So simple, but simply overlooked

[Gannon]

Ever since my TF035 turbo upgrade, ive had problems where it would pull hard till 3000rpm then power would significantly drop off after that point. made overtaking very hard.

I could actually get to 100kmh faster by changing gears before 3k rather than letting it rev out.

I changed various engine components with no change in preformance. It was throwing no codes.

Anyway, i had borrowed a fuel pressure gauge when i set my malpassi AFPR and was gonna give it back to the guy i borrowed it off, so i hooked it up once more and WHAT!!! 40 psi of fuel at idle. (supposed to be low 20s) Pulled the vac line it was nearly 50psi, Way too high. I found that i may have actually rotated the gauge face when i installed it the first time and that gave me a squewed reading.

I readjusted it to 36psi (with the vac line disconnected) and voila, instant power like it should be.

Obviously it was overfueling and hindering power.

Im a very happy man, i can boost propperly again.


BTW. The original reason i replaced the stock fuel pressure regulator was that it wasnt keeping fuel pressure in proportion with manifold pressure, i was running seriously lean on boost.

[discopotato03]

A whole 4 psi difference in fuel rail pressure and overfueling ?

I think what you'll find is that whatever pressure pump your using isn't hacking the higher pressure/workload . Fuel injection pumps react like any other pump thats asked to increase output pressure - the volume drops off .

Personally I HATE rising rate fuel pressure regulators because they create more problems than they solve particularly in a forced induced application .

If you want a better fuel supply firstly ensure the pressure pump is getting full alternator output voltage ie 13.7 volts . If thats not cutting it , particularly with a 20 year old pump , replace it . For a heads up the EFI pumps off R30 Skylines look to be a direct replacement but a VL or VLT one would do . As a last effort with std injectors you could fit an adjustable pressure regulator but not the rising rate variety . This works because in closed loop 02 feedback mode the computer will correct at light loads and switch to open loop at higher loads .

Your call , cheers A .

[Gannon]

You do realise that the stock/factory FPR is a rising rate regulator.
The factory FPR was shot and not raising in pressure when the boost came on, thats why i replaced it

I worded it wrong, the fuel pressure was 40psi at idle, and nearly 50 with the vac line disconected. Its supposed to be in the low 20s at idle (depends in manifold vac) and 36psi with the vac lione disconected

[discopotato03]

No std regulators are not rising rate . What they do is provide a constant pressure head over whatever pressure exists in the inlet manifold .

Its simpler to work in absolute pressure psia when talking about inlet manifold pressure - in other words everything above a total vacuum is positive pressure ie atmospheric pressure is 1 bar or ~ 14.7 psia .

Lets just say for example your car idles at around 4 psia manifold pressure meaning 10.7 psi below std atmospheric pressure and at idle your FPR gives 36psi fuel rail pressure . You open the throttle and manifold pressure rises 10 psi to 14.7 psia , the fuel pressure reg (FPR) will lift the fuel rail pressure to 46 psi so in other words the difference in manifold and fuel rail pressure has risen by the same amount .
Its supposed to work this way so that the injectors have that constant 36 psi pressure head over whatever pressure exists in the inlet manifold .

A true rising rate FPR such as some Malpassi's make the fuel rail pressure rise at a faster rate than manifold pressure - the idea being higher fuel rail pressure through the injectors at higher loads . The problem is that the higher pressures make the pump work harder and at some point it will trade pressure for volume .
The propper solution is larger injectors and a constant rate FPR but it means you have to do something computer wise to stop the light load fueling from being too rich .

A true rising rate FPR is the wally way to a band aid solution - if you can call it that .

Cheers A .

[Gannon]

Ah i get what you mean, i was using the wrong term for it. I thought what i have was called rising rate, but i realise now its not. Rising rate FPR's are clasified as 1:1.5 or 1:2 ect

My Malpassi FPR is fixed rate or 1:1 and for every psi my manifold pressure rises or falls, the fuel pressure rises or falls one psi. Just like the factory FPR.

Thanks A

[littlewhiteute]

No std regulators are not rising rate . What they do is provide a constant pressure head over whatever pressure exists in the inlet manifold .

Its simpler to work in absolute pressure psia when talking about inlet manifold pressure - in other words everything above a total vacuum is positive pressure ie atmospheric pressure is 1 bar or ~ 14.7 psia .

Lets just say for example your car idles at around 4 psia manifold pressure meaning 10.7 psi below std atmospheric pressure and at idle your FPR gives 36psi fuel rail pressure . You open the throttle and manifold pressure rises 10 psi to 14.7 psia , the fuel pressure reg (FPR) will lift the fuel rail pressure to 46 psi so in other words the difference in manifold and fuel rail pressure has risen by the same amount .
Its supposed to work this way so that the injectors have that constant 36 psi pressure head over whatever pressure exists in the inlet manifold .

A true rising rate FPR such as some Malpassi's make the fuel rail pressure rise at a faster rate than manifold pressure - the idea being higher fuel rail pressure through the injectors at higher loads . The problem is that the higher pressures make the pump work harder and at some point it will trade pressure for volume .
The propper solution is larger injectors and a constant rate FPR but it means you have to do something computer wise to stop the light load fueling from being too rich .

A true rising rate FPR is the wally way to a band aid solution - if you can call it that .

Cheers A .

Very well explained, good to see others understand.

[El_Freddo]

If you want a better fuel supply firstly ensure the pressure pump is getting full alternator output voltage ie 13.7 volts . If thats not cutting it , particularly with a 20 year old pump , replace it . For a heads up the EFI pumps off R30 Skylines look to be a direct replacement but a VL or VLT one would do .

The Holden VN commodore 5 litre V8 external fuel pump will do the same thing... Plus it doesn't have "turbo" in the name which usually bumps the price up a bit. You should beable to get these for about $140 -$170 depending on where you are (perth =$190 as opposed to asking for one specifically for the subi = $399).

The VN ran two fuel pumps - one low pressure in the tank to feed the external high pressure unit...

Bennie

[Gannon]

I have a bosch 770 or whatever it is. got it for about $140