Okay, I installed a s-afc this weekend, so I'm just about ready for dyno-tuning. I have one question though, and that is do I need to install an adjustable fpr before I dyno-tune?

My base fuel pressure is up to about 40 psi because of the 255lph fuel pump and no adjustable fpr. I don't see the reason for an adjustable fpr because:

1) during normal closed loop operation, the ECU compensates for the increased fuel pressure by shortening the injector pulse width based on O2 sensor information. So I don't see the need for an adjustable fpr during non-boosted operation.

2) at WOT the s-afc will be used to fine tune the AFR, so it doesn't matter if my fuel pressure is 7 psi higher than it otherwise would be.

Extra information: I have the V1 kit with 3.25" pulley and 8:1 disk, MEVI, 255lph Walbro pump, CAI per Cheston. I've been seeing 9 psi at redline, ~85 psi fuel pressure, ~1450F EGT.

I checked my fuel consumption this morning, and after a weekend of mixed highway and city driving with an occasional boost now and then, I got 26 mpg.

 

Wow. I don't get 26 mpg and I'm still NA!

I can't imagine you need a FPR. You may be a little rich down low, but that should effect much. I would have thought it could have effected your mileage a little, but I guess not.

 

I just put my SARD FPR on two days ago...I know I am running super rich, judging from mileage and a few other factors...

I am an optimist at heart and hope the adj fpr will help efficiency and track response.

 

Well there's something more to it Stephen Max. I was at 46 psi before the fpr, at 32 psi now. @ 46 psi base fp, I was getting about 180 miles per tank. Just by lowering the fp, I get on average 270 mpt.

I don't know that the ecu can compensate for that much of an increase in base fp. But hell if you're getting 26 mpg like you're saying, that's more than I've ever had, including all highway driving.

 

Yeah, I was pretty surprised, myself. This is the first time I've checked mileage since installing the SC, I figured it would be down around 20 mpg, although I'm still getting about the same number of miles per tank as before. My odometer said 313 and it took 11.7 gallons...

 

Your fuel injectors were designed to operate with the same pressure drop across them at ALL times.

Thats why your fuel pressures increases under WOT by 9-10 psi because the vacuum drops from about 20" (-10 psi) to 0" (0 psi) so the pressure drop across the injectors remains the same.

Same thing when you boost, notice that the fuel pressure is bumped up as the manifold pressure increases, again to keep the pressure drop constant across the injectors.

The only time this changes is when you use a dry nitrous setup the pressure is bumped up to ridiclous levels with same manifold pressure. The injectors were not designed to operate like this, hence the danger of the dry setup.

Therefore. I would not recommend running the fuel pressure at higher than the stock level. You are correct the ECU will try and compensate but I would feel safer with the proper pressure drop across the injectors.

You can operate out of these parameters and not have problems but its usually best not to try and out engineer the Engineers.

 

Very good point, which I did not know. Thanks for explaining that.


Here I think you are mistaken, since the amount of additional fuel pressure far exceeds the boost pressure in order to deliver enough fuel not only to compensate for the additional cfm, but also to lower the AFR to a safe level. And fuel delivered is proportional to the square root of the fuel pressure increase,reference , so for 9 psi of boost you need around 85 psi of fuel pressure.

Since the rising rate fpr is going to bump up fuel pressure to that level, then the extra 7 psi represents only an extra 10% of what it would be if I used an adjustable fpr to lower the base fuel pressure, and is within the range that the s-afc can be programmed to adjust for instead of using an adjustable fpr.

 

I was referring to cars with stock S/C's and Turbos as far as keeping the same pressure drop across injectors. The ones I have looked at follow the same logic.

Again aftermarket S/C's and Turbo's are somewhat like a dry nitrous kit in that they make the injectors do something they were not designed for.

That doesn't mean it doesn't work its just something to keep in mind when you are doing it.

 

You "might" be okay as long as you can dial enough fuel during higher boost levels.

One thing(and I think you already know this). During the warm up period, the ecu doesn't use the 02 sensor for injector pulsewidth. It uses a set program based on the assumption that the fuel pressure is XX psi. So while the car is warming up, I bet your car runs pretty rich.

 

That might explain my low gas mileage! I only drive 1.5 miles to work...
More incentive/urgency to put that SARD in with gas prices going skyhigh... I hope Thomas sends me the correct fitting soon.

 

Now I understand what you were saying. Sorry I contradicted you, Jime.