Safc And Other Afm Signal Benders

[TTT]

Bear with me on this one.

SAFC (Super Air Flow Converter) is a device alot of people take to tune their cars.

it cheap, it's effective and it's better than nothing.

however, I have a question which came to mind during a conversation in another thread about the SAFC II and it having 2 maps.

now.

SAFC (or any other signal bending device like the Jaycar digital fuel adjuster) is wired in between the AFM and the ECU.

the AFM reads the amount of air going in to the engine as it goes past the wire on it, and sends it to the ecu, the ecu then calculates how much fuel is needed for this amount of air according to it's programming from the factory.

Right so far?

As per my example in the other thread

SAFC jumps in between and goes "OMAGOD, bro, I swear ecu bro, der is only "z" amount of air cuming through uleh.. you gotta put less fuel in bro.."

ECU goes "Me so solly.. I put in less fuel"

and that is how you lean out the misture to make more power.

but the thing is.. on the SAFC's, you can only make adjustments against the RPM chart.

on the original blue screen SAFC, it is every 500rpm.

on the newer one I think it's every 250rpm.

right?

if you set your boost to 15 psi, get the SAFC tuned.

the dyno operator runs the car as is on the dyno, gets a power curve plot up and a A:F plot.

then looks at the rpm points across the bottom of the dyno printout and lines it up with the a:f points and makes adjustments on the SAFC to either make it bend the signal to make the ecu think it needs to add more or less fuel (make it richer or leaner)

right?

so what then happens if the owner of the car decides he only wants to run 10 psi during the week.

what happens at the rpm points where the dyno operator has made changes via SAFC?

at 10psi, the amount of air the AFM sees is different at each rpm point as it is when it is running 15psi.

does the safc still send the ecu a signal that has the settings from the tune?

in an example..

before tune

15psi, 5000rpm, AFM reads 300cfm of air, a:f ratio = 10.5:1... tuner plays with SAFC in to trick the ECU it's only 250cfm of air, ecu puts less fuel.

after tune

15psi, 5000rpm, AFM reads 300cfm of air, SAFC bends this to say it's only 250cfm, a:f ratio = 12.0:1

you now have more power.

driver decides to turn the boost down to 10psi

10psi, 5000rpm, AFM reads 200cfm of air.

What happens next?

does the SAFC still bend the signal at that rpm point?

does this cause a lean out?

[toffy]

i always thought that a safc would change the afr regardless of air flow, i.e your adjusting that point of air flow : 200cfm = 150cfm to the computer causing it to the same thing no matter what the boost pressure?

[ZENNON]

i always thought that a safc would change the afr regardless of air flow, i.e your adjusting that point of air flow : 200cfm = 150cfm to the computer causing it to the same thing no matter what the boost pressure?

yeah cos otherwise only turbo cars could have SAFCs...

[QWK32]

the safc adds/subtracts a percentage of the current reading of airflow at the rpm points and also taking into account your throttle %. it will still add/trim the tune percentage for that rpm point regardless of the boost pressure.

using you eg.

say at 15psi, 5000rpm, 100%Throttle, you have 300cfm and tune the safc to remove 5% at that point = 285cfm

then at 10psi, 5000rpm, 100%Throttle, you have 200cfm and the tune is unchanged (5%) you get = 190cfm

the ecu then does the rest for what its knows as to how much fuel is needing for 5000rpm, full throttle and 190cfm or 285cfm.

also take into account that you have changed one of the varibles from the original tune so there is no saying that the tune will be exactly right. on the safc2 you can have two maps, you could get one tuned at 15psi and one at 10 psi.

[Kwyjibo]

Seems pretty simple to me(hopefully I'm right), its just like a normal tune at a higher boost pressure. All thats changing is the AFM signal to make things leaner. Nothing in your car compensates for the lower boost pressure(AFM voltage will be the same as this is before the turbo) so the car will run richer after it hits peak boost.

Like has been said the SAFC II lets you have two maps, so this could be the answer for you, have one set for say 15psi and one for 10psi so it doesn't run rich at lower boost.

[TTT]

I'm not looking at buying an SAFC or any other bender.

it was from another conversation that I started thinking about this point.

Are we saying the percentage of correction it does is dependant on throttle position?

100% throttle might = 5% correction at 5000rpm but if it senses only 50% throttle it adjust by 2.5% at 5000rpm?

[BHDave]

http://www.xspeed.com.au/manuals/Apexi_S-AFC_manual.pdf

Here's the instruction manual for the old one.

There is a high/low map. the cut over point is adjustable and based on throttle position.

I see where you are coming from re the percentage reduction though as you could end up with a lean situation, particularly on something like an r33/s14 with aggressive r&r at the high airflow but not so bad at lower airflow (ie the 15 vs 10psi scenario)

Ideally, you'd have to set it up on the dyno and check it, then try a few runs at lower boost to see what happens, make adjustments accordingly and compromise your high boost tune. Or if you are a nut bag like me, you run the max possible boost all the time (except on the track because you have no fine control of the throttle on corner exit )