project diffuser... if you've made one, come on in!
project diffuser... if you've made one, come on in! The situation: cut trunk, relocated battery, tubular rear bumper support
The problem: large air pocket acting as a parachute
The solution: functional rear diffuser
The template:
Believe it or not, actual engineering went into making our cars!!! The rear bumper follows the recommended angle of 7* (7-10*) for producing actual downforce (high velocity = low pressure = downforce)!!
Fitment:
Overall the template came out great. It sits at a 20* angle where it meets the gas tank and is a perfect 7* from the ground.
What do I make this thing out of!?!? Total measurement is 54x31" so my original thought of 1/8" aluminum plate might be too heavy. Thinner aluminum might not hold the air pressure. ABS? I dont have anyone to cut it or bend it 20*. Any suggestions?
The problem: large air pocket acting as a parachute
The solution: functional rear diffuser
The template:
Believe it or not, actual engineering went into making our cars!!! The rear bumper follows the recommended angle of 7* (7-10*) for producing actual downforce (high velocity = low pressure = downforce)!!
Fitment:
Overall the template came out great. It sits at a 20* angle where it meets the gas tank and is a perfect 7* from the ground.
What do I make this thing out of!?!? Total measurement is 54x31" so my original thought of 1/8" aluminum plate might be too heavy. Thinner aluminum might not hold the air pressure. ABS? I dont have anyone to cut it or bend it 20*. Any suggestions?
Last edited by InsaneAuto86; Apr 15, 2010 at 09:03 PM.
What are you doing with the car to want/need a diffuser (i'm curious). You sound smart, so I'm sure you know by adding the diffuser you are creating an aerodynamic problem with the rear of your car...unless you have a spoiler to counter the now fast moving air under the car.
Also, you are creating probably a similar amount of drag (maybe less) by leaving the end closest to the front of the car "open" on the sides rather than shaping all the way around for a total enclosure.
there is a great thread over on frrax dot com about building diffusers with lots of engineering put into airflow and other calculations. Are you planning to just leave it smooth like that, or adding "fins" as well?
Also, you are creating probably a similar amount of drag (maybe less) by leaving the end closest to the front of the car "open" on the sides rather than shaping all the way around for a total enclosure.
there is a great thread over on frrax dot com about building diffusers with lots of engineering put into airflow and other calculations. Are you planning to just leave it smooth like that, or adding "fins" as well?
I would really like to see a Formula, a Trans Am, and a Camaro in a wind tunnel. I have considered building a flat bottom for the F-Bodies. When I did research into it, I found that it takes around 140 for it to be beneficial. I will never see that speed (137 is my best trap), so I let it go. If someone could prove that wrong in an wind tunnel, I might be in for this. You seem to have a talent for fabrication. Sheet metal would be fine and I would be one of the first in line if you chose to build this, or a full flat undercarriage kit. Keep us updated either way. This is something that I put a lot of thought into, and you have taken the step I never did: actual fabrication and fitting. I at least commend you for that.
Agree, but the angle is the most important factor. For a street car, 7* will work at highway speed (70mph +), a steeper angle (7-10*) will work better for higher speeds. GM designed the rear bumper at a 7* angle, so I assume there is a benefit on a street car. This project will make the car look a ton better too.
Last edited by InsaneAuto86; Apr 15, 2010 at 09:01 PM.
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Brett Foote No question there. Particularly on the Camaros. Even so, I would love the look on my T/A.
With fins, it could break the design, or make it amazing. There won't be an in-between! Please keep us updated. This is a pretty damned cool project!!!
With fins, it could break the design, or make it amazing. There won't be an in-between! Please keep us updated. This is a pretty damned cool project!!!
by creating an aero problem I am referring to the fast air under the car creating lift in the rear. We already have a light rear end stock, by creating a "lift" from faster air moving under it you're making the rear even lighter from aero. Adding a spoiler like one from LG or ACP would solve that problem. The parachute effect you're speaking about that we have from the factory actually helps keep the rear end planted by catching air in there; the opposite of what you will be doing with a diffuser..
TECH Regular
Joined: Jun 2009
Posts: 428
Likes: 0
From: Victoria - Vancouver Island, BC, Canada
I was wondering about all this with mine as well. I'm really wanting to chop out the trunk, but I'm worried about the massive air pocket it'll create in the rear end.
On The Tree
Joined: Mar 2008
Posts: 111
Likes: 0
From: Hampton, VA
by creating an aero problem I am referring to the fast air under the car creating lift in the rear. We already have a light rear end stock, by creating a "lift" from faster air moving under it you're making the rear even lighter from aero. Adding a spoiler like one from LG or ACP would solve that problem. The parachute effect you're speaking about that we have from the factory actually helps keep the rear end planted by catching air in there; the opposite of what you will be doing with a diffuser..
by creating an aero problem I am referring to the fast air under the car creating lift in the rear. We already have a light rear end stock, by creating a "lift" from faster air moving under it you're making the rear even lighter from aero. Adding a spoiler like one from LG or ACP would solve that problem. The parachute effect you're speaking about that we have from the factory actually helps keep the rear end planted by catching air in there; the opposite of what you will be doing with a diffuser..
Fast moving air reduces pressure, which actually will "suck" the bottom of the car downwards.
TECH Apprentice
Joined: Jun 2008
Posts: 391
Likes: 0
From: Mesa, AZ
Coming from an Aerospace Engineer here:
It's not going to make a discernable difference. That "Parachute" doesn't work the same way as a parachute, it actually will generate an eddy current that will decrease drag on the car. Adding the 'diffuser' as you call it will have a slight downforce effect, but at the expense of extremely high skin friction drag and increased turbulence and upward vortices coming off the tail of the car. These will decrease the performance of any spoiler or actual inverted wing that you may have on your car. (My Z28 has both... inverted wing with an upper surface lip spoiler... idk about everyone else's)
Long story short:
Increases drag
Increases downforce
Adding an additional spoiler would be more efficient at generating downforce with a smaller amount of drag.
Wind tunnel testing Camaros doesn't work right because of the inlet of the engine at high RPMs actually has a significant effect on the airflow around the nose of the car... and exhaust has a similar effect on flow behind the car... makes it nearly impossible to directly model, especially for wind tunnel testing at anything but a wind tunnel/dyno full scale test.
It's not going to make a discernable difference. That "Parachute" doesn't work the same way as a parachute, it actually will generate an eddy current that will decrease drag on the car. Adding the 'diffuser' as you call it will have a slight downforce effect, but at the expense of extremely high skin friction drag and increased turbulence and upward vortices coming off the tail of the car. These will decrease the performance of any spoiler or actual inverted wing that you may have on your car. (My Z28 has both... inverted wing with an upper surface lip spoiler... idk about everyone else's)
Long story short:
Increases drag
Increases downforce
Adding an additional spoiler would be more efficient at generating downforce with a smaller amount of drag.
Wind tunnel testing Camaros doesn't work right because of the inlet of the engine at high RPMs actually has a significant effect on the airflow around the nose of the car... and exhaust has a similar effect on flow behind the car... makes it nearly impossible to directly model, especially for wind tunnel testing at anything but a wind tunnel/dyno full scale test.