Close this search box.

Budget boxer build: WRX gains from chump change (Part Two)

30 January 2020



Forget big-dollar, big-power builds. We’ve smashed open the piggy bank to build a WRX for chump change and we’ll show you exactly how we do it

Getting lost in a big-dollar car build that spans years and swallows the entire contents of your wallet, time and time again, is something that is all well and good if you have the budget and patience, but what if you had limited funds and wanted the entire project to be over in a matter of weeks, not months? That was the brief given to JTune Automotive for our project, code-named ‘Budget Boxer Build’, for which we’d keep the total build cost under $10K and extract as much party power as the factory engine would allow while still holding on to its rods. 

The project is based around a 2003 Subaru WRX two-litre turbo with 240,000kms on the clock. The plan? To push the power levels to around the 170kW at the wheels mark — a point that is just about the ceiling for an unopened EJ20 of this vintage. To do this, we are going to upgrade the turbo, exhaust, injectors, and fuel pump, as well as adding a little more boost, and reflashing the factory ECU. Jacky from JTune says: “The idea came about after going overseas for work and tuning. There is a lot of cars out there with a lot of modifications, but zero tuning. Not everyone is educated or has the real experience to understand how important tuning can be. We also wanted to prove a point. Parts are cheap nowadays — it’s often cheaper to buy something that can be bolted on than it is to make it. With experience, you can get amazing results without overspending,  going fancy or overdoing it.”

We kicked off this build last issue, so we suggest you go back and read that first before carrying on here. In part two we will complete the project and detail all the little tricks involved with making power for peanuts. A word of caution: we are going to push this engine to its limits, so, if you choose to follow suit, you do so at your own peril. It is also worth noting that most shops, JTune included, would not take this job on. Why? Second-hand and cheaper bargain-basement parts just aren’t as dependable as new, high-quality items. This is intended as a DIY project, as the time and effort to make things work can result in a lot of extra labour, something that could make your eyes water when a shop is charging upwards of $100 per hour. 

Turbo: Slightly secondhand TD05-20G — paid $200, retails for $495 
Exhaust: Full stainless including front, up, and mid-pipe — $495 
Injectors: Secondhand STi 560cc — $80 
Fuel pump: DW 225 — $40 off Trade Me
Boost tap: $15
Oil: Motul 300V 5W40 oil — $160 
Spark plugs: NGK No.7 heat range plugs: — $45
Intercooler: Secondhand STi Ver. 8 — $250 
DIY parts for intercooler spray — $20
Secondhand reflash box — $50
K&N panel filter — $95
30A fused relay and cable — $50
Jaycar timer circuit kit — $28

Having maxed out the factory turbo, we managed to score a TD05-20G off Trade Me that was basically brand new for only $200. To remove the old turbo, the intercooler has to come off. The front pipe also needs to be disconnected, but be warned — the bolts are notoriously tricky to remove without snapping, as they are often seized due to rust. Thankfully for us, this time around things went smoothly, but if you’re doing the swap as a weekend job and need to get to work on Monday, having replacements on hand is a good idea. Sitting the two turbo units side by side, it’s easy to see how much larger the 20G is, having gone from a 55mm inlet to a 60mm, but the bigger frame does cause fitment issues in the engine bay. This requires a few little clearancing modifications to the block, which is why we suspect the turbo was listed for sale by the previous owner. But it’s nothing a grinder can’t sort out — just don’t message us asking how — you’ll have to put it in place and work out what needs massaging in your bay.

Before installing the front pipe, there are a few key things to point out. The new aftermarket exhaust kit that we’ve sourced came with a bigger-diameter front pipe than the turbo’s outlet. While you can bolt it on and it will function, the step in diameter will cause turbulence and will actually be worse than running the stock pipe, which is size-matched to the turbo. You could bore or machine the turbo housing out, but we chose to retain the OEM front pipe instead, which keeps things simple. We also upgraded the cheap multi-layer gasket with a steel one, as the cheap ones have a tendency to blow out causing leaks! Better to upgrade while you have it all apart, than have to tear everything back down to repair it in the future. Once this was on, we completed the new exhaust installation. We picked up the 3-inch stainless steel cat delete system brand new for $495. The kit comes complete and bolts in as a direct replacement, even matching up to our stock up pipe, so it’s an easy installation. 

With the front pipe secure, it’s time to bolt on the upgraded intercooler. Given the excessive intake temps we saw last issue on the dyno, we hunted down a bigger STi unit at the wreckers for $250. While bigger in size, it still bolts directly on, and all the piping is a direct fit, so it couldn’t be a more simple upgrade. 

Even though we upgraded the intercooler core size, we also wanted to add a water sprayer unit. This is a cheap exercise that uses the stock window washer pump and reservoir, although to keep the car up to WOF standard, the intercooler sprayer is fed off an in-expensive fish tank air line tap that plumbs into the window washer feed line and can easily be turned off come WOF time. The trick part of the system is a DIY timer circuit kit we picked up from Jaycar for $28. Although it requires assembly, the timer allows us to programme the spray to last a desired length of time, meaning you can keep two hands on the wheel while hard up it. Set to a five second spray, we calculated we can get 18 hits from it before needing to refill. 


The plumbing is a simple affair, with the small tap plumbed into the sprayer feed line, then run to the pair of wiper washer nozzles we mounted in the bottom of the bonnet scoop. These allow the water to atomise with the incoming air while at speed. It’s worth noting that the STi Spec C features a 15-litre boot-mounted tank, so you could be fancy and set up a separate system if you wanted to retain the window washer. For a complete investment of under $30, the water sprayer is cheap horsepower — take a look at last month’s instalment to see what happened when we used a handheld spray bottle to cool the intercooler core. 

To ensure we won’t run out of fuel while on our high boost setting, we upgraded the factory 420cc injectors to 565cc units from an Ej20 STi, which cost us $80. These are another direct bolt-on upgrade. We also upgraded the fuel pump to a DW 200. To do this you need to remove the mounting plate from the top of the tank, accessed under the rear seat. While bolting in the new pump, we also replaced the fuel filter, as it was filthy — cheap insurance to ensure no blockages or unstable fuel pressure.    

 Another little trick we carried out was to upgrade the power feed wire with a direct feed from the battery. When you upgrade to a bigger pump, it will have a larger draw on the electrical circuit, which the manufacturer didn’t factor into the loom. This means you may suffer issues with voltage stability. To do this we purchased a four-pin 30 amp fused relay and a coil of 5mm 25 amp wire. We ran a live wire directly from the battery to the terminal marked 30 on the relay. Then a 5 amp wire from the terminal marked 87 to the pump. The 85 terminal needs to be earthed with 5mm wire, and the remaining 86 terminal is where the existing fuel pump power connects. This wire now acts as a switch. Stock voltage at the pump was around 12.5 – 12.6 volts, but with our new direct feed, we are getting a constant 13.5 volts. As an added bonus you can simply remove the fuse from the relay as a cheap and very effective kill switch. 

As we plan to crank up the boost until we reach the point of diminishing returns, we didn’t want to risk a misfire that could easily hurt or kill the engine. The motor was running Iridium 7-degree heat range plugs during our first dyno session, and we actually downgraded to a set of budget plugs, selecting a colder 8-degree heat range plug to handle more heat. But before installing them, we have re-gapped the plugs, closing up the gap so we won’t get any misfire under high load. 

Once all the parts were fitted, we were just about ready to strap the WRX back onto the dyno and cross our fingers, hoping we wouldn’t meet Mr Conrod. But first, we had to hack into the stock ECU through the OBD port. The first job was to adjust the injector scale to match the bigger injectors. We also adjusted the ignition map to suit the laggier turbo spool from the TD0520G, and set the boost cut at 20psi. Normally peak boost is at peak torque, but because this is a stock motor, we have pulled the ignition timing back so when it gets to full boost we don’t let it ramp up or accelerate too quickly which is how you snap rods. At the end of the day we still need to play it safe — these are all cheap parts. 

On low boost we netted 130kW at 12psi, and it was obvious that we now had lag, with our  TD05-20G hitting full boost at 4600rpm, not the 3500rpm we saw with the old turbo. 

We started cranking the boost up until the motor stopped making power. This happened at 18psi, so we tried dialing in more timing, but still it made no more power. The final results, an impressive 180kW at 6000rpm. For a very small cash investment of under $3000, we have gained over 60kW at the wheels, which is on-par with a GRB WRX STi. The only downside being a small loss in low end pick up, but if you were on the track you’d never notice. While on the street this can actually save some money on fuel as you’re not hitting boost all the time as you did with the factory huffer… Not that we are doing that very often — this thing was way too fun not to mash the pedal!