Correct me if Im wrong, but if you goto larger injectors to eliminate the FMU, you run basically stock fuel pressure with a 1:1 FPR. I guess my question is: Are there any negative effects when lowering fuel pressure against the same intake pressure? I mean, boost works against the incoming fuel.. I just wonder to what significance.

hope this made sense... been a long day.

-Scott.

 

All an FMU does is increase the fuel line pressure to force more fuel through the smaller stock injectors. When you replace that setup with larger injectors, you still have to have a fuel pump that will keep up with the bigger injectors, but they will be capable to putting more fuel into the combustion chambers and will be able to do it at a lower duty cycle which will keep the injectors healthier. As far as negative effects, if the injector is on the verge of what the high impedance drivers can handle, idle quality can suffer and the car can feel kinda sluggish at times.

 

Hey 97. I understand what youre saying. I guess I wasnt too clear about my post.

Fuel pressure, or psi entering the intake through the injector has to combat intake plenum psi from FI.

So when you lower fuel pressure (with a larger injector), I wondered if that has any meaningful significance at all. As I type this Im thinking obviously not... or at least not anything very significant.

 

I don't think you could run enough boost to actually affect an injector. You're spraying a liquid at 45 PSI (liquids don't compress) into a chamber that is being filled with a compressed gas (most likely at less than 12 PSI). Also, the injector is pointed at the intake valve instead of towards the plenum which is where the air is coming from.

That would be a good question for our engineers in the peanut gallery.

 

With mid sized injectors (36 - 50) you probably won't reduce your fuel pressure. I'm running 36# injectors at 52 psi (vacuum line off) making 580 rwhp; my friend's TA is running 42# injectors at 36 psi making 600 rwhp. Both cars idle fine and pass CA smog.

 

That's what a boost referenced fuel pressure regulator is for. For every psi of boost that enters the plenum, there will be an extra psi of fuel. So lets say your base fuel prssure is at 45psi. When running say 20psi of boost, your fuel pressure will be 65psi to compensate for the 20psi pushing on the other end of the injector. So even when boosting, you'll always have 45psi of fuel pressure

 

Thanks everyone!

-Scott