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Old 05-08-2007, 04:47 PM   #1
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Default Fuel pressure: what's too high?

It's been so long since I even thought about this. On a DSM you run stock base pressure or lower and you always have a 1:1 rise so most people are running under 70psi of fuel pressure, running too high is not on anyone's minds. A long time ago I knew what was a dangerous level that could hang the injectors open. I thought it was above 90psi. How come nobody here worries about that? What is 'too much' pressure in the Miata world? How come nobody worries about hanging injectors open? What's the Miata base pressure? Does a Vortech/BEGI FMU increase the base pressure?

Sorry for the noob questions, I am having a shitty search day looking for answers here and elsewhere.
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Old 05-08-2007, 04:58 PM   #2
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no they only increase pressue in boost.

Corky suggests a <50Lb/hr. (525cc) will not suffer any sort of hazard up to about 200psi. I've run mine at 120psi for the longest time. don't worry about it.

Quote:
Originally Posted by Corky Bell
Regarding the high fuel pressures, after 30 years of playing with rising rate regs and driving them on my own cars for about 1/4 mil miles, the only injector problem ever observed was a 100+ lb/hr injector jamming shut under pressures in excess of 100 psi. Sawed the sucker in half, measured the areas and spring rates, calc'd a couple numbers, and it was clear that pressure can eventually jam any injector shut. I'd suggest not exceeding 200 psi on any injector over 50 lb/hr.

.....I've not yet seen an injector or an injector pulse hurt by too much fuel pressure. It can happen, sort of, with injectors of the 100 lb/hr size if pressures reach about 100 psi. The only problem, even then, is jamming them shut.
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Old 05-08-2007, 05:04 PM   #3
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And what is Miata base pressure? I've seen 50 used several times to calculate max pressure. This seems consistant with other naturally aspirated cars from the same era. Is it exactly 50psi?

So far on my build I've aquired a Vortech FMU/Discs and an RX-7 AFM, I want to run 305cc injectors, BIPES, O2 clamp, a heavy dose of methanol and a 14B or 16G in the 12psi range. I don't care about power, persay, but I'd like to trap in the 105mph+ range if that gives an idea about my goals.
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Old 05-08-2007, 05:10 PM   #4
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35psi idle, 50psi at 0 ~hg.
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Old 05-08-2007, 05:14 PM   #5
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Quote:
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35psi idle, 50psi at 0 ~hg.
Yeah, we always called "base pressure" whatever the FPR was with no boost/vacuum source, 50psi it is.
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Old 05-08-2007, 06:49 PM   #6
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I've seen tired injectors lock at 55psi. And since a lot of us use older injectors we run the risk, and cleaning won't fix that.

The guy that does my testing thought I was insane when I asked to test a set of injectors to 100psi. His machine wouldn't do it.

I'd stay away from much over 100psi. Most of the line in your car is rated for 125psi. I imagine there is more risk when that line is 15 years old.
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Old 05-08-2007, 06:59 PM   #7
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I ran close to 100psi for awhile, but switched to a bigger injector/lower fuel pressure setup b/c of the risk of an injector locking. Granted it never happened, but why take the risk if you don't have to.
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Old 05-08-2007, 07:01 PM   #8
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true, regular "fuel injection line" (SAE30R9) is required to have a working pressure rating of 125psi but a minimum burst pressure rating of 900psi.
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Old 05-08-2007, 07:18 PM   #9
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I didn't put a gauge in it, but I'm sure I was at the pressure limit of my walbro 190 HP for a few days last week. Still couldn't get enough fuel out of my blue tops though. I'd be more worried of a new line popping off than an old line bursting. Clamps and double clamps on fuel lines, and all should be fine.
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Old 05-08-2007, 08:08 PM   #10
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In my opinion running larger injectors with only a slight increase in fuel pressure is a better option ... of course you'd have to properly manage larger injectors but even with a VAF tune and an SAFC on the stock ECU you can run something close to 400cc injectors and still get decent mileage on the 1.8.
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Old 05-08-2007, 08:57 PM   #11
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the SAFC sucks, especially on a 1.6 without a TPS. on top of that you didn't address ignition retard.
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Old 05-08-2007, 09:32 PM   #12
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Quote:
Originally Posted by Braineack View Post
the SAFC sucks, especially on a 1.6 without a TPS. on top of that you didn't address ignition retard.
I was giving a cheap alternative to the FMU. You can run a map sensor instead of a TPS with an SAFC and it does its job.

the FMU does nothing for ignition to begin with .... so no loss there - and the BP and B6 can take quite a bit of ignition before problems appear. Lastly, you can address that with an ignition retard box. And yes I am aware that by artificially changing the load seen by the ECU you are also affecting the ignition.

I'm not sure why I'm even bothering to go on about this, you just always come across like you know best ...
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Old 05-08-2007, 09:54 PM   #13
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the SAFC was tried by a few years ago and never caught on....for the price the EMB at the time was the better route, which can also control the ignition for pretty much the same price. that's all. btw, what VAF tune do you suppose you can do for the 1.8s?
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Old 05-08-2007, 10:01 PM   #14
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inside the VAF, there is a spring that sets the flapper door tension. A cogged wheel sets the tension on this spring. This cogged wheel can be spun at a few degree intervals in either direction and can loosen or tighten the VAF door. This is effectively a crude SAFC. This applies to any VAF equipped car ... at some point the 1.8 went to a MAF right (I'm not sure about these yearly changes to the Miata)? Well, this only works for mechanical VAFs. If you open the top black cover of the VAF you will see what I am talking about.

Although this is crude, it allows some flexibility on a modified car and has shown decent results for a "Free" tuning device.

As a sidenote, the SAFC is not as good as an emanage BUT it does it's job for what it is and will allow the car to handle injectors 100cc larger than factory ones. Using a MAP sensor as the TPS is also a very good way to make it load sensitive.
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Old 05-08-2007, 10:04 PM   #15
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yes, the AFM can be modded exactly like that, not to mention the Air-Idle mixture screw and utilizing pot-resistors. However, every 1.8L miata 94-05 went MAF.
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Old 05-08-2007, 10:05 PM   #16
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Quote:
Originally Posted by Braineack View Post
yes, the AFM can be modded exactly like that. However, every 1.8L miata 94-05 went MAF.
Wasn't aware of this specific change to miatas, I unfortunately never owned one.
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Old 05-08-2007, 10:14 PM   #17
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The FMU/Bipes/FP combo has proven itself to a certain point. I was making 200rwhp by simply slapping in 305cc injectors and jacking the fuel pressures a bit. Nothing wrong with the setup, you just lack specific control.

The next obvious step that we can both agree on is some sort of managment, something that can idle larger injectors without having to drive up fuel pressure. In comes EMB, EMU, & MS. Cost effective piggybacks that have tons of support and work great.
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Old 05-08-2007, 10:23 PM   #18
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The bandaid route seems like a good road for many people.
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Old 05-09-2007, 03:24 AM   #19
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It's a progression for many ... it was for me and I won't go back from an MS but it was all worth it for the learning experience. As a note, the SAFC with a turbo (GTX) ECU and 460cc injectors worked well for what it was - daily driven and did make 270whp.

I mostly enjoyed playing with all of the above and I even made my own SAFC from scratch (as part of an engineering project) but that's a different story.
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Old 05-13-2007, 12:09 PM   #20
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Quote:
Originally Posted by Ben View Post
I'd be more worried of a new line popping off than an old line bursting. Clamps and double clamps on fuel lines, and all should be fine.
Please ignore the above.
This fuel hose vagina is the result of following my own advice. 130+ psi FTL
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