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Thread: Rear diffuser normal stick out distance (street)

  1. #1
    Senior Member Frank818's Avatar
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    Rear diffuser normal stick out distance (street)

    Hey, how far back should the rear diffuser stick out to get the best aero for street?
    Frank
    818 chassis #181 powered by a '93 VW VR6 Turbo GT3582R
    Go-karted Aug 5, 2016 - Then May 19+21, 2017
    Tracked May 27/July 26, 2017
    Build time before being driveable on Sep 27, 2019: over 6000h
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    Senior Member Canadian818's Avatar
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    I don't think any testing was done on different stick outs. They did mention that the diffuser effects the entire car and not just the rear, so just do what looks best.
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    Senior Member Frank818's Avatar
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    Yeah but what does the theory, the physics, in general says about that?
    I'll do what I do best at building this thing: googling for answers to awkward questions. loll
    Frank
    818 chassis #181 powered by a '93 VW VR6 Turbo GT3582R
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    Senior Member UnhipPopano's Avatar
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    Somewhere in the Internet is a highly technical article that looked at the are flow under cars with diffuses and their downward force. The closer the bottom of the car to the track the better / greater the force. As the bottom gets higher, they noted that diffuses still provided a small downward force if they produce turbulent eddies.

    You can go find the article and spend some time reading it, or accept what everyone has said that how much it sticks out the back will not provide enough difference for a road car to measure.

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    Senior Member Frank818's Avatar
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    Well absolutely, I read a few things and there are too many variables at stake. Since I'm street, it won't make a difference. A rule of thumb is that many cars have the farthest rear point of the diffuser equal, vertically, to the farthest rear point of the car. Most the time a rear wing or the top of a bumper. I'll follow that and will add a little more to the diffuser, hoping to disturb less the exhaust gases and prevent them from coming back into the cockpit.
    Frank
    818 chassis #181 powered by a '93 VW VR6 Turbo GT3582R
    Go-karted Aug 5, 2016 - Then May 19+21, 2017
    Tracked May 27/July 26, 2017
    Build time before being driveable on Sep 27, 2019: over 6000h
    Build Completed Winter 2021

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    The Factory Five wind tunnel "testing" was laughable. I appreciate the effort, but really wish they had saved the several grand they spent there and used it to fix some of the major 818 issues. The fact that they claimed to test ground effects (Splitter, rockers, diffuser...) without a moving ground plane boggles my mind, not to mention there's no consideration for wheel wake effects either. I'm almost equally upset at A2 for letting them test there at all and claim they got usable data.

    To try and work through some of the questions here without writing a text book: (actually it would take several anyway,)

    1.) Define "best aero." This is no different than wanting the "best suspension setting" or "best power curve."

    2.) Simple diffuser physics are pretty easy to understand. While it will affect the entire car, most of the downforce will be added to the rear axle. The highest point of downforce on the underbody occurs just before the ramp up/exit. Do some research on your center of pressure.

    3.)It's very true that the closer you get to the ground the better - to a point. But much like lowering air pressure in your tires to increase mechanical grip, eventually you hit a point where grip/downforce drastically and suddenly drops. Remember, in order to create downforce from the airflow, there must be airflow. To further this with what I was started with, there is a reason actual race cars have massive rocker extensions, turning vanes, and strakes underneath. So look that up as well, and don't trust FFR for a second when they say that the rocker extensions had little to no effect on the car. I guarantee in the real world they will.

    4.) I'm not sure what your sources are, but there are much more practical reasons behind where diffusers end. On a street car, it will probably end even with the bumper because a,) you don't want people to step/trip on it. And b,) it's probably molded into/mounted to the bumper for manufacturing purposes. On a dedicated race car, there are generally things like rules they have to follow, and they define the maximum areas aerodynamic elements are allowed to protrude from the car. Being designed by race engineers, such things must always reach to the absolute extent of the allowable space.

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    Senior Member UnhipPopano's Avatar
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    You are not being fair to FF. They did spend money on some testing, and did try to see if they could improve air flow for air to air inter-coolers. They are a business and as such they need to make a profit in order to keep the doors open and buy new toys. Race car teems justify the high cost of testing with the deep pockets of their sponsors and the recognition of winning. Perhaps if their was a financial incentive, you would see more and better testing. Perhaps they have taken what they learned from the 818 and are applying it to the next project. After all, they are in business to sell cars.

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    I installed mine sticking out about 1-1/2" from the bumper.

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    Has anyone who has installed the diffuser noticed a difference?

    I'm sort of wondering if the diffuser will help with the exhaust smell in the cabin. My thoughts are that it might reduce the high pressure area inside the engine compartment, which causes air and fumes to move from the engine compartment into the cabin (since the cabin is a very low pressure area)..... and assuming the low pressure area created by the diffuser will pull more exhaust fumes down into that area instead of back into the engine compartment.

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    Senior Member AZPete's Avatar
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    phil1734, your critique of the FFR wind tunnel testing is interesting but would be more valuable if you would give us an idea of your credentials or experience in aerodynamics.
    818S/C : Chassis #25 with 06 WRX 2.5 turbo, ABS, cruise, PS, A/C, Apple CarPlay, rear camera, power windows & locks, leather & other complexities. Sold 10/19 with 5,800 miles.
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    Senior Member Frank818's Avatar
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    Tnx coloskydiver, looking at it I will certainly install it over 2 if not 3 inches out, but it gives me a comparison.

    Phil, defining "best aero" is simple in case of the diffuser. The diffuser's job is to improve downforce while minimizing drag, therefore "best aero" means highest downforce/drag ratio.

    In the end on a street car it may not much make a difference if you install it 1.5" or 3" out, that's true.

    Good friend from Atlanta, after seeing videos and pix of how diffusers affect airflow, I think it may drag down the exhaust flow or at least prevent any turbulence around the bumper which may help to not redirect it to the cockpit. No one seems to know for sure on this car yet, but theory looks good. I also think it could help Bob's rear radiator dead airflow. I wonder if he finally tried with the diffuser on. Only Trump knows, cuz it seems he knows everything (sarcastic).
    Frank
    818 chassis #181 powered by a '93 VW VR6 Turbo GT3582R
    Go-karted Aug 5, 2016 - Then May 19+21, 2017
    Tracked May 27/July 26, 2017
    Build time before being driveable on Sep 27, 2019: over 6000h
    Build Completed Winter 2021

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    Hey, look at it this way, it only costs $550 to try the diffuser to see if it works on the exhaust issue. I'm just about desperate enough to try it at this point though.

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    How sealed off is your car? Im getting a bit of exhaust and engine heat into the passenger compartment. But I know that I need to seal off the engine compartment a bit better. Im hopeful that this will fix my problem. Currently I have a lower rear engine side firewall and the 2 sides of the upper firewall blocked off. The center is open for me. I personally haven't noticed a difference with my diffuser installed though as a S and as a C

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    Not sealed at all. Hump blockoff plates arent in yet either. I am sure that isnt helping but still wonder why so much exhaust is getting into the engine bay.

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    Senior Member Bob_n_Cincy's Avatar
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    Quote Originally Posted by Hindsight View Post
    Not sealed at all. Hump blockoff plates arent in yet either. I am sure that isnt helping but still wonder why so much exhaust is getting into the engine bay.
    Hi jeff,
    If you remember my redneck wind tunnel test. I could not even push air out the back of the car with two 12" fans.
    With your exhaust coming out the bottom screen, I suspect the wrap around air is just blowing in back into the engine bay.

    watch the yarn next to the license plat in this video. This is when I had rear radiator to 2 big fans.

    https://youtu.be/pDvr0Ic5iQs

    Bob
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  16. #16
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    Hey Bob, nice video! I do see all the tufts on the rear bumper flowing to the back in that video.... though the ones in the lower mesh aren't flowing as well as the upper ones for sure. Am I missing it?

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    Frank - in this case then you are looking for the "most efficient" aerodynamic set-up. Let that be an annoying grammar-police metaphor for how the little things like to have big affects on aerodynamic development. In any case, as you point out, on a street car 4 to 5 inches off the ground you'd probably notice a more significant change in performance by eating lunch after any hot laps than worrying about an inch one way or another.

    To go back and try and help with your original question then - as I said, the center of pressure for the diffuser should be roughly where the ramp-up begins. Given the simplicity of these parts and cars, the downforce and drag generated by it will be pretty much the same regardless of where you mount it. (In short, how far back it sticks out doesn't matter - it's where that pressure center is, and that simply goes wherever you want it/need it.)

    Hindsight - the diffuser should not be fed from air in the engine compartment. The whole concept of the diffuser is to accelerate air along the underbody of the car to generate downforce. What the diffuser may do is help direct air into the low pressure area that Bob has pointed out and make it less low pressure, but again on a street car the effect will probably be minute. If you want to try something quick and dirty, the 1/4" plywood diffuser is becoming more and more popular in the autocross scene.

    Without seeing what cut-outs you have and where your exhaust ends, I'd guess your exhaust smell is coming back through the engine bay and through where the hump closeouts are supposed to be. I'd install those and see if it changes anything. I have my own personal doubts that the engine bay is a very high pressure area because if so then the lower yarn should be flowing into the low pressure wake of the car. My guess is that at speed everything behind the windshield is relatively low pressue, and therefore has very little flow to vent exhaust, or power intercoolers/Bob's radiator (Is it all starting to come together yet? I'm getting tired of typing...) In addition, if you've ever gotten a laugh out of someone with long hair driving a convertible with the top down you'll know that the air flows back to front there, thus your exhaust gasses are ending up in the cockpit.

    I need a beer.

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    Senior Member Bob_n_Cincy's Avatar
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    Quote Originally Posted by Hindsight View Post
    Hey Bob, nice video! I do see all the tufts on the rear bumper flowing to the back in that video.... though the ones in the lower mesh aren't flowing as well as the upper ones for sure. Am I missing it?
    The ones on top are not flow much or 60 mph.
    When I had my radiator fans on, I would get about 10 MPH out of the rear vent when that car was sitting still. That number actually went down the faster I went.
    818S #22 Candy Blue Frame, Front Gas Tank, 2.5L Turbo, Rear radiator, Shortened Transmission, Wookiee Compatible, Console mounted MR2 Shifter, Custom ECU panel, AWIC soon
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    Senior Member Frank818's Avatar
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    Bob, you had no diffuser on if I remember. Did you ever re-try that yarn test with the diffuser on?

    Phil, I installed the diffuser with the ramp up around the axles, pretty much where theoretically it should be. In the end the diffuser sticks out exactly 2.5" out and that looks cool, so I thought I'd left it there since everything else will probably less matter than looks. Now it's time to fit the front splitter.
    Frank
    818 chassis #181 powered by a '93 VW VR6 Turbo GT3582R
    Go-karted Aug 5, 2016 - Then May 19+21, 2017
    Tracked May 27/July 26, 2017
    Build time before being driveable on Sep 27, 2019: over 6000h
    Build Completed Winter 2021

  20. #20
    Senior Member Bob_n_Cincy's Avatar
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    Quote Originally Posted by Frank818 View Post
    Bob, you had no diffuser on if I remember. Did you ever re-try that yarn test with the diffuser on?
    .
    Never tested with the diffuser installed.
    818S #22 Candy Blue Frame, Front Gas Tank, 2.5L Turbo, Rear radiator, Shortened Transmission, Wookiee Compatible, Console mounted MR2 Shifter, Custom ECU panel, AWIC soon
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    Bob are you getting exhaust "Smell Back" from the current side exit location?

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    Senior Member Bob_n_Cincy's Avatar
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    Quote Originally Posted by BC Huselton View Post
    Bob are you getting exhaust "Smell Back" from the current side exit location?
    Hi BC
    Can't wait to see the pictures of your finishes car.
    My exhaust is the FFR elbow, forester catalytic down pipe, and the FFR tail pipe rotated 90 degrees.
    The pipe sticks out the back about 4". I did not trim it.
    Runs at 93 decibels easily staying under the 103 required at NCM.

    I get NO exhaust smell. I do feel some heat when stopping at a red light. Since I change my roll bar, I have a couple of holes to seal up.

    Here is a picture with the diffuser taped in place. I have not installed It yet.
    Bob


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  23. #23
    Senior Member UnhipPopano's Avatar
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    Some clarification Please. The air coming into the engine compartment has to go somewhere. If we ignore the air into the engine and out the exhaust as well as air leaks, we have the table below. From the FF testing it would indicate that the airflow from the center rear vent on the engine cover that the air flows in as the area above the lid is a high pressure zone. Theoretically the area behind the car should be a low pressure zone and air should flow out the back. From the street test with yarn, can we infer that the air is being sucked out of the bottom of the engine compartment? Can we also infer from this that any plate that seals the bottom of the compartment will result in more air flow out the back? Also, as the car is low and short that stopped in tragic you will always be smelling the exhaust of the cars around you and it would be hard to tell if what you are smelling is from your car or the others?

    [See Image Below - Table did not render correctly]
    Last edited by UnhipPopano; 07-24-2016 at 11:27 AM. Reason: Table did not render correctly

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    Senior Member UnhipPopano's Avatar
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  25. #25
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    Quote Originally Posted by UnhipPopano View Post
    Some clarification Please. The air coming into the engine compartment has to go somewhere. If we ignore the air into the engine and out the exhaust as well as air leaks, we have the table below. From the FF testing it would indicate that the airflow from the center rear vent on the engine cover that the air flows in as the area above the lid is a high pressure zone. Theoretically the area behind the car should be a low pressure zone and air should flow out the back. From the street test with yarn, can we infer that the air is being sucked out of the bottom of the engine compartment?
    From what I've read as stated by FFR about their testing, that is true. Vents up high where the license plate normally goes don't have much effect due to the high pressure area above the deck lid. The best venting is from the lower rear vent.


    Quote Originally Posted by UnhipPopano View Post
    Can we also infer from this that any plate that seals the bottom of the compartment will result in more air flow out the back?
    Yes I think, and that is exactly why I asked my question in this thread to those who have run with and without the rear diffuser.

    Quote Originally Posted by UnhipPopano View Post
    Also, as the car is low and short that stopped in tragic you will always be smelling the exhaust of the cars around you and it would be hard to tell if what you are smelling is from your car or the others?
    Definitely not, in my case anyway. I am running catless and as such, the smell from my car is quite strong and unlike the other cars around me. I'm definitely smelling my own exhaust.

    Over the next couple of weeks, I'm going to change my exhaust exit so see if it helps.

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    Senior Member flynntuna's Avatar
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    " I'm definitely smelling my own exhaust. "

    Ya gotta watch what you eat!

    Sorry about that, couldn't help it. Ya can take me out of middle school but ya can't take the middle school out of me.

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    Senior Member Frank818's Avatar
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    Bob, I see your diffuser on, looks like about 6" out?
    How did you seal up underneath the car, then? Over 3" out it won't reach the belly pan and will leave a wide open hole, certainly then affecting aero for track like you.

    Smelling my own exhaust, me too I had a reaction when reading that! Jeff, we'll always remember that, now! And we'll remember "you" saying it. lolll

    The high area in the rear bumper is inefficient at speed, but at idle it will help get the hot air out by itself or with a fan pushing the air out, helping prevent heatsoaking. I have a couple of ideas to prevent what I hate most, heatsoak!

    I also plan on sealing almost everything underneath the car. Will need to fab some alu panels to seal the left and right sides area of the engine as the belly pan doesn't cover there. Also there is a hole between the rear splash guards and diffuser which brings air into the bottom bumper, which is probably why some found the Amazon louvers inserts interesting to prevent the parachute effect.
    Frank
    818 chassis #181 powered by a '93 VW VR6 Turbo GT3582R
    Go-karted Aug 5, 2016 - Then May 19+21, 2017
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    Senior Member Bob_n_Cincy's Avatar
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    Quote Originally Posted by Frank818 View Post
    Bob, I see your diffuser on, looks like about 6" out?
    How did you seal up underneath the car, then? Over 3" out it won't reach the belly pan and will leave a wide open hole, certainly then affecting aero for track like you.
    Frank
    I don't have my diffuser mounted.
    This is the back of my car today.
    20160720_235611c.jpg

    This was my gap with about 4" overhang.

    P1050708s.JPG

    I think I was supposed to get a longer belly pan.
    I am considering dropping the front edge of the diffuser to make it act like a scoop to pump a lot more air into the engine bay. Problem is, sometimes the air right off the asphalt can be pretty hot.
    Bob
    Last edited by Bob_n_Cincy; 07-25-2016 at 07:43 PM.
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  29. #29
    Senior Member Frank818's Avatar
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    Yeah no engine car, but you had the diffuser on before when you took off the engine (as seen on pic), just no airflow test.

    You definitely don't have the right belly pan. Too short. Ask Courtnie if you want a longer one. Which you won't if you want to keep an open spot to let air in.
    Frank
    818 chassis #181 powered by a '93 VW VR6 Turbo GT3582R
    Go-karted Aug 5, 2016 - Then May 19+21, 2017
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  30. #30
    Senior Member UnhipPopano's Avatar
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    Been thinking about how the rear diffuser is installed, what I have read about under car aerodynamics and the 818 frame design. You would think that the forward edge of the diffuser should mount to the steel under the gas tank. Am I missing something? Why are all of the images I have found showing the diffuser mounted to the steel of the rear suspension? The sudden change in the contour would cause turbulent eddies / drag as well as killing the aerodynamic ground effect. Could someone explain?

  31. #31
    Senior Member Bob_n_Cincy's Avatar
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    Hi Unhippopano,
    The cheap aluminum engine belly pan and the expensive carbon diffuser together make up the total diffuser.
    This picture isn't the best at showing it. I added the red line to explain.
    diffusser.jpg
    Last edited by Bob_n_Cincy; 07-27-2016 at 08:40 PM.
    818S #22 Candy Blue Frame, Front Gas Tank, 2.5L Turbo, Rear radiator, Shortened Transmission, Wookiee Compatible, Console mounted MR2 Shifter, Custom ECU panel, AWIC soon
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    At least your diffuser is real carbon fiber, I should of returned mine and switched it out while I had a chance. I also have the same belly pan as you bob and I need to contact Courtnie asking for the diffuser belly pan

  33. #33
    Senior Member Frank818's Avatar
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    The "real" 80059 belly pan is long enough to have the diffuser stick out about 2.75". Together they sort of make the diffuser, yes. It starts right after the tank at the 1.5" bottom frame tube. The belly pan is flat, but I made a small angle to it where the trans support starts and that's where the diffuser gets to play too, a few inches after the start of the trans support.

    I too have a fake diffuser with what looks likes fiberglass on one side and CF on the other. I am jealous of Bob. And he's not even using his diffuser! Bob I pay you 20 bucks to exchange our diffusers. Thanks. lolll
    Frank
    818 chassis #181 powered by a '93 VW VR6 Turbo GT3582R
    Go-karted Aug 5, 2016 - Then May 19+21, 2017
    Tracked May 27/July 26, 2017
    Build time before being driveable on Sep 27, 2019: over 6000h
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  34. #34
    Senior Member Bob_n_Cincy's Avatar
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    Quote Originally Posted by Frank818 View Post
    The "real" 80059 belly pan is long enough to have the diffuser stick out about 2.75". Together they sort of make the diffuser, yes. It starts right after the tank at the 1.5" bottom frame tube. The belly pan is flat, but I made a small angle to it where the trans support starts and that's where the diffuser gets to play too, a few inches after the start of the trans support.

    I too have a fake diffuser with what looks likes fiberglass on one side and CF on the other. I am jealous of Bob. And he's not even using his diffuser! Bob I pay you 20 bucks to exchange our diffusers. Thanks. lolll
    I have 2 of them. Might sell one for what I paid.
    Bob
    818S #22 Candy Blue Frame, Front Gas Tank, 2.5L Turbo, Rear radiator, Shortened Transmission, Wookiee Compatible, Console mounted MR2 Shifter, Custom ECU panel, AWIC soon
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    Sell the diffuser or belly pan?

  36. #36
    Senior Member Bob_n_Cincy's Avatar
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    Quote Originally Posted by 07FIREBLADE View Post
    Sell the diffuser or belly pan?
    carbon fiber diffuser
    818S #22 Candy Blue Frame, Front Gas Tank, 2.5L Turbo, Rear radiator, Shortened Transmission, Wookiee Compatible, Console mounted MR2 Shifter, Custom ECU panel, AWIC soon
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  37. #37
    Senior Member Frank818's Avatar
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    Hey Unhip, you seem good on aero, what do you think about this: http://thefactoryfiveforum.com/showt...478#post245478

    tnx
    Frank
    818 chassis #181 powered by a '93 VW VR6 Turbo GT3582R
    Go-karted Aug 5, 2016 - Then May 19+21, 2017
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  38. #38
    Curmudgeon mikeinatlanta's Avatar
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    Very generally speaking,
    All else being equal, the more rearward the diffuser sticks out the easier it will be to boost it with flow off the rear wing, and the less backflow from the dirty air behind the car will hurt it. That said, moving the rear of the diffuser aft should never be done by moving the front of the diffuser aft as well. The front should start as far forward as possible, so to any extension should be done by making the diffuser longer.
    MKII "Little Boy". 432CI all aluminum Windsor. .699 solid roller, DA Koni shocks, aluminum IRS, Straight cut dog ring T-5, 13" four piston Brembos, Bogart wheels. BOOM!

  39. #39
    Senior Member UnhipPopano's Avatar
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    Frank, If I am correct, the FF documentation shows the front splitter aluminum installed below the bottom pan for the radiator. If installed that way there would be a continuous surface from the splitter to the floor. What I have not seen is what supports these panels in the middle. I do not think that attaching the aluminum to the fiberglass body would be enough.

  40. #40
    Senior Member Frank818's Avatar
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    Mike, that's interesting, cuz the belly pan has a section that you can (I did) bend 13-deg upwards (same angle as diffuser) and it's a few inches long. This adds to the diffuser so what I did is I installed the diffuser 1/4in between the alu belly pan and frame, just to say that the belly pan's bolt holes also thread through the diffuser, but barely. You cannot do any longer without having an empty space between belly and diffuser (unless you add a panel there of course).

    Unhi, there is no front splitter aluminum, only a front splitter carbon fiber. Check out my other thread you'll see how it's fitted. It fits maybe 2in below the rad floor pan, basically the height of the first opening in the nose insert. The splitter stops before the under steering rack alu panel but since that one is downwards it pushes the air down and flows under the rest of the car. The splitter is attached to the metal supports that support the fenders and nose insert (new nose design, old one I don't know, I believe it was attached to fiberglass).

    There are 7 under body panels including the rad floor pan. 2 in CF and 5 in alu.
    Frank
    818 chassis #181 powered by a '93 VW VR6 Turbo GT3582R
    Go-karted Aug 5, 2016 - Then May 19+21, 2017
    Tracked May 27/July 26, 2017
    Build time before being driveable on Sep 27, 2019: over 6000h
    Build Completed Winter 2021

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