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Old 10-10-2001, 10:21 PM   #1
the Dabbler
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Default Injector, RRFPR, and Pump Sizing Conclusions (Long)

Between two threads that have been running recently, I've picked up a lot of good information and learned a lot. These are my conclusions, if anyone wishes to comment on my reasoning and/or math:

My Setup:
5 psi (assume 221 hp, for purposes of discussion)
Vortech FMU, set to 4:1
Walbro 255lph Fuel Pump (see pump flow data, below)
280cc/min stock injectors

Assumptions:
40 psi idle fuel pressure
80% max injector duty cycle
Assume 0.5 BSFC for normally aspirated
Target 0.60-0.65 BSFC for turbocharged engine


Stock Injectors (280cc), 4:1 RRFPR:
342.9cc/min (21.8gal/hr, total) at 60 psi
BSFC = 0.47
BSFC not optimal, especially for a turbocharged car, but not necessarily lean, if my understanding of BSFC is correct. The fuel flow should be well within the Walbro 255's capacity.

Stock Injectors (280cc), 6:1 RRFPR:
370.4cc/min (23.5gal/hr, total) at 70psi
BSFC = 0.51
Again, not optimal, but getting better. Flow still well within the pump's spec.

Upgraded Injectors (370cc), 4:1 RRFPR:
453.2cc/min (28.8gal/hr, total) @ 60psi
BSFC = 0.62
Much better. Still within pump spec.

Upgraded Injectors (370cc), 6:1 RRFPR:
489.5cc/min (31.1gal/hr, total) @ 70psi
BSFC = 0.67
BSFC starting to look excessive, which would mean too rich. Flow getting close to the pump's limit.

Upgraded Injectors (440cc), 4:1 RRFPR:
538.9cc/min (34.2gal/hr, total) @ 60psi
BSFC = 0.74
Even with only a 4:1 rise, BSFC starting to look unreasonably rich. Pump flow is actually okay, since the pressure is lower. 440cc injectors are probably overkill, for a 5psi system, unless you could go to a lower rise than 4:1.

Conclusions:
For my setup, either run the stock injectors with a 6:1 RRFPR, or go to larger injectors (370cc) with a 4:1 rise. The 370cc injector option looks like it can support a better BSFC (no surprise), but it's certainly more expensive ($90 x 4 = $360 vs. $30 for a 6:1 RRFPR recalibration kit).

Probably do the 6:1 first, to get running, then injectors later (and back to the 4:1 disk).

In any of these cases, the standard (i.e., the "non-HP" version) Walbro 255lph pump seems to be okay.
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Last edited by the Dabbler; 10-10-2001 at 10:28 PM.
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Old 10-10-2001, 10:35 PM   #2
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good stuff

care to explain what BSFC is?

-j
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Old 10-10-2001, 10:36 PM   #3
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Actually, I think my numbers may be 4% or so off, since I based the rated injector flows on 40psi (RC's injectors, at least, are spec'd at 43 psi).

Conclusions should still hold, though. I'll redo the math when I get a chance.
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Old 10-10-2001, 10:46 PM   #4
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Quote:
Originally posted by Jaxx
good stuff

care to explain what BSFC is?

-j
Brake Specific Fuel Consumption, in lbs of fuel per hour per horsepower. The "standard" assumption of BSFC for a normally aspirated engine seems to be 0.5. According to RC Engineering's site, 0.60 or higher is recommended for turbocharged cars. I'm a bit vague on the reasoning there. I don't know if they're suggesting that an (aftermarket) turbo will be inherently less efficient, or just that the higher BSFC will tend to make the car run rich (i.e., safer, given that you're compressing air into the engine to begin with).

I got most of the equations and info from RC's site www.rceng.com. Other sites had pretty good explanations of BSFC as well, but RC's site was a great resource.
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Old 10-10-2001, 11:27 PM   #5
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Nice right up. I have not had any time to do this, but it definitely was worth it. But I will say the BSFC is also dependent on the octane. Basically you won't need to run as much fuel is the Octane is a much higher rating (say 100 as compared to 91). Therefore the BSFC will be lower. I think RC Eng was covering thier butt and goign with a standard octane of 91 octane.

Now I have 440cc injectors so that I could run a much higher boost then 5 PSI. But with the Vortech Super FMU (if it actually works) you can set the idle fuel pressure as low as 25 PSI and you can have increase as low as 1:1 or as high as 12:1 depending on if you bleed the off boost pressure and other stting you do. So basically I am hoping that I can setup my car for any boost level (within reason, less then 12 spiking) and have the correct amount of fuel at that boost level with the 440cc injectors. First thing is to get this damn Super FMU working. All without the use of the Tec-II. Would be really cool if that is possible but I really doubt it. besides that i have to start worring about the spark timing. Woohoo, more things to start looking into.

It never ends.
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Old 10-11-2001, 12:03 AM   #6
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Great Write Up Dabbler, that pretty much coincides with the research I've been doing so I'll have to agree with your math. Do you mind if I steel that information for my Website?

Chad
93 1.8T
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Old 10-11-2001, 12:14 AM   #7
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One thing to consider: the MY00 ecu will be much harder to trick out when using the larger injectors. I believe people are using AFC's to pull back some of the fuel? Unless I am wrong and people are just using the larger injectors with factory ECU controlling them and a FPR keeping fuel pressures low for stoic idle.

-Jon
www.nothingserious.net
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Old 10-11-2001, 12:33 AM   #8
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I think using your stock injectors for 5 psi should be fine. When you upgrade injectors, you might as well consider anything else required to run 7-8 psi or more. I believe Skywalker's friend Mcgyver was running upwards of 12 psi briefly with nothing more than parallel fuel lines and larger injectors. Mind you, he doesn't have the ECU/piston casting problems that might arise from the 2000.
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Old 10-11-2001, 01:02 PM   #9
the Dabbler
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Quote:
Originally posted by skywalker
Nice right up. I have not had any time to do this, but it definitely was worth it. But I will say the BSFC is also dependent on the octane. Basically you won't need to run as much fuel is the Octane is a much higher rating (say 100 as compared to 91). Therefore the BSFC will be lower. I think RC Eng was covering thier butt and goign with a standard octane of 91 octane.
Could you explain this further? BSFC is a measure of fuel efficiency. I didn't think higher octane fuel burned any more efficiently. If anything, I thought it was slightly less efficient? Or, are you saying that since higher octane fuel is less prone to detonation, you generally don't have to run as rich in order to prevent it (i.e., using the lower octane fuel as a cooling agent)? That's a good point. The rich/lean factor is kind of muddled in the BSFC definition, but I think it's safe to say that, all other things being equal, a lower BSFC means a leaner mixture.

Quote:

Now I have 440cc injectors so that I could run a much higher boost then 5 PSI. But with the Vortech Super FMU (if it actually works) you can set the idle fuel pressure as low as 25 PSI and you can have increase as low as 1:1 or as high as 12:1 depending on if you bleed the off boost pressure and other stting you do. So basically I am hoping that I can setup my car for any boost level (within reason, less then 12 spiking) and have the correct amount of fuel at that boost level with the 440cc injectors. First thing is to get this damn Super FMU working. All without the use of the Tec-II. Would be really cool if that is possible but I really doubt it. besides that i have to start worring about the spark timing. Woohoo, more things to start looking into.

It never ends.
I'm concerned to hear that the SuperFMU is not working out for you. I was considering getting one myself. Please post when you get it running (or give up on it). It was either this, or the Bell unit, for me.

Regarding timing, I was thinking that the poor man's way around that would be to ignore it. Certainly not optimal from a performance standpoint, but from a safety standpoint, it seems like if you have enough fuel, you're good to go. Like I said, not optimal, but for me, anyway, I need to keep things as simple as possible until I get more comfortable with things.
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Old 10-11-2001, 05:48 PM   #10
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Quote:
Originally posted by stimpy
One thing to consider: the MY00 ecu will be much harder to trick out when using the larger injectors. I believe people are using AFC's to pull back some of the fuel? Unless I am wrong and people are just using the larger injectors with factory ECU controlling them and a FPR keeping fuel pressures low for stoic idle.

-Jon
www.nothingserious.net
Yes, that's definitely a problem. I'm hoping skywalker can get his SuperFMU working in stand-alone mode (which can set idle pressure), and report if this addresses the issue.

btw, I just ordered the Vortech recal kit, which includes all disks, to experiment with. In other words, I'm keeping the stock injectors for now, and trying for 6:1 (or maybe 8:1, to see if the pump still works -- right on the edge, there).
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Old 10-11-2001, 06:05 PM   #11
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Quote:
...higher octane fuel is less prone to detonation, you generally don't have to run as rich in order to prevent it (i.e., using the lower octane fuel as a cooling agent)?
That is what I mean. Thanks for clearing it up a bit. But as a cooling agent, not really sure about that. I have not noticed a difference in EGT temps between 91 and 91 with octane booster. Of course I have no background in chemistry let alone car mechanics so don't be surprised if my theory's are not correct. They are my theory's so I am testing them and they seem to be for the most part fact but other variables could be coming into play that I don't know about. Just trying to be an informative source on what works and what doesn't, so you don't have to waste your money.

Anyway I will be trying the Super FMU again this weekend. I am hoping it will work this time but I am not betting on it. If it doesn't I am going to send it back to Vortech and have them send me a new one or fix the one that is not working. I can't even run my car right now because I am running to much pressure at stock which is causing my oil to smell like gas. NOT GOOD... (Don't want it to sieze on me) So in any case I will be finding a solution for this problem one way or another and fast. If there is no practical solution I will be heading down the road of Tec-II. But I am hoping not quite yet.
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Old 10-11-2001, 07:26 PM   #12
the Dabbler
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Quote:
Originally posted by skywalker
That is what I mean. Thanks for clearing it up a bit. But as a cooling agent, not really sure about that. I have not noticed a difference in EGT temps between 91 and 91 with octane booster.
I didn't mean to imply that higher octane gas had more of a cooling effect than lower octane gas. What I meant was, any fuel beyond stoic (i.e., running rich) has the effect of cooling off the combustion chamber, delaying the onset of detonation. This, incidentally, is the theory behind water injection (it dumps water in the chamber, which has a much higher specific heat, and therefore cools more efficiently than gasoline). With higher octane gas, you don't have to use as much additional fuel purely for cooling's sake, since it's more resistant to detonation to begin with. Thus, the fuel efficiency is effectively increased.
Quote:
Of course I have no background in chemistry let alone car mechanics so don't be surprised if my theory's are not correct. They are my theory's so I am testing them and they seem to be for the most part fact but other variables could be coming into play that I don't know about. Just trying to be an informative source on what works and what doesn't, so you don't have to waste your money.
Well, I'm no chemist or thermodynamicist either -- just a humble engineer (not even an automotive one). And I'd be the first to admit I have quite a lot to learn about cars. All opinions welcome.

Quote:
Anyway I will be trying the Super FMU again this weekend. I am hoping it will work this time but I am not betting on it. If it doesn't I am going to send it back to Vortech and have them send me a new one or fix the one that is not working. I can't even run my car right now because I am running to much pressure at stock which is causing my oil to smell like gas. NOT GOOD... (Don't want it to sieze on me) So in any case I will be finding a solution for this problem one way or another and fast. If there is no practical solution I will be heading down the road of Tec-II. But I am hoping not quite yet.
Good luck. If it works out for you (or if I can get other testimonials), I'm going to try it myself.

Last edited by the Dabbler; 10-11-2001 at 07:40 PM.
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Old 10-12-2001, 09:10 AM   #13
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Higher octane will allow a lower BSFC due to the resistance to detonation. This means that the engine requires less fuel (ie: runs leaner) while avoiding detonation. In the real world, you will make more power with the same amount of fuel.

Let us know how well the SFMU works for you. I believe Aaron from azscooby.com is using one successfully. I will be using a fixed rate while recalibrating with the racal kit. My one problem will be stoic idle; I am running near 50psi right now and I don't know that I want to hunt down a new FPR.

-Jon
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Old 10-12-2001, 05:17 PM   #14
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Quote:
Originally posted by stimpy
Higher octane will allow a lower BSFC due to the resistance to detonation. This means that the engine requires less fuel (ie: runs leaner) while avoiding detonation. In the real world, you will make more power with the same amount of fuel.
This is what I just said, no? Maybe my meaning wasn't clear.
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Old 10-12-2001, 05:25 PM   #15
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Just clarifying. I had to state it in lamens terms to make sure I understood the concept

-Jon
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Old 10-12-2001, 05:41 PM   #16
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Quote:
Originally posted by stimpy
Just clarifying. I had to state it in lamens terms to make sure I understood the concept

-Jon
Thanks for the clarification. My verbosity gets out of hand, sometimes...
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Old 10-12-2001, 08:41 PM   #17
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I redid the calcs, correcting for the fact that the injectors are rated at 43psi, not 40psi. Also, I created a graph that shows Boost vs BSFC for various combinations of injector and fuel rise (I'll post it as soon as I can get it converted).

New conclusions:
With the graph, it's interesting to see that the character of the Boost vs BSFC curve actually changes with fuel rise. That is to say, a 4:1 fuel rise, regardless of injector size, always results in a monotonically declining BSFC, with boost. But starting with a 6:1 rise, the curve initially increases, then tapers off as boost gets past a certain point (it'll be clearer once I have a graph up).

So what does all this mean? Good question. I think what it means to me in the short term is that a 8:1 rise with stock injectors is actually looking more attractive. (Incidentally, the Vortech tech support guy was trying to decide between suggesting a 10:1 and a 12:1 for me! I don't think he really understood the application, though -- and like I pointed out to him, my pump wouldn't be able to sustain any kind of flow at that pressure anyway). With the 8:1 rise, the BSFC is always increasing in the range of boost I want to run (up to 5psi). Good thing I ordered the kit from Vortech with all the disks, not just the 6:1. 8:1 at 5psi comes out to 19.4gal/hr at 80psi, which is just under the line of what the Walbro 255 can do.

I want to restate that I'm not necessarily equating a higher BSFC with running rich. I know that BSFC is a measure of efficiency, and other factors like injector spray pattern, atomization, intake port swirling, whatever, etc. come into play. However, I am going on the assumption that, all things being equal, a higher BSFC can be said to be richer than a lower BSFC, and therefore less prone to detonation.
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Old 10-12-2001, 08:57 PM   #18
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stimpy,

Since you are running 50 PSI at idle just check your oil once in a while for the smell of gas. It will appear over time and once again you will need to chage your oil. Personally I am trying to stay awy from driving my car, because I am scared of my engine siezing on me while I drive. Just something to watch for.

I will hopefully be getting the SFMU working this weekend. Crossing my fingers.
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Old 10-12-2001, 11:56 PM   #19
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Quote:
I am trying to stay awy from driving my car, because I am scared of my engine siezing on me while I drive
welcome to my world
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Old 10-14-2001, 04:49 AM   #20
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Well I tried just about every possibility on the Vortech Super FMU and guess what i found. A small hissing sound every so often. There is a leak on one of the AN Fittings. So I am losing pressure and and wa-la it no worka!!! UGH, now I have to go to a part dealer pick up some parts and see if that might fix the problem.

In any case I seemed to be able to get it to 50 PSI if I left the vacuum line off (from the spring tension). Of course, then the SFMU will not work under boost. UGH, such annoying little things. Have to wait till Monday now. Unless people think Teflon tape might work? I doubt it and personally think it is a bad idea.

Well I am betting I can get this small issue solved this and that will fix 50% of my Fuel Problems. Still have to install the new injectors. Might not need the Tec-II after all.

Just an update,
...
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Old 10-14-2001, 04:55 AM   #21
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glad to hear you figured out what the problem was and hope you can find something to seal that stuff up. if its an AN fitting for vacuum i think you might be ok with teflon as long as you can get it so that it wont fly off and into the vac. fuel wont touch it so it wont get eaten away. im sure there is a better way to fix it though.
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