Fuel Pressure Regulator explanation

[teknicalissue]

I'm looking online for the answer and I just can't find anything.

I'd like to know how aftermarket fuel pressure regulators work. Looking at this link:

http://www.dsmtuners.com/forums/articles-tuning-ecu/366003-injector-fuel-pump-supported-airflow.html

I'm confused as to how this person came up with the numbers and I'm even more confused as to what boost does in his equation.

I'm looking for knowledge and not looking to be flamed.. I tried searching and all I came up with are threads that ask if people can run a wally without an FPR.

Reason I ask this is because I have a 255HP wally as well as a FPR and now need to know what kind of injector I should buy for my car. Since boost is part of this equation (can someone explain why) I'm running a td06sl2-20g with a 7 blade billet wheel with a 7cm hotside.

 

[Dude McGee]

Boost comes into the equation because both boost and fuel pressure rise at a 1:1 ratio. This means at 15psi of boost on a car with base pressure set to 43, you fuel pressure is now at 58psi, making your pump move fuel slower, and your injectors move fuel faster. So with that 20g at whatever boost level you have it set to, you can check what your pump and injector flow rates will be at that pressure.

Base Fuel pressure+Max boost=max fuel pressure to use as a reference when checking injector/pump flow sheets. The same type of situation occurs when vacuum is applied, only it results in a reduction of pressure.

I dont have any links for you to read in depth, Ill check around, but your trying to find the perfect injector size for your setup I take it? Id recommend oversizing them somewhat. For example, I use fic 1450cc, e85, boost controller buried for E316g, I see 67% IDCs at my redline. So the injector is too big. But it gives me room to grow.

 

[teknicalissue]

Boost comes into the equation because both boost and fuel pressure rise at a 1:1 ratio. This means at 15psi of boost on a car with base pressure set to 43, you fuel pressure is now at 58psi, making your pump move fuel slower, and your injectors move fuel faster. So with that 20g at whatever boost level you have it set to, you can check what your pump and injector flow rates will be at that pressure.

Very nice!! Ok now I understand how boost correlates with the FRP

Base Fuel pressure+Max boost=max fuel pressure to use as a reference when checking injector/pump flow sheets. The same type of situation occurs when vacuum is applied, only it results in a reduction of pressure.

so let's say my base is 37psi + 25psi of boost = 62PSI of what? Total boost? how does this change with the different pumps? in the link I posted above for example a 2g fuel pump does not pump the same as a walbro.. maybe I'm not understanding what the equation provides.

I dont have any links for you to read in depth, Ill check around, but your trying to find the perfect injector size for your setup I take it? Id recommend oversizing them somewhat. For example, I use fic 1450cc, e85, boost controller buried for E316g, I see 67% IDCs at my redline. So the injector is too big. But it gives me room to grow.

yea, I was thinking I'd go with 1250cc and have my bottle neck at the pump it self (If I understand correctly.. my 255 wally will max out at 25psi on 1000cc injectors)

EDIT: Back to the formula... I'm assuming if I got 62PSI that would be what the injector should be rated for??

 

[Dude McGee]

In your example above, it would be 62 psi of fuel pressure. You can take that number amd check what your fuel pump flows at that pressure on wallys webpage or a vendors site. More pressure is more resistance, so the pump flows less as its increased. On the injector side, you can check flow rate at said pressure, and for the injectors increased pressure results in more flow, which you can find on FIC or IDs webpage (or whoever you choose) for flow rates.

Edit: The pressure calculated is only for checking flowrates. Whatever injectors you choose will handle the pressure, the number is just for checking how much fuel can be provided at the pressues youll be hitting, and cross referencing if that fuel rate will support the airflow you plan on making.

Are you going pump or e85? 1250 will support a huge amount of power on pump, so youd be good to go there.

 

[WES_393]

The FPR used in our fuel system is called a "rising rate" regulator. Whether it's stock or aftermarket, the FPR has one basic function: To adjust fuel pressure by the exact amount of pressure in the manifold. So if you have 10psi of boost in the manifold, the FPR will increase fuel pressure by 10psi.

The purpose of this equal (or 1:1) regulation is to keep actual fuel pressure constant. Let's say you have 10psi in the manifold and 43psi in the fuel rail. When the fuel injector opens and the fuel pressure tries to escape, it's going to meet 10psi of resistance. So your total fuel pressure is only 33psi. But if you raise fuel pressure to 53psi, that 10psi of resistance from the boost will result in 43psi of total fuel pressure.

It's important to keep fuel pressure constant so that injector flow stays the same. Fuel injectors have a flow rating (lets say 850cc/min for example) which is usually tested at 43.5psi. If you change the fuel pressure, you change the flow rate. At 50psi they are actually flowing 910cc. At 30psi they're only flowing about 700cc. You've already told the ECU that the injectors flow 850cc, so any changes to the flow rate will cause an undesired change in fuel delivery.

However, that 1:1 regulation has an impact on the fuel pump. As fuel pressure increases, the pump can't flow as much fuel. Fuel pressure is needed to keep the injector flow rate constant, but fuel flow is needed to keep the injectors fed. At 20psi, a Walbro 255 can still flow enough fuel to support 55 lb/min of air. But if you raise the fuel pressure 5psi, then it can only flow enough fuel to support 50 lb/min. If your turbo flows 55 lb/min at 25psi, then you won't have enough fuel flow to feed the injectors.

Before looking at that page, you should determine your turbos flow and boost abilities. Let's say a 20g can put out roughly 45 lb/min at 24psi (which I think is fairly accurate), and you aren't going to push it harder than that. You want a fuel system that can handle more than that. So for fuel injectors, you'd want at least 850cc since they can handle 48 lb/min at 80% capacity. Then for a pump, the Walbro 255 will handle 50 lb/min at 25psi. That's cutting it a little close for my taste so I'd suggest a rewire, but it should still work.

And like McGee mentioned, the numbers on that page are for pump gas only.

 

[LandSpeed-DSM]

Very nice!! Ok now I understand how boost correlates with the FRP

so let's say my base is 37psi + 25psi of boost = 62PSI of what? Total boost? how does this change with the different pumps? in the link I posted above for example a 2g fuel pump does not pump the same as a walbro.. maybe I'm not understanding what the equation provides.

yea, I was thinking I'd go with 1250cc and have my bottle neck at the pump it self (If I understand correctly.. my 255 wally will max out at 25psi on 1000cc injectors)

EDIT: Back to the formula... I'm assuming if I got 62PSI that would be what the injector should be rated for??

You always want more fuel then you expect to need at any point in normal operation. You do not want the fuel system to be the "bottleneck" in power production, that is how things become.. melty/explodey

The best way (in my opinion) to approach this is determine first how much Power (air mass) you need for your goals with the car. Having a familiarity with engine demand flow and compressor maps is going to be key to this route however.

Then determine what fuel, we'll use pure gasoline (no ethanol "E0") for discussion.

Now pick a worst case (Rich) target AFR for that fuel at peak airflow. I usually use 15-20% margin for safety. So on gasoline (E0) If you expect to be running as rich as 11.0:1 AFR or Lambda 0.75; 15% more fuel would put you around 9.5:1AFR

Let's say your goal is 450awhp on an average setup, this will likely require about 50lbs/min mass airflow. At 9.5:1AFR, this is going to require about 5.25lbs/min fuel. Find the specific gravity of your fuel at a likely temperature and convert mass flow rate to volume flow rate. Now pick your injectors so they can support 5.25lbs/min fuel flow or ~700cc/min injectors. Recommended max injector duty cycle is 80% (industry standard) this means you would want 880cc/min injectors. Automotive fuel injectors are rated at 43.5psi (3bar) standard.

Now through assumed previous knowledge (Engine Demand/Compressor Maps, etc.), you have found a turbo that will produce 50lbs/min airflow at 30PSI boost on your setup.

This is where the 1:1 rising rate fuel pressure regulator comes in. In order to maintain that flow rate across the body of the injector the same pressure differential needs to be maintained for consistency in the tune and injector scaling. Try tuning a Turbo/SC converted Honda with a returnless system and factory FPR, and you'll grow to really appreciate our fuel systems.

So If we have 43.5PSI base pressure, and max boost of 30psi, while still needing to supply 5.25lbs/min of fuel we now have our criteria for finding a pump. The pump (flow rated by pressure and voltage) needs to be able to deliver that 5.25lbs/min @ ~74psi total fuel pressure. Pump flow falls off fast as pressure increases and/or voltage drops. This is why pump re-wires are a huge deal, as seen in the chart here:

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Look at the difference a mere 1.5V at the pump makes!


I typically put together a spreadsheet for my builds to put all this in one place that is easy to digest. Here is the sheet for the Magnafuel pump in my car now and what it can support at a given pressure and voltage for a variety of potential fuels and fuel temperatures:
https://docs.google.com/spreadsheet/ccc?key=0Ahi5eTgzfWg_dHh3RGpienAyYXRrNUF3c2Y0cEwyNlE#gid=0

Consider also that this does not account for pumping losses and pressure drop from the rest of the fuel system: fittings, lines, filters, etc.

Hope this helps.

 

[Dude McGee]

Oops, I applied the base pressure thought effecting the injector during boost. Pardon my misinfo. During boost, the injector flow rate does not change wih the increased pressure as per what Wes said. Only pump flow rate is impacted.

Sorry, still need my coffee

 

[LandSpeed-DSM]

Kinda handy how we both wound up around roughly the same mass flow figure (48-50lb/min) for our examples, Wes. Keeps our injector numbers the same too since we both chose to use gasoline.

 

[WES_393]

Kinda handy how we both wound up around roughly the same mass flow figure (48-50lb/min) for our examples, Wes. Keeps our injector numbers the same too since we both chose to use gasoline.

That's quite a coincidence. It definitely shows the consistency between each approach.

 

[athlete3344]

This thread should be saved as an Article or its equivalent. A lot of good, basic, information for selecting fuel systems to match turbos and vice versa. This is an excellent source, and I will be using it in the future if/when I can get my hands on a GSX.

 

[jdezmit]

Great Info!

 

[teknicalissue]

In your example above, it would be 62 psi of fuel pressure. You can take that number amd check what your fuel pump flows at that pressure on wallys webpage or a vendors site
.

I can't find anything on their site that would tell me what it would look like running at 25PSI of boost (62 Total PSI).. do you have a link?

Are you going pump or e85? 1250 will support a huge amount of power on pump, so youd be good to go there.

I'll be using pump gas for a short amount of time. in the future (after I get a better long block and tranny) i'll be aiming for E85.

EDIT: It's really frustrating when you have DATA that contradicts it self....

this website: http://www.autoperformanceengineering.com/html/fpspecs.html

is saying that at 17PSI of boost (with a 43 base pressure) the 255HP will flow at 61Gal/hr while the other page shows that 71 pounds per minute.. Using math
61 gallons per hour is 375.95886 pounds per hour which ends up to be 6.2 Pounds a minute..

I'm converting 61 gallons to pounds by multiplying 6.16326 (weight of 1 gallon of gasoline in pounds) and then dividing by 60 (minutes) to get the flow per minute

 

[steve]

The FPR used in our fuel system is called a "rising rate" regulator.

I could be confused but I don't think so. I think that term "rising rate" actually refers to FPR's that have a > 1:1 response (even though many use the term incorrectly) and as you note the FPR in a DSM is 1:1 so that the flow rate from the injectors remains constant per unit time regardless of manifold pressure.