Modding and performance tuning parts for the Subaru EJ25 engine!

Tuning guide "EJ25 Modifications"

EJ25 Modifications Guide

How to modify and upgrade the Subaru EJ25

"Comprehensive guide to performance tuning the Subaru EJ25 engine!"

Forums like the one at TorqueCars are always getting posts requesting info on how to improve the EJ25 from people wanting to know what are the best tips on EJ25 upgrades. So let us look into Subaru EJ25 tuning mods and outline the best modifications on this great engine and point out some potential pitfalls along the way.

The Subaru EJ25 are popular engines and with carefully chosen sports upgrades like ECU maps, turbo kits and camshafts you will positively increase your driving enjoyment.

Car Tuning Tips will review EJ25 tuning and provide tips on the optimum modifications for your car.

Just because particular mods are are common on EJ25 it doesn't mean it is good, instead we will focus on the best mods that will give your EJ25 the best value for money to power increase.

Without us subscribing to the popular mentality of 'if it's shiny and makes more noise it must be good' you seem to get in many tuning magazines and on car forums'.

EJ25 induction kits

Improving air intake on the EJ25

You need fuel and air for an engine to work. If there is little supply of fuel then it will run lean, if there is not enough air it runs rich losing power which risks damage to the EJ25. We'll pick up on fuelling later in this article but for now we shall look at air supply.

So Pulling air and fuel into your EJ25 is the aim to any engine upgrade task.

You'll commonly see there's a restriction in the air flow sensor MAF/MAP on the EJ25 when considerably more air is being drawn into the engine.

You'll see that 4 bar air sensors coping with quite large power gains, whereas the OEM air sensor was restricting bhp and torque at a much lower level.

Engines intake headers flow the on the suck phase from the filter and allow it to be fed into the engine cylinders with fuel for the squish phase.

Design and flow rate of the Headers can make a large improvement to fuel mixing and power on the EJ25.

Most intake are ripe for aftermarket tuning parts, although a few car makers provide decently flowing intake.

Larger EJ25 valves, doing some 3 or 5 angle valve jobs and porting and head flowing will also raise bhp, & importantly will make space for increasing the bhp increase on other modifications.

EJ25 performance cam tips.

Each engine responds better to mild cam durations check your engine on a rolling road.

The map and fuelling also have an effect on the power gains you'll get.

Extending exhaust or intake durations can alter the power band and on most engines the exhaust and intake durations do not need to match, although most cams and tuners use matched pairs there are some advantages to extending the intake or exhaust durations.

Significant air intake, and exhaust improvements on the EJ25 can be made from cam upgrades. Changing the cam profile alters the intake and exhaust durations on the engine and can dramatically change the engines power and power output.

NB: Fast road camshafts normally bump the torque through the rev band, you may lose a little bottom end power but your higher rpm power will be lifted.

Motorsport and race camshafts, bump the higher rpm power band but as a result the car will not idle smoothly and low end power nearly always suffers.

A Motorsport cam will just annoy you whilst driving in heavy traffic.

You should ideally optimize your engines power to your cars usage so for a road car stick with a mild fast road EJ25 cam

Stage 1, 2 and 3 mods for the EJ25

Stage 1 EJ25 parts:

Fast road camshaft, Remaps/piggy back ECU, Panel air filters, Sports exhaust manifold, Intake headers, Drilled & smoothed airbox.

Stage 2 EJ25 parts:

Fast road cam, fuel pump upgrades, Sports catalyst & performance exhaust, Ported and polished head, induction kit, high flow fuel injectors.

Stage 3 EJ25 parts:

Crank and Piston upgrades to alter compression, Competition cam, Internal engine upgrades (head flowing porting/bigger valves), Engine balancing & blueprinting, Adding or Upgrading forced induction (turbo/supercharger), Twin charging conversions.

The EJ25 power plant are fantastic to work on and we're finding that there is an increase of mods and performance parts around.

A remap should help to unlock the full potential of all the parts you've done to your EJ25.

Tuning tips and articles Engine tuning Transmission tuning Care care Intake & exhaust mods Improve handling Forums

You will typically expect to see gains of approximately 20-30% more power on turbocharged vehicles and you can expect to see around 15% on NASP engines, but power output will depend much on the parts you've carried out and the condition of your engine.

Turbo modifications on the EJ25

To get a turbo working on a NASP engine there is a lot involved, from lowering compression ratio to improved fuel supply and mapping, so we have found a great guide to helpavoid the common pitfalls or end up with an unfinished project because you were not told what you were getting into by doing this on the EJ25.

The more air you can get into an engine, the more fuel it can burn and uprating the induction with a turbocharger upgrade makes major power gains.

When your car is turbo charged tuning parts are simpler to install and you will discover turbocharged engines already contain strengthened components.

There are reliable limits for every engine, with some being over specified and some only just able to handle stock power

We recommend you find these limits and fit better quality components to handle the power.

It's not unheard of people spending a loads of money on turbo upgrades on the EJ25 only to have the car go up in smoke on it's first outing after it's completed.

Large upgraded turbos tend to experience a bottom end lag, and smaller turbos spool up more quickly but don't have the peak end engines power gains.

Thankfully the choice of turbos is always increasing and we now see variable vane turbos, allowing the vane profile is altered according to speed to lower lag and increase top end bhp and torque.

Twin scroll turbos divert the exhaust gases into 2 channels and push these at differently profiled vanes in the turbocharger. They also boost the scavenging effect of the engine.

Beefing up the air intake by adding a supercharger or additional turbo will help you achieve very large torque gains, although more complex to setup. We have this article covering twinchargers if you want to read more.

Tips about fuel supply on the EJ25

When you boost the power you will need to look at to the fuel delivery.

More power needs more fuel. At this point we flag up the need to over specify your injector capacity.

As a rule of thumb add another 20% when specifying an injector, this accounts for injector deterioration and affords some spare capacity should the engine require more fuel.

Don't forget that different fuel grades will usually require different settings, a high octane fuel will burn more efficiently than lower octane fuel for example. A car will trim fuel supply to keep within the ideal air fuel mix but at the extreme ends you may need to adjust your injector capacity, or the mapping or you'll get flat spots and potential lean or rich fuel issues..

Recommended EJ25 performance exhaust upgrades

We'd only recommend you look to replace your exhaust system with an upgraded one is if your exhaust is creating a restriction in flow.

On most factory exhausts you'll find the exhaust flow rate is fine even on modest power gains, but on a serious tuning project with a big boost in power you will certainly need to get a better flowing exhaust.

Please dont run with the widest exhaust you can get you'll slow up the exhaust flow rate - the best for power gains are usually between one and a half to two and half inches. Your exhaust design, the bend angles and construction are what have more effect on flow rates than your pipe width to a certain extent. So the exhaust design is a big factor.

Typically exhaust restrictions are traced to the emissions filters installed, so adding a faster flowing race alternative will help avoid this restriction.

Catalyst removal (NB:catalysts reduce harmful engine emissions) is illegal in most countries and regions.

Faults issues and things to look out for on the EJ25

Like most engines an EJ25, should have few issues if it gets properly serviced and looked after.

We cannot stress enough the need for oil changes with the correct grade of oil on the EJ25, failure to do this will exacerbate engine wear, on a tuned EJ25 this is more important than ever and close attention should be paid to your maintenance schedule.

If you are interested in learning more or getting impartial tuning advice for your EJ25 why not pop over to the forums at TorqueCars where you can chat about EJ25 tuning options and read about others projects who may be planning to do similar modifications on their EJ25.

We need your help to complete and improve this page, so do give us your feedback in the comments box below.

We want to know your experiences with modifications on your EJ25, it helps us improve our tips and keeps us current so please use the comments box to drop a tip.

If you liked this page please share it with your friends, drop a link to it in your favourite forum or use the bookmarking options to save it to your social media profile.

Feedback

Please use our forums if you wish to ask a tuning question, and please note we do not sell parts or services, we are just an online magazine.

Help us improve, leave a suggestion or tip

You must be logged in to post a comment.