Rear Underbody Diffuser - Pics anyone?
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Joined: May 2003
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From: Arlington, TX
A rear diffuser on the Evo can help reduce drag. The rear bumper acts as a huge catch for any air traveling under the car. It may not create race car like downforce, but it will help to reduce drag. in testing 70 - 140mph runs with test setups using underbody paneling we foudn remarkable improvements just from keeping the air flowing under the car and not getting caught behind the rear axle.
Granted a full sheet under the car from the front panel to a rear diffuser would be ideal. That may be the next experiment.
Granted a full sheet under the car from the front panel to a rear diffuser would be ideal. That may be the next experiment.
Originally Posted by Mister2zx3
A rear diffuser on the Evo can help reduce drag. The rear bumper acts as a huge catch for any air traveling under the car. It may not create race car like downforce, but it will help to reduce drag. in testing 70 - 140mph runs with test setups using underbody paneling we foudn remarkable improvements just from keeping the air flowing under the car and not getting caught behind the rear axle.
Granted a full sheet under the car from the front panel to a rear diffuser would be ideal. That may be the next experiment.
Granted a full sheet under the car from the front panel to a rear diffuser would be ideal. That may be the next experiment.
Originally Posted by RyanShanley
that one is made by VARIS...real carbon fiber at about $1200 to $1600 depending who you buy from.
they also produce side diffusers for about $1200;
wind tunnel and track tested...well accourding to VARIS.
winston
Last edited by WINSTON; May 18, 2004 at 06:43 PM.
i think that is a jic ti but i could VERY well be wrong.
additional note on diffusers, not only do you have to have a flat under tray you also have to isolate the incoming and out going air. the idea behind an underbody diffuser is that you have a dense charge of air coming in the frong of the car near the groud usually through a slit or a hole, air damn wahtever you wanna call it and if travels to the back of the car in an isolated tunnel and as the tunnel "trumpets" out towards the back the air expands creating low pressure as it goes to the rear of the car (maximum near the rear wheels) this makes the entire car one large wing. the way this was accomplished in f1 and lemans cars (when this ingenius idea was still legal, they banned it after its first year of conception) is they took the undertray and used it as the tunnel, so the car looks kinda like a wing and if the undertray which is flat from front to back is bolted up "air tight" with an air inlet in front then the geometry required for diffuser is achieved.
also the evo can't really achieve the effective speeds that are required for a diffuser to make huge amounts of downforce like f1/lemans cars do, but debateably every little bit helps and there are marginal benefits that can be realised.
lastly, the evo is a rally car... where downforce is important, but not THAT important, keep in mind most evos and rally cars spend their time losing traction, not making huge downforce to keep traction, which is probably why big steps have not really been made towards underbody diffusing and the such. if you look at the evos from hks and apexi you could prolly get some good tips on what aero parts to add, cuz in japan they are using the evo like we do here in the states (cept for the drag racing part). i'd say do waht they do, or find someone to minick products that they make. :]
additional note on diffusers, not only do you have to have a flat under tray you also have to isolate the incoming and out going air. the idea behind an underbody diffuser is that you have a dense charge of air coming in the frong of the car near the groud usually through a slit or a hole, air damn wahtever you wanna call it and if travels to the back of the car in an isolated tunnel and as the tunnel "trumpets" out towards the back the air expands creating low pressure as it goes to the rear of the car (maximum near the rear wheels) this makes the entire car one large wing. the way this was accomplished in f1 and lemans cars (when this ingenius idea was still legal, they banned it after its first year of conception) is they took the undertray and used it as the tunnel, so the car looks kinda like a wing and if the undertray which is flat from front to back is bolted up "air tight" with an air inlet in front then the geometry required for diffuser is achieved.
also the evo can't really achieve the effective speeds that are required for a diffuser to make huge amounts of downforce like f1/lemans cars do, but debateably every little bit helps and there are marginal benefits that can be realised.
lastly, the evo is a rally car... where downforce is important, but not THAT important, keep in mind most evos and rally cars spend their time losing traction, not making huge downforce to keep traction, which is probably why big steps have not really been made towards underbody diffusing and the such. if you look at the evos from hks and apexi you could prolly get some good tips on what aero parts to add, cuz in japan they are using the evo like we do here in the states (cept for the drag racing part). i'd say do waht they do, or find someone to minick products that they make. :]
Diffuser
Don't forget that ride height will also make a huge difference in downforce. In order to make any reall downforce with a diffuser, the car needs to be very low, aka 2-3" maximum, and would need to have a completely flat bottom. It could very well help decrease drag though.
I do have some experience with this FYI:
http://home.pacbell.net/tblackey/pics.html
I do have some experience with this FYI:
http://home.pacbell.net/tblackey/pics.html
you will always make downforce with a diffuser whether it is 5 inches off the ground or on the ground, like i said, the air is isolated.
if you say maximum 2-3" then the lemans cars would never be able to use it, they MUST be at LEAST 2" (lemans regulations) above the ground (well they can't use it anyway, undercar diffusing is illegal).
i will admit that the lift forces will be reduced when the car is lower to the ground and hence the downforce will be magnified. any aero philosophies are good at any height any speed but it's whether or not the the benefits outweight the costs. big wings, low ride height ect are no good if yer goin' slow, they add weight and cause scrubbing (lots of complications like wheel well clearances ect) very little downforce. but if you are going very fast and you can generate the necessary downforce to justify the added weight and complication then it's worth it.
that's why civics with big wings are ricey, not just cuz it looks terrible but because it really serves no purpose.
btw, nice road car. is that 4 throttle bodies i see?
if you say maximum 2-3" then the lemans cars would never be able to use it, they MUST be at LEAST 2" (lemans regulations) above the ground (well they can't use it anyway, undercar diffusing is illegal).
i will admit that the lift forces will be reduced when the car is lower to the ground and hence the downforce will be magnified. any aero philosophies are good at any height any speed but it's whether or not the the benefits outweight the costs. big wings, low ride height ect are no good if yer goin' slow, they add weight and cause scrubbing (lots of complications like wheel well clearances ect) very little downforce. but if you are going very fast and you can generate the necessary downforce to justify the added weight and complication then it's worth it.
that's why civics with big wings are ricey, not just cuz it looks terrible but because it really serves no purpose.
btw, nice road car. is that 4 throttle bodies i see?
Last edited by trinydex; Jun 23, 2004 at 02:55 AM.
Diffuser
Yep, Car runs a 1000cc Yamaha YZF (150hp), but only weighs 1100lbs, so power to weight remains good. Lots of downforce helps too.
I respectfully disagree with regards to the diffuser. With the car sitting, lets say 6” off the ground, there is too much gap for high pressure air to be drawn in from anywhere and nullify any attempt at making downforce. Underbody aero depends on the ground as well, not just the car. This is always a tough subject, even in the sportsracer world. By changing ride height 1”, I can greatly alter the downforce of my sporsracer. (2.0” to 3.0”) In order for the diffuser to really work only air from under the car can be used.
Agreed it could make downforce if you could limit air coming in from anywhere but the front of the car, aka tunnel cars. This just isn’t practical in a sedan. Add this to the turbulent nature of underbody airflow under a standard sedan, and diffusers just don’t work. If you watch cars with true diffusers, they will go to great lengths to keep underbody air laminar. This includes completely smooth underbodies, and side splitters to keep air from coming in from the side. For a lot of truly good discussions on underbody aero see:
http://p081.ezboard.com/fdsrforumfrm20
I respectfully disagree with regards to the diffuser. With the car sitting, lets say 6” off the ground, there is too much gap for high pressure air to be drawn in from anywhere and nullify any attempt at making downforce. Underbody aero depends on the ground as well, not just the car. This is always a tough subject, even in the sportsracer world. By changing ride height 1”, I can greatly alter the downforce of my sporsracer. (2.0” to 3.0”) In order for the diffuser to really work only air from under the car can be used.
Agreed it could make downforce if you could limit air coming in from anywhere but the front of the car, aka tunnel cars. This just isn’t practical in a sedan. Add this to the turbulent nature of underbody airflow under a standard sedan, and diffusers just don’t work. If you watch cars with true diffusers, they will go to great lengths to keep underbody air laminar. This includes completely smooth underbodies, and side splitters to keep air from coming in from the side. For a lot of truly good discussions on underbody aero see:
http://p081.ezboard.com/fdsrforumfrm20
Last edited by blackey; Jun 24, 2004 at 10:38 PM.
Evolving Member
Joined: Jan 2004
Posts: 206
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From: Concord
you cant deny the usefulness of downforce.
look at the escudo or any other fast-*** pikes peak car.
their aero mods have dust flying 40 feet in the air for a reason.
downforce = traction... even on ice.
look at the escudo or any other fast-*** pikes peak car.
their aero mods have dust flying 40 feet in the air for a reason.
downforce = traction... even on ice.
oooh i was reffering to tunnel diffusers, which is why i said the air is isolated, but you're right if you're using nothing but the undertray of the car the height makes all the difference. i guess it's next to impossible to make a tunnel car outta yer car huh? but then again why make a tunnel car if can just reap all the benefits without it.
Tunnel cars
Tunnel cars are a completely different beast. Tunnels are much less sensative to pitch and are easier to tune. They also make a lot more downforce with very little drag. Unfortunately, tunnels are banned in most forms of racing, as they created too much downforce and led to boring racing. Now if we could have a tunnel EVO...