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Old 04-13-2012, 10:47 PM   #1
the suicidal eggroll
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Default How the Fueling System Works

The fourth in the series:
http://forums.nasioc.com/forums/show....php?t=2339831


The fueling system


Also see Unabomber's Manifesto on the topic:
http://forums.nasioc.com/forums/show....php?t=1218460


Alright, this is going to be a big one. The return-style fueling system in our cars is straight forward, but there are a lot of different pieces to cover. I’m going to start out with a description of what AFR is, why it’s important, and how the ECU controls the fueling system. Then I’ll talk about the fuel system from a mechanical perspective, and how the different components work together.

So what is the air/fuel ratio, and how does the car’s computer control it? Combustion in an engine deals heavily with stoichiometry, the study of reactants and products in chemical reactions (after all, fire is a very common chemical reaction). Contrary to popular belief, while fuel is what gives the engine its energy/power, AIR is what’s really important to making power. After all, if fuel was the only thing that mattered, why wouldn’t people just throw giant fuel systems on tiny engines and make obscene amounts of power? The answer is stoichiometry. While fuel contains the energy you need to extract in order for the engine to make power, air is what allows you to extract that energy. The ratio of air to fuel is critical in this process. Too much air and the fuel can’t burn smoothly and completely. Too little air and the fuel simply can’t burn, because there isn’t enough air to burn it all. We can inject as much fuel as we need, so the focus in making power is trying to cram as much air as we can into the engine, so that we can add the right amount of fuel and burn it.

The stoichiometric ratio is the amount of air required to properly burn a given amount of fuel. The stoichiometric ratio for gasoline is around 14.7 parts air to one part fuel. This is where you get a good balance of a stable, clean burn, and relatively complete combustion of the fuel. Now while the stoichiometric ratio is the “ideal” AFR for complete combustion, due to the nature of a realistic combustion chamber in realistic conditions, at stoich you aren’t actually burning all of the fuel or all of the air, due to the simple fact that some of the air can’t “find” some of the fuel when combustion occurs. Some engines are alright with an AFR (air/fuel ratio) slightly above 14.7, allowing you to more completely burn the fuel and improve mileage, but it all depends on the design of the engine and how the air and fuel mix together. On the other end of the spectrum, adding more fuel can let you burn all of the air completely, allowing you to make some extra power, up to a point. The ideal power AFR for gasoline is somewhere around 12.5:1.

So all cars run an AFR of 14.7 at idle and cruise for gas mileage, and an AFR of 12.5 at wide open throttle for power, right? Unfortunately it’s not that simple. Most naturally aspirated cars do exactly that, but forced induction cars (turbo, supercharged) are a completely different animal. While 12.5 is still the “ideal” power AFR, most forced induction cars running regular pump gas can’t actually do it. An AFR of 12.5 is so energetic and unstable, that the heat created by compressing the huge amount of air in the cylinder of a forced induction car running high boost can actually cause the air/fuel mixture to ignite by itself, while the piston is still compressing, before the spark plug fires. This is called detonation, pinging, or knocking, and is very harmful to the engine. As a result, most forced induction vehicles have to add additional fuel beyond 12.5 for no other reason than to cool the air/fuel mixture and reduce its tendency to pre-ignite. On regular gasoline, most turbo 4-cyl cars (Evo, WRX/STi, and so on) run an AFR around 10-11. With higher octane race gas or additives (methanol injection, etc), it is possible to lean out the AFR to closer to 12.5 without running into detonation, allowing more complete combustion and providing significantly more power.

So how does the fueling system work, exactly? Air entering the engine is measured by the mass airflow (MAF) sensor. There are many types of MAF sensors, but the one used on our car is also referred to as a hot wire anemometer. This type of MAF works by electrically heating up a very thin wire which is placed near the middle of the air stream (stock location is between the air filter and the turbo). As air passes over the wire, the wire is cooled. The MAF sensor measures the amount of power required to maintain the wire at a fixed temperature, and sends this information to the ECU.

The ECU reads the output of the MAF sensor, which is nothing more than a voltage between 0-5v. The ECU then references a huge lookup table which contains the relationship between MAF voltage and air flow rate. For example, on a stock 2005 STi, a MAF reading of 3.95 volts is equivalent to an incoming air flow rate of 166.14 grams/second. Once the ECU has an air flow measurement, it then looks at the crank position sensor to get the current engine RPM, for our example say it’s 4000 RPM. The combination of the two give the ECU a value for the “load”, which is a measurement of the amount of air per engine revolution (MAF*60/RPM). For our example, this would be 166.14*60/4000 = 2.49 grams/revolution. Using the load and RPM, the ECU then references another lookup table which tells it what AFR it should run. Again for the 2005 STi, at 4000 RPM and a load of 2.49, the target AFR would be 11.33. At this point, the ECU knows the engine is currently drawing in 2.49 grams of air per revolution, and the AFR should be 11.33, so it knows it needs to inject 0.22 grams of fuel per revolution. It then adds any necessary fueling adjustments (such as adding more fuel during warmup, etc).

At this point, the ECU references another lookup table, the injector scaling. This tells the ECU how long the injectors need to be open in order to inject the necessary 0.22 grams of fuel per engine revolution. The ECU must also look at another table, which tells the ECU how long it needs to TELL the injectors to open for, in order for them to actually open the right amount of time. This value is the injector latency or dead time, and is the amount of time between when the injector is told to open, and when it actually opens, and is usually on the order of 0.5-1.5 milliseconds.

The ECU finally knows how long it needs to open the injectors, so it does, they spray the fuel, the spark plug fires, and you get combustion. Now there are two modes in which the fueling system in the ECU operates while you’re driving: closed loop and open loop. Open loop fueling stops here; the ECU injects the fuel, you get combustion, and then the process starts over for the next engine revolution. Closed loop fueling takes things one step further. After combustion, the exhaust exits the engine and passes through the exhaust system. On its way, it passes by the front O2 sensor, which tells the ECU what the ACTUAL AFR ended up being. The ECU compares the actual AFR against what the AFR was supposed to be, and modifies the fueling on the next combustion cycle to get closer to target (this is one of those fueling adjustments mentioned before). Closed loop is only used at idle and cruise when the target is 14.7, open loop is used at high throttle and when the engine is first started, before the O2 sensor has come up to temperature.
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Last edited by the suicidal eggroll; 04-13-2012 at 11:30 PM.
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Old 04-13-2012, 10:47 PM   #2
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Thatís it, thatís the fueling system from the ECUís perspective. Now itís time to look at the fueling system from a mechanical perspective. Mechanically, the fuel system is very straight forward. Following the diagram below, you can see the pump pressurizes the fuel, pumps it through the filter, the first rail, the second rail, and finally to the FPR. The FPR is a valve that keeps the fuel pressure 43.5 psi above the manifold pressure, and any extra fuel is then bled off in the return line which dumps back into the fuel tank.




The fuel pump is a positive displacement pump, just like the oil pump. A positive displacement pump is a constant-flow pump, unlike many other pumps that are constant-pressure, etc. An ideal positive displacement pump always pumps out the same volumetric flow rate of fluid regardless of the pressure required to do so. Whether itís pumping into nothing, into a fuel pressure regulator, or into a brick wall, it will always push the same amount of fluid. Now the fuel pump isnít an ideal pump, but thatís still the principle under which it works. A real fuel pump does have some pressure dependence (with flow decreasing as pressure increases) but at most normal pressures this dependence is relatively small.

Since the fuel pump is constant-flow, how is the pressure controlled in the fuel system? The answer is the fuel pressure regulator. The FPR actually looks a lot like a wastegate internally. There is a spring holding a valve closed. Manifold pressure pushes with the spring helping to keep the valve closed, while pressure at the fuel inlet pushes against the spring. When the fuel pressure exceeds the manifold pressure by 43.5 psi, the spring compresses and the valve opens, allowing the fuel to exit the FPR and return to the fuel tank. In the real world (with a positive displacement fuel pump), the FPR is almost always partially open, allowing some fuel to return to the tank while keeping the rail pressure 43.5 psi above the manifold pressure.

If the fuel pump is too large and the FPR is too small (a common problem when people upgrade their fuel pump too much without touching the rails or FPR), there is actually too much fuel for the FPR to bypass it all, and the pressure starts rising uncontrollably, causing the car to run rich (too much fuel for the amount of air in combustion). This is mostly a problem at low load (idle/cruise) when the injectors arenít pulling off very much of the fuel coming from the pump, so almost all of it has to be bypassed and returned to the tank. This problem is helped in part by fuel pump controllers, which vary the voltage or duty cycle of the power going to the pump to reduce their flow rate at low load.

If the fuel pump is too small for the setup, the injectors might pull off all of the fuel coming from the pump, so thereís nothing left for the fuel pressure regulator to regulate. In this case, the pressure will start dropping uncontrollably, which is what causes cars to run lean (too little fuel for the amount of air in combustion) at high load and high RPM when their fuel pump isnít adequate for the amount of power theyíre making.

The FPR is quite possibly the most critical part in the entire fuel system. The entire principle on which the ECU operates (opening the injectors a fixed amount of time to add the right amount of fuel) relies on the difference between the fuel pressure behind the injector and the air pressure in front of the injector being a constant. Without the fuel pressure regulator holding the fuel pressure a fixed 43.5 psi above the manifold pressure, as manifold pressure rose, fuel flow would drop, since the injectors would have to spray into a higher pressure environment. Similarly, as the manifold pressure dropped below atmospheric, fuel flow would increase, since in addition to the fuel pressure pushing fuel through the injector, the vacuum in the intake manifold would actually be sucking fuel out of the injector as well. The reference line running between the manifold and the FPR is quite possibly the most critical hose in the entire car. Whatís more, Subaru decided it would be a good idea to just slip the hose on, with no clamps. If you ever have the engine bay disassembled to the point where you have easy access to the FPR and this hose, do yourself a favor and put some zip ties or hose clamps on each end to hold it on there.

Injectors are very precise valves which are pulsed by the engineís computer to let a fixed amount of fuel enter the combustion chamber. They are rated in their flow rate (volume/time), which is an indication of how much fuel they are able to flow when running at close to 100% duty cycle. The larger the injector, the more fuel they can flow (supporting higher horsepower setups), but the less control they have over exactly how much fuel is being injected. Large injectors also tend to have worse spray patterns, so the fuel doesnít atomize as well as it should, and combustion suffers as a result. This is why larger injectors can lower fuel economy, because the worse spray pattern and worse atomization means you need more fuel for proper combustion, lowering gas mileage. Usually this isnít a big concern until you get to very large setups though, 1200cc/min and larger.

If youíre interested in fuel system calculations (hose diameter, pressure loss, etc.), check out the excel worksheet I made. Itís a little too advanced for these ďhow stuff worksĒ articles, so Iím keeping it separate:
http://forums.nasioc.com/forums/show....php?t=2158062



FAQ


Q: Will larger injectors/fuel pump add power?
A: No. Remember what I said earlier, itís AIR thatís important when it comes to adding power, not fuel. The fuel system is a support system; it adds the fuel to support the amount of air youíre bringing in to maintain the proper AFR. Larger injectors/fuel pump do nothing more than increase the CAPACITY of the fuel system, allowing you to run larger turbo setups without running the fuel system dry. The only time that upgrading the injectors or fuel pump will add power is when youíve already exceeded their limits, and have had to de-tune the engine to work correctly on the restricted fuel system capacity. In that case, upgrading the fuel system will let you tune it up to take full advantage of your turbo system. If you ever run into this situation, then you need to take two steps back and re-think your researching process, because upgrading to a turbo setup that your fuel system canít support is step 1 in the unwritten book of how to blow up your engine. If you donít know how much fuel you need for a given setup, use the calculator linked above. If you donít know what any of it means, then you should be using a professional to help you design your setup.

Q: Is the O2 sensor narrow or wideband?
A: It is a wideband. If you disagree, then just stop talking and listen. There are numerous tests, diagrams, plots, measurements not just suggesting this, but PROVING this. The problem with the O2 sensor is not its accuracy away from stoich, the problem is its installation location. When the boost rises, the exhaust pressure rises, and the exhaust pressure throws off the readings of the O2 sensor. For this reason, even though the O2 sensor reads down to 11.0, in the factory location its measurements can not be trusted once you get into boost. Many people have relocated the O2 sensor after the turbo where the pressure is more or less normal, and have had great success when compared to aftermarket widebands installed in the same location. If you havenít done this though, donít pay attention to the factory O2 once the boost starts rising.

Q: What is the closed loop/open loop delay?
A: As I said before, closed loop is when the ECU uses feedback from the O2 sensor to maintain target AFR, while open loop is just ďinject it and forget itĒ. Since the O2 sensor is only accurate at idle/cruise, thatís when the ECU is in closed loop with a target of 14.7. As you get on the throttle, at some point the ECU has to make the switch from closed loop to open loop and drop the target AFR. Now ideally this switch would occur when the RPM and/or load increased to the point where the target AFR started dropping below 14.7, and this is exactly when most tuned cars will make the switch. The problem is the factory decided to implement a delay in this switch to improve emissions and fuel economy (they didnít want the car constantly switching to open loop and dropping the AFR every time you blipped the throttle on the freeway). This is called the closed loop/open loop delay (CL/OL delay). When you get on the throttle and one of many switches is triggered (high RPM, high load, high injector pulse width, high throttle), the ECU starts a counter. When this counter exceeds the CL/OL delay, the ECU finally makes the switch to open loop and drops the target AFR from 14.7 to whatever it is supposed to be given your current RPM/load.

Unfortunately, on late model cars, this counter threshold is very, very high, and itís entirely possible for the car to be at peak boost and STILL be stuck in closed loop running 14.7. This causes significant detonation and pulled timing, and will eventually damage the engine. The 02-03 WRX had either no delay, or a delay that was so small that it didnít matter. Over time Subaru has increased the delay due to increasing emissions restrictions. Even through the 06 STi it wasnít terrible, but in 07 the counter threshold was jacked way up, and has since been believed to be the primary cause of engine failures at stock to stage 2 power levels (blaming the failures on tuned vehicles to damage that had already been done before the tune was implemented). If youíve ever noticed a hesitation on your 07+ WRX/STi at high throttle between 3-4k RPM, thatís probably the ECU pulling timing to combat detonation caused by the CL/OL delay.

Q: Is driving without a front O2 sensor bad?
A: No. When the O2 sensor is disconnected, the car is immediately switched to open loop. Unless thereís a major problem with the fueling system (untuned intake, untuned injectors, clogged fuel system, a vacuum leak, etc), the car will continue to run just like normal, maybe even better than normal (see above about the CL/OL delay). The only problem with this is that the ECU will not correct for global fueling problems, such as a change in stoich from ethanol-infused gasoline, etc. If you ever drive without your front O2 sensor operating, it would be very wise to install an aftermarket wideband sensor so you can monitor the AFR yourself to check for any fueling problems. If your O2 sensor ever fails and starts reporting incorrect AFR values to the ECU, causing bad gas mileage or stalling, it is MUCH better to unplug the O2 sensor entirely and let the car run in open loop than to allow the car to continue running under those conditions. Now keep in mind that this ONLY applies to the front O2 sensor. The rear O2 sensor is used to check for cat efficiency, and on the 16-bit ECU (02-05 WRX) it can be unplugged without a problem (other than the resulting CEL which can be disabled), but on the 32-bit ECU (everything else), unplugging it will drop the closed loop AFR target from 14.7 to closer to 13.5, hurting gas mileage.

Q: Ok, so once and for all, what mods will, and what mods wonít affect the AFR on our cars?
A: Good question me! Hereís a handy dandy list:
Stock sized intake: most likely
Big MAF intake: absolutely
Turbo inlet: it will have a small effect, it has been debated whether or not itís even worth worrying about
Turbo: same as turbo inlet
Intercooler: no
Intake Manifold: it will affect the per-cylinder fueling compensation, and maybe tip-in enrichment
Cams: might affect tip-in enrichment
Porting: same as cams
Valves: same as cams
Header: no
Uppipe: no
Exhaust: no
Fuel pump: no, not unless the previous pump was dropping pressure, AND the car was previously tuned taking that into account, or the pump is overrunning the fuel pressure regulator and causing an uncontrollable rise in fuel pressure
Fuel rails: it might affect the per-cylinder fueling compensation
Fuel injectors: definitely, even if the flow rate is the same, chances are the latency is not
Fuel pressure regulator: not unless the fuel pressure is being changed, or the previous regulator was being overrun AND the car was previously tuned taking that into account

Q: Why does a stock sized intake require tuning?
A: Remember, the MAF does not measure the air flow rate, it is actually measuring the velocity of the air that crosses the hot wire. The MAF scaling table then extrapolates that velocity to get the total air flow rate, taking into account the diameter of the tube around the MAF and the air flow pattern of the air around the MAF. While a stock sized intake is the same diameter, the filter location, bend locations, and bend radii will be different than stock. These all affect the air flow pattern across the hot wire, which will affect the MAF scaling.

Q: When it comes to the fuel system, more capacity is always better, right? It gives me more room to grow in the future.
A: Not necessarily. The fuel system is one of those things where you want to run what you NEED, and no more. As far as injectors go, as long as youíre under about 1000cc it doesnít really matter, you might as well get as big injectors as you think you might need in the future. Above 1000cc you should take a step back and look at your setup to see if you really need that much fuel, at that point youíre starting to get into negative side effects.

This oversizing applies to pumps too people! Everybody is so gung-ho about getting the biggest baddest drop in fuel pump they can find, because more capacity is always better, right? Wrong. Look at the fuel system diagram above again. What do you see? In addition to many other things, you see a series rail setup. The fuel comes from the pump, passes through one rail, then through the other rail, then to the regulator. Our fuel lines are tiny guys, ľĒ inside diameter. That causes a lot of pressure drop, and the more fuel youíre flowing, the worse it is. The pressure drop between the pump and the first injector doesnít really matter, more pressure drop just means the pump has to operate at a slightly higher pressure. As covered before, since the pump is positive displacement, this pressure increase doesnít have much of an effect on the flow rate. It hurts it a little bit, but not much.

The bad part is there is about 3-4 feet of fuel line between the first injector and the last injector. The more fuel youíre trying to force through this line with your gigantic pump, the larger the pressure drop in this section of line, and the larger the pressure difference between the first and last injector will be. This will cause the first and second cylinders to receive fuel to run rich, and the third and fourth cylinders to receive fuel to run lean. If you want to see how bad it is, use the 4th tab in the excel spreadsheet linked above to find out. All of the equations are there, just plug in your numbers and see how it looks. In my opinion, a DW 300 LPH is about the largest you should go on the stock rail setup. Above that, and the flow will start to cause some problems with the pressure imbalance in the rails. Just to be clear, if you NEED the flow, then by all means go for it, but if you donít, a huge oversized pump will just make things worse. If you NEED more than a DW 300 pump, you should really start to look into either converting your rails to parallel, or going with aftermarket rails and at a minimum a nice, fat line connecting them to keep the pressure imbalance low.

Q: Is a series or parallel rail setup better?
A: Iím not touching this one with a 10 foot pole, hehe. Both have their advantages and disadvantages, and youíll find numerous arguments on both sides of the fence. A parallel setup certainly has a smaller pressure difference between the two banks, that canít be argued, but a parallel setup can introduce other problems such as vapor lock. Speak to your tuner about this before you decide on one way or the other. For what itís worth, when I built my fuel system I went parallel. Cruise and full throttle were fine, but I ran into problems at idle. I converted to series, and it literally ran exactly the same. Turns out my problems were being caused by the injectors, but I still noticed absolutely no difference between the series and parallel setups in regards to drivability, AFR, etc. After fixing the problem, I just stayed with series, no particular reason.
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Old 04-13-2012, 10:48 PM   #3
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First, and great write up!!





















you can has sticky?

Last edited by ronzogonzo; 04-13-2012 at 11:03 PM.
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Old 04-14-2012, 02:06 AM   #4
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Holy **** sticky all of this...

Thank you great simplification!
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Old 04-16-2012, 12:11 AM   #5
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I'm no tuner but this is making perfect sense lol

Thanks for the write up!
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Old 04-16-2012, 12:16 AM   #6
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Great write up!
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Old 04-17-2012, 03:49 PM   #7
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Great stuff!!!

Question:
- Is there any negative at all to moving the STOCK O2 sensor? Why I ask this is using my aftermarket wideband has been a pain in the butt. I'd LOVE to just be able to log and have the factory O2 actually read under full boost. I have 2 bungs in my downpipe and could put the factory wideband in either. I guess the only risk is if it's in a position that isn't reading that great, you could go lean while in CL and damage your engine. Thoughts?
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Old 04-17-2012, 05:31 PM   #8
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That's a good question...the only drawback I can think of is that the delay between combustion and the measurement will be increased, which may make the CL operation a little more oscillatory/unstable.

Also the factory sensor can only read down to 11.0, which may not be enough if you're tuned on the rich side.

Beyond that...I don't know of any drawbacks. There are several people here running with their front O2 relocated to the bellmouth, you should consider asking them how their experience has been in regards to closed loop behavior, etc. I don't know any names off-hand, but you should be able to find them with some searching.
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Old 04-19-2012, 04:17 PM   #9
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Thanx for info, very good reading!
And sticky for this!
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Old 04-19-2012, 06:23 PM   #10
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Another great write up!
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Old 05-04-2012, 07:07 PM   #11
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@ the suicidal eggroll: Do you know how fuel mileage is calculated by the ECU? Is it calculated from the level of the fuel tank or is fuel consumption measured somewhere else down the line (i.e at the injectors, ect....)?
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Old 05-04-2012, 07:18 PM   #12
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To be honest I have no idea how the ECU does it. The easiest way would probably be to divide distance traveled by the integrated MAF g/s divided by AFR, but I don't know if that's what they actually do. I can't imagine they use the level in the tank, otherwise it would change every time you parked on an incline, but who knows.

Any E85 guys want to chime in with whether or not their reported gas mileage stayed the same or dropped when they made the switch? My STi doesn't report gas mileage, and my WRX is still on pump, so I'm not sure. If the reported mileage dropped accordingly, it must be using the tank level. If the reported mileage stayed about the same or even rose while the actual gas mileage dropped, it must be using MAF/AFR or injector pulse width with an assumed stoich of 14.7.
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Old 05-06-2012, 03:30 AM   #13
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I wish i have time to read all this. Unfortunately i just hate reading even though it interests me alot...
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Old 06-12-2012, 11:55 AM   #14
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i was literally just about to search the manifesto for something like this...great timing, awesome write up. walbro 255 and DW 850's will be perfect for my setup, your calculator is awesome as well.
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Old 06-12-2012, 01:51 PM   #15
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I know that my MPG guage on my 2009 STI reads optomistically now, because when i switched to 750cc DW's, it thinks I go X distance with X gas (assuming I still have my stock injectors), and it's wrong. My car tells me i get 22MPG usually, i really see about 17-18MPG. Sean Church warned me of this when he tuned my car, and told me that it would read wrong, and at that time it wasn't in his ability to talk to that part of the program from a tune persepctive.

Furthermore, great writeup man. Sticky!
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Old 06-12-2012, 03:13 PM   #16
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Great write up
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Old 06-12-2012, 03:19 PM   #17
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Just a question...don't you have the fuel feed and return line labeled backwards?

The feedline goes to the regulator correct? Return line to the tank? I've plumbed lots of stuff and never plumbed the regulator on the return side heh. Given I've not plumbed a subaru...only race cars that are carburated not efi.

Edit:

Nevermind I see whats going on..your regulating pressure after the rail instead of before.
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Old 06-12-2012, 10:13 PM   #18
the suicidal eggroll
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Quote:
Originally Posted by haberkorn View Post
Nevermind I see whats going on..your regulating pressure after the rail instead of before.
Correct. There are other ways to do it, the above diagram just shows how the stock and most aftermarket Subaru setups are plumbed.
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Old 06-12-2012, 11:26 PM   #19
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Gotcha.. I've always plumbed the regulator prior to the carb/fuel rail. That way the carb/rail sees the regulated pressure versus unregulated. Different strokes for different folks I guess.
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Old 06-12-2012, 11:36 PM   #20
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That will only work with a returnless system. Otherwise pressure after the regulator is 0.
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Old 06-12-2012, 11:40 PM   #21
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Correct. I've always used dead head regulators... Never ran a bypass style. I've always used a pump with a bypass like the magnafuel 500 pump.
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Old 06-19-2012, 09:46 PM   #22
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Could the FPR Cause a rich condition at semi-WOT and WOT?

I have a situation recently where my car suddenly quit holding fuel pressure after key on prime or when the car is off it immediately goes to 0.

Its an aeromotive pump in the tank and an aeromotive FPR.

Around the same time this has occurred my car began hitting 10.0-10.3 semi WOT and WOT
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Old 06-19-2012, 11:21 PM   #23
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A failing FPR could do any number of things. I would recommend installing a fuel pressure gauge to monitor what the pressure is doing at idle and WOT. That will immediately tell you if the problem is the FPR or not.

That said, anything that would cause pressure to drop to zero as soon as the pump shuts off SHOULD cause the pressure to be low at WOT, not high. There could be something I'm not thinking of though.
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Old 06-19-2012, 11:54 PM   #24
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Its holding steady at 43.5-44 lbs at idle, not sure at WOT as I can't see it under the hood, LOL
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Old 06-20-2012, 12:04 AM   #25
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That's high...what do you have your base pressure set at and what's your manifold vacuum at idle?

With a base pressure of 43 psi and a typical sea level idle of around -10 psi, you should be seeing around 33.
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