What does a tuneup on a Subaru Forester entail?

Depends on what you want to tune up?

Assuming a basic engine tune would be simply to replace oil / air filters using correct grade of engine oil.

Surprising but using the wrong grade of oil and a clogged air filter can result in increased fuel consumption.

Changing the set of platinum NGK spark plugs after 25,000 miles will make the engine more responsive and run smoother.

Cost of the above DIY will be about £25 plugs, £25 oil, £15 air filter, £10 oil filter, £20 Fuel filter, £15 petrol injector cleaner (Total £110).

Dealer cost anyhwere from £250 and upwards.

Next level would be ECU re-mapping to get more power but this might reduce fuel economy as will increasing the turbo boost pressure.

The Forester uses a lower boost pressure than the Subaru imprezza so modifying the Turbo or direct replacement might be worthwhile I.e. increased basic BHP from 170 hp to 250 hp+.

However if you are considering this then brake and suspension replacement will be required to keep the car on the road.

You could easily spend upwards of £3,500 including full brake, suspension and turbo upgrades and get a stunning performance 250 bhp Subaru Forester.

It really depends on how much you want to spend and what you intend doing with the car I.e. mild improvement or full blown rally performance.

If you want to go rallying suggest buying an Imprezza Turbo.