Opinions on these methods to lower knock and IDC's...

[DG]

Agreed.

dpartinvr4, how about you write a summary of the mechanism you think would cause lower IDC through higher fuel pressure. Spell it out for us.

DG

[dpartinvr4]

higher fuel pressure means your injector will flow more. if it flows more, it has to be open less time to get the same amount of fuel in the chamber. this means you can get the afr you want with a shorter injector cycle. basically more pressure means more flow which means your injector has to be "on" a shorter time to get the same amount of fuel into the chamber.

[RealMcCoy]

higher fuel pressure means your injector will flow more. if it flows more, it has to be open less time to get the same amount of fuel in the chamber. this means you can get the afr you want with a shorter injector cycle. basically more pressure means more flow which means your injector has to be "on" a shorter time to get the same amount of fuel into the chamber.

I disagree with none of that... I still haven't heard anyone explain how or why the OP expected his IDC's to drop when he raised his fuel pressure.

[dpartinvr4]

if you read what I just said, why wouldn't they?

[RealMcCoy]

if you read what I just said, why wouldn't they?

Because the ECU has no clue that you raised the fuel pressure... I ask again, would you expect the ECU to magically self adjust if you installed bigger injectors?

The only thing that will happen by increasing the fuel pressure, is the ECU sill leave the IDC's exactly the same, and you will consequently inject a lot more fuel...

[x2xtreme360]

Not quite following you Real... raised fuel pressure is going to cause the injectors to act "larger" which will cause them to stay open for a shorter period of time to inject the same amount of fuel. IDC is the percentage of time the injector is on throughout its entire cycle.

What Dpartin is saying makes complete sense. My IDC's vary when I change my fuel pressure.

[x2xtreme360]

Because the ECU has no clue that you raised the fuel pressure... I ask again, would you expect the ECU to magically self adjust if you installed bigger injectors?

The only thing that will happen by increasing the fuel pressure, is the ECU sill leave the IDC's exactly the same, and you will consequently inject a lot more fuel...

Doesn't the ECU adjust fuel based on fuel trims and O2 readings? If the higher fuel pressure increased fuel flow, wouldn't the ECU adjust the IDC's because of the increased richness in the fuel mixture?

[dpartinvr4]

no, but your ECU reads your air fuel ratio, and will realize that it has to keep the injectors open alot less to maintain 14.7, and for WOT, you have the safc to adjust things so that you can tweak the afr to where you want., and I know this from personal experience.

[x2xtreme360]

hmm... found this interesting quote on another car forum.

The stock ECU mapping does maintain closed loop fueling even under part throttle acceleration up to about 4000 RPM, if I recall correctly. So as Tim points out, it isn't just at idle and constant RPM.

I think the key point to make is that the injector duty cycle is controlled by the ECU and is not *directly* a function of fuel pressure or AFR. What IDC to use at any given moment is calculated by the ECU using lookup maps, learned trim data, the MAF sensor, and in closed loop, the O2 sensor. Once the ECU determines the current state of things and has calulcated how long to open the injector for, it opens the injector. Fuel pressure is not accounted for by those lookups and thus changing fuel pressure has no direct effect on ECU IDC calculations, and thus no direct effect on the actual IDC.

Now, in closed loop, the ECU does reference the O2 sensor to see if the IDC it is using is generating the desired AFR. If it isn't, the ECU will adjust the fuel trims which then causes the calculated IDC to change. This only happens in closed loop, so an increase in fuel pressure can indirectly alter IDC in closed loop. However, the IDC would go down if the fuel pressure increased (due to overrun or whatever) since the ECU would see the rich condition and determine that the fuel injectors shouldn't stay open as long. So in closed loop, if we have fuel overrun, then the ECU will ultimately reduce IDC. And in reducing the IDC, removing the rich condition (where rich is simply richer than target, the target itself might be considered rich too).

In open loop, the ECU is not reading the O2 sensor and therefore does not realize that it is running rich (due to FPR overrun increasing pressure for example). Since the ECU does not know it is running rich, it will not alter IDC. So here we have the engine running rich and the IDC remaining the same.

It makes more sense to me now.

Aeromotive FPR + Install pieces - Page 4 - Subaru Impreza WRX STI Forums: IWSTI.com

[RealMcCoy]

Doesn't the ECU adjust fuel based on fuel trims and O2 readings? If the higher fuel pressure increased fuel flow, wouldn't the ECU adjust the IDC's because of the increased richness in the fuel mixture?

WOT operation is a programed in map. Unless you modify it with some type of fuel control it's not going to change.

, you have the safc to adjust things so that you can tweak the afr to where you want., and I know this from personal experience.

I believe here is where the problem is... You jumped into this making that asumption, but never stated so. What is the OP using for fuel control?