Diffuser Myths Begone [Archive] - Toyota MR2 Message Board

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reflexx08-30-2006, 01:34 PMIt seems all kinds of people are slapping diffusers, canards, slats, etc on their car. If you're doing it for looks, that's one thing. Here's the scoop on what it is, what it does, and whether or not you need it.

The Venturi Principle
This principle states that as a fluid (which air is) flows through a constricted area, its velocity increases. Think of putting your finger over a water hose.

The Bernoulli Principle
As a fluid's speed increases, pressure decreases.

http://www.astrocore.com/images/Bernoulli.jpg

http://www.astrocore.com/images/flow_pattern.gif


Application in Race Cars

Venturi Tunnel
The car has a shaped underbody. The first third of the car underside slopes towards the ground, the middle third is flat, and the last third slopes away from the ground. The front compression section accelerates the air by constricting it (nozzle effect). The middle part contains the air at a rapid speed and low pressure, creating suction. The final part (diffuser) slows the air back down to ambient.

Flat Underbody Plus Air Dam
The car has a flat underbody. The car also has a low front air dam. The air dam forces incoming air under the car, which then flows at a high speed/low pressure over the flat underbody. The diffuser section at the end slows the air back to ambient.


http://www.astrocore.com/images/venturi.jpg
http://www.astrocore.com/images/flat.jpg


Factors essential to venturi principle operation

1. Measurement B must be A/3 or less. This translates to about 1-2 inches for a low-sprung race car.
2. The tunnel must be adequately shielded from spilling out on the sides. This means, fairly close to the center of the car.
3. The compression and diffusion section must be at pitched 17 degrees or less, otherwise the flow will detach and become turbulent.
4. For angle T to be 17 degrees or less, and C to be B*3, F must be at least 1/3 the length of the car.


What purpose does the diffuser serve?
The diffuser decelerates the air under the car and returns it to ambient speed and pressure. If the air isn't decelerated, it continues at a negative pressure past the end of the car. This causes drag.


Is this practical in my street car?

Probably not. In order to achieve a low enough center section ("B"), the car would need incredibly harsh suspension to control the ride height. This would be most unpleasant, and the amount of normal body roll may ground out parts of the car around corners.
The center section, if it generates enough suction to make a difference, will probably be ripped from the bottom of the car, unless it's built as an integral component and/or welded on. Then, you'll have a real fun time working on your car - with the bottom welded shut! If you hit anything or went over a speedbump, your painstakingly-attached carbon or aluminum underbody work would probably shatter or gash.

The diffuser's job is to slow down and increase the pressure of the air. If the air is already moving at ambient, slowing it down below ambient will immediately cause drag.



Does the diffuser I saw for sale help me?
Unless you have bodywork which accelerates air to faster-than-ambient, probably not. Unless the diffuser is a real 10-17 deg diffuser, probably not. Unless the lowest point on your car is 2 inches off the ground, probably not.


Here is an example of a real diffuser:
http://www.astrocore.com/images/f1diffuser.jpg

Here is an example of a gimmick diffuser:
http://www.astrocore.com/images/crappydiffuser.jpg PHOENIX08-30-2006, 06:07 PMWow, Very impressive write-up, i had no idea!

Please share more knowledge with us :) Mister2.208-30-2006, 06:28 PMGreat write up, it's something I didn't know a lot about, thank you! trdny08-30-2006, 07:56 PMreflexx I'm glad you're sharing your knowledge with us. i do have some questions that I haven't found the answers to yet.

1) For the tunnels to be effective is there a % of the cars width it should be? ALso since we are increasing velocity would 3 1" tunnels would be better than 1 3"? Also should the top of the tunnel be B+ as much as possible?

2)Which "type" of diffuser is better? The curved up to the bumber type or the straight back like F1 cars (assuming they both were done correctly)? Raptor13x08-30-2006, 08:30 PMVery interesting information... thanks for the post! reflexx08-30-2006, 08:58 PM1) The tunnel should be as wide as possible. However, two factors limit the width.
a) in F1, you have a bunch of crap in the middle of the car - the driver pod, engine, etc. This limits how close to the center the two tunnels can come.
b) air spillage. If the tunnels are too close to the outside of the car, the air spills out and causes vorticity. This is bad. F1 guys solved this by using sliding skirts (like the rubber piece on the bottom of a house door), and making full-width tunnels.

Designers put vertical slats in the tunnel to keep the flow laminar and moving upwards along the tunnel contour, instead of sideways. Making multiple tunnels (3) is basically the same general thing. The slats can be increased up to a practical limit of maybe 1-2" spacing between them, wherein a slot/edge effect actually impedes air going into the slots.


2) the f1 ones are actually curved. The curvature helps the tunnel get higher in the bumper faster, so therefore is a good thing if you have a limited amount of room for the diffuser section. Obviously, this curvature has an upper limit, beyond that and the air detaches. A gentle curve like in the F1 picture I provided is generally a good compromise.


I'm working on a similar write-up for the aerodynamics of wings, turbo wheels, and a few other things. Thanks for the positive commentary!

-D trdny08-30-2006, 09:22 PMthis info is great. keep it coming trdny08-30-2006, 09:31 PMDo you think it would be worth while to put a small "lip" behind the front wheels to help with keeping the flow from spilling under the car? I'm thinking like a dive plane but with a little curve. reflexx08-31-2006, 08:25 AMif you can get away with creating ridges or something that go from the front of the car to the back, creating a division between the middle 'tunnel' and the outside of the car.... maybe running a foot inside the edge of the car just inside the wheel-well line, yeah, that would helphttp:// Mister2.208-31-2006, 08:26 AMI'm working on a similar write-up for the aerodynamics of wings, turbo wheels, and a few other things. Thanks for the positive commentary!


That should be interesting. :D trdny09-01-2006, 10:21 AMhttp://www.f1-country.com/f1-engineer/aeorodynamics/f1-aerodynamics.html arashi85809-06-2006, 11:36 PMThat's cool Dance_Business09-06-2006, 11:44 PMif you can get away with creating ridges or something that go from the front of the car to the back, creating a division between the middle 'tunnel' and the outside of the car.... maybe running a foot inside the edge of the car just inside the wheel-well line, yeah, that would helphttp://

Any chance you can put up an illustration of this? reflexx09-07-2006, 10:18 AMhttp://www.astrocore.com/upload/rails1.jpg
http://www.astrocore.com/upload/rails2.jpg
http://www.astrocore.com/upload/rails3.jpg

red are the 'ridges'
blue is the venturi profile/aluminum underbodywork trdny09-12-2006, 03:46 PMI just sent you an email. wanna help with actual design :) Dance_Business09-12-2006, 04:02 PMWhat happens if you only have one or the other? reflexx09-12-2006, 04:13 PMone or the other what? G3aR09-13-2006, 03:01 AMgreat stuff man, i see that yr new to the boards. WELCOME! I think we are all going to love having you here. This write-up was absolutely fantastic. Cant wait for the next one. reflexx09-13-2006, 08:31 AMi will try to get one out this weekend while i am drugged and unable to work on my car due to wisdom teeth removal on saturday. feral4mr209-28-2006, 07:15 AMwould the 'gimmick' rear diffuser i made for the feral serve any purpose as a diffuser once i ghetto rig a flat bottom etc..?

hrmm.. for some reason i cannot get onto my imageshack site to get thumbnail links. :(
there are some pics of it on here, though not real close up.
http://www.sounddomain.com/ride/237058/4 reflexx09-28-2006, 08:29 AMYour rake angle is so high flow will detach and might even make more drag than without it :( feral4mr209-28-2006, 09:20 PMoh... didnt seem that much steeper than many i have seen. :(
maybe its the angle/view of them pics.

thanks. reflexx09-28-2006, 11:10 PMprobably most anything you saw on a street car is a gimmick, is why :)

Ferrari's street car diffusers are one of the only ones that actually work. Here's a 430's, it looks steep here but i think that's just the end of it plus a weird angle of the photo, i've seen it in real life and the beginning is quite shallow and gradually becomes steeper. the whole piece is maybe 2 feet long:

http://www.revozport.com/webpics/FERRARI/F430/Diffuser%20Fins/F430_press-diffuser.jpg

here is a real, 100% working racing diffuser. Note the angle. This is more like what kind of angle actually works.

http://www.astrocore.com/upload/PLM019.jpg feral4mr209-29-2006, 03:56 AMyea, i see with that last one how it doesnt kick up at the end, just a slight curve. where the 1st one does look rather steep.

got imageshack to work, just a couple other angles. (i know the cut out where my exhaust tip isnt good, gotta do something with it)... :confused:
http://img221.imageshack.us/img221/711/newsticker1zc1.th.jpg (http://img221.imageshack.us/my.php?image=newsticker1zc1.jpg)http://img212.imageshack.us/img212/1681/newzorst66se.th.jpg (http://img212.imageshack.us/my.php?image=newzorst66se.jpg)http://img84.imageshack.us/img84/2897/feralrrhs8ez.th.jpg (http://img84.imageshack.us/my.php?image=feralrrhs8ez.jpg)

was going to continue it on underneathe more. 1st attempt at something like this. :)

cheers for your info. reflexx09-29-2006, 08:12 AMHi,

the most important angle is the one where the 'diffuser' meets the horizontal line - that should be as smooth as possible. In other words, if the end of the diffuser is quite steep, that might be OK as long as the point where it meets the flat underbottom is not. In other words, this: \____ (sharp angle) should be avoided at all costs. Also, in the closeup pic, you can see that the underside of the bumper is going to catch air instead of work like a diffuser. You have to take metal sheet and seal off the bottom of the car to get anything out of the setup.

It's a street car, and it's not nearly low enough to take advantage of ground effect anyway, so who cares if it works - it looks cool! dsf3g10-23-2006, 08:25 AMprobably most anything you saw on a street car is a gimmick, is why :)

Ferrari's street car diffusers are one of the only ones that actually work.

I'm pretty sure the Lotus Exige also a functional diffuser. The whole point of the Exige was to add downforce through aerodynamics, and Lotus were the guys who inveted ground effect cars to begin with:

http://www.elises.co.uk/models/s2_exige_240R/Rear2.jpg reflexx10-23-2006, 09:11 AMexige does an OK job, it's more about handling/light weight than downforce (although it does make some).

If you want to talk real downforce, the Noble is the king. Everything in this car was designed around downforce and it generates like 1700lb (AFAIK) @ 120mph. fidget10-23-2006, 11:04 AMWhat about front splitters? Do or would they help at all? reflexx10-23-2006, 12:45 PMfront splitters do a great job, IF

1) the splitter is very low to the ground. we're talking 1-2 inches.

2) the rest of the car is relatively smooth and low to the ground. underbelly trays, 2 inches clearance. steven072010-24-2006, 01:21 AMhmmmm , very interestin , so where can i find one? reflexx10-24-2006, 08:30 AMGotta build one :)

Might as well relocate your exhaust while you're at it.

Mach2 on the boards is fabricating a full front splitter, undertray, diffuser after a few conversations with me. Might want to ask him about his project.

Carbon fiber or aluminum are good choices, aluminum might be better for the undertray as it won't break if you bottom out or hit a speedbump (not to mention easier to work with). PearlJammzz02-01-2007, 11:07 AMAny progress made in this? trdny02-19-2007, 03:10 PMI'm pretty sure the Lotus Exige also a functional diffuser. The whole point of the Exige was to add downforce through aerodynamics, and Lotus were the guys who inveted ground effect cars to begin with:

http://www.elises.co.uk/models/s2_exige_240R/Rear2.jpg

Just read a article about the car and they say it makes 90lb @ 100mph reflexx02-19-2007, 03:23 PM90 is kinda crappy... the ferrari 360 makes a few hundred lbs at that speed IIRC and the Noble makes 550? Gotta find the numbers. PearlJammzz02-19-2007, 03:38 PMpretty good for a 40k dollar car I think... Mark_Dorman07-21-2008, 06:39 PM90 is kinda crappy... the ferrari 360 makes a few hundred lbs at that speed IIRC and the Noble makes 550? Gotta find the numbers.

Yeah, but for a car that weights ~2000lbs compared to the ferrari at ~3000lbs it's still pretty good. Plus the lotus is tiny and doesn't have an much to work with to create downforce. The noble is just ridiculous... Raptor13x07-21-2008, 06:44 PMAlso keep in mind the average car creates a rather large amount of lift, so just getting lift down to zero is a pretty big accomplishment for your typical street car, let alone generating downforce. Basement Digital07-21-2008, 09:19 PMI'd love to see the visual aids restored on this post. Dance_Business07-21-2008, 09:57 PMI believe Reflexx was banned for being the boldest liar I can remember. The Mods would have to have the images from a back up.
I would like to revisit this thread, too. I have some honeycomb AL that could work as a splitter... Basement Digital07-22-2008, 12:07 AMI believe Reflexx was banned for being the boldest liar I can remember. The Mods would have to have the images from a back up.
I would like to revisit this thread, too. I have some honeycomb AL that could work as a splitter...

Crazy. Lying about aerodynamics? :) DenverMR207-22-2008, 12:24 AMNa I think it was performance and about photoshopped dyno sheet Mark_Dorman07-22-2008, 12:50 AMHow low does your car actually have to be to benefit from a splitter? I parked next to an Lotus Elise this weekend and it wasn't much lower than your average dropped MR2...it's a good bit smaller though. If the lotus can benefit from a splitter and diffuser I don't see why the MR2 couldn't either. Raptor13x07-22-2008, 01:01 AM^ Your car will probably benefit from a properly-designed splitter- it's just a matter of to what degree. Mark_Dorman07-22-2008, 12:26 PMDo you think the two would benefit from a completely flat bottom? Like adding the flat bottom panels and making a rear diffuser that covered the bottom of the engine bay? I feel like the side scoops would provide sufficient air flow for the engine (NA at least). Thoughts??? b.18tuner587507-22-2008, 01:44 PMive heard the air scoops actually disrupt the airflow coming in from the side vents. afaik, the side scoops bring the air and push it up towards the hood where it vents out, and the air coming in from the scoops just disrupt it. only benefit would probably be to make a custom airbox for your air filter directly connected to the scoop Mark_Dorman07-22-2008, 02:58 PMI was referring to the side vents when I said scoops...sorry for the confusion. I know having the air come in from two directions would cause some turbulence. I know that air does come up through the bottom of the engine bay and go out the engine cover...but if that was covered to improve underbody aerodynamics (reducing drag behind the car) and aid a little in down force do you think the side vents would provide enough air for the engine? I figured it would. And if you added a engine lid (see pic below) cover that help direct the incoming air towards the rear I think you could reduce the amount of airflow disruption caused by the "notch back???" design. I'm not a physicist so I couldn't say for sure but from my basic understanding of physics it seems logical.

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