95% Injector Duty on Stock ECU - is this right?

[crispyduck]

All,

I reflashed my car back to stock to get a 4th gear WOT run as a baseline to compare against. As you will see from the attach log my injectors are at 95% on the stock ECU. I can't see that this is correct. Car runs great although its much slower than my modified map

For your information, my car is a EURO MY03 2.0 WRX with a decat downpipe, decat center pipe and Prodrive back box from an STi. Surely this would not make 95% IDC.

Outside temp this morning when taking the log was about 3 degress C, fuel is Optimax (in the UK its around 98 or 99 octane).

Does anyone think this could be a problem in ecuExplorer's conversion from say pulse width (mS) to IDC (%)?

Any advice here greatly appriciated as this seems very odd.
-Steve.

[Kha0S]

Sounds more normal than you'd think. With the same modifications on stock mappings, I would occasionally see IDCs as high as the mid 90% range on cold (< 0C) days.

Combination of turbo efficiency in cold weather and stock rich fuel mappings end up hitting the top of the fuel system pretty quickly.

[Spiider]

95% idc is very "normal" on the stock ecu program.


I think one of the best mods to fix this is a walbro and some Perrin parallel fuel rails, and I'd think about bigger injectors too since scaling them up is no problem.

[ahains]

I think that with a "good" fuel map your IDC's will be fine. I wouldn't bother upgrading the entire fuel system until your IDC's are high with a good fuel tune.

[JimbobUK]

I also took a log on my standard maps a couple of days ago...

My IDC is going up to 108% !!!

Temperature was about 2 deg C... Optimax... Decat downpipe, straight center pipe and H&S backbox.

Load was about 200 g/s at 6000 rpm (I guess that means 4 g / cyl).

The reported AFR by the ECU was 11.25 (although the standard map specifies about 9 ish for this load / rpm zone).

Could it be that the AFR sensor is knackered, and the ECU is adding more fuel to attempt to reach target AFR? Perhaps it doesn't correct in this way (I don't know about fueling).

Or could it be that the fuel pump / injectors are not working optimally?

Or is this just normal(ish) behavior?

How does the ECU calculate IDC anyway? Does it slowly correct it based on some feedback of AFR / something else?

What are the various AirFuel learning / correction values that can be logged? Would one of these help me to identify the problem (if there is one)?

Many thanks,
James

[FrSTi]

James,

First of all, you must consider the stock WB is quite narrow as it is mostly dedicated to close loop follow-up, in the 14.7 region.
The richer value you'll get is something like 11.3:1 AFR or so, as you noticed. But this is not really accurate when you get away from the 14.0...

On the IDC side, you should pay attention to the fact that you are actually above 100%... but...

My experience revealed that the fuel system -running a walbro 255- is still matching targeted AFR until you are reaching 120% or more...
Don't consider this as a rule, it may vary from car to car, cam timing, fuel system set up, etc... (disclaimer )
By the way, I would higly recommend every one here to use a real WB system + detcans, it is worth the investment.

[crispyduck]

Thanks everyone for your feedback so far. Important points noted:

* Stock ECU + decat pipes commonly get 95%+ IDCs

* A 'good fuel tune' could make all the difference. Can one assume around a 10% reduction in IDCs with a good tune? Does this mean getting a wideband sensor rigged up so it can be monitored whilst being leaned out some.

* Interesting feedback on the Walbro 255 pump. I'd like to investiage that further... To this end perhaps someone can help me get my head around this one. See example below:

Upgraded pump raises fuel supply pressure to the regulator. What is the percentage increase using a WB 255 to a stock EURO MY03 WRX (15-20%)? Also does the WB 255 have the three speeds like ours currently?

Assume stock fuel regulator raises pressure on fuel rail given the increase in supply pressure. Assume this not quite proportional (assume 10% increase?).

So injectors at the same IDC should flow more fuel given inceased pressure. In the example will this be 10% more?

Assuming stock fueling was fine, may be still slightly rich even at high (95%+) IDCs, can one simply scale back fuel maps by 10%, thus reducing IDCs by 10%?

Appriciate anyone that has read this far and that can comment on my logic above!
-Steve.

[Kha0S]

I'm running an upgraded pump (the Walbro 255lph high-pressure pump) with the stock fuel pressure regulator, and there was little to no change in injector duty. It is my understanding that the stock RRFPR is all that's managing the pressure, and that the pump is simply capable of flowing at higher pressures. My long term fuel trims are under +- 5%, so that leads me to believe that the upgraded pump alone isn't changing fuel pressure at all, and shouldn't change IDC.

The Walbro pump does, indeed, operate at the three duty cycles that the stock pump can operate at.

The rule of thumb that I've heard (take it with a big grain of salt) is that a regulator tuned for 10psi over stock will yield approximately a 10% drop in injector duty, or, if the duty is kept constant, corresponds roughly to a 1 point drop in AFR (ie, 12:1 to 11:1).

[ahains]

I could be mistaken, but I had the impression from the DSM world that you do not see the inadequacy of a stock FPR unless you are flowing great amounts of fuel.

The weakness would generally be the return line / route of the FPR, so it is able to control the pressure just fine until the rate of flow is great enough that the resistance of the return line would cause the pressure to increase out of control.

Because of this, I don't know that fuel trims would be an adequate measurement. I would think that a fuel pressure gauge or a wideband O2 sensor would be the best tests to see that the pressure is under control.

Again, I could be wrong...

[JimbobUK]

Surely if the pressure fed to the injectors is raised, then this has a similar effect to using higher flow injectors, and you would need to alter the injector scaling value to compensate?

BTW. What does the injector cc rating actually mean? I'm guessing that it means that the injector will pass x cc of fuel per minute (under standard conditions - a duty ratio of y, and a stable fuel supply pressure z).

Hence for a higher pressure, the cc value is effectively increased.

I may well be totally wrong!!

[Spiider]

My belief is that it has more to due with volume at higher rpms not pressure.

Volume and pressure do have a relationship to each other in this system and what I think happens is that although pressure may still be adequate, it is volume that suffers. The stock pump just can't move enough volume of fuel in the high rpm range, especially if you have power increasing mods.

Plus, the walbro is only $70. Cheap insurance really.

STi pinks, an aftermarket FPR, and Perrin fuel rails might be another $800-900 if you want much better IDC numbers.

[ahains]

Spiider,
I'm not sure I understand your post 100%, I could totally be misreading it.
If you're thinking that the stock fuel pump IS maintaining the correct fuel pressure to the fuel rail at high RPM, but is NOT able to keep up with the volume needed - this statement is not correct.

Pressure and volume of flow are 100% related. If the pump can maintain the proper pressure on its output then it is absolutely able to "keep up" with the volume. When you get to the point where it cannot keep up with the volume, then you will absolutely observer that the pressure is lower than it should be.

JimbobUK,
Yes, if the input pressure to the injectors is raised, it is exactly like having larger injectors. In fact this is a hack that many folks use (not specifically WRX owners) when they need a bit more fuel but don't want to upgrade injectors. You can find some useful technical data here:
http://www.rceng.com/technical.htm
They have the formula to determine how much increase in cc/minute you get by a given increase in pressure. It is not linear, it is the square root of the new/old pressure ratio..
So for example if you increase pressure 50% from 40psi to 60psi, the new/old ratio is 1.5. Square root of 1.5 is 1.225, so new flow rate is 22.5% increased over the old flow rate.

[Kha0S]

What's important to realize is that, no matter what the capacity of your pump, if the FPR doesn't change, the pressure at the injector will be the same, and the flow will not change.

[ahains]

What's important to realize is that, no matter what the capacity of your pump, if the FPR doesn't change, the pressure at the injector will be the same, and the flow will not change.

You probably mean "if the fuel rail pressure doesn't change"?
If so, you are exactly right.

But if you are saying that aftermarket pump + OEM fuel pressure regulator will always give you the same output pressure, then this is not necessarily true. See my previous post. I'm not positive, but I think the issue is that when the pump is running at high duty cycle you can overrun the return line of the FPR, so fuel rail pressure is no longer properly controlled (it is higher than desired).

I'm not saying this is definitely a problem on a WRX, I'm only saying that observing fuel trims will not generally detect this kind of problem. It may very well be that the WRX OEM FPR can handle an upgraded pump just fine.

[Kha0S]

I'm not saying this is definitely a problem on a WRX, I'm only saying that observing fuel trims will not generally detect this kind of problem. It may very well be that the WRX OEM FPR can handle an upgraded pump just fine.

Perhaps, but my injector duty cycles and AFRs didn't change when installing a high-pressure capable Walbro 255... so it's empirical evidence that with the common pump upgrade on these cars, it doesn't buy you anything without changing the regulator.