A bit confused on fuel requirements and tuning

[Calan]

See mods in profile (except for Walbro and AFPR, which aren't installed yet)

While I'm waiting for my fuel goodies to get here, thought I would try to learn a bit about something I don't quite understand.

Basically... if I can adjust to run rich (even to the point that the car stutters, black smoke, etc), why/how would the 255 FP and AFPR upgrade help? How do these mods affect fuel-cut, MAF-T tuning, and performance in general?

I understand that the MAF-T is "lying" to the ECU about airflow, causing the ECU to do it's thing. But what I don't understand is ... for a given amount of airflow and the ECU obviously managing to get gas into the cylinders, how can additional fuel pressure or flow improve things?

Without the fuel mods yet... O2 sensor cycles as expected on logger, my o2 trim is hovering around 100% at cruise, and mid/high trims are around 105-120% after short amount of drive time since reset. I'm running 12-15 PSI boost, although I haven't really gotten on it due to some knock issues. (I replaced my knock sensor and still have some PK ... but that is another story).

I'm an old V8 shade tree mechanic and I'm still learning this DSM tuning.... just need some gaps filled in...

Thanks as always!

Calan

 

[romeen]

Actually with the 255 fuel pump you will need the afpr to decrease (not add) fuel pressure as the pump will overrun the stock regulator causing you to run rich.

 

[Calan]

yeah... that's what I've heard (and why I bought it)... but why the bigger fuel pump in the first place, if I'm already able to run rich?

 

[Gromit]

See mods in profile (except for Walbro and AFPR, which aren't installed yet)

While I'm waiting for my fuel goodies to get here, thought I would try to learn a bit about something I don't quite understand.

Basically... if I can adjust to run rich (even to the point that the car stutters, black smoke, etc), why/how would the 255 FP and AFPR upgrade help? How do these mods affect fuel-cut, MAF-T tuning, and performance in general?

I understand that the MAF-T is "lying" to the ECU about airflow, causing the ECU to do it's thing. But what I don't understand is ... for a given amount of airflow and the ECU obviously managing to get gas into the cylinders, how can additional fuel pressure or flow improve things?

Without the fuel mods yet... O2 sensor cycles as expected on logger, my o2 trim is hovering around 100% at cruise, and mid/high trims are around 105-120% after short amount of drive time since reset. I'm running 12-15 PSI boost, although I haven't really gotten on it due to some knock issues. (I replaced my knock sensor and still have some PK ... but that is another story).

I'm an old V8 shade tree mechanic and I'm still learning this DSM tuning.... just need some gaps filled in...

Thanks as always!

Calan

OK, 1st question is why are you running so rich right now? Have you done a boost leak test? Typically, boost leaks are the #1 cause of ruuning rich. The ECU thinks there's more air in the system than there really is & adds fuel accordingly, causing you to run rich. If you haven't done 1 on your 15 year old car, do 1 now while you're waiting for your fuel upgrades.

As for why the larger pump & AFPR, depends on your goals for the car. Do you plan to run more than 12-15 psi & upgrade injectors over stock? If not, a rewired stock fuel pump may suit your needs. If so, a larger pump is really a necessity. The stock pump won't deliver enough fuel much past stock boost settings. The AFPR is necessary for the 255 pump because it will overrun your stock regulator. This makes tuning your fuel trims near impossible. At WOT it may not be an issue, but without the AFPR you have no way to insure you are getting a 1:1 rising rate, i.e., 1 psi of fuel pressure per 1 psi of boost.

 

[Calan]

Well I'm not running that rich... but I could be

I did a boost leak test a few days ago when I first got the car, and it was horrible. I was blowing air all over the neighborhood. I fixed most of them, and it now pretty much holds at 15-20 PSI, with just a small leak in the TB somewhere around the throttle cable at the back.

When I first got the car, it was running "that" rich because of this very thing. After I fixed the boost leaks (amoung other things) and upgraded injectors, I'm now trying to retune it.

 

[CanadianTSi]

Once you start really modding the car and flowing alot of airflow you are going to need more Fuel, the stock pump would not be able to handle the demand hence the upgrade to the 255.

 

[romeen]

The stock 450cc injectors can't flow enough fuel for anything more than a few psi above factory set levels. So put in larger injectors. But now the stock pump can't deliver enough fuel to the injectors and at the proper pressure to work properly. And on and on...

The air leak at the TB you're describing is from the shaft seals and can only be fixed by disassembling the TB and replacing them. It's pretty common. Do a search on it for tons of info. Hope this helps.

 

[kenamond]

I'm limited by my rewired stock FP at 15psi above 6k rpm. I'm at 7500ft elevation, so things are a bit different, but that gives you one more data point on where the limit of the stock (rewired) FP/FPR is.

If you're like the rest of us, you'll want to go beyond what the stock FP can support anyway , so this is a necessary step in your upgrade path.