acrylic hood protector and air flow - good or bad

Discussion in 'Fuel Economy' started by alcom44, Feb 24, 2013.

  1. alcom44

    alcom44 Al

    An acrylic hood protector disrupts the smooth flow of air over your car. Does this help or hinder fuel economy at highway speeds?

    The Subaru Crosstrek XV has a tiny rectangular bit at the rear of its roof rails that wind tunnel tests have shown to improve highway fuel efficiency by 1 mpg. So small details count.

    Disrupting smooth airflow at the rear of a car can reduce suction/drag and improve efficiency at highway speeds. But what about the rest of the car?

    An aerodynamics engineer would be the best source for this. But has anyone done a controlled experiment with and without a rigid hood protector? If it hurts fuel economy, there are film protectors that can be used to prevent stone chips. If it helps fuel economy, I'll leave mine where it is.
  2. 08EscapeHybrid

    08EscapeHybrid Moderator

    I used to have a pickup truck that got 2 MPG better once I removed the bug deflector.
  3. herm

    herm Well-Known Member

    If you slow down a tiny bit, lets say 5mph under the speed limit, you will recover the economy and then some.. pump up your tires also, safer, longer tire life and more economy.. its like free welfare money!
  4. WriConsult

    WriConsult Super Moderator

    My understanding has been that the only time you don't want to keep the airflow as smooth as possible is where the vehicle's shape is tapering too dramatically for the airflow to remain attached without creating a vacuum -- such as at the back of a car. It's possible that it doesn't hurt much, but I'd be extremely skeptical that it helps.
  5. RedylC94

    RedylC94 Well-Known Member

    Over 30 years ago, there was a story supposedly out of GM that they had discovered by accident, in the course of road-testing unrelated components, that certain aftermarket front-of-hood bug deflectors reduced aero drag of their pickup trucks. I'd guess that's no longer true today, or at least less true, since today's trucks are better aerodynamically in unmodified state.
  6. donmk8

    donmk8 Member

    I have a hood protector on my Edge. I think it is a trade off of MPG vas rock chips on the hood and windshield. It also helps eliminate bug smear on the windshield. There are some positive factors to the hood protectors. If my MPG has decreased as a result of the protector I haven't noticed it.
  7. ALS

    ALS Super Moderator Staff Member

    Depends on the type of protector. If it is one the new molded types that wrap over the hood than the fuel economy hit would be minimal. If it was one of the older types that was a three to four inch vertical lexan panel then you would see a major hit in your fuel economy. The other issue is as you increase speed the amount of stone and bug damage incurred on the hood edge and bumpers also increases. I learned that one from a friend of mine that owns a body shop. It isn't as much as the time spent on the highway as it is the amount of time spent over 55-60 mph. Tiny and small debris that would harmlessly bounce off at 50-55 mph will easily penetrate the clear coat and chip the paint at 65 mph and above.
    You can walk though a parking lot and easily see who and who doesn't speed on the highway just by observing the front end of the car and the amount of paint damage.
    Last edited: Apr 20, 2013
  8. priusCpilot

    priusCpilot George

    Go clear bra!
  9. DaveJ

    DaveJ Well-Known Member

    I had a GM pickup about 15 years ago I put a shaped wind deflector on. It lowered my mileage 2 mpg, from about 19 down to about 17. I used it 2500 miles and removed it. My mileage went right back up. never again for me.
  10. herm

    herm Well-Known Member

    You also notice who tailgates, they pick up all kinds of flying stuff kicked up by other cars..
  11. The Doctor

    The Doctor Old Dude, New Car

    I just put a new, plastic, bug deflector on my Kia Soul.


    I'm hoping that it will deflect the bugs and rocks enough so they will miss the windshield.

    With a car that's as aerodynamic as a cement block, I'm not too worried about the MPG suffering much. I'm more worried about damage to my car from road debris.

    I actually went with the plastic deflector, because it was cheaper than a bra. Eh?

    The Doctor :cool:
  12. herm

    herm Well-Known Member

    The headlights are bigger than the hood :)
  13. DaveJ

    DaveJ Well-Known Member

    Nice looking car!!
    That is the same style as I used on my pickup, and it cost me 10% =/- on my mpg(from 19 down to 17), and my windshield washer would not even hit the windshield with it on.
    As long as you are happy, that is what counts.
    Do you have any MPG history on this car to compare with and without. It looks new.
  14. Mightymouse13

    Mightymouse13 Member

    I once had an 1989 S-10 Blazer with the 2.8l V6 when I was in college. It had one of those Lund SunVisors on the top of the windshield and a bug deflector on the hood. At speeds above 35mph when I turned the windshield washer on the fluid went FORWARD towards the front of the car.

    The two components were creating a low pressure area. Even after I took the SunVisor off it still did that, but to a lesser extent. So my opinion is that yes, it disrupts airflow. And that its not good for mpg.
  15. bullwinkle428

    bullwinkle428 Well-Known Member

    Word! I got the 3M paint protection film covering the front 2/3 of the hood along with the entire front bumper/fascia, headlights, and side-view mirrors. Not cheap, but from what I've observed/researched, apparently does wonders in preventing rock and bug damage, without affecting the aerodynamic properties of the vehicle.
  16. Aero Mind

    Aero Mind New Member

    I am an amateur aerodynamicist, and will say my thoughts on this. A small bug deflector likely has only a small effect on aerodynamic resistance, and an even less significant affect from its weight. The effect can be positive or negative, depending greatly on the model deflector, and most importantly, the vehicle design. Bug deflectors tend to generate a vortex over the hood, rotating clockwise if viewed from the left side of the vehicle. This vortex often reduces the pressure against the windshield, as I have personally tested in CFD. However, the deflector induces turbulence early on, increasing surface friction (not really a big problem, a minor fraction of the pressure resistances). Turbulence is not always bad, although it is usually detrimental if caused long before the flow begins diffusion (yes, diffusion) at and over the rear of the vehicle. The main factors to consider, are: how "bluff" is the windshield? How large is the deflector? How long is the distance between the deflector and the windshield? Generally, the worst aerodynamic effect would come from a medium height deflector at the front of a long hood, with a smoothly curved streamlined windshield... In this case, the turbulence caused by the deflector would outweigh the pressure reduction on the windshield. A deflector could reduce drag, if the windshield is very vertical and bluff, the hood is short and the deflector is tall and inclined to direct wind over the windshield. This will cause a large vortex over the hood, and a reduced pressure on the windshield. In no case, however, have I found a deflector to change the aerodynamics greatly... Mainly because it causes both positive and negative effects, which relatively cancel each other. As for these statements about losing several mpg, I'd take those comments with a dash of salt.​
    BillLin and xcel like this.

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