The Automotive Aftermarket Suppliers' Association in their 2011 annual status report estimated that in 2009 - in a down economy - aftermarket auto parts were a $182 billion industry in the US alone[2016].  In such a large and lucrative market there are bound to be charlatans who stand to benefit financially from pushing misinformation that moves products.  Couple that with good old-fashioned speculation, rumor, and ignorance, and  you arrive at a rich set of myths about ways to improve your car's performance.

These can be dangerous myths.  Dangerous to your wallet if you waste money, and more so if your vehicle gets damaged as a result.  There's even potential risk to your personal safety - what if your Fuelotronic Torquinator 2000tm  - a slab of plastic reputed to double your horsepower - breaks loose while you're merging onto a freeway?

In this article I'll address some of the most pervasive automotive performance myths and attempt to shed some light on them.

Myth: Bigger exhaust pipes equal more power

Reality: It's not so simple!

Exhaust pipe overkillCredit: Beaterz.comSome people seem to be under the mistaken impression that they can achieve power gains by replacing a stock tailpipe with a bigger one, or swapping in a free-flowing (i.e. loud) muffler, or, ridiculously, by bolting on a noisy and silly-looking resonator tip.  This "loud means fast" mentality is especially pervasive among teen import enhusiasts, though it's by no means exclusive to that crowd or representative of all of them. 

It's true that modifcations to a car's exhaust system can increase its performance by allowing gases to vent more efficiently from the motor.  But this isn't always the result, and all too often the fact that it's an exhaust system gets lost when you're browsing through the catalog of bolt-on upgrades.

There's at least one "cat back" exhaust kit available for just about every model car around - you can even put one on a Prius!  A cat back system essentially replaces everything along the exhaust path after the catalytic converters; it's the most popular type of exhaust modification because it's relatively easy to install and because emissions laws in most states prohibit other exhaust mods on street-going cars (though that's often not much of a deterrent). 

When considering one of these kits as a performance upgrade, there's one primary question to be asked:  Is the factory cat-back restrictive?  If the answer is yes, then there's likely power to be made (with some noise level and price trade-offs along the way).  But automakers have gotten extremely good at squeezing the maximum efficiency out of their exhaust systems without compromising noise or emissions requirements.  So the newer your car, the less likely you are to see big gains from a cat-back upgrade.  That won't stop dozens of companies from trying to sell you a system, though - but the more honest ones will tell you if their kit is more about looks and sound than about power gains, or that benefits are likely to only show up in the presence of other supporting modifications.

The best way to tell if an exhaust upgrade will benefit your particular vehicle is to locate before and after dynomometer results for the kit you're considering on an otherwise stock version of your car.  Sometimes manufacturers will provide these, but even better are test results published by an independant tuning shop.  These results are usually presented in the form of a graph like the one below.

Dyno resultsCredit:

This graph shows the before and after horsepower and torque compared across the RPM range.  For a street car, the ideal result is to see  the "after" lines (shown here in red) consistently above the "before".  Running exhaust pipes that are too big can actually result in a loss of power at lower RPMs - not a good thing if you ever have to pull into traffic from a parkling lot!

Myth: High octane fuel is better

Reality: Shut up and do what you're told!

Fuel octanesThis is less of an issue when fuel prices are high, but there are some people who will unnecessarily spend extra money on high octane fuel in the belief that it's better for performance or mileage or engine life.  The best advice on this matter is simple and straightforward:  use the grade of gas the manufacturer recommends. 

If your car requires high octane gas, use it.  This is especially important if it's a supercharged or turbocharged car - improper fueling under these conditions can cause serious damage to your motor!  Note that some turbo cars, especially newer models, don't require the expensive stuff, and a lot of recent vehicles are smart enough to adjust to whichever grade of fuel you're running.

On a vehicle that's not calibrated for it, running too high an octane can have another down side.  Unburnt fuel can travel through your exhaust system, increasing wear on emissions equipment that leads to the lighting up of the dreaded check engine light.

Myth: Oil additives will make your car bigger, better, faster, stronger

Reality: Your oil is already on steroids!

Magical motor oil additives?Credit: WikipediaIt began, if I'm not mistaken, with a product called "Slick 50" back in the 80s.  Its ingredients could make your motor run cooler, last longer, resist rust, and achieve better performance and gas mileage - these were among the claims made for Slick 50 and the many copycat products that sprang up and are still around today.

In most cases, these claims about the ingredients of these magical elixirs are, believe it or not, absolutely true.  And that's why those same ingredients are already in every major brand of motor oil.

Many of the companies involved in marketing these products have come under investigation for some of the more extreme claims in their ads.  One additive-maker said an engine treated with their additive would still run even with the oil drained out.  Car & Driver magazine decided to test this claim, and found that yes, the drained motor would run - for about 11 seconds.[2035] 

Myth: Bigger wheels are better wheels

Reality: What are your priorities?

Bigger wheels are not necessarily better wheelsCredit: Hummerguy,netAlmost every car on the market has one or more optional wheel sizes available from the manufacturer, and replacement wheels remain one of the most popular ways of customizing a car.  It's common to see cars cruising around with giant wheels wrapped in tires that look to be little more than wide rubber bands.  But bigger wheels aren't necessarily better - there are a complex set of trade-offs involved in wheel selection.

A set of low-profile tires mounted to a large-diameter wheel can improve cornering performance because there's less flexing of the tire side walls, meaning less weight shift from all those lateral g-forces.  The thin tires won't be able to absorb impacts as well as better "padded" ones, though, so the driver feels a rougher ride, and potholes are more likely to cause tire and even (expensive) wheel damage.

Likewise, a wide rim and tire setup can mean better traction through larger contact patches - the areas over which the tire touches the road.  But if you ever have to drive on snow with wide tires, you'll quickly find their disadvantage - spreading the weight they support across a wider patch of snow means they're less likely to dig into it for traction!

Large diameter wheels and wide wheels (and, let's face it , many aftermarket wheel buyers go for a combination of both) have one particular disadvantage in common:  weight.  Unless you're replacing a heavy steel wheel with an expensive lightweight alloy one, a wheel size increase is probably going to add at least a few pounds of mass to each corner of your car.  This weight gain isn't the same as tossing a bag of sand into your trunk, either, because wheel mass has two special properties:

  • It's unsprung, meaning that it is not supported by the car's suspension.  More unsprung weight means more wheel bounce - a rougher ride and potentially worse handling.
  • It's rotational mass.  The quick summary of what this means is that the motor has to work harder because not only does it have to move the increased weight, it also has to overcome rotational inertia to spin it.  There's an old hot-rodder's axiom that one pound of rotating weight is worth ten pounds of dead weight.  The ratio itself is probably way off, but the concept behind it holds true; auto manufacturers trying to eek out the best gas mileage from their motors often do so by using lightweight alloy internals to take mere milligrams of mass out of the spinning bits inside.  Increase in rotating mass means a decrease in power to the ground and a drop in gas mileage.

When considering a wheel swap, it's a good idea to do some research first.  See if your car's manufacturer offered different tire sizes for your vehicle from the factory.  If they did, you might be able to get gas mileage figures for the different sizes to estimate the possible impact, or find reviews of differently equipped versions to compare what they said about handling and ride quality.

While you can get better handling and traction from a new set of tires, don't forget to take the potential downsides into consideration, and keep in mind that there are diminishing returns even for the good aspects of an upgrade.  An inch of wheel diameter is often considered a reasonable change with reasonable trade-offs.

Just make sure the new wheels will fit on your car!

Myth: Transform your car with [insert cheap car gadget of the week here]

Reality: No.

If it sounds too good to be true...If it sounds too good to be true, it probably is.  So when someone tries to sell you a 30-cent magnet for 15 dollars with claims that strapping it your fuel line will boost your performance through some highly sceintific sounding process involving ions and quantum physics, be suspicious.  When they try to sell you a $70 bit of twisted sheet metal to stick into your intake that will create a power-and-mileage-boosting "vortex effect", don't take them at their word.  Instead, ask yourself this:  in a highly competitive auto market, if these very inexpensive-to-build items really had any positive effect, would any new car on the planet be sold without them

Popular Mechanics has tested a variety of these add-ons at price levels up to about $400.  The best case scenario resulted in no power or mileage change at all; most of them resulted in a power loss, and one of them required use of a fire extinguisher.[2044]

So do your research before plunking down $300 for that "electric supercharger" that looks suspiciously like the leaf-blower in your garage.