Fuel Pressure Regulator Question

I've been dealing with a multitude of issues around the fuel system on a Mitsubishi Evo X.

Can anyone shed some light on why my fuel pressure is not rising 1:1 with boost pressure? The car is running 50psi base pressure (vacuum line disconnected) and the engine is running 25psi of boost. My FPR only goes up to 65psi and holds steady.

Theoretically it should go up to 50psi + 25psi, 75psi.

Is this something normal to see it not working 1:1?

Every single thing has been checked in the fuel system, multiple regulators used, it's not a regulator issue.

You have the factory in tank venturi jet and an upgraded internal fuel pump by any chance?

The factory fuel system uses a venturi jet pump to syphon fuel from the opposite side of the saddle tank to the side the pump is on. This venturi requires a small orifice to cause the venturi. This orifice means also means that your return line is only effectively 3mm in diameter the tank.

If you have an adjustable reg, you will find that with the engine running, there will be a point that you can wind the base pressure to where trying to lower it any further will see no change from the fuel pressure. This is because you are unable to bypass/return fuel any faster to the tank due to the return line restriction (also in some instances it is the regulator is too small internally to bypass enough fuel which is common with the factory regulators on bigger pumps)

You will probably find if you have 50psi as a base, and connect the vac line, it may not be reducing 1:1 either.

You can buy adjustable jet pumps (from memory radium and aftermarket industries sell these) designed to maintain the venturi operation in the tank but with more flow from the return line in mind. You can also adjust the venturi port diameter with a drill bit, but if you go too far, you will reduce the systems ability to scavenge fuel from the other side of the tank.

Nathan thanks so much for that insight.

The car is running a higher flow drop in AEM pump using the factory housing. It is also on an aftermarket, adjustable FPR because the base pressure was very high with the stock regulator fitted.

Yes, it cannot get below 50psi even with the regulator wound all the way out. Also yes, when I connect the vacuum reference, it does not budge, remains at the same 50psi as with the line disconnected.

I fully understand what you're saying, but if this is something native to the evo X, surely not many people probably even realize the issue in the first place as I have done. Should I then be ok tuning around it and leaving it be?

Hey Chris, this is not native to the evo X. I've had the same issues with evo 4-9. I usually use a 9/64 bit on the evo 4-9 to drill the venturi. Are you using an AEM regulator by chance? If so you could also try a larger orifice to help gain some control when setting the base pressure of the regulator.

I've tried different regulator, same thing. I will try tuning around it before having to resort to enlarging the venturi. Have you done this on your Evo x @gerrard?

How old is your fuel pump and fuel filter?

@chris no I haven't done this on a evo X. but on a couple evo 4-9's