Fuel Cut Defenders, Are They Really Bad?

[tek_01]

Just been reading a bit here and there and came across a thread stating why a FCD is not good for your car

basically it clamps voltage at say 4.8v (5v max) so you computer is thinking its only getting xxx amount of air and throws in the correct amount of fuel

HOWEVER it is really getting more than xxx air but is only getting the enough fuel for xxx and not the real amount, hence the engine running lean which causes all sorts of dramas

now my query is, when you are on a dyno and you are getting your perfect AFR's with a FCD installed. Is that reading the actual amount of air and fuel or its reading what the computer THINKS it is getting?

if it is that latter than yes it is dangerous but if thats the case why sell the damn things in the first place???

also from what i know with a SAFC you can control the amount of air/fuel mixture your getting. If the FCD is bad is the AFC better in the way it works?

experts please help

[45ULT]

yeah FCD's clamp the voltage as u stated causing mixtures to go lean but also the timing is advanced in some cases at the same time which is not the best thing to have happen. SAFC's are not so much a clamp as a manipulator varying the voltage reading of the airflow meter to either richen or lean out a/f's.

[Lazy-Bastard]

now my query is, when you are on a dyno and you are getting your perfect AFR's with a FCD installed. Is that reading the actual amount of air and fuel or its reading what the computer THINKS it is getting?

If it's being tuned on a dyno, the tuner would/should be using a wideband o2 sensor, completely independant of the ECU (and the factory o2 sensor), so if the AFR's are perfect on the dyno then I'd say it should be fine.

[tek_01]

so when measuring the air on a dyno, where does it take the reading from?

[tek_01]

ah just replied as i wrote

tuning was done a dyno dynamics at both UAS and CRD

any idea if they use a WB o2 sensor?

[Lazy-Bastard]

Never used either, but I know they are both reputable workshops, so I'd say yes.

[tek_01]

very good, yeah i thought it was a bit bogus saying theyre bad

sure theyre bad if you dont use the correct means of adjusting them but other than that i think its fair to say theyre ok

[45ULT]

a business not using a wideband sensor would not last very long.. im pretty sure that dyno dynamics has a built in wide band which logs each run on top the previous. if used for what they're designed for then both safc and fcd are quite good gizmo's once u start trying to do too much with them thats when u start breaking things. trust me i know from experience.