From 2000 to 2005, Subaru carried on using the same basic engine that had powered the first generation of Impreza Turbo 2000 for the WRX. It's a 1,994cc four-cylinder horizontally opposed 'Boxer' design with aluminium head and block. It has a 92x75mm bore and stroke and compression ratio of 8.0:1. A turbocharger is fed by an air-cooled intercooler, while multi-point fuel injection takes care of the rest of the induction side. There are four valves per cylinder and double overhead camshafts.
The 2.5-litre engine that replaced the venerable 2.0-litre in late 2005 is a 2,457cc unit of the same construction and layout. The main differences are the capacity and an active valve control system. Many prefer the lag and turbo rush of the 2.0-litre to the more linear power delivery of the 2.5.
Both engines will run on 95 RON fuel but prefer 98 RON. Economy will come in at around 30mpg on a run and low 20s when using the performance.
Another key difference to look out for is the five- and six-speed manual gearboxes. The five-speed is perfectly well suited to the standard engine, but the six-speeder was made stronger to cope with the added power of the SIi models it was fitted to and is consequently better able to cope with tuned engines.
Both the 2.0-litre and 2.5-litre engines are generally very reliable, though the 2.5 has earned a reputation for being more fragile. However, several specialists have told us they reckon this is unfair and has more to do with owners thrashing the car. It pays dividends to see how a seller treats the car from cold, whether they let it warm through fully before using full revs and let it cool off with a gentle drive or a minute of idling before switching off.
The turbocharger is oil-cooled, so needs to be warmed up and cooled down properly, and it also requires service intervals to be adhered to for a long life. Service intervals are every 10,000 miles in the WRX, with a minor service costing around £200 and a major one up to £500.
Checking under the bonnet is essential. It should be free of oil leaks and the intercooler's aluminium fins should be straight and unbent. Any car with bent intercooler fins may suffer from compromised performance and point to a careless owner. While under the bonnet, check all of the fluid levels. An oil leak is probably the cam cover gasket and will be obvious by a smell of burning oil as it drops fluid on to the exhaust.
With the engine running and bonnet open, look and listen for any signs of the engine vibrating on its mounts. Subaru switched from metal to plastic engine mounts for the 2006 model year and this type deteriorates to let the engine vibrate. Most will have been changed to metal mounts under warranty.
Check the history for evidence the car has been serviced properly and all four spark plugs have been replaced at the required 10,000-mile intervals. Access to the spark plugs is tricky and some owners will skimp on this to save money.
Clutch judder is a relatively common affliction for the WRX and some owners reckon it can be sorted with a full-on standing start to clean up the clutch plates, but others have reported needing a completely new clutch assembly to fix the problem. A new clutch assembly fitted will cost around £800.
With three differentials, it's important to make sure they are all functioning as they should. Listen for any whines and feel for clunks or shunts in the drivetrain as you pull away, change gear and lift off the throttle. The front differential is open on the WRX and a limited-slip version on the STI, while the rear is a limited-slip differential for all models. In the middle is a viscous coupling that helps give the Impreza is superb all-weather grip. The rear differential is easily seen without jacking up the car and should be free from any leaks or drips.
Later STI models have Subaru's DCCD (Driver Control Centre Differential). This is operated by a switch between the front seats and can send as much as 65 per cent of the engine's power to the rear wheels.
Many Impreza WRXs have been modified to some extent, with freer flowing air filters and exhaust the most common changes. If a car has been uprated, ask to see receipts for the work. Better still, seek out an STI model for more power and pace as standard, while the Prodrive Performance Pack upgraded power for the WRX to 265hp and the STI to 305hp for cars up to 2005 and 270hp and 320hp respectively from 2006-on.
"A lot of these have been modified and/or thrashed, so as always look for a history that shows the car has been looked after and any mods have been carried out by someone who knows what they're doing - there are plenty of them around."