Boost referenced SAFC is a HUGE improvement!

[H@xtGSX]

If you're like me and use an SAFC to compensate for larger injectors, you've probably discovered that the SAFC really only works "well" in a narrow band of the engine load. Most people compensate for this by spending weeks messing around with the settings to get the most power when they are going at full throttle. This generally means you also run rediculously rich for a vast majority of the normal daily driving. Mine was so rich at one point when I was tuning it that I could light my exhaust on fire as it left the tailpipe and it would maintain a flame... not a good thing. It's also rather painful at the gas pump. Well, I got sick of dealing with this and did some researching to see if there's a fix. I found one, and it works. First, I found out that the SAFC was originally designed for naturally aspirated motors running high compression and bigger injectors. Because of this, referencing the fuel correction off the throttle position is roughly in accordance with the engine load, at least it is moreso than any boosted engine. We use them on boosted motors with a much more dynamic range of conditions at the exact same throttle position, thus they really only help for a narrow portion of those conditions. So how does one fix this? Stop basing the correction on a useless figure like the throttle position and use something representative of the actual engine load... like the absolute intake manifold pressure. This is how speed density operation calculates fuel, so why not do the same with the SAFC? This thread by a guy with a boosted 420a outlines how to do just that.

2GNT.com - SAFC_fuel_Compensation_Based_on_Boost

I basically followed his advice and ordered the MAP sensor mentioned in that write up. It was about $20 shipped, and it is wider range and more sensitive than a GM MAP sensor for roughly 1/4 the price. This is it:

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It's about the size of a quarter. I soldered on some wires for the power and signal, attached a hard tube and T to splice into my boost gauge line, stuck it inside a little project box from Radio Shack, and installed it. This is what it looks like right now.

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It reads absolute pressure, so that means it can read vacuum also. I attached the gray SAFC wire to the signal wire from the MAP sensor, and this is what it reads this morning with the engine off and at normal atmospheric pressure.

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So I make "0 psi" on my boost gauge the low throttle point, and set the high point for what the sensor should be at for around 20 psi. The SAFC will self adjust to higher boost and set the 100% point higher if you don't take the car out and run it at your absolute highest boost pressure at least once, so I did, then backed it off to where I want the normal maximum boost to be at. This is what I wound up setting it at.

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Now, the SAFC makes it's fuel corrections based on the engine load, and it works MUCH better. My gas mileage has improved, the engine responds faster, and the turbo spools a bit quicker. It also allows me to set my corrections fairly aggressively for 550's, still keep the fuel trims close to 0, and the ECU basically self tunes everything for you from that point. It beats the heck out of messing around with it for weeks, trying to find that "sweet spot"! It's a major upgrade if you're using an SAFC and I highly recommend it.

Enjoy!

 

[4Lurker4]

Very interesting, I still have my Apexi Neo hooked up so I might want to give this a try.

 

[pats16valve]

Genius. This'll be a life saver for guys struggling to tune with these.

 

[H@xtGSX]

It is a thousand times easier to set up and tune, so long as you understand that the throttle percentage shown on the SAFC screen is just a representation of the MAP signal voltage, and that you need to set things in relation to ambient pressure and your maximum boost pressure. Once you have those points figured out, set your correction points a bit richer for idle and progress to slightly beyond your injector size difference percent. For me, with 550's, that equates to around 10% larger than stock (depending on conditions), so the low settings at 3k and up are around -10% (adjusted slightly for fuel trims where needed). At high settings, 3k and up are around 18%, so between hi and low, the engine load determines the corrections, the ECU compensates on the fly for quick adjustments, and the overall tune is set slightly leaner than stock. It has been amazing how much better of a system this is. You don't have to fiddle with it much at all... A few minor adjustments at first, and then it works with the ECU and self adjusts. It is a night and day difference compared to the endless farting around you have to do with a normal SAFC.

 

[1998_gst]

this exact set up will work with the NEO?

Very interesting, I still have my Apexi Neo hooked up so I might want to give this a try.

 

[H@xtGSX]

I would think so, if the NEO is still using the signal from the TPS as a reference. If it does, I can't see how it'd be any different in calculating fuel from the MAP instead.

 

[4Lurker4]

I would think so, if the NEO is still using the signal from the TPS as a reference. If it does, I can't see how it'd be any different in calculating fuel from the MAP instead.

It does use the TPS, same as the other SAFCs just more data points and in color So it would work the same with this mod.

 

[H@xtGSX]

So I have had a full week of commuting back and forth to work now with this MAP installed. I know that on average I would burn roughly 10 to 12 gallons of 91 craptane just commuting. I kept track of the additional miles from cruising, tuning, and going to stores so I could judge the difference based off of a known value. I am happy to report that the difference as of 1 week is almost 2 gallons less. Calculated MPG I am getting according to the "full tank" reference point now is 24.8 mpg highway.... Up from roughly 20 mpg previously!!! That alone just paid for this mod, not to mention the car runs and drives soooo much better now.

 

[1998_gst]

New MAP sensor ordered, lets give this a try thanks man!

 

[fonzi90Tsi]

Interesting!! Thanks for showing us!!

 

[Gruff511]

this exact set up will work with the NEO?

I did this on my buddies 420a running the NEO and works pretty good...

 

[talonESturbo]

will it work for a afc o the one with nubs

 

[Chicken Patty]

This is really good stuff man, thanks for posting and sharing. At least for those running S-AFC's that are on a budget and really are limited by it, they can do this and get away with it a bit longer. Great stuff!

 

[H@xtGSX]

I don't know about the older afc, but again, if it is making its calculation based on throttle position, I think this will work. I don't know anything about the old style afc though.

I think those of you that try this will be surprised at how much different it will respond. I love it. I have mine pretty well dialed in now, and it is just like driving it when it was stock. Very predictable, more efficient, and no soggy initial boost with the resulting severe kick in the ass when it catches up. It is smooth and even. I hope you all enjoy it!

 

[pats16valve]

Any updates on how this is going?

 

[H@xtGSX]

Nothing to update from me. It works great and makes the old way of using the SAFC obsolete, in my opinion. There are still a few issues that will never go away using the SAFC, like injector dead times, but other than the timing and latency issues which can't really be helped, it runs loads better than the normal way. I have gained gas mileage, the car is quicker in response, and it is easier to tune. It is completely worth the effort.

I will only be driving my MAP referenced SAFC setup for a few more days. I bought a flash ECU and a Tactrix 2.0 cable and will be flash tuning the car from now on. My 1g CAS made it necessary to pot mod my baro sensor, and that coupled with my SAFC made my timing problems unmanageable. My tune literally changes with the weather, so much so that what is driveable in the morning may only barely work in the afternoon. I got tired of having to constantly adjust because I had no accurate barometric compensation, so I am moving on to flash tuning. If you are staying with the SAFC though, the MAP mod is the best thing you can do to make it respond better to engine load.

 

[1fastlaser]

I did this back in the day with my HKS VPC, and wired the TPS signal wire of the SAFC to the VPC map sensor signal wire. It works wonders and adds fuel based on load not TPS.