in advance.

1) It is most commonly recommended to add the STFT and LTFT and try to get the sum of the two values to zero (+/-5 commonly viewed as acceptable).

Having said that, one of the site's Wisemen (a fellow Canadain ) has suggested using the STFT value only. Make SAFC corrections to get the STFT's to zero and the LTFT's will eventually follow. One member who has tried this stated that it worked well for him.


2) You're only partially correct here. Yes, if you were to run 720cc/min injectors it would put you in a higher timing map. However, the higher timing will make the motor more susceptible to knock which would cause the ECU to pull timing in response (not to mention the potential damaging effects). So you can end up chasing your tail so to speak. I'm not saying you can't tune the 720's with the SAFC. But it won't necessarily net you more power for the reason that you listed.

As far as tuning a particular injector size relative to emissions, I would think that as long as it is tuned for the proper air/fuel ratio and assuming everything else is operating properly that the injector size shouldn't have an effect on emissions. In other words, a 14.7:1 A/F ratio is the same regardless of injector size. Someone correct me if I am mistaken on this.

 

[daver36]

Thanks for the response. Ya, i new too much timing would lead to knock, high egt's, etc., I forgot to mention that. Your comment about a/f leads me to another question. You mentioned about 14.7 a/f ratio. That seems normal to me for what i know about the NA V8 world but everone who knows what they're talking about seems to be running 11:1. Is this a forced induction thing, or because of large timing advance, or something else i'm missing?

 

[spyderturbo007]

The ECU has two different AFR targets depending on how you are driving the vehicle. During light cruise and idle, the ECU is shooting for an AFR of 14.7:1. This is the stoichometric point of gasoline and leads to maximum efficiency through complete combustion of the fuel, i.e. best gas mileage. When you go WOT, the ECU attempts to hit an AFR of 9.5:1 when you're running on the highest airflow map.

The additional fuel richens up the mixture and is injected in an attempt to keep the motor from knocking. Running at 14.7:1 during open loop (WOT) would blow your motor.

So basically, you want to shoot for an AFR of 14.7:1 (FT's = 0) during cruise / idle and an AFR of 11:1 during WOT.

Here is a write-up I did awhile ago in response to a similar question. See post #20.

 

[mikelv]

Advanced tuning. I just wanna make sure I'm understanding kpt4321 in his tutorial under the Timing vs. Airflow section. What I'm getting out of it is that is if I run bigger injectors (let's say 720's instead of 550's) on the same setup I could potentially see more power b/c there is more correction from the safc, meaning less air the ecu sees, meaning more timing advance? Obviously there's a limit, you don't want knock, high egt's or anything else bad, but am I understandng this correctly?

If you look into mine or kpt4321, timing advance is a result of the concept that less air is entering the engine. If the motor is getting more, but the power control module (ECU) is "seeing" less, it'll advance as if it's actually what the value "seen" is.

Since you get more timing advance for an actual physically higher amount of air, you'll get more power. And like you said, this will happen to a point of detonation.

Bigger injectors will allow you to flow more air before you start to knock, so in a way, yes you're correct.

 

[daver36]

Thanks guys, as you can see from my profile i don't have a dsm yet. I stupidly convinced myself a truck was more practical when i was younger and now with wife and a kid on the way, it's gonna be a bit. But i'm completely obsesst with dsm's and plan to get one in the future.