Perdue
10-16-2008, 09:18 PM
Intro
There are a few different ways to milk extra horses out of your Subaru EJ Series motor. The basics are Bolt-ons, Aftermarket Turbo Kits, and even Nitrous. You could even swap the drivetrain all together for that of a WRX or STi. Since aftermarket turbo kits can be unreliable in the wrong person’s hands, swaps are expensive, and I know not a thing about nitrous, I’ll be covering the bolt-on/naturally aspirated side of things in this thread. Nothing here is absolutely concrete, but isn’t up for argument here. I’m simply writing this so that I can share a little bit of the knowledge that I’ve gotten through years of Subie ownership and research with all you noobs who keep asking the same question over and over (I kid, I kid. :lol )
Before you start, consider some pros and cons of NA Tuning. The later model year you own, the harder it is going to be to tune. After about 1999, Subaru started making an effort to make their ECU’s, “better.” This isn’t good for tuners, as the ECUs became increasingly harder to tune using piggyback type tuners, as they eventually start to learn around them. Know that you will not be beating any STi’s with a NA motor (in a straight line) unless you drop big money. There are some NA Subies that have broken the 300hp mark (see: Zzyzz Motorsports 2.5RS), but it costs money and takes a lot of work, not to mention might not be very streetable. If you’re using your car for a daily driver and maybe the occasional auto-x, then NA might just be for you, as you might be able to maintain better fuel economy and not have to wait for a turbo to spool up. Another plus, you can do NA tuning in installments, so if you find yourself unable to save $5k+ for a swap...well, you get the picture.
With that, I’m sure everyone here will probably have something to add to this, or challenge something I say here. To them, bring it on. This is basic, so bear with me. If you want 300hp or better out of your current motor, stop reading now and start saving for a swap.
Intake
Any intake that is meant for the earlier model N/A Impreza will pretty much work here. It might take a little bit of work, but it will work. Figure out if your car is run by a MAF or MAP sensor, and buy an intake that is meant to work with that sensor.
Cobb Tuning, Weapon-R, and Injen are some big name companies who make or have made intakes for our cars. Both will run over $250 new, but can be picked up for cheaper used. The Cobb has been shown to display the best horsepower and torque numbers.
There is also a generic short ram intake that is sold on ebay for like $30. They usually take a little bit of modification to work and most replace the cone-filter with a K&N, but they show pretty good results and offer really good bang-for-your buck. Who would’ve known?
Some people argue that the torque box is needed however. This little contraption apparently hold extra, clean air for immediate demand. Some say it’s better to leave it in place, some say it’s not. I’ve not seen any dynos that prove one or the other correct, so hopefully someone can post some up. If you would like to keep the “torque-box,” look into a Ganzflow style intake.
Group-A also makes a throttle body spacer. These are available for about $100, and even from our very own Reason. A plus to these little things is that aside from the 2-4hp that most dyno runs are showing, most people report fuel economy increases of 4mpg or even better, so it can easily pay for itself.
The intake manifold can also be port-matched and polished to the heads, which will supposedly help with flow. I’ve not seen a lot of numbers here, so I can’t really comment. Also, in 2005, Subaru released an improved intake manifold on the Impreza 2.5i. It flows better, but takes a good deal of work to mate it up to the older motors and wiring harnesses.
Exhaust
Borla, Cobb, Rallitek, MRT, OBX, TWE, and probably even more make headers for our cars. There are two major types: Equal-length (EL) and Unequal-length (UEL).
Borla and OBX both make UEL headers, and are pretty much of the same design. While EUL headers exaggerate the renowned boxer rumble, they aren’t very good for power gains. They will however bolt up in place of your stock exhaust manifold. Borla is also the only one to make a header for the single port heads.
TWE offers a EL header that will bolt up in place of the stock manifold. This will help with the step by step process I spoke of in the intro, but these are extremely expensive for a set of headers.
Cobb, Rallitek, and OBX all offer EL header sets that will replace not only the stock manifold, but the stock cat-pipe as well. Cobb and Rallitek run $500+ depending on options IIRC. Base sets come with resonators rather than cats, which is a concern for those trying to meet emissions. Cobb will option in a cat for around $100. The OBX EL header comes with a resonator as well, but at the cost of around $275, which is much cheaper. For around $80, you can pick up a high flow cat and have a shop weld it in place of the included resonator for just a few bucks. These headers are all of similar design. Patrick Olsen did a dyno test between the MRT header and the OBX header a while back and the OBX showed much better gains…again, for less money. Who would’ve thought?
TWE and Random-Technology are two companies that make aftermarket cat-pipes. They will replace your stock catpipe with an all-stainless unit. “Track pipes” are also offered, which is nothing more than a cat delete pipe. These are usually also made of stainless and will bolt up in place of the stock cat-pipe. A cheaper means would be to buy a high flow cat and have an exhaust shop fabricate a replacement cat-pipe for you.
As far as I am aware, Stromung is the only aftermarket company currently offering a catback for the N/A legacy. These run $500-600 new, and can of course be found cheaper used. SPT made an axleback as well, but these are made with 2” pipe, or maybe even smaller...I can’t remember. These are also stainless units. Another option is to buy a muffler, tip, and a resonator if you wish, and have an exhaust shop fabricate an exhaust for you just like the cat-pipe.
Also, the general consensus is that 2.25” exhaust piping is the best size for a N/A motor, as it will open up the exhaust while maintaining needed back pressure. If you are running Cams, ported and polished heads, and so on, a 2.5” exhaust will usually work out, as you will be moving plenty more air.
Cams
Cobb Tuning and TWE have both offered cams for the SOHC and DOHC EJ25 in the past, and may still currently. Both offer different stages of cams. Sport/Stage 1, Spicy/Stage 2, and Race/Stage 3. The first stage maintains drivability and keeps power low. The second stage will push power higher, but still maintain drivability. Some people complain of CEL’s after installing stage 2 or spicy cams, but this seems to be few and far between, as well as in the later model year ECU’s. Stage 3 or Race cams are not streetable, as they push the power very high and make stop and go driving a relative pain. They also require stronger springs, shims, buckets, and so on...I’m not going into all that.
Delta Cams also offers regrinds of most Subaru cams in Stages 1 and 2, which are comparable to the Cobb/TWE stages. Delta requires that you send them your cams. While they have them, they weld new material onto the cam lobes and regrind them to the new profile. This company is pretty good about their turn around times, usually getting the cams back to you in under a week. This leaves your car out of commission for a few days, but is much cheaper at about $75/cam.
Ported and Polished Heads
This is where you will start spending big money. Porting and polishing your heads will allow more air into the motor. Cobb and TWE both do headwork...Crucial may also still do it. They will require you to remove and disassemble your heads, then ship them to them, so again, you will suffer downtime. The stages here run just like that of the cams. Stage 3/Race will usually leave a car almost unstreetable as well as probably require stand-alone engine management. This costs anywhere from $1-3k.
Pistons
In N/A Subaru Tuning, this is it. Aftermarket pistons, usually made of forged aluminum, are lighter and stronger. They will also give you the ability to bump up your Compression Ratio (CR), which runs around 10:1 stock. A CR of 11.2:1 will usually give you a very streetable upgrade, while 12:1 seems to be about the safe cutoff for 93 octane. The higher compression ratio will require good fuel as well as good engine management and tuning. Aftermarket pistons will also require a buildup of the bottom end, including a forged/balanced crankshaft, bearings, and so on.
Rotational Mass
The Subaru drivetrain itself is parasitic. AWD...it’s what makes a Subaru a Subaru, right? Unfortunately, it takes more power to drive four wheels than it does to drive two wheels. Power is lost through multiple differentials and rotational mass...it’s just the way it is, so deal with it. One way to combat this is by lightening rotational mass in the drivetrain...this runs from the crank to the tire. A lightweight pulley is easy to install with basic tools for most people and will free up a few pounds of rotational mass. A lightened flywheel will free up much more weight and rotational mass, allowing the motor to rev quicker. Using lighter wheels will also help out. While these things do not necessarily add power, they do help the motor put the power it has to the ground.
Just remember that doing a lightened pulley as well as a lightened flywheel can have adverse effects such as rough idle and CEL’s.
Electrical
Some dyno graphs can be found that suggest boosting your electrical grounding in the engine bay can net 2-4hp. A small gain, but a gain nonetheless. Paranoid Fabrications makes a kit that runs around $50, but you can find tutorials for a DIY grounding kit on here, NASIOC, and RS25. Just use the search button.
You can also change out plugs, but for N/A applications, I personally believe that anything more than a copper plug is a waste of money. Plug wires are also available for our motors, but at a hefty price.
I’ve also read about swapping the stock Subaru coil pack for that of a dodge neon. I’m not sure how, why, or anything else, as I haven’t researched it. I’ll do so, and update as I find out.
Headgaskets
Some people have told me in the past that they’ve tried to boost their compression a little more by using thinner headgaskets, be they from the STi or whatever. I’ve heard of people on RS25 doing this and ended up with piston and head damage. If you look, the N/A piston will actually protruded form the block a little at top-dead-center. This can be worsened if the deck has ever had to be planed on either the heads or the block, causing the piston to contact the head. Personally, I’d stick with the stock OEM headgasket to avoid such issues...just in case...
Frankenmotor??
It is October as I write this, but no, it’s not Halloween. Commonly referred to as a Frankenmotor, the use of a Phase 1 EJ25 shortblock with Phase 1 or Phase 2 EJ22 heads will yield a higher compression ratio, supposedly around 11-11.5:1. It’s basically like doing high compression pistons, but much cheaper and without all the work in strengthening the bottom end of the EJ25. Again, you’ll need to run premium fuel and good engine management is highly recommended.
Engine Management
I’m saving the most important for last here. Engine management is basically using whatever means to trick the ECU into giving up the fuel, timing, and everything else that you want it to, or replacing the ECU all together. Using a piggyback is the most common way to tune, especially when dealing with a mildly modified N/A motor. These range from the Apexi SAFC to the Greddy E-Manage to the Perfect Power 6. All are good units when used correctly and tuned by a professional. They do require you to splice into your wiring harness, so be careful. The SAFC will only control fuel, which isn’t a bad thing considering how rich the EJ’s run anyway. The E-manage and PP6 will control fuel, timing, and other things.
You can also go with a stand-alone engine management, which replaces the entire ECU all together. This stuff is really expensive, but allows you pretty much unlimited control over your motor. I’m not exactly full of knowledge here, so I’ll allow someone else to chime in.
There are a few different ways to milk extra horses out of your Subaru EJ Series motor. The basics are Bolt-ons, Aftermarket Turbo Kits, and even Nitrous. You could even swap the drivetrain all together for that of a WRX or STi. Since aftermarket turbo kits can be unreliable in the wrong person’s hands, swaps are expensive, and I know not a thing about nitrous, I’ll be covering the bolt-on/naturally aspirated side of things in this thread. Nothing here is absolutely concrete, but isn’t up for argument here. I’m simply writing this so that I can share a little bit of the knowledge that I’ve gotten through years of Subie ownership and research with all you noobs who keep asking the same question over and over (I kid, I kid. :lol )
Before you start, consider some pros and cons of NA Tuning. The later model year you own, the harder it is going to be to tune. After about 1999, Subaru started making an effort to make their ECU’s, “better.” This isn’t good for tuners, as the ECUs became increasingly harder to tune using piggyback type tuners, as they eventually start to learn around them. Know that you will not be beating any STi’s with a NA motor (in a straight line) unless you drop big money. There are some NA Subies that have broken the 300hp mark (see: Zzyzz Motorsports 2.5RS), but it costs money and takes a lot of work, not to mention might not be very streetable. If you’re using your car for a daily driver and maybe the occasional auto-x, then NA might just be for you, as you might be able to maintain better fuel economy and not have to wait for a turbo to spool up. Another plus, you can do NA tuning in installments, so if you find yourself unable to save $5k+ for a swap...well, you get the picture.
With that, I’m sure everyone here will probably have something to add to this, or challenge something I say here. To them, bring it on. This is basic, so bear with me. If you want 300hp or better out of your current motor, stop reading now and start saving for a swap.
Intake
Any intake that is meant for the earlier model N/A Impreza will pretty much work here. It might take a little bit of work, but it will work. Figure out if your car is run by a MAF or MAP sensor, and buy an intake that is meant to work with that sensor.
Cobb Tuning, Weapon-R, and Injen are some big name companies who make or have made intakes for our cars. Both will run over $250 new, but can be picked up for cheaper used. The Cobb has been shown to display the best horsepower and torque numbers.
There is also a generic short ram intake that is sold on ebay for like $30. They usually take a little bit of modification to work and most replace the cone-filter with a K&N, but they show pretty good results and offer really good bang-for-your buck. Who would’ve known?
Some people argue that the torque box is needed however. This little contraption apparently hold extra, clean air for immediate demand. Some say it’s better to leave it in place, some say it’s not. I’ve not seen any dynos that prove one or the other correct, so hopefully someone can post some up. If you would like to keep the “torque-box,” look into a Ganzflow style intake.
Group-A also makes a throttle body spacer. These are available for about $100, and even from our very own Reason. A plus to these little things is that aside from the 2-4hp that most dyno runs are showing, most people report fuel economy increases of 4mpg or even better, so it can easily pay for itself.
The intake manifold can also be port-matched and polished to the heads, which will supposedly help with flow. I’ve not seen a lot of numbers here, so I can’t really comment. Also, in 2005, Subaru released an improved intake manifold on the Impreza 2.5i. It flows better, but takes a good deal of work to mate it up to the older motors and wiring harnesses.
Exhaust
Borla, Cobb, Rallitek, MRT, OBX, TWE, and probably even more make headers for our cars. There are two major types: Equal-length (EL) and Unequal-length (UEL).
Borla and OBX both make UEL headers, and are pretty much of the same design. While EUL headers exaggerate the renowned boxer rumble, they aren’t very good for power gains. They will however bolt up in place of your stock exhaust manifold. Borla is also the only one to make a header for the single port heads.
TWE offers a EL header that will bolt up in place of the stock manifold. This will help with the step by step process I spoke of in the intro, but these are extremely expensive for a set of headers.
Cobb, Rallitek, and OBX all offer EL header sets that will replace not only the stock manifold, but the stock cat-pipe as well. Cobb and Rallitek run $500+ depending on options IIRC. Base sets come with resonators rather than cats, which is a concern for those trying to meet emissions. Cobb will option in a cat for around $100. The OBX EL header comes with a resonator as well, but at the cost of around $275, which is much cheaper. For around $80, you can pick up a high flow cat and have a shop weld it in place of the included resonator for just a few bucks. These headers are all of similar design. Patrick Olsen did a dyno test between the MRT header and the OBX header a while back and the OBX showed much better gains…again, for less money. Who would’ve thought?
TWE and Random-Technology are two companies that make aftermarket cat-pipes. They will replace your stock catpipe with an all-stainless unit. “Track pipes” are also offered, which is nothing more than a cat delete pipe. These are usually also made of stainless and will bolt up in place of the stock cat-pipe. A cheaper means would be to buy a high flow cat and have an exhaust shop fabricate a replacement cat-pipe for you.
As far as I am aware, Stromung is the only aftermarket company currently offering a catback for the N/A legacy. These run $500-600 new, and can of course be found cheaper used. SPT made an axleback as well, but these are made with 2” pipe, or maybe even smaller...I can’t remember. These are also stainless units. Another option is to buy a muffler, tip, and a resonator if you wish, and have an exhaust shop fabricate an exhaust for you just like the cat-pipe.
Also, the general consensus is that 2.25” exhaust piping is the best size for a N/A motor, as it will open up the exhaust while maintaining needed back pressure. If you are running Cams, ported and polished heads, and so on, a 2.5” exhaust will usually work out, as you will be moving plenty more air.
Cams
Cobb Tuning and TWE have both offered cams for the SOHC and DOHC EJ25 in the past, and may still currently. Both offer different stages of cams. Sport/Stage 1, Spicy/Stage 2, and Race/Stage 3. The first stage maintains drivability and keeps power low. The second stage will push power higher, but still maintain drivability. Some people complain of CEL’s after installing stage 2 or spicy cams, but this seems to be few and far between, as well as in the later model year ECU’s. Stage 3 or Race cams are not streetable, as they push the power very high and make stop and go driving a relative pain. They also require stronger springs, shims, buckets, and so on...I’m not going into all that.
Delta Cams also offers regrinds of most Subaru cams in Stages 1 and 2, which are comparable to the Cobb/TWE stages. Delta requires that you send them your cams. While they have them, they weld new material onto the cam lobes and regrind them to the new profile. This company is pretty good about their turn around times, usually getting the cams back to you in under a week. This leaves your car out of commission for a few days, but is much cheaper at about $75/cam.
Ported and Polished Heads
This is where you will start spending big money. Porting and polishing your heads will allow more air into the motor. Cobb and TWE both do headwork...Crucial may also still do it. They will require you to remove and disassemble your heads, then ship them to them, so again, you will suffer downtime. The stages here run just like that of the cams. Stage 3/Race will usually leave a car almost unstreetable as well as probably require stand-alone engine management. This costs anywhere from $1-3k.
Pistons
In N/A Subaru Tuning, this is it. Aftermarket pistons, usually made of forged aluminum, are lighter and stronger. They will also give you the ability to bump up your Compression Ratio (CR), which runs around 10:1 stock. A CR of 11.2:1 will usually give you a very streetable upgrade, while 12:1 seems to be about the safe cutoff for 93 octane. The higher compression ratio will require good fuel as well as good engine management and tuning. Aftermarket pistons will also require a buildup of the bottom end, including a forged/balanced crankshaft, bearings, and so on.
Rotational Mass
The Subaru drivetrain itself is parasitic. AWD...it’s what makes a Subaru a Subaru, right? Unfortunately, it takes more power to drive four wheels than it does to drive two wheels. Power is lost through multiple differentials and rotational mass...it’s just the way it is, so deal with it. One way to combat this is by lightening rotational mass in the drivetrain...this runs from the crank to the tire. A lightweight pulley is easy to install with basic tools for most people and will free up a few pounds of rotational mass. A lightened flywheel will free up much more weight and rotational mass, allowing the motor to rev quicker. Using lighter wheels will also help out. While these things do not necessarily add power, they do help the motor put the power it has to the ground.
Just remember that doing a lightened pulley as well as a lightened flywheel can have adverse effects such as rough idle and CEL’s.
Electrical
Some dyno graphs can be found that suggest boosting your electrical grounding in the engine bay can net 2-4hp. A small gain, but a gain nonetheless. Paranoid Fabrications makes a kit that runs around $50, but you can find tutorials for a DIY grounding kit on here, NASIOC, and RS25. Just use the search button.
You can also change out plugs, but for N/A applications, I personally believe that anything more than a copper plug is a waste of money. Plug wires are also available for our motors, but at a hefty price.
I’ve also read about swapping the stock Subaru coil pack for that of a dodge neon. I’m not sure how, why, or anything else, as I haven’t researched it. I’ll do so, and update as I find out.
Headgaskets
Some people have told me in the past that they’ve tried to boost their compression a little more by using thinner headgaskets, be they from the STi or whatever. I’ve heard of people on RS25 doing this and ended up with piston and head damage. If you look, the N/A piston will actually protruded form the block a little at top-dead-center. This can be worsened if the deck has ever had to be planed on either the heads or the block, causing the piston to contact the head. Personally, I’d stick with the stock OEM headgasket to avoid such issues...just in case...
Frankenmotor??
It is October as I write this, but no, it’s not Halloween. Commonly referred to as a Frankenmotor, the use of a Phase 1 EJ25 shortblock with Phase 1 or Phase 2 EJ22 heads will yield a higher compression ratio, supposedly around 11-11.5:1. It’s basically like doing high compression pistons, but much cheaper and without all the work in strengthening the bottom end of the EJ25. Again, you’ll need to run premium fuel and good engine management is highly recommended.
Engine Management
I’m saving the most important for last here. Engine management is basically using whatever means to trick the ECU into giving up the fuel, timing, and everything else that you want it to, or replacing the ECU all together. Using a piggyback is the most common way to tune, especially when dealing with a mildly modified N/A motor. These range from the Apexi SAFC to the Greddy E-Manage to the Perfect Power 6. All are good units when used correctly and tuned by a professional. They do require you to splice into your wiring harness, so be careful. The SAFC will only control fuel, which isn’t a bad thing considering how rich the EJ’s run anyway. The E-manage and PP6 will control fuel, timing, and other things.
You can also go with a stand-alone engine management, which replaces the entire ECU all together. This stuff is really expensive, but allows you pretty much unlimited control over your motor. I’m not exactly full of knowledge here, so I’ll allow someone else to chime in.