Testing aerodynamic modifications

Discussion in 'Gen 3 Prius Accessories and Modifications' started by Vman455, Sep 28, 2020.

  1. Vman455

    Vman455 Active Member

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    I've made a few aerodynamic modifications to the Prius this summer that I thought I would report here along with the methods I used for testing their effectiveness, in case anyone is interested and wants to try for themselves. First, here's what the car looked like this spring:

    [​IMG]

    As it stood then, I had made the following changes:
    -lowered ~35mm (Tein H-tech springs)
    -partial cooling air intake block (aluminum sheet behind stock grill)
    -rear wiper, washer nozzle, antenna removed
    -exterior mirrors removed
    -fairings from a C-max added in front of wheel openings, and front wheel air dams enlarged with outdoor rubber mat
    -external air curtain ducts added

    [​IMG]
    [​IMG]

    I was not happy with the front air curtain ducts, as they made the car less stable. I had modeled them on the ducts of the Hyundai Ioniq, which are angled downward from horizontal, and I think this was producing an upward force on the ducts themselves. So, I redesigned them and made a new set to test. Version 2.0 are taller, have horizontal vanes inside, and have a 12mm wide outlet, down from 15mm in v1.0, against a 30mm wide inlet--so the speed of the outlet flow has been increased from the first version.

    [​IMG]

    After fitting them, the difference was immediately apparent; the car was noticeably more stable, which I theorize is because the yawing moment has been decreased as a result of the faster-flowing air down the sides of the body. (More on this in a bit).

    What effect did this have on drag? Well, I tested them by going out to a flat section of road, 3 miles long, in the middle of the night when winds were low, temperature was constant, and traffic was nonexistent. I measured fuel economy on the Scangauge with no ducts, front ducts, and front and rear ducts over multiple runs in both directions (to average out the effects of wind). Result: with front and rear ducts fitted, fuel economy improved 1.2% at 55 mph.

    Next, I tried an experiment to reduce intake air temperature, since Toyota saw fit to redesign the intake tube from the 3rd gen to 4th gen with a pickup in front of the radiator, taking in ambient air rather than warmer engine bay air that has passed through the AC condenser and radiators. I fitted a NACA duct in the front bumper cover:

    [​IMG]

    2.5" tubing connects it to the air filter housing, with a secondary inlet at the low point of the tubing that also serves as a drain point for water ingestion (my first road trip with the new intake I ended up driving through a series of thunderstorm downpours; the filter and box were completely dry afterward when I stopped to check them).

    [​IMG]

    I measured IAT on the Scangauge with no tube, tubing run to the stock inlet location behind the headlight, and tubing run to the new inlet and found, at 90 degrees Fahrenheit outside temperature:
    -no tube: IAT 12-15 Fahrenheit degrees above ambient
    -stock location: IAT 10-12 Fahrenheit degrees above ambient
    -bumper inlet: IAT 6-8 Fahrenheit degrees above ambient

    Finally, a couple weeks ago I purchased a manometer and pitot tube, and fabricated a pressure sensor disk, in order to measure panel pressures on the road.

    [​IMG]

    I thought I would see how effective a 21mm tall lip spoiler is as my initial test, but first I taped yarn tufts to the back window to see the airflow pattern with no spoiler, the spoiler at the top of the window, and the spoiler at the back edge of the factory spoiler:

    [​IMG]
    no spoiler

    [​IMG]
    roof spoiler

    [​IMG]
    rear spoiler

    As you can immediately see, the roof spoiler causes flow to separate over the entire window--not good for lift or drag! What would pressure readings show?

    I took readings at 4 centerline locations on the roof, window, and base and found the following differential pressures from ambient (all values in Pa):

    [​IMG]
    no spoiler

    [​IMG]
    roof spoiler

    [​IMG]
    rear spoiler

    As expected, the roof spoiler decreased pressure over the window substantially, but with a pressure increase on the roof ahead of the spoiler; the spoiler in the rear location had an ambiguous effect on rear window pressure but measurably increased pressure on the roof, a phenomenon I did not expect.

    To further investigate, I measured pressures in ten locations on the stock spoiler, window and roof outside of the centerline, with and without the lip spoiler at the rear and found:

    [​IMG]
    no spoiler

    [​IMG]
    spoiler

    [​IMG]
    change in pressures from fitting a lip spoiler

    Surprise! A substantial and consistent increase in pressure simply from fitting a 21mm lip spoiler, all the way up to the middle of the roof.

    So, I fitted the lip spoiler permanently to drive for a few weeks and see how the car feels and if there is any measurable impact on fuel economy, which would indicate higher or lower drag:

    [​IMG]

    Next up, I will measure pressures on the side of the car with and without the air curtain ducts I wrote about previously; I expect to find lower pressure (higher speed) with them fitted. Then, I will be measuring pressure on the rear diffuser with and without diffuser vanes to see what effect, if any, they have.

    All these techniques are covered in an inexpensive book by Julian Edgar, former editor of Autospeed, called Modifying the Aerodynamics of Your Road Car; I highly recommend it.

    I'll reply to this thread as I complete more tests.
     
  2. ASRDogman

    ASRDogman Senior Member

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    mmm Why didn't Toyota think of doing this?????? (n):whistle:
     
  3. Vman455

    Vman455 Active Member

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    Well, manufacturers have to optimize under a variety of constraints: regulations governing car design, crash test performance, looks, cost. As modifiers of personal vehicles, we don't have to operate under those constraints.:D
     
  4. TMR-JWAP

    TMR-JWAP Senior Member

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    +1% mpg
    +starting to get that MadMax look..........
     
  5. Ed Beaty

    Ed Beaty Active Member

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    "+starting to get that MadMax look.........."

    But it won't be complete until he gets that big blower sticking out of the hood....
     
  6. PriusCamper

    PriusCamper Senior Member

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    Any plans to add a trailer hitch and build out a teardrop shape on back of the car? From what I read extending the back end is the #1 greatest thing you can do to improve aerodynamics and boost MPG... Also more places for storage too.
     
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  7. Vman455

    Vman455 Active Member

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    Funny you should ask that--I ran the 2018 Green Grand Prix like this:

    [​IMG]

    And prior to that, I built a temporary corrugated plastic fairing for a trip to Connecticut and back:

    [​IMG]

    However, I found that these did not make as much difference as most people think they will. At the GGP the car got 62.8 mpg with the tail, and 59.2 mpg the next year without (in much worse conditions, with standing slush on the track). Plus, it was awful to park in Chicago and New York and limited hatch access. So now I'm more interested in reducing drag and lift, measuring changes, and doing it without extending the footprint of the car.
     
  8. PriusCamper

    PriusCamper Senior Member

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    Interesting, did the tail with the gap between rear of car work as well as tail without gap? Seem you could eliminate the awkward parking challenge by designing a tail cone that flips up on to the roof or folds up into the back inside of the car? Maybe something that's inflatable would be even more ideal?

    And since you've done so much... Tell us about the underside? Have you experimented with putting plastic underbody covers on too?
     
  9. 2012 Prius v wagon 3

    2012 Prius v wagon 3 Active Member

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    First off ... Great work here. Very impressive experimentally driven development.

    But I'll ask - especially considering how you found how the slight variations in the rear spoiler would affect flow stability and pressure, wouldn't you expect similarly dramatic changes depending on exactly how you transition from the car body to the long tail? Gap vs. no gap seems to be huge. And beyond that, there's surely a lot to be optimized.

    Completely understand the possible loss of practicality. But hey if you could figure out a good solution, providing MPG and some extra storage, it could be a big deal.

    I remember one of the places I used to work, one of the guys had invented and patented the thing you see on all the 18-wheeler big rigs on the freeway. Sits on top of the cab, helping to efficiently transition the gap between there and the box trailer behind it. Friend of mine told me the inventor basically gets (or got, since the patent has probably expired by now) $50 in license fees for each one of those you see out there. Not bad.

    Seems obvious that those are needed ... now that you see them everywhere. But they used to not be there.

    And on the cold air intake for the engine air ... no doubt that will improve the max power, at wide open throttle. But unless you are there (and I'll guess that if you're aiming for efficiency, you will not spend much time at WOT), would it actually do much to improve efficiency? If the air is hotter and less dense, the throttle will just need to open marginally wider to allow the same mass of air through. The air will be a little hotter when it enters the cylinder, but how much does that matter? Of course, once you are at WOT, you can no longer just open a little wider, so that is where this can be clearly beneficial.
     
    #9 2012 Prius v wagon 3, Sep 28, 2020
    Last edited: Sep 28, 2020
  10. Vman455

    Vman455 Active Member

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    Oh, for sure more optimization is possible. At this point, however, I'll leave that for someone else to test unless I suddenly find myself with a lot more time on my hands.

    My reasoning was, it mattered enough for Toyota to consider the extra expense engineering and then manufacturing an over-the-radiator ambient air intake justified. It may be that cooler intake charge produces slightly more torque and allows the engine to run at a lower RPM (which was one of the reasons engine size increased from 1.5L to 1.8L from 2nd to 3rd gen). It might be possible to test by measuring mass air flow and fuel economy in different configurations.
     
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  11. Vman455

    Vman455 Active Member

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    Not yet! But I am thinking of covering some of the center exhaust tunnel with aluminum sheet. The rest of the underside is pretty well covered.
     
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  12. PriusCamper

    PriusCamper Senior Member

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    Such good answers I feel the need to ask more questions...

    1) Have you ever given much thought to driving habits like Pulse and glide? Also, on long roads trip I realize that other vehicles, especially 18 wheelers can play a huge role in boostingMPG. And while slipstreaming way too close behind a 18 wheel truck is not sustainable or safe, I've found that headwinds are rarely coming head on and if there was a way to get accurate readings of direction of prevailing winds you could basically find a way to drive along side or back behind an 18 wheeler in a different lane to boost MPG. Thoughts?

    2) Tire pressure? What kind of tires do you prefer and how do you find the balance between higher PSI giving a Big boost to MPG versus loss of traction. I prefer 47F and 45R PSI in this regard. What say you?

    3) Have you considered air bags so you can lower the car on smooth roads and raise it on rough roads to further boost MPG?

    4) Your thoughts on other Gen3 engine mods from Japanese websites? My buddy got an aftermarket sololenoid that boosted his 1/4 mile speed by a second, as well as a larger throttle body that boosted horsepower a bit.
     
  13. Kenny94945

    Kenny94945 Active Member

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    Your topic and testing is outstanding.
    Worth the time for me to re-read and give some thought to.

    I did not see any mods to the undercarriage, such as flattening the floor and some type of diffusor aero.
    Further I wonder if some pressure relief vents at the top of the fenders or on the hood would reduce lift and increase your "slippier" goals.

    Again great post.

    Thank you.
     
  14. Vman455

    Vman455 Active Member

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    There's always this route:

    [​IMG]

    But I never understood why, in a post-apocalyptic wasteland where oil is scarce and controlled by capricious baronets, all the cars have huge, gas-guzzling engines.
     
  15. Vman455

    Vman455 Active Member

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    I did a lot more pulse-and-glide on my previous car, a Civic with a manual transmission and an engine kill switch. I'll make sure to coast up to stoplights and such, but on the highway anymore I just drive at a steady speed. I don't get too close to trucks, but their wakes are huge; you can get an idea how long they are if you look at one on a wet road where water spray visualizes the wake. Even a fair distance back you might see some benefit from following a truck.

    Unfortunately, there is no standard in the US for low rolling resistance tires (Europe gives a letter grade compared to a reference tire, IIRC). I think I have Continental TrueContacts right now, about 45,000 miles in; I keep them at max cold pressure on the sidewall, 44 psi.

    That would be nice, but I've never really looked into what it would take--time or cost.

    I don't have any experience with the "Red Bullet" solenoid or larger throttle bodies, so I can't comment on them. I did put a larger BBK throttle body on my Ram SRT-10 when I had it; it was one of the first ten made that we had a group buy on. That changed throttle response drastically. But, that was a vehicle with mechanical throttle linkage, not the electronically-controlled throttle that most cars have now.
     
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  16. Vman455

    Vman455 Active Member

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    You're welcome! You're right, I haven't done anything to the underside aside from a catalytic converter cover plate. I do have some cheap eBay diffuser vanes that I'm going to throw on and measure pressures under the rear of the car to see if it makes any difference.

    I'm considering hood vents; a couple of years ago I used a Magnahelic to measure the pressure differential between air in the engine bay and on top of the hood in several locations and found consistently higher pressure under the hood. Venting the hood might relieve some of that pressure, improve airflow through the radiators, and possibly reduce lift. This fall or next spring I'd like to get some second-hand ride height sensors so I can measure lift directly.
     
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  17. PriusCamper

    PriusCamper Senior Member

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    Thanks for all your replies!!!
     
  18. fuzzy1

    fuzzy1 Senior Member

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    upload_2020-10-1_13-19-18.png
    Can you get screws or rivets with a much lower profile, for less added drag? While it is unlikely you can make them equivalent to aircraft flush rivets, it would seem there is room for improvement here.
     
  19. Vman455

    Vman455 Active Member

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    Yeah, I might see if I can come up with a more elegant attachment in future. These have rivnut inserts in the bumper cover and machine screws so I can remove them easily for testing.
     
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