Blowtus

Ok. I know that it modifies the afm output, and is used to modify the air / fuel ratio.

Now any reading I've done has suggested the pressure sensor doesn't do particularly much, so I can't see why it would cause trouble having it at it's maximum allowable voltage output (fcd) as long as everything else in the system was still compensating? Or are the other components not designed to function properly with more than 8psi of boost? I'd have thought everything should be able to compensate fine... but obviously not.

Ridge Tech

your pressure sensor is also what controlls fuel cut. When you add an fcd you are stopping the voltage coming out of the pressure sensor just before fuel cut, which allows the boost to keep going, but heres the thing. Sense you are lieing to the ecu about how much boost your giving it, it doesn't know how much gas to give. You could be running 17psi and the computer would think your running 7psi. The SAFC allows you to kind go behind the ecu's back and richen the fuel mixture. You can get away with doing this up to about 10psi on stock equipment safely, but you still might want to upgrade your fuel system. If you plan on running higher than stock boost, rewire your fuel pump first and start looking around for some larger injectors to use with your SAFC. Unless you have a down pipe and perhaps an intake you shouldn't be hitting fuel cut yet.

Chris

Blowtus

But the pressure sensor isn't where the ecu gets it's main information from is it? Shouldn't it be able to tell from the amount of airflow how much fuel is needed...

NZConvertible

The pressure sensor doesn't control how much fuel is injected. The primary load sensor used to calculate fuel injection is the airflow meter, and as boost increases so does airflow. Up to a point the ECU measures this extra airflow with the AFM and adds the right amount of fuel accordingly.
The pressure sensor controls ignition timing by retarding the spark as boost rises to prevent detonation. Lying to the ECU with a FCD means the ignition isn't being retarded any more once you go past ~8.5psi. Adding extra fuel via the S-AFC to make the mixture richer (as oppsed to just keeping up with the extra airflow) can suppress this detonation, but at the expense of some power.

chris_stampe

He is right tho, the ecu gives fuel depending on the afm signal, not the boost sensor. The boost sensor only retards timing if im not mistaken. The main problem is just that the stock ecu wasnt made to run w/ high boost/fuel. The afc allows to patch up where the stock ecu isnt compensating for the added pressure correctly.

Ridge Tech

I'm just spitting out what I was told

Blowtus

thanks nz, explains it perfectly.
So some variety of thing that retarded the spark based on the output of the pressure sensor before it got to the fcd would be much better basically?

capsoval

S-AFC are cool.

RylAssassin

wow this has been very informative,
Can you guys some up what your saying please in laymens terms (sp?) cuz i think im...aaalmost getting it

Blowtus

What don't you understand Ryl? I wanted to know why an s-afc was used after adding a fuel cut defender, because as far as I could see the airflow meter should account for how much air flow there is and richen the mixture accordingly. Now according to mr convertible, it does, but because the pressure sensor is no longer outputting the correct voltage above 8.5psi, (to retard the spark timing) more fuel has to be added to avoid detonation. I now want to know why spark timing modification isn't used instead of adding more fuel.