I'm getting ready to go turbo hopefully later June or early July. I'm working on upgrading my fuel system at the moment I have 450cc injectors and walbro 255 fuel pump. I have been reading a lot of articles in the search and have come to the conclusion that that megasquirt is better then a SAFC. I have been offered a AFPR for $50 is this worth getting and what should i have a FMU or SFMU? Am I forgeting anything any help would be greatly appreciated

Red 97 Eclipse GS (420a) MTX
Custom Garrett T3 Turbo system
Megasquirt w/52 lb/hr injectors & a Walbro 255
Kicker/Alpine Audio (3 amps, 2 subs)
90 Cav 2.2 TBI auto (winter) <- yeah, it's fast

So let me get this straight if I get a SFMU I do not need a FPR.

Correct.

The SFMU will raise the fuel pressure under boost in a certain proportion, say 12:1. For every 1psi of boost, it will raise the fuel pressure 12 psi.

The FPR does the same thing, only it's a 1:1, so for every 1 psi of boost it'll raise it 1 psi.

So why do you need an FPR with the MS? Great question! I'm glad you asked

Imagine the fuel injector:

Outside

____________________
____________________ <--fuel rail with 50 psi
/ /
/ / <--- Fuel injector (crappy, but it'll work)
/-/---------
/ /

Manifold
-----------

The fuel injector sticks into the manifold to shoot the gas in.

Okay, let's say you have 50 psi of fuel pressure in the fuel rail and you're shooting into a manifold with vacuum. No problem! The gas goes in without a hitch since there's practically no resistance as it gets fired in.

Now---you go under boost. That means you have pressure in the manifold, let's say 10psi. Well now when the injector opens, there's only a 40 psi difference between the rail and the manifold. Uh oh! That means for a given injector time opening, less fuel will be added. This couldn't come at a worse time too, because under boost, you want MORE fuel, not less!

Enter the FPR.

The FPR raises the fuel pressure 1 psi for 1 psi. Same scenario, 10 psi of boost. Now with the FPR in place, it raises the pressure in the rail to 60 psi, so you still have a 50 psi difference!


With the SFMU, since it is doing that already, you wouldn't need both the SFMU and a FPR. You'd have two devices doing the same thing.
Make sense?

Brian

Red 97 Eclipse GS (420a) MTX
Custom Garrett T3 Turbo system
Megasquirt w/52 lb/hr injectors & a Walbro 255
Kicker/Alpine Audio (3 amps, 2 subs)
90 Cav 2.2 TBI auto (winter) <- yeah, it's fast

Yup thanks for the help

Originally posted by ForceFed420a
Correct. The SFMU will raise the fuel pressure under boost in a certain proportion, say 12:1. For every 1psi of boost, it will raise the fuel pressure 12 psi. The FPR does the same thing, only it's a 1:1, so for every 1 psi of boost it'll raise it 1 psi. So why do you need an FPR with the MS? Great question! I'm glad you asked Imagine the fuel injector: Outside ____________________ ____________________ <--fuel rail with 50 psi / / / / <--- Fuel injector (crappy, but it'll work) /-/--------- / / Manifold ----------- The fuel injector sticks into the manifold to shoot the gas in. Okay, let's say you have 50 psi of fuel pressure in the fuel rail and you're shooting into a manifold with vacuum. No problem! The gas goes in without a hitch since there's practically no resistance as it gets fired in. Now---you go under boost. That means you have pressure in the manifold, let's say 10psi. Well now when the injector opens, there's only a 40 psi difference between the rail and the manifold. Uh oh! That means for a given injector time opening, less fuel will be added. This couldn't come at a worse time too, because under boost, you want MORE fuel, not less! Enter the FPR. The FPR raises the fuel pressure 1 psi for 1 psi. Same scenario, 10 psi of boost. Now with the FPR in place, it raises the pressure in the rail to 60 psi, so you still have a 50 psi difference! With the SFMU, since it is doing that already, you wouldn't need both the SFMU and a FPR. You'd have two devices doing the same thing. Make sense? Brian

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Red 97 Eclipse GS (420a) MTX
Custom Garrett T3 Turbo system
Megasquirt w/52 lb/hr injectors & a Walbro 255
Kicker/Alpine Audio (3 amps, 2 subs)
90 Cav 2.2 TBI auto (winter) <- yeah, it's fast