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Aerodynamic Effect of Sunroof & Rear Spoiler

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  • Aerodynamic Effect of Sunroof & Rear Spoiler

    Hi All,

    I noticed that my fuel consumption when travelling was worse when I didn't use my air conditioner. It then dawned on me that the cause was my sunroof in the vent mode. When travelling without a/c I use it in the vent mode......I noticed that immediately I closed it fully there was an immediate improvement in response. This is very noticeable when exceeding 150 km/h. The difference in fuel consumption is also considerable. So, from now on when I travel in the M3 the sunroof remains closed unless I'm doing less than 100km/h.

    Now, bouncing off this I was considering adding a rear spoiler extension......similar to the version that has those BMW M Power stickers on it. All of you that have spoiler extensions do you notice a drop in fuel consumption at top end speed ? I figure that due to the shape of those extensions the added drag will be considerable the faster you go....or am I wrong regarding this?

    Click image for larger version

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    This is what I'm referring to.



    Just want to hear from members experiences with the additional rear spoiler before I go for one.

    Cheers

  • #2
    A few racers in the northeast did some real world testing on the Evo 3 wing at Watkins Glen back in the 90s I think it was and found that it generates alot of drag as you start raising the flap up to position 2 and 3.
    Remember it was designed for cars with 300+ HP.

    With that giant DTM parachute in the picture will make it WAY worse. Those larger DTM flaps didnt hit the scene until the DTM 91-92 cars and those were pushing 400HP.

    If you want to try an experiment take the rear wing off completely.
    When the racers tested the car with no wing they achieved their fastest lap times (at the cost of some stability).
    Last edited by jimmy p.; 11-13-2021, 11:58 AM.
    jimmy p.
    87 E30 M3 Prodrive British Touring Car
    88 E30 M3 Zinnoberot - Street
    88 E30 M3 Lachsilber - Race (#98 SCCA SPU)
    92 E30 M Technic Cabrio - S14 POWERED!
    98 318Ti M44, Base - Morea Green
    04 Ford F350 - V10

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    • #3
      It has never stopped irritating me that these cars came with sunroofs. They are heavy, take away headroom (helmet room), I never use mine, and they are completely out of character for the car.

      Comment


      • #4
        LOL while I agree with you 100% in theory, in practice I have to say I love the sunroof on my street car, I use it every single time I drive it. If its nice enough for me to drive it I have it open in some manner.
        It somewhat was the inspiration to buy my cabrio from Tim, because I love having it open.
        It was however a gigantic PITA while building the race car and I would have loved a no sunroof shell for that
        jimmy p.
        87 E30 M3 Prodrive British Touring Car
        88 E30 M3 Zinnoberot - Street
        88 E30 M3 Lachsilber - Race (#98 SCCA SPU)
        92 E30 M Technic Cabrio - S14 POWERED!
        98 318Ti M44, Base - Morea Green
        04 Ford F350 - V10

        Comment


        • #5
          I usually have my sunroof in vent mode. I can't say that I run around at 93 mph often, but 75-80 isn't uncommon. I don't have the late DTM flap, but I do have the Evo version. I always run it fully extended. I've done a few long distance tests to see what would give me the best fuel economy, the flap fully closed, or where I have it normally. Oddly enough, I got better mileage with it up then I got with it down. Go figure. These tests were over a 4 hour distance drive each way. I saw roughly 26 mpg with the splitter and spoiler retracted (low drag position) and 29+ extended. Also, it didn't seem to matter if the sunroof was cracked in vent position or closed. I do have to say the airflow around the sunroof in vent mode is interesting. The boundary layer will curve around the back, then smooth out when it hits the rear window ridge, then fall down onto the spoiler. I discovered this when I was driving my car on Saturday. Its a bit dusty (I've done 1000 miles in the past 5 days in it, 1400 in the past month.) and it was really foggy causing water to condense then beed up and make trails as the water beads rolled through the dust. Looking at the patterns in the dust was pretty fascinating.

          Will
          '69 Datsun 2000 Roadster vintage race car (Street driven on a regular basis :taz
          '59 Alfa Romeo 101 Sprint (HUGE project :uhoh
          '88 M3

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          • #6
            Originally posted by Ironhead View Post
            It has never stopped irritating me that these cars came with sunroofs. They are heavy, take away headroom (helmet room), I never use mine, and they are completely out of character for the car.
            I'm with you Bro.

            Boils my piss.
            Sport Evo No.47

            My Sport Evo Restoration

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            • #7
              lol....fuel consumption is nowhere in the realm of my thoughts when I am driving an E30 M3...
              75 M2 Zinno
              88 M3 Henna

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              • #8
                Originally posted by mlytle View Post
                lol....fuel consumption is nowhere in the realm of my thoughts when I am driving an E30 M3...
                Most valuable post in this thread
                jimmy p.
                87 E30 M3 Prodrive British Touring Car
                88 E30 M3 Zinnoberot - Street
                88 E30 M3 Lachsilber - Race (#98 SCCA SPU)
                92 E30 M Technic Cabrio - S14 POWERED!
                98 318Ti M44, Base - Morea Green
                04 Ford F350 - V10

                Comment


                • #9
                  Originally posted by mlytle View Post
                  lol....fuel consumption is nowhere in the realm of my thoughts when I am driving an E30 M3...
                  Clearly you aren't trying to drive from California to Washington State without having to stop in Oregon for fuel. They don't let you pump your own gas in Oregon. The gas station attendants pumping the gas usually don't give a shit if you have a nice, rare car or not. I speak from experience. It is possible to make it from Yreka Ca (The last place you can get fuel before crossing the Oregon border) to Vancouver Washington on a single tank of fuel if you don't get screwed by stop and go traffic through Portland. That said, I don't think it can be done going south as the climb from Ashland to Yreka will leave you short by a good bit.

                  I have made it from the Canadian border to the SF bay area on 3 fuel stops (883 miles) pretty easily.

                  Will

                  '69 Datsun 2000 Roadster vintage race car (Street driven on a regular basis :taz
                  '59 Alfa Romeo 101 Sprint (HUGE project :uhoh
                  '88 M3

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                  • #10
                    Something something drag force goes up as the square of the velocity something something. Decrease speed, decrease drag force, improve fuel economy.
                    Dave.
                    76 M2

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                    • #11
                      Very interesting responses !

                      I too don't really worry about fuel economy when driving the M3 ....this was more of an academic exercise. Not just the fuel economy but in my case the movement of the car is much better with the sunroof closed.

                      I figured that if the sunroof in vent mode could contribute so significantly to the cars performance what exactly would a rear spoiler addition do. I also noticed that with the sunroof in vent the high speed handling of the car is also enhanced...go figure. Can't have everything so I guess one just has to pick a poison..

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                      • #12
                        Keeping it academic, don't discard the effect of air flowing through the inside of the car and then out the tilted sunroof instead of passing along the outside, when you have the fan off and all vent controls to the left (or recirc on on A/C cars).

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