How not to make your car go faster, stop better, handle sharper, or save fuel.
We've all looked into modifying our cars at some point. As enthusiasts, we all want our car do go faster, corner better, and stop quicker. There's a big aftermarket out there for just bout every brand, but it's unfortunate that unscrupulous sellers are out there happy to take advantage of the young, the naive, and the plain gullible by using dubious claims to sell junk. Some will do very little to aid a cars performance, some will do nothing, and some will even have an adverse effect. These are the performance modifications out there everyone should ignore.
The theory of a short ram air intake system is that you're placing the air intake into the path of moving air, then that air is forced, or rammed if you prefer, into the engine to make a bigger bang and, as a result, more power. Unfortunately, the speed you would actually have to be traveling for that to happen and be noticeable is very, very, very fast indeed.
While a short ram intake can help get a more direct path for the air into the engine, the air being pulled in is still from the hot engine bay where air pressure is lower than outside the car. While the benefits of a proper cold air intake system can include a small horsepower bump, this isn't that and tests consistently show either no gain in power when a short ram air intake system is used on a road car, or a gain so small it's negligible at best. Sometimes, there is even a loss recorded in the mid-power range.
Lowering a car makes a big difference in performance. It lowers the center of gravity and, if done properly, will lessen the amount the chassis rolls through a corner and help make the amount of grip on offer more predictable and consistent. However, every car model is different and has a different balance and weight to it. The best way to go about lowering a car is to find a set of springs and shocks or struts matched together for the car in question designed for the particular make, model, and year of the car.
Simply adding a set of random springs to lower the car will cause all sorts of problems. The first probably being to blow the stock struts, which control the vertical motion of the springs and leave you bouncing down the road as you feel every single ripple in the road.
In the history of scientific quackery, the fuel line magnet rates as one of the all-time greats. The claim is that the magnet helps by aligning fuel molecules as they pass by. However, gasoline is non-polar so there is absolutely no effect a magnet can have. It has been debunked many times over the years, but there are still companies peddling their nonsense.
The other claim is that ultra-powerful magnets allow the "hydrocarbon clusters" to vaporize more easily, and that's also complete nonsense. We don't even need to apply Chemistry 101 logic to that idea because if simply adding some cheap magnets to the fuel system would help get cars higher EPA fuel economy figures, every car would have them by now.
All spark plugs may not be created equal, but all spark plugs do the same job and are controlled by the car's ignition system. A spark plug is binary. It's either on or off - sparking or not sparking. That means if someone is claiming just changing to their spark plugs alone will improve power, they are flat-out lying. The only difference you'll find in performance is in how long they may last depending what the conductive materials are made from. The bottom line is that as long as you get the plugs recommended to operate in the heat range your car needs, then you're good to go. Replacing a set of worn plugs will restore performance, but there's no power gain to be had and the main brands selling spark plugs will tell you exactly that.
This can be done using what's known as a test pipe. A test pipe replaces the catalytic converter for, as the name suggests, testing. It's quite illegal to use on the road, and all it will achieve on a modern car is to turn the check engine light on. Modern catalytic converters aren't as restrictive as they were in the 70s and 80s and sap very little power but, more importantly, most stock engine computers don't expect it to disappear and won't adjust for the change in exhaust flow.
The way to get actual measurable gains in flow here is to buy a high flow catalytic converter along with a larger diameter exhaust and have the ECU tuned properly.
For the average car, research and experience have landed us on the 17-18-inch wheel being the sweet spot for grip, braking performance, steering feel, and ride comfort on a car. At 19-inches and above, the extra weight affects fuel economy, ride quality, and 0-60 mph time negatively. That effect will vary depending on the size of the car, but there's always a point of diminishing returns.
If a car has been designed on 20-inch wheels and tires to match, that's one thing. But, even then you'll see a lot of luxury cars running on big wheels and thin tire walls that hurt ride quality because of the perception rather than the reality.
Drilled rotors can look pretty cool, but the reality is that they have very little benefit for modern cars. The origin of drilled and slotted rotors goes back the 1960s when brake pads were made of asbestos and the adhesive used would break down as the pads heated. That caused a layer of gas to form between the pads and rotor and create resistance to the two surface areas meeting. The answer was to drill through the discs so the gas could escape, and cars could slow down properly again.
These days we don't use asbestos in brake pads and the science of adhering the pad to the plate has improved. You'll hear claims that the holes reduce the rotating mass of the rotor, but it's a big compromise to add a series of stress points to your brakes rather than use smaller discs. Slotted rotors can have an application in racing in the wet, and they also can shave down glaze from overheated pads, but on a road car... it's quite unnecessary.