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

Originally posted by Madoc05
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

Okay, let me see if I can help.

1. Keep the injectors and 255.

2. If you buy the Megasquirt, also get the AFPR to act as a 1:1 FPR.

3. Given the option, get the SFMU since it allows for finer adjustment. It acts as an FPR, so you wouldn't need the seperate AFPR with this one.

Some people use the SAFC, but only in conjuction with the SFMU. They use it to fine tune even farther. It's not necessary, and many will tell you that the money would be better spent on other things. I don't have one, so I'm not speaking from personal experience.

Hope this helps,
Brian

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

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

THANK YOU ! That whole fuel stuff was getting confusing....

Originally posted by aivanne182
THANK YOU ! That whole fuel stuff was getting confusing....

Glad it helped. :)


Brian