It depends on a ton of things and it won't be linear in most cases. Every car has different things that wear out at different rates and those things affect horsepower differently as they age. Not everything that wears affects power. A coolant hose that wears out might cause the engine to overheat, but it won't affect power until then. But vacuum hoses can control things like turbos which might reduce power as the car fails to control boost and puts the car in some sort of safety mode to avoid catastrophe.
Some things, like valve lash on solid lifter engines, will degrade over time but can be adjusted back to normal as part of regular maintenance. Poorly maintained brakes can drag and reduce power. Poorly maintained differentials and transmissions can lose power to friction. Poorly adjusted cam/crank sensors can affect timing.
Off the top of my head:
- As rings and cylinder walls wear, it reduces cylinder sealing and
increases blowby, which reduces power.
- As camshafts wear, valves get less lift and/or duration, which
decreases power.
- If your car is burning oil (either from worn rings or leaking valve seals/guides in the head), you can get a carbon crust behind exhaust
valves, which can hurt cylinder sealing, hurting power.
- Leaks in intercooler piping can hurt peformance on forced induction cars.
- A filthy intercooler might not adequately cool the air entering the
engine.
- A clogged catalytic converter might choke off power as well.
- Worn out plug wires or distributor or coils would reduce power as
ignition becomes inconsistent.
edit- The only sure way to spot power loss over time is to dyno the car periodically and see how much power it makes. It's hard to provide solid proof of power loss over time unless it's a large amount because cars make different power depending upon the prevailing conditions. So a particular car might dyno 230whp on one day and then make 235whp on another day. It might mean it's a cooler day or barometric pressure is better or the humidity has changed, or the dyno is calibrated slightly differently from one time period to another. That being said, if your car dynos 100whp one day and then makes 75 hp another day, that's probably a legitimate difference. You can use things like compression tests, leakdown tests, laboratory examination of oil and coolant, etc to diagnose why the car is down on power. If you've never dynoed your car before, it makes it a lot trickier unless your car is a car for which stock dyno figures are commonly available. For example, cars used in competition or commonly modded by hobbyists are likely to be extremely common (a fox body mustang, honda civic, mazda mx-5), while rarely modified or raced commuter cars are likely to be rare. You can still diagnose a loss of power even without dyno numbers, you just won't know for sure how much power you're down from the original figures.
