#76627, "SFMU Slope Tuning" Sep-20-04 08:23 PM by Eclipse2NR
My appologies if this has been discussed before, I've never seen it.
It just occured to me that when everyone in here talks about changing SFMU discs for bigger injectors, that doesn't make any sense at all.
Think about it.
If you buy a 40% larger fuel injector, then your fuel FLOW at the fuel injector is going to be 40% greater at any given fuel pressure. Assuming that one of these SFMU discs mimics our engines fuel needs, why in the world would you want to change that? You would only want to lower the static fuel pressure (y-intercept) to compensate for the 40% greater flow.
If you were to draw a graph of fuel flow over fuel pressure with two different injector sizes, you would get something like this:
If your slope was working fine before, its still working fine with a bigger injector. If you left your system exactly the same, except you changed your injectors to a 40% larger set, the only thing that happens to your flow is that you've moved every point on the y-axis up 40%. So lower your y-intercept by 40%.
Leave the slope. Get me?
Debate.
1996 Eclipse RS / 5-Speed / Aerogear Body / Coated in Magnetic Red Metallic rev.2 / T3 Turbo Intercooled @ 8 psi / More at http://home.insightbb.com/~eclipse2nr/ Mark Galyan - Indianapolis, Indiana
#76631, "RE: SFMU Slope Tuning" In response to Reply # 0 Sep-21-04 05:09 AM by thedawg
The difference between a static FPR and a RISING RATE FPR is not exactly a debate.
The SFMU is a rising rate FPR. The discs change the rate.
Turbos need more fuel once they're spinning fast and forcing more air into your motor.
The graph you drew is of the difference between the potential flow rate of a small vs. larger injector and has nothing to do with Fuel Pressure Regulators whatsoever.
#76647, "RE: SFMU Slope Tuning" In response to Reply # 1 Sep-21-04 01:01 PM by Eclipse2NR
Originally posted by thedawg The difference between a static FPR and a RISING RATE FPR is not exactly a debate. The SFMU is a rising rate FPR. The discs change the rate. Turbos need more fuel once they're spinning fast and forcing more air into your motor. The graph you drew is of the difference between the potential flow rate of a small vs. larger injector and has nothing to do with Fuel Pressure Regulators whatsoever.
Ok tell me something I don't know. You didn't exactly prove anything I said wrong, or even try to say it was wrong. Turbo's don't need any fuel. Yes my graph illustrates flow rate over fuel pressure.
1996 Eclipse RS / 5-Speed / Aerogear Body / Coated in Magnetic Red Metallic rev.2 / T3 Turbo Intercooled @ 8 psi / More at http://home.insightbb.com/~eclipse2nr/ Mark Galyan - Indianapolis, Indiana
#76658, "RE: SFMU Slope Tuning" In response to Reply # 2 Sep-21-04 05:28 PM by thedawg
Originally posted by Eclipse2NR Ok tell me something I don't know.
Sorry, I missed the 2NR part the first time. Lets try this...
The 2GNT comes stock with a static FPR (Dark Blue) The 2G Turbo (4g63) comes with a 1:1 rising rate FPR (Pink) The SFMU with a 3:1 disk (Yellow) The SFMU with a 5:1 disk (Cyan) And the turbokit FMU with a 10:1 rise rate (Purple)
To make 8psi boost on 235cc injectors would require 120psi of fuel pressure.
To make 8psi boost on 440cc injectors would require only 65psi fuel pressure.
Therefore larger fuel injectors require less pressure.
This post was written after three bowls of chron. If any of this is found to be incorrect, inconsistent, inconspicuous, or incoherent, I take no responsibility for the ToonerRev0lut10n that may or may not occour. Yes I know I should have posted the head-slap smiley. Fuck off, who reads the fine print at the bottom of things anyways?
#76662, "RE: SFMU Slope Tuning" In response to Reply # 3
Originally posted by thedawg
Originally posted by Eclipse2NR Ok tell me something I don't know.
Sorry, I missed the 2NR part the first time. Lets try this... The 2GNT comes stock with a static FPR (Dark Blue) The 2G Turbo (4g63) comes with a 1:1 rising rate FPR (Pink) The SFMU with a 3:1 disk (Yellow) The SFMU with a 5:1 disk (Cyan) And the turbokit FMU with a 10:1 rise rate (Purple) To make 8psi boost on 235cc injectors would require 120psi of fuel pressure. To make 8psi boost on 440cc injectors would require only 65psi fuel pressure. Therefore larger fuel injectors require less pressure. http://www.2gnt.com/www/corbin/choosefpr.html This post was written after three bowls of chron. If any of this is found to be incorrect, inconsistent, inconspicuous, or incoherent, I take no responsibility for the ToonerRev0lut10n that may or may not occour. Yes I know I should have posted the head-slap smiley. Fuck off, who reads the fine print at the bottom of things anyways?
I agree, RRFPR's are a terrible way of tuning. I am just addressing what I believe to be a common misconception of the way the are used.
440cc injectors are 87% bigger than 235cc injectors.
"To make 8psi boost on 235cc injectors would require 120psi of fuel pressure. To make 8psi boost on 440cc injectors would require only 65psi fuel pressure. Therefore larger fuel injectors require less pressure."
Obviously. It just so happens, if you increase 65 psi by 87%, you get 121.55 psi. It's a linear increase. I think you might be misunderstanding the purpose of my original post. Heres my point:
If we are able to find the correct rising rate (slope, disc, same damn thing) for our engine (yes, there is only ONE correct fuel slope, regardless of the injector size), then that slope is the perfect slope. Doesn't matter what size injectors you have.
In the case of the 235cc injectors and the 440cc injectors, if you were running the proper slope with the 235's, there is absolutely no reason the change the slope for the bigger injectors. On an SFMU, you would simply decrease the STATIC fuel pressure by 87% and voila. You are still running the same ammount of fuel at any given boost level, you are just at a lower pressure. That means you can run higher boost at more reasonable fuel pressures (the purpose of buying bigger fuel injectors).
1996 Eclipse RS / 5-Speed / Aerogear Body / Coated in Magnetic Red Metallic rev.2 / T3 Turbo Intercooled @ 8 psi / More at http://home.insightbb.com/~eclipse2nr/ Mark Galyan - Indianapolis, Indiana
#76687, "RE: SFMU Slope Tuning" In response to Reply # 5
With all these new fuel systems out, it seems no one has any interest in RRFPR's. Fine by me. I hate em too.
1996 Eclipse RS / 5-Speed / Aerogear Body / Coated in Magnetic Red Metallic rev.2 / T3 Turbo Intercooled @ 8 psi / More at http://home.insightbb.com/~eclipse2nr/ Mark Galyan - Indianapolis, Indiana
#76719, "RE: SFMU Slope Tuning" In response to Reply # 6
Dude, you can't run the same fuel pressure with larger injectors. The car will just choke itself out with fuel. You need to tone down the fuel pressures on the larger injectors, I don't care what kind of graph that you draw.
#76728, "RE: SFMU Slope Tuning" In response to Reply # 7 Sep-22-04 08:17 PM by Eclipse2NR
Originally posted by Scandelous Dude, you can't run the same fuel pressure with larger injectors. The car will just choke itself out with fuel. You need to tone down the fuel pressures on the larger injectors, I don't care what kind of graph that you draw.
I never once said to run the same fuel pressure. I said to run the same disc, and lower the fuel pressure (static). By lowering the static fuel pressure, you lower every single point on the graph by the same ammount. You know, this is why I don't post here anymore because if I try to talk about something even remotely technical I get a thousand replies from people who don't know wtf they are talking about. Read more carefully before replying please.
1996 Eclipse RS / 5-Speed / Aerogear Body / Coated in Magnetic Red Metallic rev.2 / T3 Turbo Intercooled @ 8 psi / More at http://home.insightbb.com/~eclipse2nr/ Mark Galyan - Indianapolis, Indiana
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