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Old 04-28-2011, 05:47 PM   #1
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Default NB - Injectors, Pressures & pumps (Warning: Math inside)

So, someone correct my "facts"/math if I'm wrong here... I've been doing some reading, but everything seems to be based on an NA Miata return type fuel system using a rising rate FPR (BEGi, Vortech) and very little solid info about the NB returnless system. Hopefully someone with more experience can sort me out here.

Injectors:

I have a nice fresh set of 460cc RX7 injectors, cleaned and flowed. A 460cc injector is 460cc @ 43.5psi. As I have an NB, I should be at right around 60psi with the factory regulator, regardless of pump. A 460cc injector running at 60psi should mathamagically become something like a 540cc injector? As long as the pressure is maintained, as in the NB system, can this number be counted on as a constant?

For 250hp at the flywheel, that works out to a minimum injector size of 492cc @ 80% DC, but at 43.5psi. At the 43.5, I'm undersized. At 60, I'm golden, with an extra 50cc (+/-) of headroom. (I doubt I'll make 250hp, but its nice to know I could support if I wanted to, extra headroom FTW!) Thus the basis of my question.

Pump:

Conventional wisdom is that the factory pump is unable to keep up with the factory injectors on a stock NB at full tilt. I would imagine almost doubling the output of the fuel system at the same pressure would only exasperate things, so a new pump is in order. While the 11 year old factory pump has performed its function adequately, its time for it to move on.

This is where my meager skills and understanding start to break down...

Assuming ideal injector sizing, (492cc/min) 46.875 lbs/hr x 4 injectors = (1968cc/min) 187.5 lbs/hr -or- 31.25 g/hr -or- 118.28 lph would be the required volume of fuel at 250hp, regardless of pressure, correct?

As seen here in green at the bottom of the page, a (non-HP) Walbro 190lph pump is supplying almost 10g/hr more @ 60psi than my calculations require, yet I keep hearing that the Walbro 190hp or 255hp is the ONLY choice for an NB. I dislike the HP option due to the extra 5 to 35 amps that the 190HP or 255HP draws through the fuel pump relay and circuit compared to the non-HP 190, not to mention the fuel pump whine that the HP series Walbro pums are famous for. I don't mind over engineering things, but sometimes too much is simply too much.

In summation:

Based on my math and anecdotal internet information, 460cc injectors @ 60psi w/Walbro (non-HP) 190lph pump should be just peachy on a NB @ 250 crank hp.

SO, if you managed to wade through that, and you understand what I'm asking about, AND you have some experience with the NB and the returnless system, feel free to jump in here and correct me. I'd prefer to only do this once.

Thanks!
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Old 04-28-2011, 06:21 PM   #2
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NB needs 255HP, no other pump maintains 60psi to redline.
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Old 04-28-2011, 06:44 PM   #3
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Ok, I get it... I'll do some more research on the pump.

Does my injector math look kosher?
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Old 04-28-2011, 07:09 PM   #4
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(zack morris's phone)

Pretty much the max limit. I had to add the 255 to drive up fuel presses on my rail and drop my dc% at 230rwhp
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Old 04-28-2011, 07:36 PM   #5
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Sounds like its a limitation of the system, possibly the line diameter in the NB? With the regulator back at the tank, I would imagine there is some pressure/flow lost in that line all the way to the injectors...

Internet gave me this: http://www.autoperformanceengineerin...l/fpspecs.html

I'm still seeing 32-42gal/hr @ 60psi out of the little guy. Is this just a limitation of the returnless NB fuel system? Fuel line size? Sun spots? Lupis?

I'm not trying to be an ***, I swear! The manufacturers data obviously does not match up with reality. No amount of reading can replace real world experience. That is why I ask questions here, to try to leverage that experience. I do appreciate the answer, but I'm trying to understand the 'why'

Looking up info on line losses right now Fluid dynamics... how does it work?!
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Old 04-28-2011, 07:38 PM   #6
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Well on NA pressure increases due to small return size and so much volume flow. Lowered dc% from 95 to 80.
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Old 04-28-2011, 08:49 PM   #7
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Quote:
Originally Posted by Braineack View Post
Well on NA pressure increases due to small return size and so much volume flow. Lowered dc% from 95 to 80.
Higher pressure would definitely drop the DC, what pump were you running before the 255HP? How much of a pressure bump did you see? Lower DC everywhere, or just up top?

If the pressure starts to fall off at redline, but is fine everywhere else... that sounds like the pump is undersized and can't keep up, or the lines are too small to flow enough volume to maintain pressure? I'd be interested to see if anyone has data-logged there pressures. With unlimited budget, I'd buy one of each and log them at the rail and at the pump, but I'm sure this can somehow be calculated. I does love me some data!

...but that can't be right, unless all these guys running 250-300+hp in there NB have all gone with larger fuel lines? Fae states in his Bosch 044 thread that he topped out at 390hp in his NB with a 255HP, and it looks like a stock regulator and lines. Regardless, thats a lot of damn fuel. I just can't help but think I should be able to get away with a smaller pump @ 200-250.

This is all pretty much academic at this point. If the 255HP is the smallest thing that will actually get 60psi worth of fuel to the rail @ redline on an NB, there is not too much real debate to be had. I'd still like to know why though.
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Old 04-28-2011, 09:19 PM   #8
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I have a 190hp and it works just fine on my nb. The only time you even hear it whine is if you have the top and windows up and the radio off.

When I first installed it, I rigged up a fuel pressure gauge under the wiper arm and took it out for a test drive. It idled at 62-63 psi and even under boost it never dropped below like 59 psi.

I have the supra greentops and at 10 psi I see 90% DC on the emb logs with afrs in the mid-hi 11's. I've actually been trying to lean it out a bit lately to stay at around 12-12.2 in boost.

I believe the problem with using the 255 is overpowering the fpr and causing rich idle issues. Probably even more so with larger injectors.
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Old 04-29-2011, 12:17 PM   #9
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Quote:
Originally Posted by EO2K View Post
As long as the pressure is maintained, as in the NB system, can this number be counted on as a constant?
No. What you're missing is that the pressure in the injectors has to fight against the pressure in the manifold in order to spray fuel. NAs have a fuel pressure regulator which is relative to the manifold pressure, so as MAP goes up 1psi, the fuel pressure goes up 1psi to match. NBs don't, they have a constant pressure, so when running 15 psi of boost, the rail is effectively 15 psi lower than is when running WOT when naturally aspirated.

265 rwhp on my NB with stock pump and 550cc RCs, and the stock pump was the limit. I put the 255 walbro in, but haven't been back to the dyno since.

As for the 255 overpowering the stock FPR -- FM says that's known to be an issue when using it on an NA, but that NB regulators are usually fine.

--Ian
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Old 04-29-2011, 12:28 PM   #10
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the issues is not the FPR, the issue is the return line size. There's too much fuel being supplied by the 255 to make its way back through the tiny FPR outlet. Since pressure is a measure of restriction, pressure increases.

This is only an issue on return systems 90-97.

I used it to my advantage to increase the capacity of my injectors. I dropped my DC% from 96.5% to 80% at the same boost/AFR level.

and like codrus said, as boost goes up so does the pressue in a 1:1 manner, on the NB it is static.

So with my stock pump I see around 55psi before boost, then it increases 15psi in boost. so 70psi total, but since the air pressure increased as well, the injector capacity would be equvielent to the 55psi level.

My 460cc injectors were maxed at 230rwhp.



POPSTOY did the same pressure test with his stock pump, a 190 pump, and then a 255HP and found the stock and 190 both dropped off pressure in high rpms, where the 255 held constant. It's possible the 190 he tried wasnt the HP model and that's the key to keeping the pressure up.

But as intake pressures increase, the fueling capacity actually decreases 1:1...so youre pushing your luck with the 460cc injectors and that power goal.
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Old 04-29-2011, 02:14 PM   #11
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Quote:
Originally Posted by codrus View Post
No. What you're missing is that the pressure in the injectors has to fight against the pressure in the manifold in order to spray fuel. NAs have a fuel pressure regulator which is relative to the manifold pressure, so as MAP goes up 1psi, the fuel pressure goes up 1psi to match. NBs don't, they have a constant pressure, so when running 15 psi of boost, the rail is effectively 15 psi lower than is when running WOT when naturally aspirated.
That is exactly the information I was looking for, I had not counted on that.

Also: nice tool here: http://www.witchhunter.com/injectorcalc1.php4

For the sake of argument, 8psi would put me at 52psi fuel making my injectors 502cc and that still puts me slightly over my 492cc (.60 bsfc) or 451cc (.55 bsfc) requirement for 250 crank hp. I've been calculating all my DCs at 80% FYI.

Now, using Emilio's numbers here for a 211hp rotrex car, I get 475cc @ .58 bsfc for my application. Granted that is not my car, but that puts me almost back where I started with the 460cc injectors. As I do the math I start to understand the "marvelous efficiency" Emilio is talking about. The car he pulled those numbers from is running a wound-up C15-60, and I should be right in the meat of the efficiency map for my larger C30-74. If it looks like I'm running out of fuel, I can always chop the redline back and get a set of 550's.

Looks like for my application, my 460's should be good to go. Good tune and good management are going to be crucial, as with anything.

< poke Braineack about MS2 >

Man, this is gonna be FUN!
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Old 04-29-2011, 02:26 PM   #12
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My car had no probems at 250 with rx7 460's. STOCK pump. You will be fine.

Last edited by 18psi; 04-29-2011 at 02:46 PM.
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Old 04-29-2011, 02:45 PM   #13
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Quote:
Originally Posted by Braineack View Post
POPSTOY did the same pressure test with his stock pump, a 190 pump, and then a 255HP and found the stock and 190 both dropped off pressure in high rpms, where the 255 held constant. It's possible the 190 he tried wasnt the HP model and that's the key to keeping the pressure up.
My testing showed very different results than his.
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Old 04-29-2011, 02:51 PM   #14
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that doesnt surprise me.
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Old 04-29-2011, 05:20 PM   #15
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Is that the guy who can't spell a single word right?
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Old 04-29-2011, 10:09 PM   #16
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Over at miata.net, Bob (pops) kept trying to tell me that brake clean isn't flammable.
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Old 05-02-2011, 05:23 PM   #17
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Over at miata.net, Bob (pops) kept trying to tell me that brake clean isn't flammable.
Well, tell him to keep smokin' whatever it is he is smokin' and have him go clean his brakes.

But seriously, thanks guys. All that info is very useful. As soon as I figure out my $&@%ing injector issue, I might actually be able to drive this thing.
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Old 05-02-2011, 05:43 PM   #18
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there are non-flammable brake cleaners; you actually have to go out of your way to find flammable ones now.
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Old 05-02-2011, 07:23 PM   #19
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there are non-flammable brake cleaners; you actually have to go out of your way to find flammable ones now.
CRC non chlorinated brake clean is flammable, and can be had at most any parts store.
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Old 05-02-2011, 08:23 PM   #20
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there are non-flammable brake cleaners; you actually have to go out of your way to find flammable ones now.
Not around here. Hell, we've used them to spray in dead fuel pump vehicles from the parking lot to a bay in our shop.
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