How Much Horsepower Does a Car Lose Over Time (Explained)

Updated: Oct 16, 2021

Interestingly, during the first 10,000 miles, you may notice a little increase in power as the engine settles in and polishes all of its surfaces, decreasing friction and oil usage.


Ultimately, the amount of horsepower a car loses over time depends a lot on how often the car is serviced, how it is driven etc.


It also depends on what's been replaced or changed. A new set of spark plugs or fuel injectors would sort of negate the power loss due to the vehicles age and mileage.


Providing the car is driven well, maintained well and serviced regularly, a car may lose around 3-5% power by around 100,000 miles.


At 200,000 miles, around 4-7% power loss. Around 300,000 miles would see around 5-8% power loss.


Anything beyond this is going to be less and less percentage power lost per year unless something detrimental happens to the engine.


Table of Contents:


car horsepower loss over time

Why Do Cars Lose Power as They Age?

There are multiple causes of power loss in cars as they age.


Usually wear and tear over time is the main culprit, though failing to service your car will increase friction, micro-scratches and damage, increasing power loss.


The most common culprits for age-related power loss are as follows.



Piston Rings

The pistons inside of an engine have several rings of metal around them known as piston rings.


Piston rings are metallic split rings that are connected to the outside diameter of a piston in an internal combustion engine.


piston rings

There are many purposes of piston rings, including the following.


  • Creating a tight seal around the combustion chamber to ensure that only the smallest amount of gas escapes into the crank case (piston blow-by), maintaining cylinder compression.

  • Heat transmission from the piston to the cylinder wall.

  • Engine oil consumption is controlled by scraping oil from the cylinder walls and returning it to the sump.

  • Keeping the appropriate amount of oil between the piston and the cylinder wall.


The majority of piston rings are constructed of cast iron or steel.


Over time these piston rings wear down due to friction on the cylinder walls, causing more and more gap between the piston rings and cylinder wall.


This increased gap between the piston rings and cylinder wall increase the amount of piston blow-by, leaking compression and gasses into the crankcase.


Compression and gas leak will reduce slightly performance and efficiency.


You can easily diagnose a compression leak using a compression tester kit.


However, compression loss may also be caused by a cracked cylinder liner or cylinder head, head gasket failure, valves and possibly a couple of other worn out parts.


Worn out piston rings is an expensive fix but usually isn't necessary.


Head Gasket Failure

A failed head gasket is another cause of compression loss.


Compression loss from a head gasket can be tested in the same way as worn piston rings by using a compression testing kit.


Usually a head gasket failure is also noticeable by a white, blue-tinted or grey smoke coming from the exhaust.


Checking your oil cap for a milky white fluid can also be a tell-tale sign of a blown head gasket, depending on which part of the gasket has failed.


It usually manifests itself much more quickly than worn piston rings, if it's bad enough the engine will refuse to start and have no compression.


If it does run still, it will have terrible fuel economy and massively reduced power.


There may also be misfiring, oil leaks and overheating.


Usually a head gasket replacement is an expensive job but less so than piston rings.


head gasket failure diagram

Worn Spark Plugs

Spark plugs are considered a service item and should be replaced every so often.


However, if they are not replaced they may become worn out, clogged up with carbon and fail to spark.


worn spark plugs

Usually spark plugs need replacing between 30,000 and 90,000 miles, the sooner the better.


When they fail to spark the engine will begin to have the following symptoms.


  • Misfiring

  • Rough idle

  • Trouble starting the engine

  • Lower fuel economy

  • Reduced power


Spark plug replacement is usually an inexpensive job and can be done at home in your driveway with the proper tools and knowledge.


If a used car is between 60,000 and 100,000 miles the spark plugs may not have been replaced and will likely begin displaying symptoms of worn spark plugs and reduced power soon.


Sometimes spark plugs may be worn due to other issues such as too rich or too lean of a fuel mixture.


spark plug wear chart
Spark plug wear chart.

Block/Clogged Fuel Filter & Fuel Injectors

A blocked or clogged fuel filter or fuel injector can also be a reason of power loss over time.


It can lead to similar symptoms as worn spark plugs, such as the following.


  • Issues starting the engine

  • Reduced power and acceleration

  • Rough idle, spluttering and misfiring


Similarly to spark plugs, a fuel filter is a service item that needs replacing.


It should be replaced between 20,000 and 150,000 miles, depending on the age of your car.


Newer cars won't need a fuel filter replacement nearly as often as older cars do, this is due to dirt, debris and rust building up faster over time.



clogged fuel filter

A clogged fuel filter will be much darker and dirty than a new fuel filter would be.


Blocked or clogged fuel injectors can also be caused by an un-replaced fuel filter not doing its job.


Fuel injectors which are blocked or clogged will have the same symptoms as a dirty fuel filter.


How to Stop This Power Loss Over Time

Servicing your car regularly, maintaining it well and driving it properly will help reduce this power loss over time.


Servicing Regularly

By servicing your car regularly you will extend its life massively.


Over time your engine oil and coolant will pick up tiny pieces of metal from the engine and its moving parts and debris from outside.


The oil has a certain debris capacity that it can effectively sustain in the oil without damaging the cars internal parts.


Once too much debris has accumulated, the oil can non longer do its job of lubricating the engine.


If the oil can no longer lubricate the engine, it will wear itself down very quickly.


That is why it is essential to replace your cars engine oil every so often, usually every 5,000-15,000 miles.


Usually around 7,500 miles is a good time to change your engine oil.


Changing your engines coolant is also essential, every 30,000 miles is recommended for a coolant change.


Changing your spark plugs, air filter and fuel filters is also essential, check your vehicles service information about changing intervals for these components.


Maintaining Well

Maintaining a car is also going to help extend its life and reduce power loss with age.


Using the proper oil, proper coolant, storing the car right and using high-quality fuel will help massively.


Covering the car and storing it right will help prevent rust buildup, reducing power loss through indirect means.


Using the proper oil and coolant is essential for the proper functioning of the engine.


High-quality fuel will help reduce the occurrence of engine knock and preserve the engine greatly.


Driving Properly

Driving at redline all the time isn't great foe the engine, but driving below 2000rpm also isn't great for the engine.


Occasionally taking the car to 4000-5000rpm a few times will help remove any built up carbon deposits and extend the engines lifespan.


This will also possibly increase power by a few percentage points temporarily.


Petrol vs Diesel Cars

Diesel cars generally last longer than petrol cars.


You regularly see diesel cars on 300,000 or even 400,000 miles.


You don't see many petrol cars with that mileage.


There is a reason for this, petrol cars are generally built to lower strength and durability standards.


Petrol is also a non-lubricating type of fuel.


Diesel engines are usually much stronger and they also don't have spark plugs.


Diesel fuel is also a lubricating type of fuel, meaning it lubricates the engine and its moving parts.


This results in diesel cars usually lasting a lot longer than their petrol counterparts.


Should You Remap an Old Car?

You may be tempted to remap an older car to extract the power from it that it once had.


But should you do that?


Remapping an older car is not necessarily something you shouldn't do.


Just be careful as a remapped car puts much more strain on the engine and drivetrain components than a stock car would.


Providing the car is capable of withstanding the power from a remap and has a good service history and has been driven well, then a remap should be just fine.


Diesel cars are also usually better off when it comes to remapping.


Overall, just be careful and get a good remap from a professional, a dyne tune is always best for both performance and safety.


Conclusion

Overall, a car can lose power over time from a multitude of reasons, including the following.


  • Worn piston rings

  • Head gasket failure

  • Worn spark plugs

  • Clogged fuel filter and injectors


Maintaining, servicing and driving a car well will help reduce any power loss as a car gets older.


On average, 3-5% power loss is expected after about 100,000 miles, an extra 1-4% loss of power every extra 100,000 miles is also expected.


But the power loss over time depends massively on services and usage.

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