Thread: Running AFPR with no vacuum line?

Hey guys,

Are there any downsides to running an aFPR without the vacuum line to the intake manifold? My understanding is that it adds fuel pressure as the pressure in the IM rises. But if you are tuning with a standalone and you are tuning for the right fuel then is there any problem running no vacuum line?

You kinda have the right idea. Just backwards. The base static pressure of the fuel system is 3 bar,43.5 psi. The vacuum reduces the fuel pressure under light load. So at WOT the fuel system is operating at its base pressure. Close the throttle, and the vacuum inside the manifold reduces the fuel pressure.

As for your tuning question, I have no idea. Why do you want to run it this way? Are you runnin a turbo?

No downside.

All of my NA cars operate that way.

Technically you could probably tune around this sort of situation, and N/A would make it easier... But this idea just sounds silly. I can't imagine why you wouldn't want constant fuel pressure (relative to the intake pressure).

By not connecting the vacuum line you will have WAY more pressure at idle and obviously more fuel. The more fuel part you can tune out with a stand-alone, if you're lucky enough to have one with that kind of resolution, which I doubt... And on top of that, you're risking pressure-locking the injectors as they will be running way higher relative pressure at idle than they are designed for.

As you get to 100 kPa things are better.

Above 100 kPa (boost) you will be losing relative fuel pressure. This is something you can tune around I guess, but again not a situation you want to be in.



I would highly recommend you keep the FPR vacuum line connected. I see no benefit, and only downsides. I would call them major.

What is silly about it? You by no means need pressure rise relative to intake vacuum (in the case of an NA application)

You will not have way too much fuel at idle and you do not need tons of resolution either. Even At 50psi static pressure, there is no way you will "pressure lock" an injector, THAT is just silly. Most injectors today are designed to run at a 4bar base pressure anyway.

In the case of an ITB setup with no vac provision, this is the way you would go about it anyway. All who do this have 0 issue in mapping and or drive-ability. I will screen shot the raw fuel values from a static pressure setup and you can be the judge.

My car runs good at 70-80psi with no vac line connected. Running a big pump and injectors at that NA.

I admit, I exaggerated the risk of pressure lock. It is not really a concern. I hadn't had my Wheaties yet.

With ITBs you have to make all sorts of concessions, I understand. With ITBs you do what you have to.


With a proper manifold, I still think it's silly balls. It makes no sense to eliminate this simple, reliable, effective physical control device in favor of a weird-ass electronic solution that an EMS is not really designed for.
The downsides still consist of:

• Potato
  

In all honesty having the regulator un hooked in my na car with a manifold made tunning part throttle easier and I was able to run small o2 feed back + - values as low as 2-3%

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Thanks for the input guys.

Right now I'm running a plenum but I'm switching to itb's soon, hence my question.

Would you say having it hooked up will be beneficial to keeping the tuned map correct for small changes in altitude? That would be a benefit (and good enough reason to add the clutter of a lengthy vacuum line to my bay) to me as our local tracks all vary by a couple hundred meters altitude.

If you are going to ITBs I can see the appeal.