As a particular car becomes more popular, the accepted wisdom as to the mods that will make it go hard spreads wider and wider. Got a VL Turbo? Then intercool it, exhaust it and boost it. Got a later model WRX? Then upgrade the 'cooler, big turbo it, exhaust it and chip it. But the trouble with these 'package' approaches is that it can be pretty hard to see which of those steps is actually the most significant in the resulting performance gains. Was the big turbo the revolutionary change? Was the chip just used to wind up the boost - something which could have been done pneumatically for a tenth of the cost? And what if you don't have the cash to get the burger with everything?
While we can't answer all of those questions, here's a step in the right direction.
Mick Jones owns a modified '99 Impreza WRX, a car that he bought new. Over the period since purchase he's performed a number of modifications, dyno'ing the car after each change. So, while we can't see how each of the changes would have worked if done individually, we can see the results of the sequence. And, along way, we can also see some of the problems that Mick had - some of which had the potential to very expensive...
Exhaust
The very first step was the zorst, which included a twin dump pipe (ie a main pipe for the turbine gases and a secondary, smaller pipe for the gases that have passed through the internal wastegate), a new cat converter, and a single rear muffler. The pipework was three inch all the way through, with the tip a four-inch jobbie. This system cost about $1300.
So, apart from the much louder exhaust note, what did Mick notice about the performance? The turbo spooled up at part throttle far more easily (and remember, the oft-quoted dyno figures are all done at full throttle, ignoring this important aspect of driveability), while at full power the rollers showed a lift from a standard 110kW at the wheels to a new peak of 118kW - a 7 per cent improvement. However, more important than the peak power gain was that the power curve was improved all the way from about 4000 rpm upwards. And the noise? "It's quite acceptable around town and on trips," says Mick. "It's a sedan so it's not as bad for inside exhaust noise as the wagon," he added.
Intake
The next step was to run a 90mm PVC pipe cold air intake to the standard airbox, the duct replacing the in-guard resonator box. The air filter element was replaced with a 02 Rush element. However, this was the start of some major problems. The hot wire airflow meter died, and was replaced by Subaru under warranty. Then Mick decided to go to a Kaze pod filter - and another airflow meter died. Again, Subaru came to the party but they added a rider - it was their strong advice that he return the intake system to standard. (The reason for the failure is provisionally being attributed to oil being drawn from the filters and deposited within the airflow meter... something to think about.)
Following these unhappy episodes, and having left the intake standard for several months, Mick decided to fit an APS cold air intake system (around $450), which mounts a conical K&N filter within the guard. This was done at the same time as the boost was raised, so Mick can't comment on the power differences that resulted, however he says that all of the non-standard intakes resulted in more induction noise, with the turbo able to be heard spooling up.
Boost
The first approach that Mick took to lifting boost was to use a simple wastegate hose bleed, controlled by a valve usually used in a gas system. This approach gave a peak boost pressure of 1.3 Bar, falling to around 1.1 Bar. The cost of the system was only about $25, but the boost control that resulted was "peaky and changeable". With the exhaust, intake and boost now done, the car went back on the dyno. Power was now at 140kW, up by 27 per cent over standard. Even better, the power curve remained progressive - there was no sudden change in torque at full throttle as the boost suddenly hit, for example.
Turbo
Next, Mick decided to upgrade the turbo. He swapped to a VF22, one of the most common aftermarket fitments. This cost $2000 supplied and fitted, and gave an immediate and noticeable benefit. Onto the dyno and a new peak power of 168kW was realised - that's now a fairly serious gain of 53 per cent over standard! But what about the downside of a larger turbo - the loss in bottom-end response? Well, at full throttle anyway, there wasn't any! At 3500 rpm with the exhaust, intake, boost and small turbo there was 80kW at the wheels, while with the same mods but now with the larger turbo, there was 88kW! And by the time you get to 4500 rpm, the gains are immense - 112kW before and 148kW after - a 32 per cent lift. At the top-end the better airflow of the larger turbo really shows up - at 6500 with the small turbo there was 130kW, compared with the large turbo's 160kW.
It's no wonder that Mick rates the fitting of the larger turbo as the single biggest performance jump. But what about part throttle, especially around town? "It's a little bit laggy - but still quite driveable," he says. "It definitely comes onto boost harder than the standard turbo - it can now easily break traction cornering as boost arrives."
But all was not wonderful after the VF22 was bolted on. The turbo's shaft broke on a one-two gear change - a sudden change from lots of power to no boost, no smoke - and no go. Two thousand dollars later, another VF22 was back in place. Mick blames the lack of sufficient blow-off valve capability for the catastrophe, and added a second AVO blow-off valve to the first that was already in place.
Intercooler
Deciding it was time to upgrade from the underbonnet charge pre-heater (oops, sorry, underbonnet intercooler), Mick went for a PWR front-mount system. Picked up secondhand for $1800 fitted, he could notice little difference in driveability on the road. "It's more for insurance," he says. However, the dyno picked some improvement, with the power curve beefed up a little through the top half of the rev range. Peak power was up a tad at 171kW - but more importantly than what was found on the dyno, real-world measured intake air temps were well down. Previously, in city traffic Mick saw a peak of 70 degrees C, while after the front-mount was in place, that dropped to 50 degrees C. Even on the track, club sprinting on a hot day, the intake temp stays below 50 degrees.
Boost Again
Finally, Mick decided to get rid of the gas valve bleed and use a more professional bleed - a Turbosmart valve. This still allows a spike to 1.2 Bar, but holds 1.1 Bar more consistently than the previous system.
Conclusion
Mick's WRX has done more than 86,000 kilometres, all on the standard clutch and gearbox. He says that the single best move during the modifications was the fitment of the larger VF22 turbo, and reckons that the next step might be to look at having a new ECU (or chip for the standard ECU) put into place. But his most earnestly held piece of advice for would-be WRX modifiers? "Be prepared to lose your warranty!" he says, giving the strong implication that he thinks himself very lucky to have had those multiple airflow meters replaced gratis by Subaru....
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