Everyone who cares about, and monitors the performance of their engine, knows that as you put more wear on the engine – whether you measure the engine’s use in hours, miles, or passes – that engine’s performance degrades over time. Once again, our friend Jason Fenske at Engineering Explained is able to take a complex subject and break it down Barney-style so that it is easy to understand as he explains ten reasons why the engine loses power over time.

As Fenske breaks it down, there are four categories of areas in which the engine can lose power. The traditional “fuel”, “air”, and “spark” comprise the first three categories, with the addition of “compression” as the fourth category. While some seem extremely basic, some of these reasons might completely slip your mind.

AIR

Within the first category of “air”, there are four components which can contribute to a loss in performance. The first seems obvious–the air filter. A dirty air filter will reduce flow, which increases pumping losses in the engine as it struggles to get the volume of air needed. At the extreme, it can actually limit the absolute airflow and force the engine to act as if it were at part throttle.

A dirty air filter seems like a basic cause of engine power loss. It is so simple, that it can be overlooked, especially with an aftermarket filter.

Next is one which you might not consider as part of your regular maintenance, and that is the throttle cable itself. If it has stretched or become loose, not only will you lose throttle response as the first part of pedal travel will be dedicated to taking up the slack, but it can also prevent you from opening your throttle all the way even with your foot matted to the floor.

Next on the list is the catalytic converter. While big power can be made through cats with today’s advancements in materials and design, they can still foul with excessively-rich air/fuel mixtures flowing through them. With the extremely small passages in the substrate, even the smallest amount of exhaust particulate can build up over time and can clog them, causing a reduction in flow and an increase in pumping losses from the increased backpressure.

The last on Fenske’s list in the “air” category is something gearheads often think about, but not in the same way–your exhaust. While performance mufflers are probably less susceptible to the points in his video, they are still wear items and should be inspected for leaks and blockages, since either of those can reduce flow and performance.

FUEL

The first subject requiring attention in the fuel system is the fuel injectors. After being excessively heat soaked or subjected to bad gasoline they can become fouled internally. That can also happen over time through general use, which is why there are not only a number of injector cleaning products on the market, but also injector cleaning services offered by higher-end injector manufacturers. Any reduction in an injector’s performance will be readily apparent in the engine’s performance.

Next on the list is the fuel pump itself. As a mechanical part, it will wear over time. As the internal components wear, and the internal clearances increase, the efficiency of the pump will be decreased. While these are not something that the average person considers a wear item, pump rebuild kits exist for a reason and should be part of your maintenance schedule with a high-performance engine. You can also replace the pump outright as many in-tank pumps are reasonable in price unless you’re working with a big-dollar race pump.

Fuel injectors and pumps can wear over time, even if not abused. If you are running close to your fuel system’s maximum output, you’ll notice that reduction in performance a lot sooner.

No included in Fenske’s list, but something we feel is worth mentioning, are the fuel filters themselves. Like an air filter, the fuel filter can become clogged with sediment and buildup, which will reduce its performance over time. Whether high performance reusable units, or inexpensive paper-and-plastic pieces, they should be regularly maintained in a high-performance application.

With fuel being the lifeblood of the engine a reduction in performance of any of these components can–at the mild end of the spectrum–simply cause reduced engine performance. In the worst case scenario, a fuel system failure can cause complete engine failure. Since we’ve seen a lot of high-performance combinations running at or near the limit of at least one of the components in the fuel system, there is really no margin built in to meet the engine’s demands with any reduction of fuel system performance. Take this into account with regard to your maintenance schedule.

COMPRESSION

The third category is probably the most time-consuming (and expensive) area to maintain and refresh, and that is compression. The biggest culprit in a case of lost compression is worn piston rings. The constant friction of the piston ring against the cylinder wall, even in a well-lubricated, well maintained engine, will cause the ring to eventually wear to the point of not sealing properly. An improper seal allows power-robbing blow-by, effectively lowering the compression ratio of your engine.

Buildup on the valves can cost you horsepower in two ways: It can cause sealing issues between the valve and seat, which leaks combustion pressure, and it can disrupt intake flow when the valve is open.

Another component crucial in the compression category is the valve and how it seals in the valve seat. Poorly seated valves will leak compression into the runners. In addition to causing sealing problems, carbon deposits can build up on the back of the intake valves. Heavy deposits can disrupt air and fuel flow and will rob power.

SPARK

The final category of “spark” seems relatively obvious, since the most basic tune-up consists of spark plugs and plug wires. Spark plugs can lose their efficiency by incorrect air-fuel mixture causing fouling of the electrode, or by simple wear through use. When not at their peak, they may not provide complete ignition of the air fuel charge, leaving power on the table and unburnt fuel in the engine. As has been noted throughout the article, unburnt fuel can cause a reduction in performance in number of the previously-addressed systems.

Another one that seems simple. Making sure your spark plugs are in good condition is essential to maximizing your combustion event. An incomplete burn not only robs you of power, but can lead to other issues downstream.

All told, most of the power-robbing issues affecting engines can be avoided or remedied with regular attention and care. Most of the listed components are inexpensive enough be replaced before failure, as part of a regular maintenance schedule. Those which aren’t easily replaced or serviced, like the piston rings and the back of the valves, are easily addressed through regular fluid changes and chemical treatments as well as informed parts selection (recall the EngineLabs article on the different types of valve retainer grooves), and regular engine refreshing in race applications.

While this may seem like basic knowledge, it never hurts for a quick reminder, since it seems a shame to allow something easily-remedied to reduce your engine’s performance… or even worse.