WHY FUEL SYSTEM MODS ARE NEEDED

 

For those of you who are new owners of a new or pre-owed Subaru, you may have chosen this platform because of the legacy, notoriety, or the rally history of it’s all-wheel-drive performance. Although rally racing may have taken a backseat in the limelight of motorsport, understand that we are in a different world now, when compared to 20 years ago when the Subaru STI and the Mitsubishi Evolution were at peak rivalry, and the automotive aftermarket raced to produce basic performance modifications. Basic Performance Mods or BPU as known in the MKIV supra world, and FBO (full bolt-on) in the GTR world includes the (3) common mods which is intake, exhaust, and tune. These mods are relatively safe within the confines of the stock ECU that has been programmed by the factory to control your stock injector size. It’s when you surpass the programmed limits of the ECU is when things get hairy.

The basic rule of thumb is that anytime you add more horsepower to anything whether it be a WRX or a Corvette, you need to make sure that you have the fuel delivery to supplement that horsepower otherwise, you will create a lean condition in your cylinder. In Layman’s terms, if you don’t upgrade your fuel system you will melt your motor.

 

WHAT IS A LEAN CONDITION?

A lean condition is simply where your cylinder is not getting enough fuel for the air/fuel mixture. When running lean, your cylinder will get very hot, and the cylinder pressure increases. For Subarus that are synonymous with ring land failure, this means that the piston gets so hot that the ring lands on the piston crack, chip away, or smash together. WTF are ring lands? Ring lands are the grooves at the top of the pistons that hold your piston rings. Your piston rings are responsible for generating compression. Low compression means that you either have a broken piston ring due to ring land failure, damaged cylinder walls that have enough clearance to allow compression to escape, or you have bearing damage on the rod. Heat caused by a lean condition will heat up a piston enough to where it will melt, or beak apart at the edges because there is not enough fuel delivery and mixture in the combustion process to cool the cylinders.

 

WHAT CAUSES A LEAN CONDITI0ON THEN?

It could be irresponsibility such as running a downpipe without a tune. It could be turning the boost up with pump gas. Yes, pump gas does not have the octane rating that is needed for higher boost so your cylinder will get hot. And there’s not having a proper tune. Can you run a downpipe and an intake on a WRX without a tune? No, you cannot. Performance mods such as a downpipe and a cold air intake simply do one thing, and that is free up any restrictions found in the OEM design. The OEM downpipe typically features 2.25”-2.50” piping, and there are many sharp bends. A good aftermarket downpipe will feature 3” piping, a large opening connection to the exhaust housing of the turbocharger, and it’s going to have smoother bends. Naturally this will allow your turbocharger to spool faster because there is less turbulence for the exhaust gases to escape. Now the OEM intake features a box and a paper filter. It’s like trying to breathe with a cloth over your mouth. A cold air intake allows the turbo to inhale a larger volume of unrestricted air which will increase the throttle response while allowing the compressor wheel of the turbocharger to react quickly because you guess it – less restriction.

Cool, but why does this cause a lean condition? Less restriction means that your turbo and engine can breathe much easier. However, not according to your ECU. When your turbo can inhale and exhale easier, this means that naturally the turbo will make more boost. The ECU is programmed to control only a certain amount of boost, and the ECU controls the OEM injector sizes. The ECU is also tuned to increase the fuel output to match increased throttle and boost tables in the map. Increased horsepower surpasses the limits of the ECU. This is why literally every modded Subaru will have a COBB Accessport that has downloadable maps to work with certain upgrades.

If you installed an intake and a downpipe on a WRX and started driving it hard by flooring it everywhere or at a track even, the engine would misfire because the boost will go beyond the programmed limits of the ECU, and the duty cycle of your injectors will also be pegged. It will feel faster for a few seconds but just before red line you are going to hit boost cut and fuel cut. More horsepower and more boost will demand more fuel for combustion, and if you do not have enough fuel for a proper air/fuel ratio, you’re going to have a lot of pre-ignition and a lot of detonation because you are getting combustion in the cylinder at the top of the piston stroke. With OEM cast pistons, the motor won’t live that long. Like if you tried doing a long pull on the highway at the top of 4^th or 5^th gear you will send a piston out of the side of your block. All that excess heat in the cylinders and the excessive combustion puts too much pressure on your rod bearings and it’s time for a new engine.

 

THEN HOW DOES MORE FUEL KEEP THE ENGINE SAFE?

Having good fuel delivery and a proper air/fuel ration in the tune allows the cylinder to cool faster. So, with mods, you need to have the basic fuel upgrades which we will get to in a minute, and you need to have the right tune based on your injector size, fuel type, and boost levels. The higher the octane in your fuel, the better the combustion. This is why at the very least, you need to be running high octane fuel whether it be 91 or 93 depending on what state you live in. And E85 is pretty much race gas with 100-105 octane rating. Because the higher the octane rating, the more boost you can safely run. But we need to keep things in moderation here because you can have all of the fuel upgrades in the world. But if you are running high boost levels on pump gas then guess what? Melted motor is what. This is because with lower octane rating, the hotter the cylinder can get when trying to run bigger boost on pump gas. If you have a bigger turbo and you have larger injectors and a parallel fuel rail kit, larger fuel lines, and a higher volume pump and you’re trying to run 25 psi, melted motor will be the bill. A lower octane rating in pump gas will create a lean condition with modded cars with higher boost levels. As another rule of thumb if you are running pump gas, then you can only run so much boost before you start detonating the motor.  

 

I THINK I GOT IT. TOO LONG DIDN’T READ WHAT DO I NEED?

Cool so realistically, look at what you’re really going to do with your Subaru. Are you daily driving it, or are you trying to run a fast time at the drag strip with a dedicated race car? For those of you with stock engines, know that your engine won’t really live past 400 WHP. If you have a built and forged motor, you’re already familiar with the dynamics of fuel upgrades and tuning. For now, let’s focus on those who want a spicy daily driver.

 

 

FUEL PUMP

When upgrading your fuel system, you need to start with a fuel pump. When looking at a fuel pump, it will have the model, followed by LPH. This stands for liter per hour. The higher the LPH, the higher the volume and pump capacity of that fuel pump. If you have an intake, cat-back exhaust, and a tune, you will also need to upgrade to a more powerful fuel pump to make sure you avoid boost cut, fuel cut, and most importantly, a lean condition. Get yourself a fuel pump that is E-85 friendly, and most units are direct drop-in replacements for your Subaru WRX, STI, and we have universal pumps for those of you who want to wire-in a better unit.

 

 

FUEL INJECTORS

Next, you will need to upgrade your injectors because a stronger fuel pump makes sure that the fuel will efficiently travel to your injectors in a higher volume, but if your injectors are limited to size that cannot keep up with your boost levels, then you need to upgrade them. We have top feed units as well as side feed examples to suit your make and model. We also have top feed conversion kits for those of you who are upgrading your fuel rails which we will speak on next.

 

 

FUEL RAILS

Now you’re starting to make some power. You will need to upgrade your fuel rails when you start getting to a higher volume injector such as 1000CC and higher. You do not want to limit the duty cycle of your injectors with OEM rails, and aftermarket rails allow uninterrupted fuel flow to your injectors to ensure for proper furl delivery.

 

 

FUEL LINES AND FITTINGS

Very important especially if you plan on using ethanol. People often forget that ethanol is highly corrosive, and you cannot use conventional rubber fuel-approved lines. With ethanol, you need to use fuel lines that are PTFE-lined so that the ethanol does not eat through conventional rubber hose. Additionally, the options that we offer also slightly larger to accommodate a higher volume of fuel especially if you have a forged motor, bigger turbo, and running a moon tune.

 

 

FUEL PRESSURE REGULATOR

Because you cannot let all that unregulated fuel pressure dump into your motor. Otherwise, you’ll run rich which is the opposite of running lean. Too much fuel will wash pass your piston rings and it will contaminate your oil. As another rule of thumb when running a built fuel system, the static fuel pressure at idle should be around 42 psi. Any experienced engine builder or tuner should know this to help get your AFR’s (air fuel ratios) in the right strike zone.

 

 

 

FLEX FUEL

One of the best fuel system modifications especially if you switch between E85 and pump gas. With a tune from your Accessport, or from a standalone ECU, flex fuel measures the percentage of ethanol in your fuel system and will limit your boost accordingly. So, if you’re running 80% ethanol, then you can run your highest boost setting. With 50% ethanol and diluted pump gas your ECU and tune will adjust accordingly and lower your boost settings. And for those of you who store your Subarus during the winter, know that it is a bad idea to store your car with ethanol in the tank because it is corrosive. Running a tank full of pump gas before storage cleans out your fuel system, and it is a good idea to store your vehicle with pump gas and some stabilizer if you choose to store your vehicle.

 

 

We hope you have a better understanding of why fuel system upgrades are of high significance. But can easily be overlooked by novice enthusiasts because fuel injectors, fuel rails, and fuel pumps aren’t as attractive as a cat-back exhaust, a blow-off valve, or a turbo. One thing is for certain, is that boost is addictive, and if you want to run more power and more boost, then you need the fuel delivery to supplement the power, you cannot just rely on hopes and dreams as Dom said on the F&F.

At the same time, be sensible about your fuel system upgrades because the majority of Subaru owners are not going to be running 600+ HP cars on the street. Meaning don’t try running a 2000cc injector with just basic mods. The goal is to keep your car safe, while keeping it driving OEM-like with upgrades. Enjoy your Subarus responsibly without allowing your right foot to blow up your motor without the proper fuel system upgrades.