Driveability versus Performance

The problem that most young owners face is not understanding that modifying a car has an impact upon driveability.  What is driveability you ask?  Driveability is the degree of smoothness and steadiness of acceleration of an automotive vehicle.  In other words, the ability to move within traffic with a level of control that would not end in an accident.

Engine modifications

You want a fast car.  The first step in making a car fast is improving its efficiency and potential.  There are a number of different ways this can be done.  On a normally aspirated car, you can increase power by:

  • Increasing airflow into the intake
  • Improving airflow cooling
  • Improving the flow of the exhaust
  • Increasing the compression of the engine
  • Adjusting the timing
  • Increasing the size and duration of the valves (valve timing)
  • Increasing the displacement volume
  • Adding a catalyst (Nitrous Oxide)
  • Adding forced induction
  • Engine tuning

In addition to the list above, you can further increase the power produced by your turbo/supercharged engine by increasing the size of the turbo/supercharger.

How do engine modifications affect driveability?

For all engines, changing the factory specification means changing the powerband to a point in the RPM range that it will do the most good.  Most manufacturers design their engines to operate well below the redline as this is where most daily driven cars operate.  Your modified engine will most likely be optimized to produce the most power much higher in the RPM range.  What this means is that you will tend to rev higher, make more noise, and use more fuel than the average commuter car.

Powertrain modification

The goal of the powertrain modification is to transmit as much of the power made by the engine to the wheels as possible while maintaining the best fuel mileage possible.  This is a balancing act for the manufacturer, but if there is a bias, it will always be towards mileage as opposed to power.  Why is that?  Because car manufacturers have their hands tied by the US EPA that monitors and enforces US Corporate Average Fuel Economy (CAFE) standards.  This requires a certain fuel economy average for a manufacturer’s fleet of cars.

The modifications generally applied to the powertrain to improve performance are:

  • Changing the gear ratios
  • Improve torque handling limits
  • Lightening the turning assemblies
  • Installing a limited slip differential
  • Stiffer mounts

Most powertrain modifications will add noise and vibration to the car since this and the engine is where most of the spinning parts are located.  Additionally, the manufacturer generally installs the parts rated for the OEM HP rating and maybe a little more.  This means components like clutches and torque converters may not be able to handle a substantial increase in the torque load.  For manual transmissions, this means a more aggressive clutch and pressure plate.  This single modification will increase the harshness and weight of the clutch engagement.  It will make the clutch pedal much more difficult to push and the engagement threshold much smaller. Going to a taller gear ratio will increase top end performance, but reduce low-end performance and vice versa.

Suspension modifications

The suspension on the average commuter car has pretty much one goal:  reduce the amount of noise, vibration, and harshness that is generated   The suspension is one place where performance is always the opposite of comfort.  A stiffer suspension will result in better road grip, but always at the sacrifice of making the ride just a little less pleasant.

Suspension modification can include, but are not limited to:

  • Suspension lowering
  • Stiffer/Adjustable springs
  • Stiffer/Adjustable dampers
  • Lighter control arms
  • Lighter wheels
  • Adjustable connecting links
  • Stiffer sway bars

Suspension modifications are ALWAYS give and take.  From a driveability standpoint, a very stiff suspension can make the car unbearable on public roads that have uneven surfaces, potholes, and other imperfections.  There is also a point where adding stiffness can actually hinder performance.  Your suspension must be able to react to the conditions of the road underneath it.  If the surface dips, your tire must be able to follow the contour… if you hit a bump, your suspension must be able to compress to absorb the bump.  If your suspension is so stiff that your tire is no longer making full contact with the road when you drive over a dip or a bump, you are sacrificing grip and actually making the car more dangerous to drive.

When modifying your stock suspension, always consider where you will be driving the car.  If you primarily drive public roads, consider the condition of the roads you tend to drive on.  The Genesis Coupe comes with a fairly stiff suspension for daily driving.  Lowering the car or installing stiffer springs will have a significant impact on ride comfort and may not provide you with the performance outcome that you were hoping to get.

A word about lowering springs…

One of the most popular ways to lower the car is by installing lowering springs.  Springs are the component that holds the weight of the car; thus, installing shorter springs will lower the car’s ride height.  While this will lower the center of gravity of the car, making it more stable in turns, it also has some drawbacks.

The other component in your suspension that works with the springs are the dampers or shock absorbers (shocks for short).  The shocks work by modifying the compression and rebound rate of the spring.  In simpler terms, it slows down how fast the spring would normally compress or bounce back in order to prevent the spring from bouncing and to stiffen up the reaction time of the spring.  Because the shocks are designed to work with the spring, the full travel of the shock is designed to allow the spring full travel.  Lowering the car and keeping the stock shocks means that the stock shock is going to be compressed more when the car is not under load.  This means less travel of the shock strut within the shock body.

At the base of the shock arm is a device known as a bump stop, which is essentially just a rubber bumper that is installed on the arm to prevent the shock strut from over compressing and damaging the shock.  When the shock body is fully compressed and makes contact with the bump stop, your shock becomes the limiting factor in terms of how far the suspension will compress to absorb a bump.  In other words, you have effectively converted your car into a Conestoga wagon with no shock absorption capability.

Coilovers vs lowering springs…

The most common complaint from those who have installed lowering springs is that the ride of the car gets “rough”.  There are two reasons for this.  First, the shock body is compressed and almost riding on top of the bump stop.  Second, the control arm is angled up, reducing the overall amount of travel that the suspension has to absorb the bump.  What the suspension cannot absorb, the car and your body do.

A workaround that will fix the bump travel issue is to install fully adjustable coilovers that can be shortened to allow for full shock travel.  This will still not help with the suspension angle issue, but it will greatly improve the ride quality.  Additionally, most coilovers also have adjustable damping, which will allow you to customize the compression and rebound rate.  Higher end coilover systems go one step further and allow you to adjust compression and rebound separately.  The more you can adjust, the better the chance that you can have that smooth daily driver feel when you are going to and from work and still be able to have that body roll resistance when you are driving hard in the mountains.

The bottom line…

From a consumer marketing perspective, driveability is king.  The vast majority of the car buying public would like a high-performance vehicle, but not at the sacrifice of their own comfort.  As a result, most car manufacturers always err on the side of comfort when making design decisions.  What we have to remember is that car manufacturers are in the business of selling cars.  While there are some manufacturers that cater to a niche tuner market, most do not; hence, the reason why the aftermarket exists.  What you have to remember is that not all modifications are necessarily “good” depending on your own standards as to what you think the car should be.

ALL modifications are give and take, meaning that you will typically trade ride comfort for increased performance.  While there are exceptions to this rule, most of us don’t have hundreds of thousands of dollars to overcome this particular limitation.  Before you modify, do your homework.  It will save you a lot of time and money to modify once and enjoy than to have to continually tweak your modifications to reach a point where you can tolerate it.

 

 

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About the author: Bob

I am a 21 year retired Army veteran with a background as a CH-47 helicopter Flight Engineer. For the past 5 years I have been a Genesis Coupe enthusiast and am currently a Moderator and Contributor for GenCoupe.com Forums, author of the "The Ultimate Genesis Coupe Newbie Guide to Modding", and the "Ask a Geek" sub-forum. Additionally, I am an administrator and event organizer for GenSport United as well as several other groups on Facebook. While I do not pretend to know all things with regard to the Genesis Coupe, I have dedicated a substantial effort to learn as much as I can and share it with whoever will listen.