SFAC Links - GM Performance Tuning

jeremy96gase · #1 (**permalink**) 02-22-2006, 02:09 PM

These aren't GM specific, but they've got some good info in them.

http://www.2gnt.com/www/corbin/foolafc.html

http://www.roadraceengineering.com/newafc.htm

Matt95GT · #2 (**permalink**) 02-22-2006, 06:02 PM

On that first link "Fool S-AFC"...
I do not feel that's necessary unless you are running rather high boost, and it's coming from a turbocharger that spools slowly - if that's the case I can see where the S-AFC (well, it's inputs actually) will show it's limitations simply because the TPS input and boost pressure will not be linear nor synched up with each other very well. When applied to most of us Grand Am'ers who run mostly low boost, the "fool" mod for it shouldn't be necessary.

Sidenote: The S-AFCII has been a real pleasure to use... a neat gizmo that has far exceeded my expectations. Some RTFM'ing, install, initial setup, and tuning only took an afternoon. Afterwards I had a nice flat A/F ratio across all RPMs at all throttle scenarios. It has also been a great tuning tool (combined with Wideband O2) to tweak at the track - I can quickly make adjustments between runs, no need for a laptop. (*sigh* now just need to get the car on the road again, and to a track that's not 3000' up in the mountains)

Matt95GT · #3 (**permalink**) 02-22-2006, 06:13 PM

Also... wanted to comment on something from the same first article...

Quote:

Don't rely on the S-AFC to do all of the work, you really do need to use a rising rate fuel pressure regulator [like the Vortech S-FMU]. Since the PCM calculates how much fuel to inject based on several sensors, not just the MAP sensor, you need to adjust fuel flow externally [with fuel pressure]. If you keep your fuel pressure static and use the S-AFC to make all fuel adjustments from idle to full boost, the car will bog when you quickly hit the gas. It injects too much fuel and you won't go anywhere.

Don't forget that the GM engines that didn't come with boost from the factory only have 1-bar MAP sensors and lack fuel maps for boost. Therefore, you cannot have a static fuel pressure, you need an FMU to up the fuel pressure in proportion to boost pressure.

Also... some GM engines (like the Quad 4/Twin Cam) are speed density, only a MAP sensor, no MAF (aka "extra sensors") - so in that case the PCM does calculate the amount of fuel to be injected solely on the MAP (and S-AFC piggyback output) reading.