We all want to make our cars perform better, faster, tougher. That’s why some of us are obsessed with buying car accessories to make our vehicles better. There is nothing wrong with this approach, as long as you keep your priorities straight: first, take care of vital and safety systems, and then you can go for upgrades to improve looks and comfort. 
Cars have many moving parts and they simply require regular maintenance to function properly. Some of the issues are straightforward and there are even some auto parts that you can upgrade yourself, while others require elaborate diagnostics to determine the source of the issue. However, when it comes to performance there are three vital parts that every mechanic recommends to keep in-check. 

Car Gaskets

Mechanic take off old car gasket
Source: cashcarsbuyer.com

Gaskets are one of those car parts beginners don’t think about until something goes wrong. So, if you are one of those novices scratching their head regarding the function of the car gasket, the following information can be a real eye-opener. Gaskets are the seals which provide a barrier needed for certain components of the engine. 
Usually, when someone talks about automotive gaskets, they talk about the ones placed between the cylinder heat and the intake manifold. These parts are responsible for delivering air and fuel to the cylinders, so gasket issues significantly affect car performance. Since plastic is generally cheaper, it’s widely used in manufacturing. But if you are getting an aftermarket car gasket, it’s best to focus your search on those that are made from stainless steel and can offer durability. 
There are several symptoms which point to the need for gaskets replacement. Be mindful that the gaskets are only some of the possible reasons you are experiencing these issues, and other components might be responsible too. The most obvious symptom is coolant leak. It’s hard to miss this with all the resultant steam or the dripping coolant liquid under the hood. Coolant leak happens when the seal fails to fulfill its purpose properly. The coolant enters the engine bay or the combustion chamber. Both scenarios are bad and should be addressed immediately. Failing to do so will lead to overheating of the engine. If the leaks were hard to notice (which can happen), then the bad intake can lead to serious engine damage.
Other, less urgent, but still serious gasket issues can be noted by changes in performance. The symptoms are loss of acceleration, poor power, bad fuel economy and stalling. These types of issues arise when the intake manifold car gaskets wear out. The engine vacuum and pressure is disrupted (primarily the air-fuel ratio) and this creates significant hindrance to optimal engine performance. Staying vigilant and observing symptoms of diagnostic importance is your best defense. As soon as you notice something is off balance, head straight to your mechanic.

Crankshaft

removing  crankshaft
Source: diainsurance.co.uk

This is yet another part that is vital to the optimal functioning of the internal combustion engine. While fairly complex, when stripped to its fundamental role – the crankshaft serves to turn the energy coming from the pistons into rotational energy for the wheels. The pistons in a typical four cylinder combustion engine move vertically. Your crankshaft turns this movement into horizontal rotation. To do so, they are connected with the clutch, gearbox, and driveshafts in an efficient transfer of power. Day to day exposure to this power increases the risks associated with crankshaft maintenance.
One crankshaft issue is vibration in the engine. It results from misfiring cylinders because the position of pistons in the crankshaft is misaligned. Probably the sensor is to blame, though other issues (like faulty spark plug timing) can be also in play. Same goes for intermittent starting issues. Sometimes the car starts, sometimes it doesn’t. This also can be caused by problem with the crankshaft position sensor.
Inconsistent engine performance is also indication you might need to head to the mechanic soon. If your vehicle stalls or the acceleration is uneven and unreliable, then your crankshaft is the issue. Poor fuel efficiency can also be attributed to similar causes, however, as you may well know, a number of other problems may be responsible for it, so first identify the source and then address the issue.

Exhaust

close up picture of car exhaust system
Source: carfromjapan.com

Your typical car owner will not be bothered to double-check the exhaust set up on their vehicle. If you happen to be one of them, beware. Stock parts are made using the crush tube bending process and while it does serve to cut on costs, it also affects performance. It has to do with the constriction of the flow within the tubes and pipes and contributes to inefficiency. The alternative to these stock parts are the aftermarket exhaust systems. A good chunk of them are made using the mandrel bent which eliminates the hindrances in the tubes.
This is true in general terms. The exact benefits you will reap from replacing your factory exhaust with aftermarket system depend on the type of setup you already have. Aftermarket exhaust parts are usually customised and they can have different components suitable for gas (petrol, gasoline) or diesel engines