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979 Posts
got a quick question for ya....is the fcd suppossed to affect air fuel at all?..
when adjusting mine it severly affected the fuel....i started at 10 and air fuel was low 13s....when i went to 9 and 8 fuel jumped to high 13s and 14s under boost...

i set it all the way to 12 and its back into the 12s under boost....any ideas?...
 

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2JZ NA-T MKIII
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666 Posts
^^Yes, an FCD will affect your AFR's.^^

here is the basic idea, I'll just quote myself from a different post:

The map sensor tells the injectors how much fuel to dump.

For example.. we'll say that the map sensor read's "boost" since thats the easiest way to describe it. You're running 20 PSI, the FCD only allows the ECU to see 14 PSI... therefore the ECU is only going to put in fuel for that 14 PSI of boost.

Adding fuel with an SAFC will not help (I've tried on the dyno), because what you're doing with the SAFC is telling the ECU how much fuel to dump in, by modifying that MAP signal. Even if you add 5% fuel, you won't get anything because the stupid FCD is only allowing the ECU to see "14 PSI", no more.

You can use an FCD, but you need to be smart about it. When using an FCD the easiest way to tune is with the fuel pressure (yes, you'd need a fuel pressure regulator). This way you can mechanically add fuel without going through the ECU. The ECU will automatically adjust for cruising, so rich on cruise shouldn't be a problem. A wideband O2 is required to run an FCD (for anyone who has any sense at all).
 

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MK3 Owner For Life
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1,822 Posts
yes it does, when you raise the boost limit it sees higher boost at certain volts. So when it reads 5v ( 14psi) stock and you raise your boost cut it sees 18psi at 5v's but you only get enough fuel for 14psi.
 
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