I'm starting to imerse myself in the knowledge of the Megasquirt. But if the MS can control injector duty cycle, why the RR FPR? THanks

Alvin

Because the fuel pressure has to be raised 1 psi. for every 1 psi. of boost. If you don't, then you would lose 1 psi. of fuel pressure for every psi. of boost. I hope that makes sense. :-) But if you are N/A I would think that you could still run the stock fpr.

Originally posted by BOOSTED ECLIPSE
Because the fuel pressure has to be raised 1 psi. for every 1 psi. of boost. If you don't, then you would lose 1 psi. of fuel pressure for every psi. of boost. I hope that makes sense. :-) But if you are N/A I would think that you could still run the stock fpr.

Well...kinda. 15lbs of boost = 15lbs of fuel, that's kinda low. Okay, I'm still trying to get this whole MS thing down: running big injectors at stock fuel pressures (43psi)and decreasing the duty cycle VIA MegaSquirt will make it idle good. And using the software you adjust your AFR to what you want. The MS uses the different sensors to determine the rate at which it should fire, electronically. Now fuel pressure is raised or lowered mechanically by the FPR, but how do they relate.
Maybe I'm still in the SFMU/ bigger injectors thoery since I'm running it. Someone really should post a VFAQ thing on how it works. Thanks




Alvin

http://www.cardomain.com/id/96rst

I know what angle you are coming from Alvin... you are thinking, since the MS can just tune the injector pulse lengths, then what else could you possibly need? As boost goes us, just raise the pulse width, right?....

Here's the catch- by how much? Not only do you have to compute the additional fuel needed to combust the extra oxygen in the chamber, you ALSO have to take into account that the delta in pressures between the fuel rail and intake manifold is changing. A changing difference in pressure across the fuel injector makes the amount of fuel given at a certain pulse width different.

A 1:1 FPR makes the situation simpler. It automatically compensates for the raising manifold pressure, hence taking the manifold/fuel rail pressure change COMPLETELY out of the equation. it's done for simplicity.

If you had extremely high fuel rail pressures all the time, you could go without a FPR, if you were a genious and figured out the algorithm to change pulse lengths by "x amount" given "y pressure difference between rail and manifold"

hope this made sense

You need the 1:1 because of fuel pressure on one side, and intake pressure on the other. Lets look at it this way. Let's say you have 45 psi of fuel pressure in the fuel rail, and X amount of fuel flow at that 45 psi fuel pressure. Now add 10 psi boost pressure to the other end of the injector(intake side), and now you have reduced the fuel flow from the injector, theoretically, to 35 psi, making up for the 10 psi difference in pressures from the fuel rail to the intake. A 1:1 regulator will raise fuel pressure by 1 psi, for every 1 psi boost, to keep fuel from the injector more consistant. Another example is if lets say you ran large injectors on the stock ecu, and ran 15 psi of fuel pressure, without a boost referenced fuel pressure regulator. Now you run 15 psi boost. In essence, you would have no fuel flow out of the fuel injector, because pressures are even on the fuel rail side of the injector, to the intake side of the injector. Just a little simple physics going on, thats all :)

Originally posted by XtremeRS
You need the 1:1 because of fuel pressure on one side, and intake pressure on the other. Lets look at it this way. Let's say you have 45 psi of fuel pressure in the fuel rail, and X amount of fuel flow at that 45 psi fuel pressure. Now add 10 psi boost pressure to the other end of the injector(intake side), and now you have reduced the fuel flow from the injector, theoretically, to 35 psi, making up for the 10 psi difference in pressures from the fuel rail to the intake. A 1:1 regulator will raise fuel pressure by 1 psi, for every 1 psi boost, to keep fuel from the injector more consistant. Another example is if lets say you ran large injectors on the stock ecu, and ran 15 psi of fuel pressure, without a boost referenced fuel pressure regulator. Now you run 15 psi boost. In essence, you would have no fuel flow out of the fuel injector, because pressures are even on the fuel rail side of the injector, to the intake side of the injector. Just a little simple physics going on, thats all :)

Ohh, I never thought of the pressure of boost acting on the fuel pressure to slow it down. I get it now. So now when you have the regulator when you boost the MS knows the fuel pressure by??
Thanks for info guys, the more and more I learn about this thing the more I like it.



Alvin

http://www.cardomain.com/id/96rst

The MS doesn't care what the fuel pressure is, it's not the MS's job. The MS sort of expects the differential fuel pressure (pressure in fuel rail - pressure in manifold) to always be the same, most people set it to 43.5 PSI because that's what most fuel injectors are raited at. Otherwise your VE table would look pretty weird. With a 1:1 you KNOW you will get X amount of fuel flow at Y duty cycle regardless of manifold conditions assuming the regulator is doing it's job by keeping the fuel pressure on target.

Okay, I get it. Thanks
Alvin

http://www.cardomain.com/id/96rst

Originally posted by ner947
The MS doesn't care what the fuel pressure is, it's not the MS's job. The MS sort of expects the differential fuel pressure (pressure in fuel rail - pressure in manifold) to always be the same, most people set it to 43.5 PSI because that's what most fuel injectors are raited at. Otherwise your VE table would look pretty weird. With a 1:1 you KNOW you will get X amount of fuel flow at Y duty cycle regardless of manifold conditions assuming the regulator is doing it's job by keeping the fuel pressure on target.

...43.5psi makes sense, but at what rpm are you aiming for that? in a perfect world, would you have 43.5psi of fuel pressure at any given point between idle and redline (assuming you aren't boosting anything)?

Ideally, and what the 1:1 FPR does, is keep your fuel pressure at that 43.5 psi, regardless of boost. Lets say you boost 10 psi, adn then the FPR will raise fuel pressure to 53.5 psi, so fuel pressure remains constant, while the acutal injector pulse width will add fuel instead of more fuel pressure. Even though the fuel pressure is 53.5 psi, the differential pressure between the intake side and fuel rail side of the injector, is still 43.5 psi.

Originally posted by XtremeRS
Ideally, and what the 1:1 FPR does, is keep your fuel pressure at that 43.5 psi, regardless of boost. Lets say you boost 10 psi, adn then the FPR will raise fuel pressure to 53.5 psi, so fuel pressure remains constant, while the acutal injector pulse width will add fuel instead of more fuel pressure. Even though the fuel pressure is 53.5 psi, the differential pressure between the intake side and fuel rail side of the injector, is still 43.5 psi.

yeah that explanation makes sense, I understood that part totally.
but should I have 43.5psi even at idle? I am not sure if I am stuck in the FMU school of thought, where they have really low idle pressures and insane boost pressures, or if I am just not understanding something really basic here.

Originally posted by BumpinTalon
yeah that explanation makes sense, I understood that part totally. but should I have 43.5psi even at idle? I am not sure if I am stuck in the FMU school of thought, where they have really low idle pressures and insane boost pressures, or if I am just not understanding something really basic here.

Yeah, I usually tune fuel pressure to a fully warm idle. Even stock FPR that are vac referenced, will increase fuel rail pressure a little bit, as vac gets closer to 0(WOT for a non boosted engine). But since there is no positive pressure acting on the injectors, fuel pressure stays fairly consistant. The thing about FMU's and the like, is that when you want to run larger than stock injectors with a stock ECU, you have to lower the fuel pressure in order to get less fuel spray out of the injector, since the actual injector pulsewidth(injector "on" time) remains the same from the stock ECU.

Originally posted by XtremeRS

Originally posted by BumpinTalon yeah that explanation makes sense, I understood that part totally. but should I have 43.5psi even at idle? I am not sure if I am stuck in the FMU school of thought, where they have really low idle pressures and insane boost pressures, or if I am just not understanding something really basic here.

Yeah, I usually tune fuel pressure to a fully warm idle. Even stock FPR that are vac referenced, will increase fuel rail pressure a little bit, as vac gets closer to 0(WOT for a non boosted engine). But since there is no positive pressure acting on the injectors, fuel pressure stays fairly consistant. The thing about FMU's and the like, is that when you want to run larger than stock injectors with a stock ECU, you have to lower the fuel pressure in order to get less fuel spray out of the injector, since the actual injector pulsewidth(injector "on" time) remains the same from the stock ECU.

Ahh alright thanks for clearing that up.

The thing that got me was: The (x)psi of boost is acting on the injector, slowing down the flow of fuel. So if you have 45psi of fuel in the rail and you boost 10psi; that 10psi is going to act on the injector, lowering the fuel pressure to 35psi. It's physics like said above. So you need that 1:1 regulator to balance the fuel psi to boost psi, keeping the FP constant. I know where you're comming from, SFMU School of Thought.

ALvin

Good read! :thumbsup