Fuel Pressure Adjustment

[phunny]

OP, would you be willing to tune my car?

 

[steve]

Two fundamental problems with the idea. Running higher base pressure and tuning to deal with it.

The formula is NewFlow = SQRT( NewPressure/OldPressure ) * OldFlow like you posted.

A 2G runs a factory base fuel pressure of 43psi (from FSM, but I assume it's really 3bar), raising the base pressure to 65psi give an effective flow rate of 553.27 cc/min. Let call it 553 cc/min

First problem is this is the base value, The actual fuel pressure at any given point is not a fixed value, it tracks manifold pressure 1:1. This is part of what bastarddsm is getting at.

At 15 psi of boost the fuel pressure is going to be 80 psi. Most fuel pumps have a pressure relief valve that opens at around 70 psi causing their flow rate to drop off. You'll need a high pressure pump to run more than about 5 psi of boost if you up your base to 65.

At 65 psi base your fuel pump will need to deliver 2.212 l/min (133 l/h @ 65psi) to support 100% IDC.
At 80 psi you'll need 2.456 l/min (147 l/h @ 80 psi) to support 100% IDC.
Assuming a 80% IDC you need to find a pump that can deliver at least that to provide about 20% margin for the pump. The stock pump doesn't come anywhere close to that.

Remember that the base + boost pressure is limited by the relief valve pressure - any pressure losses in the plumbing when working out what you can run. We've ignored the losses so far but they come into play in the real world.

Next issue is that the injectors are going to deliver about 18% more fuel all the time than what the ECU is expecting causing the AFR's to be even richer that stock. You need to correct for this by either reprogramming the ECU or lying to the ECU about the incoming airflow. Without a way to retune the car cranking the base pressure up doesn't solve the problem of needed more injector it just causes it to run rich until it runs out of fuel.

Adjusting the base pressure works as a tuning aid when your trying to adjust for say a MAF that reads high or perhaps overly rich factory tuning but creates issues when you trying to stretch the injectors and lower the IDC.

IDC is just a value between how long the ECU want the injector to be open vs how much timer there really is at this instant to be open.

At factory boost levels in stock trim I expect a 2G doesn't go much over 80% IDC but we aren't talking about a fully stock car and on stock injectors I've seen logs of well over 100% IDC which means not only are the injectors static, which isn't good, but they still can't deliver the fuel that the ECU requires and the car will lean out.

Log your car and find out what your IDC's are. What everyone else sees means nothing to your car. A simple boost leak can drive the IDC's over 100%.

 

[Dark_Horse]

OP, would you be willing to tune my car?

If you ain't running 100psi base pressure, you ain't a true DSM'er.

 

[nightstocker99]

Finally someone actually said something worth while, thank you.

I know that I need a fpr and a tune. I also know that fuel pressure over 70-75 psi will damage the injectors or leave them stuck open. I have walbro 255 lph fuel pump already, i dont think that will begin to fail at 70psi of fuel.

Two fundamental problems with the idea. Running higher base pressure and tuning to deal with it.

The formula is NewFlow = SQRT( NewPressure/OldPressure ) * OldFlow like you posted.

A 2G runs a factory base fuel pressure of 43psi (from FSM, but I assume it's really 3bar), raising the base pressure to 65psi give an effective flow rate of 553.27 cc/min. Let call it 553 cc/min

First problem is this is the base value, not a fixed value so it tracks manifold pressure 1:1. This is part of what bastarddsm is getting at.

At 15 psi of boost the fuel pressure is going to be 80 psi. Most fuel pumps have a pressure relief valve that opens at around 70 psi causing their flow rate to drop off. You'll need a high pressure pump to run more than about 5 psi of boost if you up your base to 65.

At 65 psi base your fuel pump will need to deliver 2.212 l/min (133 l/h @ 65psi) to support 100% IDC.
At 80 psi you'll need 2.456 l/min (147 l/h @ 80 psi) to support 100% IDC.
Assuming a 80% IDC for need to find a pump that can deliver at least that to provide about 20% margin for the pump. The stock pump doesn't come anywhere close to that.

Remember that the base + boost pressure is limited by the relief valve pressure - any pressure losses in the plumbing when working out what you can run. We've ignored the losses so far but they come into play in the real world.

Next issue is that the injectors are going to deliver about 18% more fuel all the time than what the ECU is expecting causing the AFR's to be even richer that stock. You need to correct for this by either reprogramming the ECU or lying to the ECU about the incoming airflow. Without a way to retune the car cranking the base pressure up doesn't solve the problem of needed more injector.

Adjusting the base pressure works as a tuning aid when your trying to adjust for say a MAF that reads high or perhaps overly rich factory tuning but creates issues when you trying to stretch the injectors and lower the IDC.

IDC is just a value between how long the ECU want the injector to be open vs how much timer there really is at this instant to be open.

At factory boost levels in stock trim I expect a 2G doesn't go much over 80% IDC but we aren't talking about a fully stock car and on stock injectors I've seen logs of well over 100% IDC which means not only are the injectors static, which isn't good, but they still can't deliver the fuel that the ECU requires and the car will lean out.

Log your car and find out what your IDC's are. What everyone else sees means nothing to your car. A simple boost leak can drive the IDC's over 100%.

Do you know what the max amount of cc's you can get out of the stock 450 injectors

 

[steve]

I know that I need a fpr and a tune. I also know that fuel pressure over 70-75 psi will damage the injectors or leave them stuck open. I have walbro 255 lph fuel pump already, i dont think that will begin to fail at 70psi of fuel.

Do you know what the max amount of cc's you can get out of the stock 450 injectors

Unless you have a 255HP (the standard 255 isn't the HP pump) your relief valve will pop over around 70 psi.

Injector flow rates are based on how big the orifice is and the pressure difference between the two sides. So the limiting factor is how much pressure the injector will work with. (how high before it can't open the pintle).

I don't know what the spec is for the Denso 450cc injectors. You seem to think it's
70-75 psi and you can calculate the flow rate at that pressure using the same formula as before.

The point to all of this is once you have the required parts (about $150 to $200 for a AFPR and hoses plus $150 to $600 for tuning control), the injectors become a minor expense. EVO VIII 560cc injectors can be had for about $100, 650cc to 750cc injectors for about $125 so why crank the base pressure up and limit your boost range when you can cheaply get more fuel and have headroom.

 

[nightstocker99]

I am not sure if it is 70-75 psi of fuel will damage the injectors that is just the information I have been giving in My Power and Performance class. these are the things I am trying to figure out. I can dyno tune the car for free at my school. I can buy the tuning program after I have enough money.

I am not sure what FPR I should get I dont know what too big or too small.

Thats why I started this thread. to find out what pressure will be achieved before I waste my money buying too small of a FPR.

I have been told to buy and FMU because they are better for boosted application then the FPR are. An FMU is often referred to as a boost-dependent fuel pressure regulator. The FMU is essentially a variable fuel-pressure regulator that automatically raises fuel pressure as boost rises.

Unless you have a 255HP (the standard 255 isn't the HP pump) your relief valve will pop over around 70 psi.

Injector flow rates are based on how big the orifice is and the pressure difference between the two sides. So the limiting factor is how much pressure the injector will work with. (how high before it can't open the pintle).

I don't know what the spec is for the Denso 450cc injectors. You seem to think it's
70-75 psi and you can calculate the flow rate at that pressure using the same formula as before.

The point to all of this is once you have the required parts ( about $150 to $200 for a AFPR and hoses plus $150 to $600 for tuning control), the injectors become a minor expense. EVO VIII 560cc injectors can be had for about $100, 650cc to 750cc injectors for about $125 so why crank the base pressure up and limit your boost range when you can cheaply get more fuel and have headroom.

 

[bryanwheat]

You are getting some bad information along the lines. You DO NOT want to run the fmu. That is for boosting a non boosted engine and running the stock ecu and injectors.

Your factory regulator and all you buy aftermarket are going to raise the fuel pressure 1 psi for every 1 psi of boost. Buy a 1:1 regulator, cheap Evo injectors and an SAFC if you want more fuel. IT will be FAR more reliable than what you are wanting to do.

Listen to the people on this forum, there is a million times more information on this site than you will learn at any tech school.

 

[nightstocker99]

The problem on here is knowing whats correct and what is incorrect. When I was a newbie and knew nothing about cars I thought that everything was right. Not the case.

Knowing is half the battle - G.I. Joe

So what FPR should I get?

I plan on running no more then 20psi. I already own the 550cc injectors, I forget what brand they are but they are blue on top.

You are getting some bad information along the lines. You DO NOT want to run the fmu. That is for boosting a non boosted engine and running the stock ecu and injectors. Your factory regulator and all you buy aftermarket are going to raise the fuel pressure 1 psi for every 1 psi of boost. Buy a 1:1 regulator, cheap evo injectors and an safc if you want more fuel. IT will be FAR more reliable than what you are wanting to do. Listen to the people on this forum, there is a million times more information on this site than you will learn at any tech school.

For every 15 pointless post
I get 1 or 2 positive replys

Thanks for telling my why I dont need a FMU and should stick with the FPR

 

[bryanwheat]

I would look into the Aeromotive or Fuel Lab regulators. For tuning, you have a flashable ECU in your car so you just need a 150 dollar cable to flash it. The tuning software is free, but the logging software is like 20-30 bucks.

 

[Beastd01]

Do you have an adjustable fuel pressure regulator? you set the base fuel pressure at 43 PSI at ldle with the vacuum line off, then your fuel pressure regulator will raise the fuel pressure as needed.

I'm tuned with more mods and set mine at 42-43 and once I connect the vacuum line mine drops to 36psi. I saw you said that it would typically increase with vac source connected. I run lean and lose fuel pressure at high rpm.

 

[steve]

It's stated earlier in different words is that when using a 1:1 ratio fuel pressure regulator the actual pressure at any given point in time is equal to the base fuel pressure + manifold pressure.

Manifold pressure goes both ways, negative (vacuum) and positive (boost) so at idle when there is a vacuum in the intake manifold the fuel pressure drops from the base setting. Because the vacuum helps "suck" the fuel out of the injector but when there is boost it's raised so the fuel can "push" against the increased pressure.

The goal is no matter what the manifold pressure is the same amount of fuel is delivered in a unit of time. If you didn't you would get more under vacuum and at some point the fuel would dribble to a stop when the manifold pressure equaled that in the fuel rail which would make it hard for the ECU to know how much to inject.

 

[Beastd01]

It's stated earlier in different words is that when using a 1:1 ratio fuel pressure regulator the actual pressure at any given point in time is equal to the base fuel pressure + manifold pressure.

Manifold pressure goes both ways, negative (vacuum) and positive (boost) so at idle when there is a vacuum in the intake manifold the fuel pressure drops from the base setting. Because the vacuum helps "suck" the fuel out of the injector but when there is boost it's raised so the fuel can "push" against the increased pressure.

The goal is no matter what the manifold pressure is the same amount of fuel is delivered in a unit of time. If you didn't you would get more under vacuum and at some point the fuel would dribble to a stop when the manifold pressure equaled that in the fuel rail which would make it hard for the ECU to know how much to inject.

Thank you, after further reading and installing an AEM fuel pressure sensor I found two issues.
1) The FPR gauge on the regulator was bad and not reading correctly. Sensor allowed me to reset my base fuel pressure accurately via ECM link.
2) My new wideband is acting like a heart monitor at WOT. (enter eye roll)