COS idea

[diynoob]

I'm sure this has been discussed before...

While doing my VE tuning, I've noticed that even after the car is warmed up (180 ECT), the car is not "fully" warmed up. I have an 03 Z06 with the GM FFS fuel system, which means the fuel pressure regulator is in the fuel tank. I haven't touched it at all, which means my fuel pressure should ride around 58psi. When the car is totally cold, I'll see 60psi of pressure on my gauge. When it's warmed up from 30 minutes of spirited driving, my idle fuel pressure is closer to 55psi. I am guessing this is happening due to changes in the temperature of the fuel in the tank.

So I can't really do any good VE tuning until my pressure has hit "bottom", as cells that reported .99 or 1.0 the day before will report 1.05 the morning after now that the car is cool again. The only thing really affecting it seems to be fuel pressure, and obviously a 5psi change from 60psi base could easily cause a 5% fueling change (I am surprised it's not more).

It would be slick if there was a table in a COS for a fuel multiplier based on fuel pressure. That way as pressure changed the tune wouldn't have to. With changes as small as 5% I know the PCM can more than compensate through trims, but ideally trims would only come into play when something else that cannot be easily measured happens (clogs in fuel filters, clogs in air filters, decreased cat performance for those who run them, etc.)

I understand some of you guys are logging fuel pressure using an Autometer sender and wiring it in to the EGR circuit -- this would be perfect for that.

Just a random thought.

[Tordne]

You'd be surprised how variable your fuel system pressure *actually* is. A bunch of us have recently installed a fuel pressure sender and logged the data into EFILive. I am seeing 52-53PSI at 0 vacuum (WOT) and up to 62 PSI at 80 vacuum (decel). As a result of this logging I have changed the whole slope of my IFR table, and subsequently the VE values have become more realistic

I've not seen this fuel pressure drop that people talk about. Next ti I drive I'm going to see if I can see that pattern.

[joecar]

Andrew,

What is your fuel pressure changing as a function of...?

FP vs MAP
FP vs RPM
FP vs Batt. Volts
FP vs INJPW
FP vs INJDC

I'm thinking it could be the last two, but I can't log FP [yet].

[joecar]

Hmmm... so is a 3-D IFR table required...?

[Tordne]

Fuel pressure doesn't have any relationship to INJPW and INJDC. the PW is the required on time to flow the fuel to achieve the commanded AFR, and DC is simply a calculation on that.

It relates to Manifold Vacuum, which is basically 100 - MAP. The AVG fuel pressure is almost linear like you'd expect the IFR table to perhaps be. The difference is that the slope angle changes based on the 53 -> 62 PSI variance, as opposed to calculating on a flat 58 PSI expectation.

This is just my $0.02, but if you want to have a tune with accurate values the IFR table is critically important ans it affect basically everything fuel related calculation wise. I wish I'd done this mod ages ago, and would recommend it (and consider as important) as using a wideband

[Tordne]

I don't think a 3D table is required, personally. And it would be a hell of a job to change the OS code at that level. Bags not ask Ross to do that!

[diynoob]

Tordne, you bring up a great point, and one not contemplated within my original hairbrain idea. If there were a single 2D fuel pressure vs. IFR multiplier table, that would take care of the broad environmental issue of dropping fuel pressure during extended driving (even normal driving!) and dropping fuel pressure at WOT.

Personally, I have no fuel pressure drop at WOT courtesy of a boost activated Bosch 420 pump, but on the contrary... I have a fuel pressure INCREASE at WOT... which could take advantage of pressure vs. IFR table. With the amount of air going through the motor the subtle increase in pressure is not problematic and it can be tuned around, but the other fuel pressure problem is a bit difficult to tune around.

I wish there was a way to keep everything cool so I could tune that way. Then everything is slightly rich and the PCM wants to pull fuel instead of having to add it.

[PladdPezzPunk]

what are you guys using to monitor FP and then implement a tuning function. I've seen how some of the fords work and they actually have a FP sensor. Is there a way to integrate this into programming or not within reason?

above, i was under the impression that FP would affect duty cycle, higher fuel pressure, the injector dosn't have to open as long to add the same ammount of fuel as a lower pressure. Is this correct or am i compleatly out of line here?

[diynoob]

As of right now, I don't think there's a way to make the LSx PCM aware of fuel pressure in a way that would affect the tune (intentionally). The PCM can auto tune itself around fuel pressure changes through long and short term fuel trims, but some fuel pressure events occur so quickly, even a short term fuel trim is probably useless.

[TAQuickness]

Andrew,

What is your fuel pressure changing as a function of...?

FP vs MAP
FP vs RPM
FP vs Batt. Volts
FP vs INJPW
FP vs INJDC

I'm thinking it could be the last two, but I can't log FP [yet].

It's a function of dyncylair. RHS has a good write up on it. There's a link to it in the FP tutorial thread. I'll dig it up for you in a few minutes.