How to get a s-afc to adjust for boost pressure, info inside.
 

I went searching for information about the apexi s-afc, since i'm about to get one... and i came across this site.

It instructs you how to use a motorolla MPX 4250 AP pressure sensor that reads from 2.9 to 36.3 psia and gives a 0.2 to 4.9 volt output, which the safc can use. You connect this pressure sensor to where the safc should connect to the throttle position sensor.

the link is here

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

Has anyone tried this? From reading the website it seems like it will work. But being an internet junkie, it has taught me to not take everything seriously all the time.

Any comments?

hrmm, sounds interesting, if you could get it to work right, you have the s-afc set itself for the higher boost levels it the high rpms after you car in modified, seems like a good idea, I'm not going to test it out though......... I'm not to good with direction........

from what i read from the website is that the tps outputs a voltage depending on how far you plant your foot down. which is 5V when floored and 0 volts when coasting.

so the motorolla chip outputs 0V at zero atmospheric pressure... and then 5V at X psi above atomspheric (gauge pressure), but i'm not sure how you can calibrate the chip.

Since it can read from 2.3 psi to 36.3 psi

so it has 22 psi range of gauge pressure... i guess that is up to teh safc? i'm not sure... i don't have an safc yet.

here is the data sheet for the motorolla sensor

http://e-www.motorola.com/brdata/PDF...s/MPX4250A.pdf

bump

looks interesting. Is there any turbo fc owners that have down this?

Isn't a rising rate fuel pressure regulator safer/easier/cheaper than this method?

RRFPR's are very crude compared to an S-AFC.

I guess it would depend on how radical the enrichment would need to be.

I just want to know if there are any actual FC owners that have actually done this. I'm not really comfortable having my motor be a guinea pig....ah screw it :devil:

I think the stock boost sensor is also 0-5v so you can just use that. some people do.

thats what i was thinking

mike

I was about to ask that since I think that is for dsm's...
If the boost sensor reads 0-5V, then it would be really simple to read boost voltage instead of using the TPS. Which leads into the next question:
If it's so simple why aren't more people doing it. I would imagine it would be much better to have fuel control be dependent on boost and not rpm.

edit: if the stock boost sensor sends beween 0 and 5 volts - what are the corresponding pressure? I would imagine 0 volts = 0 psi...but how high will the stock pressure sensor read to.

stock reads to 15 psi. but if you have a FCD. you'd have to splice in between the boost sensor and FCD. other wise you'd never see over 8.

0v would be vacum. how much I have no idea.

the FSM probably has the voltage for 0psi listed.

Yep good call on the FCD. Hmmm well I don't imagine I'll be running over 15 pounds on the stock turbo anyway...so that shouldn't be a problem. I'll check out my FSM and see if I can find what 0Volts correlates to as far as a vacuum reading. My main beef with the safc has always been the rpm dependent fuel adjustment, but maybe now I'll settle for the safc. I've decided that RRFPR's can do a decent job, but they are really sloppy.
I really think I'd be super happy with 275-300 rwhp and I'm not so anxious to splurge 700 bucks to jump on the e6k/microtech bandwagon just yet. Maybe after I blow up this motor I'll learn the next time ;)

the stock t2 boost sensor should be at about
-15psi@0v, and +15psi @5v

mike

gracias, I didn't make it to the FSM tonight...but I think I'll be trying boost dependent vs. rpm dependent safc style now if I go that route...

You mean boost instead of throttle position. It'd never work without an rpm signal.

Hmm... just a dumb question here, but what if your not on the boost? How will the s-afc compensate for the amount of fuel u need?

:dunce: yes

0volts != 0psi. 0 Volts is -15psi which is a vacuum pressure. So even if I'm not on boost the safc would still be seeing some amount of voltage between -15 psi and 15psi.

After tooling around for 500 miles trying to break in my motor with little boost and a S-AFC hooked to the tps, I can definately see an advantage to hooking it up to the pressure sensor.With TPS dependant, the only time the lo settings are good is when your cruising and the only time the high is good is under full boost.

This is my planned setup:
Low High
1000 set for good idle same as low

1600-4500 13 A/F cruising same as tps dep.(11.5 A/F)
+ a little from 2500-3500

5000- same as high same as tps dep.(11.5 A/F)

By keeping the settings the same above 5000, this should protect the motor in the event of a pressure sensor signal failure.(and nobody should be just cruising above 5000 anyway) Does anyone see a reason why this setup could go lean?

Sorry, chart was hard to read
Low ................................High
1000 set for good idle .......same as low

1600-4500 13 A/F cruising........... same as tps dependent.(11.5 A/F) + a little from 2500-3500

5000- same as high ..................same as tps dependent.(11.5 A/F)

I have set my idle on low throttle in the neg. percentage, but I left all the other areas on the low map untouched. Basically I just have a stock fuel map until around 8 pounds I have it switch to the safc values.

I need to get it onto a dyno with my 720's.

Working the AFC off the Boost Sensor signal works fine. The only problem is that you have to be in boost for the S-AFC to switch to the high map.

This mod WILL NOT WORK with NA's.

If you have an S5 turbo, there is no need for this since you have a full range TPS. This mod is only recommended for the S4 cars.

The voltage for the Atmospheric Pressure Sensore at 0 PSI is somewhere in the 2.3v range as per the FSM.

Now correct me if I am wrong, but that pressure sensor signal is still useable (and god knows it probably wont go to full throttle at 1/8th pedal like my damn SAFC does). If you change your High/Low throttle points, you can tune for a signal, even if its clamped by an FCD.

Jarrett