Last Updated: 12/28/2024

I purchased the car for $3000 sight-unseen in Texas, and drove it 1000 miles home to Illinois. Unexpectedly, it made the trip without any real hassle. However, as expected with a cheap car, it was going to need some serious TLC. The car had clearly been driven for many miles with a failed PCV system, and lots of repairs would be required to bring the engine back to life. The following items were replaced due to failure or future failure avoidance:

• Turbocharger

• PCV system (Fix Kit from Cruzekits.com)

• Valve Cover

• Oil Cooler

• Every Oil Seal Except Valve Stem Seals and Piston Rings

• Every Coolant Hose

• Thermostat

• Engine Oil, Oil Filter, Transmission Fluid, Antifreeze, Brake Fluid, Air Filter

• Front and Rear Brakes

• Rear Tires and One Wheel Bearing

Total Spent on Overhaul: $3200 and 3 long weekends

My first modification was a large rear diffuser, which in theory would reduce lift and drag. I referenced SAE technical paper 2013-01-0952 "Performance of an Automotive Under-Body Diffuser Applied to a Sedan and Wagon Vehicle" which showed a flat rear underbody diffuser could provide a 0.03 reduction in Cd for sedan body types. I also referenced low-drag modern electric sedans, such as Tesla Model 3 and Lucid Air, which have large, flat underbody panels in the rear. However, the effectiveness of this rear diffuser entirely depends on the airflow staying attached in the front 2/3 of the underbody. With the stock airdam and largely uncovered front underbody, my diffuser would be mostly ineffective until I made better flat paneling on the front  2/3 of the underbody. In hindsight, I should have started modifying with a front-to-back technique, instead of starting in the rear.

The SAE technical paper utilized for the rear diffuser also mentioned the effect of flat wheel covers on overall drag coefficient. In addition to a flat rear diffuser, the researchers added full, flat wheel covers to the test sedan. They found the greatest drag reduction on sedan body types came from a fully covering over the rear wheel only, so that is what I decided to do first. 

I experimented with different designs, front covers, and various attachment methods. The best attachment method I had at the time was zipties around the spokes, which stuck out on the wheel as shown. Not only did this damage my wheels' finish through vibration, it also detracted from the aerodynamic benefit of the covers. As I came to find out, attachment method is one of the biggest engineering challenges when implementing wheel covers. It is no easy feat to design an attachment method which does not protrude from the wheel face, and allows for easy access to the lug nuts and valve stem. I did not see my MPG rise very significantly with a combination of full rear / partial front wheel covers and the rear diffuser, maybe 0.5-1 MPG total

Prior to altering any front-end undertrays, I needed a method for ensuring the vehicle’s cooling requirements would be maintained. In the Cruze, the cooling air which passes through the radiator is exhausted through the underbody and the wheel well gaps. So, covering up more of the underbody under the engine bay would effectively reduce the area of the cooling air exit path. Restricting the cooling air exit too much can raise the pressure in the engine bay, and reduce the pressure differential across the cooling package, reducing its cooling potential. Therefore, it was critical I obtain pressure readings in front of, and behind the cooling package in its stock form. 

I used a differential manometer, which provided me with the difference in air pressure of two different ports. So, I would get a positive or negative pressure in comparison to port 1, which read pressure in the vehicle cabin (with windows up). I fed the second port’s silicone hose from the manometer, through the door panel, through the front fender, and into the engine bay. Once in the engine bay, the hose was secured in various locations, with the hose’s opening perpendicular to the direction of air to get static pressure readings.

I measured the static pressure in front of the radiator, behind the cooling fan, and at the top of the engine bay at various high speeds (40MPH-80MPH). My use case of concern was highway speeds only, as this is when pressure may build up in the engine bay and limit cooling. Under 45 MPH, the Active Grill Shutter opens and practically doubles the cooling capacity at low speeds and idle. With these highway pressure measurements, I can now accurately assess the restrictions in cooling capacity of my underbody modifications.

With the rear underbody diffuser in place, it was time to increase airflow under the car. The large front airdam was removed and a prototype coroplast board with 3-Dimensional Wheel spoilers was put in place. The prototype was a pretty rough job, with lots of duct tape to seal the front edge.  Despite this, I saw a measurable increase in MPG over about 2,000 miles with this attached. 

Thermal considerations are one of the biggest challenges of making underbody aerodynamic pieces, as the exhuast system generally occupies the same real estate. In the Cruze, this was especially challenging, as the exhaust manifold faces toward the front of the car. Since the exhaust runs underneath the engine, I had no choice but to use metal, not plastic, to make my engine undertray. I chose 5052 aluminum sheet of 0.050 inch thickness, as it was the cheapest aluminum sheet option at my local metal supplier. To attach it to the car, I utilized the existing undertray and cut the middle section out. Plastic rivets were used along the sides, and the rear was attached using existing, but unused, threaded holes in an exhaust support.

To fasten the new panel to the car, I used 4 existing screw locations along the subframe and 3/8" blind push-type fasteners along the leading edge of the panel. I chose blind push fasteners because they have an extremely flat head that would not protrude and disturb the airflow. To give the panel a slight downward bend, I used a heat gun to heat the ABS and bend it into the desired shape. The thermoplastic ability to permanently bend ABS is one of the main advantages of using it as an aero panel material.

Preventing air from hitting the front tires is key to a low-drag car, and typically small deflectors are used. A cheap, but effective option is using a flat plate to block the air, but this is not ideal as it adds to front lift and drag. A 3-Dimensional deflector would be an ideal way to redirect airflow around the tire and wheel wells. In my research I've found most low-drag production cars block the tire with a deflector which is, at lowest, 100mm-120mm from the ground. My prototype deflector blocks an area that is 55mm tall, 200mm wide, and leaves 105mm of ground clearance. It is constructed with folded coroplast board and duct tape, and secured with duct tape.

In my case, I had lightweight forged aluminum 17x7 wheels with 215/55/R17 tires, which together weighed around 42.5lb. This OEM setup was fairly good for MPG, but I think it could be improved. I decided to change to a 16x6.5 steel wheel with 205/65/R16 tire, which together weighed around 40.5lb. Despite steel wheels weighing more than forged aluminum wheels, the reduction in wheel width and diameter allowed me to lose 2lb per wheel. Also, the overall diameter was very similar to the OEM setup, so I didn't have to worry about odometer/speedometer errors. Finally, the new tires are slightly thinner, which may slightly reduce aerodynamic drag. 

Using Low Rolling Resistance Tires

I chose to buy 205/65/R16 Michelin Energy Saver A/S tires, because they had a good reputation for low rolling resistance and they were on sale for $119 each. From my research, the tires best for maximizing MPG were the Michelin Energy Savers and the Bridgestone Ecopia EP422+, although their wet performance and noise characteristics were average at best.

Making the Face of the Wheel Smooth as Possible

Attaching a smooth wheel cover was an unanticipated engineering challenge. Finding a fastening system which does not protrude from the wheel face is critical, as any protrusions will rotate very quickly and thwart the effect of a smooth cover. My solution was buying 16-inch mooneye hubcaps, which are designed to go over steel wheels. These metal disks domes come in two attachment styles: Screw-on or Clip-on. The screw-on hubcaps require drilling/tapping threads into the steel wheel, which I was not comfortable doing. The clip-on disks have a ring of hooks on the backside which clip into place without the need for fasteners. Despite some people saying they do not stay in place, I decided to buy the clip-on versions. Within 500 miles of driving, I lost my first wheel cover. So, I decided to make a more secure attachment method, which can be seen in the pictures to the left. I cut the OEM hubcaps and screwed them to externally-threaded lugnuts. Then, I secured a bolt to the OEM hubcap, drilled a hole in the center of the new wheel cover, and fastened the new wheel cover to that bolt using a low-profile specialty nut (with med. strength Loctite). 

Though smoothing the front half of the underbody generally has larger drag coefficient reduction potential, adding smooth middle underbody panels still reduces drag and is essential to creating an effective rear diffuser. I utilized the same 0.050" 5052 Aluminum sheet as used previous panels. I used a series of brackets to make the middle panel flush with the existing front underbody tray. The large middle muffler is incorporated into the undertray, to make one relatively flat panel and allow for exhaust cooling.

I installed a Glowshift temperature sensor kit, which is installed into the drain plug, to monitor transmission temperatures. Since my aerodynamics modifications limit engine bay cooling, I need to make sure my manual transmission does not overheat - particularly during road trips in the summer. For manual transmissions, a steady-state operating temperature of 175-215F is desired for maximum longevity. I can now see real time temperature data for transmission fluid, antifreeze, and engine oil, to make sure nothing is overheating.

I created full wheel covers for my OEM wheels/tires so that I could have a second set for winter. Once I use up these all-season tires, they will be replaced with true winter tires. The covers are made of 1mm ABS and are slightly convex. The rivets on the outside edge hold brackets to the backside of the wheel. Zipties then loop through the brackets and around the spoke to hold the cover to the wheel. I used cheap, small zipties and had a few of these fly off on the interstate, but after upgrading to heavy duty, thicker zipties they have remained attached for months.

Interested to determine the behavior of my OEM grill shutter system, I wired a rolling ball tilt switch to a 12V powered circuit with an LED. When the AGS is open, a blue LED in the cabin lights up, and when the AGS is closed, the blue LED turns off. 

Although it is effective, I would not recommend using a rolling ball tilt switch, as the blue LED does flicker when going over large bumps in the road, which can get annoying at night. To minimize annoyance, I reduced the brightness of the LED so it is barely visible during the day and dim at night. Brightness of the LED is simply adjusted by adding resistance to the circuit, higher resistance results in a dimmer LED. So, I continued to solder in larger and larger resistors until I was happy with the brightness.

The rolling ball tilt switch is ziptied to an angled ABS bracket in such a way that the ball falls away from the wire contacts when the grill shutters are closed. I used heat shrink to encase the tilt switch so water would not get in and corrode the delicate circuitry.

I added  2 directional vents to the underbody trays: one vent is placed directly below the flex pipe, and one placed directly below the secondary catalytic converter. They are about 5" x 10" and were purchased from the local hardware store. Ideally, these vents would be inverted NACA ducts, as the ultimate low-drag vent. However, I could not find a single metal NACA duct for sale!

The purpose of these vents is to draw air out of the engine bay using the low pressures under the car, thereby lowering the pressure and temperature in the engine bay. Lower engine bay pressure increases the effectiveness of the radiator, and lower engine bay temperatures increase the lifespan of plastic/rubber components. 

If cooling becomes a problem during the upcoming summer months (75-90F Temps) I will likely need to add more vents, and may choose NACA ducts made of plastic, and locate them away from the exhaust.

I wanted to make a removable wheel fairing, without drilling into the sheet metal. To do this, I created 4 aluminum brackets which attach to the plastic fender liner. Since the fender liner is not super strong, I had to make the fairing as light as possible. I used 2mm ABS as it was the perfect balance of rigidity and lightness.

Wool tuft testing later showed attached flow across the fairing, and much more flow attachment behind the rear wheel. This should reduce the size of the wake, and reduce turbulence created by the rear wheel. Since more flow is staying attached to the end of the car, I will need to create a new separation edge below the tail light to create a clean separation of this new flow.

In addition to being functional, the black color of the fairing blends in well with the rest of the wheel well. It is certainly noticeable, but this mod will not attract much attention, particularly police attention. I think it looks pretty slick too - very retro futuristic.

Last summer, I used the previously-mentioned mooneye-style hubcaps which looked cool, but were too convex and stuck out from the side of the car. Although they were fully smooth, I think they added drag overall. So I decided to create a completely flat wheel cover with a similar look. 

I fastened a circle of .025" thick aluminum sheet to the OEM hubcaps with M4 rivnuts and stainless steel low-profile fasteners. I then bent the edge of the aluminum sheet over the rim of the hubcap such that it wouldn't flap in the wind. This design has proven secure at highway speeds, effective at reducing drag, and visually appealing. This will be the final iteration of wheel covers for the Cruze, and this design would be my recommendation to anyone looking to do this in the future. 

On my first highway trip with these covers installed, the Cruze averaged 48.8MPG, and the odometer crossed 200,000 miles. While a number of factors led to this fantastic result, it is a personal record for a full tank of gas, indicating these modifications are indeed working.