unfortunately i cannot search using the word STOCK . so what is stock fuel pressure on a s4 t2. i saw one post saying something about 40. arent injectors flow tested at 43.5? this is fastly getting complicated im in the process of figure actual fuel flow for my car. im running and rtek 1.8 with 750 primaries and 860 secondaries and s-afc2 i re evaluate my fuel every mod or two so im doing it again. but yeah whats the factory fuel pressure? argh nevermind me ..... 39.8 per fsm

 

The stock FPR is rated from 35.6 to 37 psi.

 

im turbo i see 34.1 to 39.8

 

I see you edited your original post. So... I guess now you know. Btw, I got the FPR info from the Technical Data section. That section also states the fuel pump output pressure as 71.1 to 92.4 psi, for what it's worth.

 

i got mine out of the fuel and emmisions tech section. i saw 71.1-92.4 lbs for the pump output. this is from the s4 FSM manual.. are we studying FSMs? Dorks

 

I'm guessing you wrote this before I edited my numbers. I put down the N/A numbers at first so yeah, 71.1 to 92.4. And yes, I have the FSM open. At least it's not my printed version.

 

The FSM says 35.6-37.0psi on page 4B-9 and 34.1-39.8psi on page 4B-70. I usually just say 36psi since I doubt anyone's going to use a gauge that can measure to a tenth of a psi.

If you want to compare the flow rates to the factory figures then they should be tested at whatever the stock fuel pressure is. If you know the flow at one pressure you can estimate flow at different pressures, as flow increases to the square root of pressure. So 550 x sqrt(43.5/36) = ~605cc/min.

 

so is that at 0 vacuum manifold pressure?
I have never really understood this.

I have set mine to 32-33 psi at atmo pressure and see 3.2 bar (46.4psi ) fuel pressure at 11.3psi boost (.8 bar )
I sometimes see as low as 1.1- -1.3 bar ( 15-18psi ) at full vac under decel ( 30 mmHg )

is this right?

 

.
That's either with engine off, key ON, the 35-36psi. Same thing if the engine is running and the vac line to the boost sensor is off the sensor and plugged up (the vac line).

At idle with the vac line connected and the rpms in the 750rpm range it should read 28psi but mine reads 31-32 psi.

Let off the throttle after a hard run and it should read 28 or so psi. I need to go check this one out again. I'm pretty sure that is as low as it gets. Any thing lower and it's not stock FPR.

 

No its not a stock FPR. thats probably why. ally my solenoilds are gone. Seems to be doing ok tho.
So max fuel pressure at 10psi ( asuming the stock reg is 1:1 ) should be approx 45-46psi?

 

Yep.

 

Quick FPR lesson. The FPR's job is to maintain a constant pressure differential across the injectors, so that as manifold pressure rises when you open the throttle, the fuel pressure rises by the same amount. This means that the injector flow rate is constant throughout the load range.

Static fuel pressure refers to the pressure when manifold is at atmospheric pressure, i.e. gauge pressure (what boost gauges show) is zero. On a non-turbo engine this only occurs when the engine is not running or when the engine is running and the throttle is fully open. On turbo'd engines the fuel pressure needs to be able to go above static pressure because manifold pressure can go above atmospheric.

Fuel pressure should always be static pressure plus manifold gauge pressure. So if your static pressure is 33psi then at 11psi you should see at least 44psi fuel pressure. Your static pressure is a little lower than stock, I'd raise it to 36-37psi unless you have fuel control of some sort, in which case I'd go up to ~40psi.

I think you've got your units messed up there. 30mmHg is only ~0.6psi. If you mean 30inHg, that's actually physically impossible since it's below absolute zero pressure.

 

"Fuel pressure should always be static pressure plus manifold gauge pressure. So if your static pressure is 33psi then at 11psi you should see at least 44psi fuel pressure. Your static pressure is a little lower than stock, I'd raise it to 36-37psi unless you have fuel control of some sort, in which case I'd go up to ~40psi."""""

huh i set my fuel pressure with the pump on and car off. i set it for exactly what i want.... i have more than enough fuel. maybe im just reading what you said wrong but that looks funky

 

Thanks for the lesson. Its much appreciated and helps my general knowledge grow just a wee bit more.

thats a really confusing statement but since you didnt correct my fuel pressure reading then i'll assume there ok.

By fuel control i assume you mean a fuel computer of some sorts? If so then yes i do. I got a good old PFC FCON. im sure my regulator is a 1.3:1 ratio, i might be wrong tho but im sure. if i crank the static pressure upto 40psi then im gona be seeing 54-55psi of fuel pressure which will be making my injectors run like 650cc units and cause major over fueling, which is fine coz i dont mind cranking the boost up some more to try and compensate heh heh, although i wont coz i dont have a wideband or EGT gauge to measure my ratios/temps to make sure everything is honkey dorey

 

How else would you set static pressure?

Well I wasn't commenting on what yours is set at. I was just advising that IMO static fuel pressure should be at least as high as stock, or a bit more if you have the ability to tune the mixtures.

Vacuum under decal is usually 20-25inHg. You said 30mmHg (millimetres of mercury), which is really tiny, but I think you meant 30inHg (inches of mercury), since that what non-metric boost gauges use for the vac scale. But 30inHg of vac is impossible, so I'm not really sure what vac you were actually seeing.

That's why you have a fuel controller. You can tune the mixtures to what ever you want. Higher pressures result in better fuel atomisation, which is beneficial everywhere from idle to full load. You rising-rate FPR will only give you those pressures if your pump is capable of keeping up at high flow.

It doesn't work like that. More boost means more airflow, so the ECU will add fuel accordingly by using larger injector pulsewidths until either the AFM maxes out of the injector duty cycle hits 100%. Then it'll start to lean out...

Sounds like you need to pay for some professional tuning. There's no point in having fuel control if you don't use it!

 

you can do it with the engine running also, just pull off the vacuum line to the FPR and cap it. sometimes this is easier since even with most EMS systems the pump only primes for a short period.