Why VAC reference on FPR?

[Munchhausen]

A lot of people running aftermarket EFI systems dont use the VAC reference on their FPR cause of better tuning specialy at idle.
But what is the andvantage of it that most stock engines use it? Thought i read once here at stf about a good reason to use the Vac ref. but cant remember it or find it using the search function
(Just talking about NA)

Munch

[Piledriver]

A lot of people running aftermarket EFI systems dont use the VAC reference on their FPR cause of better tuning specialy at idle.
But what is the andvantage of it that most stock engines use it? Thought i read once here at stf about a good reason to use the Vac ref. but cant remember it or find it using the search function
(Just talking about NA)

Munch

If you have old school or hugemungous injectors with poor low PW operation, that are running at the lower limit of the electronics capability to contol them at idle, the vac referenced FPR regulates the fuel pressure dependent on manifold vacuum, making the injectors effectively "smaller" under high manifold vacuum conditions.

...not to be confused with almost the same setup (add check valve and pinhole leak) providing more fuel pressure under boost, which may be required depending on the setup, although I'd personally just prefer something than simply has the flow/pressure capability to deal with any non-linearity in electronics/software... although I still love CIS. (go figure)

A LOT of stock engines had this because a LOT of stock engines were running L-Jet or its children, with the crappy 40 year old injector design.
(Digifant/Digijet/LH-Jet and all the Japanese and US variations on the theme)

I'd assume no OEMS are still using vacuum referenced FPRs in 2011---thinking it's no longer needed with modern injectors etc, but I haven't really researched it.

[Munchhausen]

thanks for the fast answer.
will disconnect the VACref then. My idle is allready a kind of stable but it could be better

I started a CIS conversion on my T4 in 2008. A friend of mine saw that an said to me, what are you doing there, never heard of MS? Thats the way i got in tough with MS first time. Iam still fascinated from that CIS system but the parts are allready hard to find on junk yards. And used parts in good condition are away from cheap these days.

Munch

[MinamiKotaro]

Now that I'm back to NA, I may try unplugging the vacuum reference on the FPR and see if it stabilizes idle.

[MinamiKotaro]

Now that I'm back to NA, I may try unplugging the vacuum reference on the FPR and see if it stabilizes idle.

Well, I tried this. I've only tried it out for one drive, so far, but the engine seems to run smoother, especially on the low end. Can't really tell a difference in idle stability.

I'm concerned that my fuel pressure is only 40psi. It used to be 45psi static. My pump may be going out; it's been sounding strange lately.

[miniman82]

The one thing I noticed about vac referencing the reg is that it made tuning the accel settings a b*tch. In a perfect world the accel shot would be easy to set up, because the engine would need the same amount of fuel when the throttle opens at all times. But it's not a perfect world, and the VE of the engine is constantly changing. That means the amount of air entering the engine is different at different RPMs, even if the opening speed of the throttle plate remains constant. My situation is even worse, due to the turbocharger. At 20 kpa with the throttle closed between a gearshift, a very small movement of the throttle is enough to cause a big change in VE, due to the still spinning compressor forcing air into the engine. Best I have been able to do is a rough compromise, where there's a slight bog off boost due to too much fuel, but it's just enough that it doesn't lean pop during hard/fast boosted shifting.

What needs to happen is TPS enrichment needs to be based off of MAP as well, and there are 2 ways: get a MAF, or use the MS hybrid TPS/MAP accel stuff. I don't use MS2 though, so I'm stuck with my compromise.

[supaninja]

A lot of people running aftermarket EFI systems dont use the VAC reference on their FPR cause of better tuning specialy at idle.
But what is the andvantage of it that most stock engines use it? Thought i read once here at stf about a good reason to use the Vac ref. but cant remember it or find it using the search function
(Just talking about NA)

Munch

If you have old school or hugemungous injectors with poor low PW operation, that are running at the lower limit of the electronics capability to contol them at idle, the vac referenced FPR regulates the fuel pressure dependent on manifold vacuum, making the injectors effectively "smaller" under high manifold vacuum conditions.

...not to be confused with almost the same setup (add check valve and pinhole leak) providing more fuel pressure under boost, which may be required depending on the setup, although I'd personally just prefer something than simply has the flow/pressure capability to deal with any non-linearity in electronics/software... although I still love CIS. (go figure)

A LOT of stock engines had this because a LOT of stock engines were running L-Jet or its children, with the crappy 40 year old injector design.
(Digifant/Digijet/LH-Jet and all the Japanese and US variations on the theme)

I'd assume no OEMS are still using vacuum referenced FPRs in 2011---thinking it's no longer needed with modern injectors etc, but I haven't really researched it.

And most are using returnless fuel setups too. Modern injectors have come a very long ways in the last 40 years. On my civic, I ran a ish ton of fuel pressure(70psi base), ID1000cc injectors, e85, and 25psi dropping to +20psi at 8600 rpms. those injectors were maxed the f out(89% IDC at redline), but my idle was buttery smooth, even smoother then when it was stock.

[Piledriver]

And most are using returnless fuel setups too. Modern injectors have come a very long ways in the last 40 years. On my civic, I ran a ish ton of fuel pressure(70psi base), ID1000cc injectors, e85, and 25psi dropping to +20psi at 8600 rpms. those injectors were maxed the f out(89% IDC at redline), but my idle was buttery smooth, even smoother then when it was stock.

Hmmm... I paid a whole $10 for a GSXR 750 pump>regulator /fuel guage sender unit (it almost looks like it would fit a Bug tank), but elected not to use it due to long loop and hot engine compartment. (it's returnless)
---I paid another $10 for the complete wiring harness and $50 for the TBs w/injectors, worth every penny just for the 8 injector pigtails and TPSs, both usable... Not to mention 2 sets of 4 usable injectors. (I would like to know the flow ratings)
All of the above were off a 2008 GSXR750 with <2K miles.

Maybe I should rethink that, maybe I could get by with a restricted return loop...
Concern-- Hot restarts and vapor in the line.

[supaninja]

The biggest drawback of being returnless is if you are boosted, since the fpr is before the rail and boost will cancel out some of the fuel pressure across the inj. The other big disadvantage of being returnless is the rails need to have pulse dampening or your IDC will go up. I have seen that countless times in the honda world, kids put on engine bling (fancy fuel rails) and wonder why their ish is running worst. Those are 2 reasons to stay with a fuel return IMHO.

[MinamiKotaro]

I'm concerned that my fuel pressure is only 40psi. It used to be 45psi static. My pump may be going out; it's been sounding strange lately.

It wasn't my pump making that disconcerting "whoosh" noise and causing low fuel pressure, it was the fpr. I replaced the Bosch fpr with my old rising-rate* to see if it would get pressure up to 45 psi. It did, and the "whoosh" noise, which I thought was the pump maybe cavitating or sucking in air somehow, went away. Kind of weird, because I'd swear to you that the "whoosh" was coming from the front. Now, I wonder why the Bosch one was making noise and barely 40psi of pressure. E85 ate up the diaphragm? Guess I need to go back to gas or get a regulator rated for alcohol.

*I quit using it because it pushed fuel pressure from 45psi to over 90psi at only 10lbs boost! "Rising rate" indeed! It should work just fine at static pressure.

EDIT: Well, never mind. After a little bit of driving, the "whoosh" came back, from the fpr. Also, my fuel pressure started dropping and settled at about 40 psi by the time I got home. I guess the pump really is going out. I find it unlikely that two completely different fprs would develop exactly the same problem.