Technical Documents

WRX And STi 2.5L Conversion Tips


Have a pre MY05 WRX or STi and thinking that you’d like the extra torque and bottom end of the 2.5L engine, but are still really enjoying your car and do’t want to trade up? You’re in good company, and like the many others who feel the same way, we bet you have or are considering converting your engine for 2L to 2.5L.

At MRT, we have done a lot of 2.5L conversions for the WRX and STi, and get asked regularly for advice and tips on what is best practice when doing the conversion.

We have compiled a list of things we recommend that anyone thinking of performing a 2.5L conversion should consider, and believe it to be invaluable knowledge to have BEFORE you start.


Empty engine bay

There are two (significant) differences between the 2.0L and the 2.5L heads. Based on what we’ve seen and experienced here the 2.5L head has smaller ports more designed for emissions than power, the second being the chamber size (and shape) between the 2 and 2.5L variants (The 2.5L chamber being larger to suit the larger bore / pistons). The benefits of the free’er flowing 2.0L heads would be marginal but worthwhile over the 2.5L heads. If funds do permit we recommend that the 2.0L heads be modified to have the chamber machined to replicate that of the 2.5L heads. This way you have the best of both worlds. Any good machine shop should be able to do this, even if you have to lend them a 2.5L head to replicate the machining.

Always have the cylinders serviced before refitting including replacement of the valve stem seals and triple check with your machine shop that they test each valve vs. valve seat for the best seal possible as this is easily overlooked and can be the difference between a good new motor and a great one.


The head gasket thickness is’t something that we can specify for all conversion examples – the best gasket thickness is the right one!

When your new engine goes back together, your builder should be measuring (or cc’ing) the head chambers and the pistons. With this they will be able to calculate the gasket thickness required to achieve the correct compression ratio. Generally speaking we’ve found that most 2.5’s with these types of mods should be shooting for around 8.2-8.5:1 compression ratios, but again this is also best discussed with your builder and your tuner also as they will both need some input here.

Another point to take into account when using non genuine head gaskets, is the cam timing. As the stacked height of the block, heads and gaskets are ‘in theory’ now different, the cam timing will no longer be spot on. Make sure whoever is building your engine is experienced in degree’ing cams – as again this is another difference between a good motor and a great one.


Getting the 2.5L engine in

We recommend that you take the opportunity while the engine is out to replace the factory rods and pistons with heavy duty items. Most people use the EJ257 block for the 2.5L conversion, and the factory pistons are prone to ring land failure in modified state and the connecting rods are’t consistently reliable over 17 psi either. Again if budget allows, get yourself a piston that replicates the factory piston design that matches the heads you use. That is to say, some 2L heads are used on engines with a  square cut out on the piston whereas the 2.5L heads are a domed design with cut outs for the valves. These designs are there for a reason and specifically to match combustion chamber design. If budget dictates that you must use the factory pistons, no worries. Just remember not to push the car beyond 17 psi!


Get your crank checked by your machine shop before re using. Have your engine builder at least check it for wear and again if budget allows, a crack test will ensure you wo’t have problems with it –  Also leave it up to them to pick your main cap bearings as it’s often not as simple as buying a ‘set’ of a single size. Do’t re use the factory head bolts – these are designed as a stretch bolt for a single use and are not re useable. The best bet would be a set of ARP 12mm head bolts – these are a direct fitment into the block without mods and are reliable up to around 22 psi. If you and you tuner have ideas of pushing past this you will need to look at a 14mm stud conversion, but this is a complete other story as heads, turbo and cams would need to be re-evaluated.


EJ257 Engine Block

There is no correct or perfect turbo for a 2.5L conversion, as it all depends on what you want from the car and how you drive as to which is the better turbo for you. The best compromise better low down power and top end are the newer twin-scroll turbos found in the latest WRX, STi and Liberty models, but to use a twin-scroll turbo you will need to replace you engine oil sump, exhaust manifold and turbo up pipe.

Most conversions stick with the single-scroll turbos that fit in the factory position. For best driveability and where the engine is limited to 17 psi, the VF34 is still a good and popular choice. More increased mid-range power, the APS SR40 turbo is very popular and for good mid and top range power up 400 kw at the engine, the APS SR56 is a good choice.

For those who want more, rotated Garrett monsters are the way to go – best talk to you engine builder about this as the list of supporting mods will be significant (including some serious drivetrain enhancements).


You should be ensuring you replace the oil pump and the head exchanger also if your existing engine failed. MRT do this as the debris from the failure could be any where in the pump or the cooler / heat exchanger and you can never guarantee to get it all out – risking your new engine.

Virtually everything else is interchangeable in the conversion such as manifolds, ancillaries etc so you should’t have any issues with those. Your timing belt is a personal one – We generally recommend replacing them simply so you know you can officially ‘reset’ your service schedule and not have to worry about it for another 4 years or 100,000kms.


Upgraded 5 speed gearbox

The Subaru 5 speed gearbox found in the WRX and STis pre the MY02 is considered fragile, but is it really that bad and will it be the first thing to break after a 2.5L conversion?

Our experience is that the 5 speed gearbox in the MY99 WRX and STi models and later are fine with modest power increases, indeed pretty much the same box is used today in the WRX and Forester. As long as the car is treated sympathetically (i.e. no repeated launches at 7000rpm) with no hard shifting between gears (i.e. shifiting too fast for the synchros), then the gearbox should last with no issue for conversions where the engine power levels do’t exceed 250-300kw. A golden rule is also to ensure that the clutch has a bit of slippage so it can take some of the strain – button clutches are not recommended!

For serious conversions, where you know you intend to drive the car with little to no mechanically sympathy, or just for insurance then we recommend either upgraded PPG gearsets or a 6 speed conversion.


We are more than happy to assist. Contact us on 02 9767 4545.

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