I Seek Fuel Pressure Clearification

[CajunKenny]

I’ve been doing a little research on fuel pressure and need some clearification. This is how I understand it; but, please correct me as needed…

The stock fuel pressure regulator is 1:1 in relation to fuel and boost. Stock fuel pressure is at ~37 psi with a stock boost of ~6.5 psi. So, for every psi of boost over stock the fp should be increased one psi.

I have an Aeromotive AFPR w/SS Lines (the Driftmotion Kit), 550’s, MAFT (AFC Mode), and a Walbro 255. Is the ratio still 1:1 with this setup?

I am currently trying to tune for 15psi. My fp is currently at 35 psi (vac off). At 15psi of boost, should I set my fp to ~43.5 psi?

Another question: In relation to a properly functioning oil system, lower pressure indicates higher flow. Is this the same with fuel pressure? Does higher fp = more flow or less flow?

I’m trying to understand this concept better. Mods are in my sig.

Thanks everyone!
-Kenny

 

[jdub]

When talking about the 1:1 FP rise with boost, you have to talk about FP with vac on. The Aeromotive AFPR controls FP (it is 1:1)...the rest of the mods you listed is irrelevant when it comes to the FP rise ratio.

This statement/question:

My fp is currently at 35 psi (vac off). At 15psi of boost, should I set my fp to ~43.5 psi?

Is comparing an apple to an orange...since the car is actually run with vac attached, the vac off FP is not what the AFPR references for the 1:1 rise. You want to set the pressure at idle using the TSRM pressure range (vac on) and let the AFPR do it's job...a wideband O2 will confirm that it's working correctly.

Lower pressure does not = higher flow, in oil, fuel, or any other fluid system being driven by a pump. Think of pressure as rate/velocity...the higher the pressure, the faster fluid moves through a given diameter tube. This is true only to a point...when pressure rises above a certain value, you get much larger pressure increases vs the amount of additional flow. The viscosity of the fluid has a huge impact as to when this point is reached...oil for example will reach this point much sooner that would gasoline for the same diameter tube. It becomes self defeating to continue increasing pressure in a system due to the increase in velocity (flow becomes turbulent)...this is where "pressure is resistance to flow" really comes into play.

 

[ForcedTorque]

Jdub look at the numbers. They prove out that he left out the value for vac on. With a 1/1 increase from 35, 15 would put him at 50. He stated 43.5, so obviously the pressure vac on he has must be 28.5.

 

[jdub]

I know Scott...but, that is not what he said
An AFPR references vac...that what FPs we need to be talking about too...who cares what vac off FP is in this context. If you want someone to help you, you need to proof what you write to make sure it makes sense or some grumpy old guy is going to call you on it (not to mention causing confusion)

 

[crisp]

I know Scott...but, that is not what he said
An AFPR references vac...that what FPs we need to be talking about too...who cares what vac off FP is in this context. If you want someone to help you, you need to proof what you write to make sure it makes sense or some grumpy old guy is going to call you on it (not to mention causing confusion)

Grumpy old guy???:sarcasm:



NAAAAAWWWW!





-crisp

 

[CajunKenny]

I didn't drive my car to work today; but, when I get home, I'll post my vac on idle fp and make any needed adjustments per the TSRM.

I read so many posts where folks were adjusting their fp to this or that and wanted to be clear on the subject.

Great info so far... Thanks!

 

[crisp]

I read so many posts where folks were adjusting their fp to this or that and wanted to be clear on the subject.

Great info so far... Thanks!

You make a great point here. So much information on such things, and often leads to confusion! I set my AFPR up at first with WAAAY low fuel pressure (wrong line, incorrect vac!:cry and had poor fuel rise to boost, on TOP of the low pressure... then couldn't figure out why I was only turning 300hp!

I keep learning more about it as I read these threads... kudos to jdub and others who keep teaching OVER and OVER sometimes!


-crisp

 

[jdub]

Kenny - Look at FP as a global correction...it applies to the fuel being supplied across the board (for given size pump and injectors). Every car is different, so you will see a guy with this FP, another with that FP...even though both have near identical set-ups. Look at numbers like this as an approximation, your optimal setting will very likely be different.

What you need to be paying attention to is what AFR is at a desired boost level and the EGT that goes with it. FP has a direct affect on AFR and EGT...for a street tune, these are what's going to tell you if your FP is where it needs to be.

Thanks Crisp - seems I'm just an arrogant, grumpy old dickhead (who really doesn't know shit) to a lot of guys around here.

 

[CajunKenny]

So, being that my EGT probe is plumbed post turbo (just below the flange shown in the pic) what is the recommended 'safe' EGT range?

I'm pretty sure it's right around 1150*F at a given WOT pull to 4th gear. Currently, I have no lean issues. She's at around 10.8 for now.

 

[jdub]

I see 1350-1450 deg F EGTs, but my sensor is in the turbine housing. A 100-150 deg drop for post turbo would not be unusual.