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Old 11-16-2012, 12:44 AM   #1
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Default adjustable fpr vs oem pressure

So let's say I have two fuel systems which are identical other than the regulator ... ignore the obvious filter difference:
- c5 oem regulator (no vacuum reference)
- aeromotive adjustable fpr with vacuum reference

The aeromotive is adjusted to the same base pressure at idle as the c5 regulator and sees atmosphere pressure on the vacuum line at all times. Will the fuel pressure be roughly the same for both regulators at all points on the VE table? Just to be clear, I'm not asking if fuel pressure will always be the same exact value across all entries in the table with one regulator.

I want to say no since the regulators can react at different speeds during changes in flow. I'd also like to be wrong about this so the fuel tables don't need to be updated on the street a few days before a track day just because I decided to cleanup an already functioning fuel system.
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Old 11-16-2012, 12:57 AM   #2
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So, if I understand you, the question is:

1: Will the two regulators "react" with equal speed to transients in rail pressure and bypass volume due to changes in the amount of fuel flowing through the injectors, and

2: Will the two regulators, having been set to the same pressure under identical flow conditions (idle) maintain either linearity or identical delta at lower rates of bypass owing to increased fuel flow through the injectors?
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Old 11-16-2012, 01:12 AM   #3
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Right. (1) may or may not be important, but I can see how they are probably not the same. If they both react fast enough then it doesn't matter if they react at different rates between each other. (2) is my bigger concern, but really only in open loop at or around WOT. I can come up with convincing arguments (at least to me) for them to match and ones that say it's not guaranteed.
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Old 11-16-2012, 09:30 AM   #4
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If one is vac referenced and one isnt then they are only going to have the same pressure at one manifold pressure. At all other times the c5 unit will have the same pressure relative to the atmosphere and the aermotive pressure will follow the manifold pressure.

Is this on a v8 swap? I dont know the c5 computer too strongly but if the car was designed to not run a vac reference on the fpr then there will be a table in the stock ecu called something like "fuel injector multiplier vs vac" and it will basically skew your VE data so that the VE map makes sense with changing flow rates through the injectors. When switching a car like this to a vac referenced fpr you need to make this table all 1s.
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Old 11-16-2012, 11:05 AM   #5
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Obviously, You can adjust your VE table to compensate for the static pressure regulator, but if the intended purpose is forced induction then the manifold pressure referenced regulator would keep the fuel atomization optimal at x number of psi above manifold pressure.
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Old 11-16-2012, 11:07 AM   #6
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Quote:
Originally Posted by sixshooter View Post
Obviously, You can adjust your VE table to compensate for the static pressure regulator, but if the intended purpose is forced induction then the manifold pressure referenced regulator would keep the fuel atomization optimal at x number of psi above manifold pressure.
Most certainly. Its much easier to tune a boosted car with a manifold referenced fpr than one that sits at a single pressure.
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Old 11-16-2012, 11:38 AM   #7
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I should have made this more clear: the vacuum port on the aeromotive is disconnected at all times so it sees atmospheric pressure. The question is not how to compensate for differences in fuel pressure, but whether there will actually be differences between the two regulators. Joe did a good job of summarizing my question.
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Old 11-16-2012, 11:42 AM   #8
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If both see the same pressure at their reference they'll output more or less the same fuel pressure if they were set to the same base pressure and have the same reference ratio (normally 1:1).
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Old 11-16-2012, 12:32 PM   #9
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Quote:
Originally Posted by orion4096 View Post
The question is not how to compensate for differences in fuel pressure, but whether there will actually be differences between the two regulators.
This is pretty much an impossible question to answer.

In theory, both regulators should maintain the same pressure at all operating conditions.

In practice, no two devices are exactly alike. Two brand-new OEM regulators will exhibit some minor differences relative to one another.

My intuitive reaction is that the differences between the two should be sufficiently small as to not cause a performance degradation. Broadly speaking, the volume of fuel passing through the rail is sufficiently large as compared to the volume of fuel flowing through the injectors that variations in the latter should not seriously impact the former.

But can I say this with absolute authority? Of course not. I haven't tested it.


Quote:
Originally Posted by sixshooter View Post
but if the intended purpose is forced induction then the manifold pressure referenced regulator would keep the fuel atomization optimal at x number of psi above manifold pressure.
^ This.

I'm not entirely clear on the application here. Is this a Corvette engine? Will the engine be running in a fairly stock configuration (naturally aspirated, stock ECU, etc)?
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Old 11-16-2012, 12:43 PM   #10
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It's a naturally aspirated v8 ls6. The car already has the aeromotive with stock miata filter and uses the stock hard lines. I deleted all my old logs, unfortunately. The hard lines get close to the exhaust in several spots and I've been ignoring it for a while. Also, I don't need the return line to run the length of the car. These are the "problems" I want to fix. Given more time I would just log the current pressure, install the c5 regulator, and adjust the tune for any differences. But I want to go to the track soon and probably won't have time to do this. That's why I decided to ask my internet friends. So if I can convince myself that the regulators will be close then I'll go ahead and install the c5 one before my track day.
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