Rear Diffuser

[Bryce]

thanks greg, let me rephrase my last statement. You can reduce aero drag at any speed, but to get enough pressure differential to obtain any downforce you will need to be approaching 100mph. See this thread here: http://www.pro-touring.com/showthrea...h-Aerodynamics

Originally Posted by Bryce

John,

I think your goal should be reducing drag, unless you are doing a lot of driving over 100mph, you will not see a lot of down force with simple body mods.

1. The air dam needs to be very close to the ground to be effective. I would extend the air dam in the vertical direction only. A front slitter will work by creating a pressure differential between the top and bottom, this can be tested using pitot tubes or monometers. If you build a splitter you should be able to stand on the thing without any deflection. If it deflects it will be les effective.

The nose on the car should be blocked off as much as possible only have as much air as needed going through the radiator.

The stock air dam is there to prevent air from coming into the engine compartment from underneath the car.

2. Do you have a picture of the hood, most hoods I see have intakes just for looks, most of the time the intake is in a low pressure area the hood. The base of the windshield is the second highest pressure area on the car. You cannot vent air into a high pressure area.

3. That may work, but without testing it would just be a hypothesis. In theory vortex generators work, nut its main use is to get air onto the spoiler. What you always want to consider is how do I make the air transition back into the flow stream easier. Trailing edges that are squared off rejoin easier with less turbulence (drag) than curved edges. Those trailing edges should be vertical and/or horizontal. The roof could benefit from a horizontal spoiler, think SUVs.

4. Spoilers add drag when they add down force. A diffuser slows down the air so that is can rejoin the flow stream with less turbulence. But a diffuser needs to be designed not just for looks. To be the most effective the car should have a full belly pan. I am designing a rear diffuser for the falcon right now.

[GregWeld]

That's good info... most of us don't think about all that stuff.

I actually always wondered what the diffuser actually did.

[skatinjay27]

Originally Posted by GregWeld

I actually always wondered what the diffuser actually did.

look super cool duh!!!

[GregWeld]

Originally Posted by skatinjay27

look super cool duh!!!

DOH!

That's exactly what I figured they were for... 'cause they're kool....

[Bryce]

Originally Posted by GregWeld

DOH!

That's exactly what I figured they were for... 'cause they're kool....

They do look cool! I am building one because I am removing the rear bumper and I need to add something back in. The lateral face of a diffuser should be as long as possible going as far forward (think rear end housing) as possible and even protruding past the rear body lines of the car (that is determined by wind tunnel testing to determine the flow patterns and the best location to rejoin the two flow streams). The lateral face should also be at an angle up towards the aft end of the car, 10 degrees is a good starting point. The vertical plates should run parralel to the length of the car, assuming the lateral face has enough up angle. If the lateral face is horizontal then the vertical faces need to be wider at the aft end of the diffuser than at the forward end of the diffuser. The vertical plates are there to try and straighten the air as much as possible.

A diffuser is the opposite of a nozzle, think of your sprayer on your hose. To get the jet it goes through a nozzle. A diffuser wants to slow down the air.

[cjperry81]

Thanks guys for all of your imput. The main reason I wanna do one is just for look. Now if its functional then thats a total bonus. I just wanna make sure that im not doing something that is gonna negatively affect my car. Plus i was interested in how some of you may have made yours.

Bryce im a huge fan of your build so thanks for the input. I love small body fords. I think that removing that clunky rear bumper will be awesome for your car. Yours will definitely be one of a kind. Thanks again

[Sieg]

I believe the Roadster Shop has built some great looking rear diffusers. Go to their site and check the build gallery.

The benefits of aerodynamics is typically under-rated IMO, Formula 1 is a great example of the advantages. Ever year the FIA enacts regulations to slow the cars down and the aerodynamisists and seem beat them on a regular basis with very subtle yet intensively researched modifications.