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Old 05-03-2010, 03:48 PM   #1
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Default AFPR: questions about yours vs. mine

I've searched here, Miata.net, and ClubRoadster and found nothing that answers my questions. I'm trying to wrap my head around some of the differences b/t turbo Miatas and my turbo Talon before I change platforms.

I fully understand the need for an AFPR, as I have one on my DSM. An OK quality fixed base pressure unit comes on factory turbo'd cars and works fine for lower pressure pumps, but I bumped up to an Aeromotive adjustable one b/c it can handle higher pump pressures. Either way, both are 1:1 rising rate fpr's, so when it sees positive manifold pressure it raises fuel pressure accordingly (duh). But at that 1:1 ratio. All the fpr's I've read about for Miatas are listed as having a ratio of 4:1, 6:1, 8:1, ect, and installation includes retaining the stock fpr.

Can someone help me understand why these raise pressure at such a high rate? Or am I missing something in translation about these fprs + the stock fpr working in conjunction with one another. And as a corollary, why can't the stock fpr be set aside for an adjustable/rising rate unit alone (unless this was answered by the first question)?

Is there any relationship between these differences due to the fact that DSMs have different fuel maps programmed into the ECU which react to engine load? If so, do Miatas running standalone ECUs just use a straight afpr since they can (theoretically) be programmed to respond to varying loads?
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Old 05-03-2010, 03:51 PM   #2
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your DSM has a "A"FPR. a = adjustable

your miata would have an "A"FPR. a = auxiliary
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Old 05-03-2010, 03:54 PM   #3
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OK, there's one distinction. But I still don't understand why the Aux fpr would dump way more fuel per psi into the system than my Adj fpr. Is it the ECU differences?
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Old 05-03-2010, 03:54 PM   #4
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The FPRs that rise at 8:1 10:1 12:1 are typically used as the sole means of increasing the amount of fuel in forced induction applications when the stock ecu (Which won't increase fuel when MAP is increased) is used. This is because the stock ecu won't increase injector duty cycle, and oftentimes, the stock injectors won't flow enough fuel at the base/stock fuel pressure. This is known as one of the "bandaid" fuel mods.
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Old 05-03-2010, 03:56 PM   #5
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OK, that makes sense. So then a standalone (like MS or AEM) can be used with only an Adj fpr, 1:1 rising rate, if I'm thinking correctly. It's the stock Miata ECU that's the limitation.
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Old 05-03-2010, 04:00 PM   #6
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Correct.
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Old 05-03-2010, 04:02 PM   #7
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Yes, an aftermarket ECU will only need a 1:1 fpr.
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Old 05-03-2010, 04:02 PM   #8
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Quote:
Originally Posted by matthewdesigns View Post
OK, there's one distinction. But I still don't understand why the Aux fpr would dump way more fuel per psi into the system than my Adj fpr. Is it the ECU differences?
It wont per say. The aux. fpr will only add extra fuel pressure in boost and then increase it at a rate per it's ratio.

If you have your 1:1 FPR fitted on a miata,with the base pressure increased from 50psi to 65psi, running on the stock ECU, the car will not run well at all.
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Old 05-03-2010, 04:19 PM   #9
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OK, it's all clear now. Thanks for the info guys!
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