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Old 06-09-2006, 02:21 AM   #1
Scooby Specialist
Member#: 62629
Join Date: May 2004
Chapter/Region: MAIC
Location: Richmond,VA
1996 OBS in Green
2.5 N/A w/ full exhaust

Default N/A power white paper, long, please critique.

This is long you may want to print it out and read at your leisure. I will revise as new information is brought to my attention (note: red text is the revision)

August 07, 2006 Update: Rallitek Header + Exhaust Dyno

How to get power out of a N/A Subaru motor.

This information is based on a reasonably extensive search of this site and specifically the N/A forum. Thanks to Matt Monson, Patrick Olsen, SOLUTION, Slack and several others for there pioneering efforts.

Im sure many will chime in with corrections but here goes:

Cold Air Intake:

Cobb’s Intake is the consensus leader with clear HP gains in the 10 hp range.
Cobb sells the intake for $279 and they can be found used in the $100 range.

Injen makes an intake as well, they have models that fit 2.5L 98-05 RS’s
The only dyno I could find was for there WRX intake which gave +12 hp.
Prices are between $250 and $280 new and in the $150 range used

Ebay Intakes are cheap and have been dyno’d to be competitive with the Name Brand Intakes.
They generally require some minor corrective work to fit properly.

There are numerous exhaust and muffler combos out there. The general consensus is that the largest diameter you want to use is 2.25”. The typical power gain is in the 10hp range. The biggest Issue is really how loud a particular combo is.

The best bang for the buck is to find a reputable custom exhaust shop and work with them to build an exhaust to your specs. Quotes I have gotten are in the $300 range. Keep in mind this is a mild steel piped exhaust with a Magnaflow or similar muffler and hi flow cat.

Cobb / TWE and Rallitek along with several other suppliers offer Stainless Steel setups for $500 - $800 dollars new. The benefits are obvious. These setups can however be found used on NASIOC.COM and fairly frequently so some patience will save you some money.

OBX on ebay, Sparktec is the store, sells a Cat Back exhaust for $275 + $75 shipping. IF it is built like their other products then it will likely need some work to get everything to fit. This post illustrates the possible issues.

Headers are the next step in the exhaust / power upgrade ladder. There are two types generally. Unequal length headers, used on most N/A subarus until the ’06 model. These are attributed to giving the Subaru its distinctive “boxer rumble”. They are generally believed to be less useful for generating power.

Equal Length headers apparently improve exhaust pulses and significantly improve power. The loss or reduction of the boxer rumble has kept many Subaru owners away from EL Headers. Typically a good resonator in the midpipe will restore the “rumble”.

Borla sells both a single port and dual port unequal length header. The Borla header is renowned for its enhancement of the “rumble” that makes so many Subaru owners pants tight J . Early versions had some issues with cracked welds but newer generations have apparently solved this problem. The Borla header is available for around $300 new and $150 used.

COBB / Rallitek sell an EL header that appears to be of a similar design. Dyno results indicate an avg gain of 12 - 15 hp but gain results for bolt ons depend heavily on supporting mods, intake, exhaust, cams, etc. Both of these headers replace the stock cats with a new cat section as the routing of the pipes is changed significantly from stock. Cobb gives some dyno information here:
Cobb’s EL Header runs $650 w/o a cat, add $100 for hi flow cat
Rallitek’s EL header costs $700 not sure on the cat status.
Used both can be found in the $400 -$500 range.
Picked up a COBB header w/ hi flow cat and thermal coating for $400 from

Rallitek EL Header Dyno

TWE sells a EL header designed to work with the stock exhaust routing which can be handy. Power gains appear to be on par with the competition. Cost of these headers new is listed at $950 plus $460 for a hiflow cat section if you want it. Used price is commonly around $750.

MRT also sells a EL header on there site. After doing the conversion it appears to be $440 US. I dont believe this includes shipping as you have to put in credit card info before that is calculated.

Patrick Olsen's MRT vs OBX dyno thread, good read.

EM comes in a couple of forms piggyback, standalone and air-fuel controller. Each provides diffirent benefits and different costs.

S-AFC is the most common air/fuel controller I see used. This device is relatively easy to install some splicing may be required and only controls the air / fuel ratio at given RPM levels. As Subaru’s apparently run pig rich this little device can yield gains and provides some level of engine safety by giving the user the tools necessary to avoid running to rich or too lean. Prices new run around $250 and used ones run in the $150 range.

Perfect Power Pro is the most widely mentioned piggyback system for N/A Subaru motors. The PPP allows the user to control a/f ratios, and timing to further optimize your Subaru’s power. The biggest benefit to the PPP IMHO is the ability to adjust the timing and a/f as you add mods to your motor. The PPP comes with software to use a laptop to adjust engine maps to meet new requirements. Also Rallitek will provide you with 2 maps if you provide your cars specs to them when ordering. The PPP costs $440 from Rallitek and this includes 2 maps and tuning software. It is generally believed that the PPP adds about 10 hp to your current setup. Warning: the following thread should be considered before purchasing / installing PPP.

One word of caution the PPP requires that you splice the piggyback into your ECU wiring harness. This may deter some from using it but I believe a competent shop or friend could help if this is the case. Rallitek does offer a PnP harness, this is only available for ’05 / ’06 models according to their site.

Standalone systems such as linx and hydra give the user full control over the engine. These essentially replace the stock ECU. These systems require a fair bit of user knowledge from what I can tell and are quite expensive, $2000 - $4000 depending on capability.

[color=red]For the those of us who like to tinker, there is the megasquirt system. Essentially a stand alone system that you can build from scratch to meet your exact needs. The basic kit starts around $250 and provides full control over a/f ratios, timing advance, IAC control etc. Check out for more information.

I forgot the I-speedusa ECU reflash. This seems to be effective for many ppl. You give I-Speed your ECU and mods and they reflash the chip for additional power. As you add mods you will have to update the reflash. The initial license for N/A apps is $350 with the updates at $125.

New information is out on the COBB Accessport for NA cars. Se the following threads, we'll be watching.

On to bigger projects

Cams provide more aggressive power curves by changing lift and duration of the valvetrain. They generally come in 3 flavors: Street/Stage 1, Spicy/Stage 2, Race/Stage 3. I will really only address the first two and the race versions tend to make the car less than streetable. Changing cams also requires a decent level of comfort with pulling your engine part. Generally it helps to remove the engine and partially disassemble the heads. Power gains are significant, 15 – 18 HP.

Where the gain is realized is determined by the “stage” of the cam. Street cams provide power in the mid and upper range of the redline. Spicy cams provide most of their gains after 4000 rpms and can benefit from raising the redline. This however requires additional upgrades such as stronger springs and EM. There is some debate about this however. I have not seen a definitive answer to this issue.

COBB and TWE both offer cams at both levels. Both require a core return. Your intial outlay will be $900 for the COBB cams, then a $400 refund when you return your old cams to COBB. TWE sells both levels for $540 for SOHC versions.

A common route for cams is to have them reground to specific lift and duration. Deltacams is the most common and trusted supplier of regrinds. Keep in mind this requires you send them your cams (engine no worky) and they send them back modified. The process requires they weld new material on to the existing cam and regrind the material into a new lobe. Delta carries two grinds last I checked that correspond with the “street” and “spicy” monikers. The beauty of the regrind is that it cost about $75 per cam. So a SOHC owner spends $150 for improved performance. An interesting thread on Delta's 220 grind

Labor for cam work can be expensive so search around a fellow NASIOC member in your area may be able to help.


Port and polished heads provide better and greater airflow for the combustion process. This translates into more power. Coupled with a/f control, cams, intake and exhaust mods significant power gains are possible. Generally port and polish assumes a multi-angle valve job.

Again this procedure requires that you remove your engine from the car and take the heads completely off the motor and disassemble the heads completely. Then send the heads to folks like Crucial racing for the work.

This procedure follows the typical 3 stage format. As before Stage 3 is intended for the track and is likely to make the car difficult to use as a daily driver. It also is likely to significantly reduce low end power and require stand alone EM to be worthwhile.

COBB sells stage1 heads for $1375 assuming you send them your heads otherwise there is an $800 core charge.

TWE offers all 3 levels of head work. Stage 1 is $1500 and Stage 2 is $2000. The valve job is extra at $250.

Most other vendors charge between $1000 and $2000 for similar work. HP gains are highly dependent on supporting mods. EM is also very helpful as this work coupled with intake can begin to make Subaru’s running very lean (not good) at higher RPMs.

I have seen used heads with this work already done for sale but generally they are not any cheaper than having the work done to your existing heads. I have also pulled good heads from my local junk yard for $100, so you may be able to avoid having the car out of commission for extended periods.

One final note on this subject, I have spoken with ExtrudeHone about their process and its cost. They claim similar gains and charge $600 to ExtrudeHone a set of 4 cyl Subaru heads. I don’t have any information on power gains, anecdotal or otherwise.


Swapping the intake manifold is another power gainer that is available to the owner willing to put in time to figure out how to do it. The consensus is that the ’04 and ’05 intake manifolds provide better air flow to the cylinders and therefore more power. Pricing on this is all over the board as these are OEM parts and also available from salvage yards.

The biggest hurdle appears to be the retro fitting of the newer intakes to older EJ25 motors. For the ’05 intake look here:

Extrudehone also works their process on intake manifolds. If you search COBB’s site you will see that their initial N/A build project car used an ExtrudeHoned Manifold. Again I don’t have any data on the benefits of this process. The pricing from ExtrudeHone was $300.


This is the big dog of mods. Not necessarily because of power gains, of which there are good ones, but because of the work necessary to do this. The pistons raise the compression of the EJ25 from its decent 10:1 ratio to some where near 11.2:1 for street purposes. Anything beyond this requires high octane race gas to run correctly as well as standalone EM. At the 11.2:1 level the motor will require good quality premium all of the time. It has been brought to my attention that 12:1 compression may be the safe limit for 93 octane use. But a quality EM system is highly recommended, otherwise engine may BOOM!

This mod usually requires that the bottom end be beefed up. This means better main bearings, better rods and forged crankshaft for reliability. The pistons themselves cost around $700 -$800. TWE shows $719 on their site just for the pistons. The additional block work and parts will likely run another $700 - $800 assuming you do the assembly.
Additionally some owners are using the Sti’s EJ257 block as it has the necessary reinforcement already. New Sti Blocks cost about $1700 though used ones can be had for around $1000.

To be continued...
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Last edited by Rally_wgn; 08-07-2006 at 10:34 AM.
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