fuel pressure

[dan2288]

I want to run 22 psi daily. I have 950cc injectors. I heard that its 1 psi of fuel for every psi of boost ? The stock 2g is 43.5 psi, would 65.5 psi be ok ?

 

[CanadianTSi]

Your FP rises 1:1 with boost so at a base of 43.5 with 22psi your FP should be 65.5 assuming your FPR is working correctly.

 

[romeen]

Are you asking if the fuel system can handle that much pressure or what to set your base fuel pressure to? The fuel system can handle that pressure and then some.

Do not change your base fuel pressure. It will remain the same regardless of what boost you will be running or what size injectors you have (there are some exceptions as skilled tuners can play around with it a little). Once you set your base fuel pressure the FPR automatically will raise the fuel pressure relative to boost level.

 

[dan2288]

ok, so I should leave it at 43.5 psi ? It will adjust by itself ?

Kinda OT, but one more question. Im trying to get on the dsmlink forums but registration is closed. Do they give you a user name and password when you purchase dsmlink ?

 

[Forsakened]

If you turn up the boost the FPR will not adjust itself...you need to do that manually.

 

[romeen]

Once you set the base fuel pressure to about 43 psi with the vacuum hose disconnected and plugged then all raising/lowering of the fuel pressure is controlled by the FPR via the intake manifold vacuum/pressure. So you set it, reattach the vacuum hose to the FPR and then leave it alone.

 

[lordnikon_04]

I don't wanna sound stupid but if stock is 43.5 then why would we set our base pressure on the AFPR to 43? I thought base was supposed to be adjusted to be higher on an AFPR when you start to raise the boost.

 

[CanadianTSi]

I don't wanna sound stupid but if stock is 43.5 then why would we set our base pressure on the AFPR to 43? I thought base was supposed to be adjusted to be higher on an AFPR when you start to raise the boost.

No you want your Fuel Pressure to be 43.5 at Idle. The only reason people install an Adjustable Fuel Pressure regulator is to lower there fuel pressure at Idle as Larger pumps generally overrun the stock regulator and cause your fuel pressure to be higher than 43.5.

Fuel Pressure rises in a 1:1 Ratio with boost on a stock or aftermarket FPR setup.

 

[lordnikon_04]

So running like a Walbro 255 and afpr i should set my afpr to 43?

 

[romeen]

Yes, for a 2G the base fuel pressure is always the same regardless of how much boost you will be running, injector size, fuel pump size, etc.

Some skilled tuners will raise the bfp above stock levels to get a little more fuel delivered per injector pulsewidth as well as better atomization of the fuel. But for most of us who are still using the factory ECU to control things it is best to adhere to the factory recommended level.

Under heavier engine loads/higher boost it is the amount of time that the injector is open that allows more fuel to be delivered, not the increase in the fuel pressure. The basic principle as I understand it is that for a 2G there is a 43.5 psi differential pressure (37 psi for a 1G) between the fuel system and the air inside the intake manifold. This pressure difference should always be 43.5 psi regardless of the operating conditions of the motor. It is this constant, unchanging differential pressure between the fuel in the fuel rail and the lower pressure air inside the IM that allows a predictable amount of fuel to be delivered per injector pulsewidth. Since the ECU is programmed with this value as a reference, changing it will make the ECU's calculations for fuel delivery inaccurate.

So whether the intake has 20 inHg of vacuum or is under 20 psi of boost the FPR raises or lowers the fuel pressure at a 1:1 ratio to maintain that constant 43.5 psi pressure differential.

This is why the bfp is set with the vacuum line disconnected. In essence this mimics a condition where the IM is at atmospheric pressure. When you reattach the vacuum hose the fuel pressure drops to a predictable value relative to IM vacuum (it drops 1 psi for every 2 inHg of vacuum). Since idle vacuum varies from car to car depending on many factors this direct line between the FPR and IM will keep the differential pressure constant. Conversely, when the IM is pressurized during boost the pressure signal to the FPR will raise the fuel pressure once again maintaining the 43.5 psi differential pressure.

For example:

At 20 inHg of vacuum the fuel pressure should be 33.5 psi which correlates to a differential pressure of 43.5 psi (2 inHg of vacuum correlates to 1 psi of pressure)
At 0 psi the fuel pressure should be 43.5 psi (the same conditions under which the bfp was set)
At 20 psi of boost the fuel pressure should be 63.5 psi. Again since the pressure inside the IM is 20 psi this would mean a differential pressure of 43.5 psi.

So again, the differential pressure between the fuel system and the inside of the IM is always the same and the amount of fuel delivered is controlled only by the length of time that the injector remains open.

If anyone sees any inaccuracies in this or would like to add to it please feel free to jump in.

 

[snox135]

Yes, regardless of what pump you run, set your base fuel pressure to 43.5. CanadianTSi was right on why we upgrade our fpr's. The return outlet is to small for high flow pumps. Which is also why the B&M fpr is no good. It allows you to adjust your fuel pressure (not needed for most of us) but does not fix the small return outlet issue.

edit: beat me to it romeen, well said

 

[steve]

So whether the intake has 20 inHg of vacuum or is under 20 psi of boost the FPR raises or lowers the fuel pressure at a 1:1 ratio to maintain that constant psi pressure differential.

This is exactly right for all generation DSMs. The Base Fuel Pressure is defined as that constant pressure differential. If you keep in mind that 1 inHg vacuum is about half a psi pressure and that vaccum is negative pressure then the rest should make sense.

Fuel Pressure = Base Fuel Pressure + Reference Pressure

I thought base was supposed to be adjusted to be higher on an AFPR when you start to raise the boost.

Nope, you don't have to change anything. The MAF will tell the ECU your using more air due to the higher boost and the ECU will inject more fuel to keep the programmed AFR, within the limits of the MAF, injectors, and fuel pump. If you raised the Base FP without telling the ECU it would cause the AFR to be richer than the ECU intended. Since the factory maps are very rich to start with your just blowing fuel out the back.

Steve

 

[lordnikon_04]

Wow that helps me out sooooo much. +100000000000 rep points for romeen. thank you very much guys.

 

[romeen]

Wow that helps me out sooooo much. +100000000000 rep points for romeen. thank you very much guys.

Sounds good. Now if I can talk Chris into matching a dollar per point.....

 

[dan2288]

anybody have any info on the dsmlink forums ?

 

[DSMunknown]

anybody have any info on the dsmlink forums ?

If you're still having problems registering, contact the DSMLink team here.




Good stuff Romeen. Just to emphasis, as that infomercial ad for the Rotisserie goes, "Set it and forget it!" (Of course, we have a FP gauge just to be sure. )

 

[PRESSURIZED]

My fuel pressure has been all over the place. I think my fuel regulator is a pos.