5 Popular “Performance” Mods That Make Your Car Slower

If you spend any amount of time in social media car groups, you’ll come across tons of people looking to do performance modifications to their cars, or showing off the ones they’ve already done.  Sometimes, this can be cringingly bad, as a fundamental lack of understanding on how these systems work contributes to performance mods actually slowing their car down.  Since the number of people actually taking their freshly modded cars to the racetrack is extremely small, the placebo effect takes over at this point.

I want everyone to get the most out of their cars, so avoiding a few of the pitfalls can make a big difference.  Now, if aesthetics are the only thing you are interested in, then if your car looks good to you then you’ve achieved your goal and frankly that’s a win.  But if you are looking into actually making a fast tuner, don’t get caught up.  Here’s five of the most popular performance mods I see that will actually make your car slower.

Huge ‘Sport’ wheels

18in are the minimum size to clear the brakes in our IMSA Audis. These exact wheels are mandated to disallow teams to gain an advantage by choosing a lighter wheel.

High trim level sports cars, as well as many race cars, have huge wheels.  This is mainly for two reasons.  The first is that large wheels and low profile tires will help with more responsive handling and minimal sidewall flex.  The other reason is to clear extra large brakes.

Because aesthetically, wheels have one of the largest impacts on how a car looks, usually the first upgrade people make is to put on large sport wheels with low profile tires.

There’s a catch though, large wheels are often very heavy or very expensive.  You only get to choose one.  Obviously, there are people that will spring for the 19lb magnesium racing wheel (and probably pay upwards of $900 per), but most will just get the big wheels that look cool.  For example, your BMW may come with some plain 17in Style 394’s which weigh a modest 22.5lbs.  Not exactly light to start.  Now go shopping for your new 19in wheels.

Screenshot from Tire Rack

If you picked up these very stylish Model Six’s, you’d have added around 8.5lbs per wheel.  Take into account that the 19in tire is around 5lbs heavier than the 17 on average, you’ve added around 54lbs of unsprung weight to your car.  That’s a catastrophic amount.  I have a more in depth answer to why that is here, but for now just know that unsprung weight (weight before the springs, like the knuckle, brakes, hub, wheels, and tires) has a substantially higher negative effect than normal weight. So before you spring for 19’s, take into account the unsprung weight you may be adding and drop the coin on those spiffy OZ Alleggeritas to get a real performance gain.

Verdict: If you want to go fast, stick with smaller and wider wheels, and pick the lightest one you can.  That’s the true performance upgrade.

Straight Pipe Exhaust

Alright, now that your car looks the business, now it needs to sound like it.  Who wants a wheezy, quiet sports car?  Let’s take the mufflers off and add a large straight exhaust pipe because that’s how race cars do it, right?  Well…sort of.   Race cars generally have large straight pipes because they are specifically tuned to work with a pipe like that.  To get performance out of an exhaust, you have to take advantage of scavenging, which is a reverberating pressure wave in your exhaust that helps suck exhaust out of the cylinder.  Really fat books have been written on the fluid dynamics behind this, which I won’t get into.

Racing cars have exhaust headers and pipes meant to allow that pressure wave to scavenge at very high RPM, near redline.  In order for them to work right, they need to live in that range.  For the street however, the high RPM tuned exhaust will do nothing but hurt off the line performance, lowering your exhaust velocity and making your engine slow to gain rpm.  This applies mostly to Non-Turbo cars.  On turbo cars, a large diameter downpipe will help turbo spool up, and you aren’t going to be enjoying much scavenging effect through the turbine wheel.

This Audi Mototec catback is properly designed for cars with street tunes

If you have a ripping tune and plan on moving a ton of exhaust gas, then the better flowing pipe would go with that.  However, if you have a stock car, that fat straight pipe will only make your neighbors hate you.

Verdict: Specific street tuned exhaust’s flow great with proper diameter pipes in order to place the powerband in the zone you use most often.

Slammed Coilovers

Race cars and sports cars are low in order to maintain an exceptional center of gravity and also aid in aerodynamics, so its natural when you are sport tuning to want the car to be very low.  Generally when you slam a car, you have to make it very stiff in order to keep it from bottoming out and scraping over bumps, so coilovers are very appealing here.  The problem is that very few people can actually work an adjustable coilover style suspension.

When you have ride height settings for every individual corner, you are able to set it up through a process called corner-weighting.  By making small increments in ride height, you can effectively move the weight around and control how the car’s balance feels.  On top of it, many coilovers have bound and rebound adjustment as well, giving even more precise control.  Adjusting them all to the lowest setting is not how you tune a well handling sports car.

Photo from Stancenation.com (looks good)

When you stiffen a car, you lower the total amount of travel the suspension can use for mechanical grip.  If you haven’t substantially lightened the car, then you are asking a lot of a little, and the car will be very susceptible to bump steer and other poor handling characteristics.  Since roads do conventionally have bumps, you’ll find a very uncomfortable ride paired with sloppy cornering.

Verdict:  For most street situations, sport springs and upgraded struts/shocks (Koni or Bilstein are both excellent choices) will give you a significant boost in handling over poorly setup coilovers.

Cold Air Intake

When it comes to the design of the engine bay in your production car, chances are that someone that knew what they were doing is responsible for your air intake.  That doesn’t stop people from assuming that their air intake system is restrictive and robbing precious horsepower.  Often referred to as a cold air intake to fool the casual viewer, stuffing a cone filter in your engine bay without any heat shielding might make your car sound a little better, but you will find your power levels tanking fast.

Engines hate hot air.  It’s less dense, so in order for it to achieve the proper air-fuel mixture the car injects less fuel.  Less air, less fuel, less power, easy.  The most restrictive part of your air intake will always be the filter (not the location or the piping), and K&N makes high flow drop in replacements to remove that restriction and let you focus your tuning elsewhere.

Verdict: If you want the throaty intake sound, make sure your open air intake is located somewhere far from the hot engine.  In the front fender well ahead of the front wheel is often a good place to find nice cool air for the engine to enjoy, just be careful about water.

Big aerodynamic mods

I see improperly understood aero so often I felt compelled to writing this blog to describe the difference between a Wing and a Spoiler, two things people often mix up.  The thing about Aero is that almost everything you put on to manipulate the direction of the air as it flows over the body, you’re going to increase drag.  Spoilers are added to very specific parts of the car to condition the air in order to reduce drag and lift, often in the form of a small lip.

However, small spoilers don’t look that cool and race cars have big aerodynamic elements all over to help them go faster around a race track.  Just keep in mind that all of those elements add considerable drag.  While they are able to corner faster, the different aerodynamic devices attached to the car actually slow the car down on the straight away.  So before you put that big spoiler or wing on your car, keep in mind that if it isn’t perfectly designed, the only result you will get is adding weight some drag.

Verdict:  A big wing is like bolting a parachute to the back of your ride.  If you are looking to improve MPGs slightly, try a small lip spoiler on the rear deck and see if it helps.  Otherwise, leave the Aero to the pros.


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8 thoughts on “5 Popular “Performance” Mods That Make Your Car Slower
  1. Kendall Linzee

    Are you saying that an NA motor that has maybe ported heads improved intake and headers will lose off the line acceleration? If there’s no monitor, like pre OBD I. Then you can get away with a cam grind etc. But isn’t my question most likely answered in the big book you mentioned?
    Just wondering how much actually works and doesn’t, What that depends on, stock motor to begin with etc?
    I have one of the last Saab NA motors and I would like to see what’s possible. 98’ 900s 2.3.
    Any advice on where I can begin research? I know a Trollhatten trained native but I’ve asked him many questions already.

    • Adam Goral

      NA Saab tuning is a hole you can fall down and never get out, but in a good way. I’m not an expert on the B234, but I do know it is a very basic engine and the science will apply. That informations is out there, don’t be afraid if there’s a big American V8 on the cover. Muscle car guys really know how to squeeze power out of NA engines, knowing the math is the biggest step when you start designing intake and exhausts.

      In general this is the order I would go in. #1 will be compression (higher the better). You can raise the compression with flat top pistons and valves, a very thin head gasket, and a shaved cylinder head. If you are buying pistons and rods, lightweight forged style will help free up some power in reducing rotating mass. With the right connecting rods you can even increase stroke, I know someone that built a 2.5NA with Chevy rods. #2 would be a well designed tubular exhaust header that promotes scavenging in your intended RPM range and a matched exhaust pipe (IE not a 3in straight pipe). Short tube if you are looking for high RPM horsepower and long tube if you want down-low grunt. If building a stroker engine, down low would be better. #3 would be a matching intake manifold, also built to promote scavenging at your target rpm range. With the intake and exhaust working together, your engine will sing as pressure pulses both suck exhaust out the combustion chamber and cram air into the intake. The stock NA T5 cams are very good for lift and duration. The T7 head (like from a 9-5) will flow better, but also might decrease compression which would be bad. It would need to be shaved considerably. All of this paired with the right T5 engine tune and you can make turbo power without a turbo, which results in a much cooler running, responsive, and simpler setup. Good luck!

  2. Adam, you said something very important there: knowing the math. That’s a huge part of engine building, suspension tuning and so many other aspects of modifying cars for added performance. Buying and trying is too expensive and ineffective these days.

    • Adam Goral

      Thanks Kevin. I’ve spent some considerable time thinking about designing an intake and exhaust manifold for a 2.1 NA Saab engine, mulling over runner lengths, diameters, collector configuration and design. Not even getting into compression ration calculations, internal component lightening… I learned a lot about how much I didn’t know, mostly from muscle car resources.

      However, many performance companies DO have options for tuners out there looking to just buy something to bolt on, leaving the math to the professionals.

  3. Jeffrey Oswald

    Any update on the impact of your intake changes on the Mini? You switched from an unshielded cotton filter right over the exhaust manifold (the “mo hotta mo betta” setup) to a commercially available cold-air system.

    As a resident on a US highway just south of a drive-in and popular car show venue, I’d like to thank you for not giving in to temptation, as shown in this article. For working to separate all the marketing BS and pseudoscience from real speed gains. Speed should be more important than looks.
    Another aftermarket supplier that also sends me e-mails will gladly sell anyone thousands of dollars’ worth of garbage. Fender extensions, tire stickers, clutch pedal stops, push handles, I mean gigantic rear wings, soft-metal rims, and vacuum-actuated exhaust ports, just for starters. In a street setting, the fact that your license plate is mounted on the front towing eyelet makes absolutely zero difference in speed. All the neighbors think of these modifications is “Compensate much?”
    “Seriously, tire stickers?”

    As you’ve pointed out, the original designers of your vehicle weren’t idiots. They had to design to probably a dozen factors, while many aftermarket parts pick one or two. Yes, you can manipulate your engine management program to favor different things, but how are your exhaust valves going to last through all of that? Just one example. There are always trade-offs.
    If I’d wanted a Cooper Works car, I would have ordered one. All the changes in the factory JCW models, compared to a Cooper S with the JCW kit installed, show how interrelated all those systems are. You can’t just bolt on an air filter and go faster. Keep buying into fad diets and weight loss pills if you think a K&N cone will speed you up.
    Yes, I have changed a few of the intake hoses on my Mini, but because it allowed me to remove a noisemaker that made no positive impact on the car. It also dropped a few pounds. F = m*a no matter what you do under the hood.

    Thank you for staying focused in reality, on actual gains. Thank you for avoiding the lure of taking people’s money that they don’t know how to spend appropriately. Thank you for supporting the people who simply want to keep their cars on the road or track. And, if you could, please update on the intake modifications on your Mini racer.

    • Adam Goral

      Thanks Jeff, it’s easy to get lost in the sea of bogus aftermarket, and we try to weed out that stuff in our catalog to keep people from spending money and not seeing results. We don’t like that. That’s why our performance vendors like Krona Performance exhaust, Do88, Koni, Mishimoto etc all have been vetted to providing high quality parts with real world gains. I will ping Mike Kenific on the air filter.

  4. David Church

    Very good article, with very good advice. My “high performance” car is a 1967 MGB/GT. When new 51 years ago, it came out of the factory with great brakes, and exceptional handling. It’s acceleration was ‘peppy’ at 0 to 60 in 12 seconds. And the optional overdrive made it a lovely high speed touring car in a world where 70 mph was about as fast as anybody drove long distances.

    From its beginning in 1929 through the early 1950s MG was a company run by hotrodders. Then controlling interest was bought by accountants. The first thing the accountants did was kill the MG 1600 Twin Cam engine. BMC (the accountants) were okay with boring out the block to 1800 ccs. But wasting any more money on an aluminum 8-port cross flow head with twin overhead cam shafts was not needed for the Companies’ line of sedans, lorries and tractors.

    So, the new B Model had to make do with the old 5 port head of cast iron. The hotrodders at Abingdon did all they could to the lump The cast iron header with it’s Siamese center port flows as well as ANY aftermarket header up to the stock 6500 RPM redline. It also produces identical Horsepower to the Maniflow Racing Header up to 6000 RPM, and slightly more torque below 6000 RPM. Likewise, the twin SU 1-1/2″ Carbs deliver all the fuel and air an MGB engine can swallow from 800 RPM to 7200 RPM, giving nothing away to any other carburetor which can be fitted to the venerable B Head. Cam Shafts start at about 265 degrees and with stock header and carbs, and an 8.0/8.8 to 1 compression ratio, delivers 95 honest horsepower, in a one ton sport car. GTs weigh 2,200 pounds,

    So how are you going to make your fifty year old technology keep up with 50 years of progress? Spend a lot of money. But you don’t need to spend money on expensive aftermarket replacement parts on the brakes, suspension or steering gear. Drive train is up to you. You can rebuild your engine. This positively the least expensive way to go. And who can resist going for a bit more punch and grunt. So along with all the gaskets and bearings and seals, you can put in High Compression Pistons to about 10-1. And you can put in a higher duration and high lift camshaft. (I’m running a 285 degree Piper I bought for $200). These two changes from OEM parts (which after 50 years are probably worn out) will deliver the biggest improvement in performance you will ever see. You can see bigger gains and retain ‘drive ability’ by the MG Tradition of adding a Supercharger. That’s not cheap at $4000, but worth the money. So that’s what I did. Now I have a car which can run pretty well against a Kia Soul (the hip-hop hamstermobile) in a drag race. But that is not important to me. I know that my 50 year old car has me behind the wheel. Of course you can update the drive train (suspension and steering) like Frontline did. You will end up with a $100,000 car which can run with the big boys in any venue and still look 50 years old. Or you can go V8, V6 and change every thing and spend roughly the same amount of money.

    • Adam Goral

      Excellent insight and great writing David. Have you published any British Roadster books by chance? Good points about souping up a car that really doesn’t want nor need it. Many of those old school engines were really pushed to the limits of what they could achieve as they evolved through the years, ending with basically a performance modified setup stock from the factory building on overbuilt elbowroom in the initial design. Take the Viper for instance, the engine in that is an evolution of the Chrysler LA family. With some thought, the engine was transformed from a truck engine meant for heavy duty towing to a fire breathing upper RPM powerhouse by just tweaking a few things like material construction and some internal changes.

      It’s hard to determine what potential an engine (in particular) has these days due to all the ancillaries modern lumps have (electronic ignition and injection, heavy handed emissions systems, the demand for low rpm power). One thing is for certain, the balance of performance vs. economy will always exist no matter what the generation, and in order to get the most out of anything you have to think harder than just bolting things on from the internet.

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