The Diffuser


GTFUN

GT Owner
Aug 19, 2009
271
Atlanta,Ga
I wanted to explain what job the diffuser does because I get asked often and wanted to know myself...

I found a very detailed explanation on wikipedia that was pretty complex and then I found another that was more straight forward:

"The purpose of the diffuser is to allow the air that has been accelerated (so that its higher speed can produce a lower pressure) under the car to decelerate back to close to the same speed and pressure it was before the car ran into it. The better the diffuser works, the less drag the underwing produces."

You also see lateral fences or strakes in the diffuser area at the rear of the car. These have been banned from Indy cars (that means they worked), but Indy Lights and F1 cars still use them. The strakes probably do several things, but one of the most obvious is that they prevent low-speed, high-pressure air getting in along the sides of the car from disturbing the flow in the central area of the diffuser.

( http://www.insideracingtechnology.com/tech110underwing.htm)

My questions are, how much "drag" does the car have without an underwing and is the rear body designed to produce a simular effect of an underwing?

Thanks
 
Nice work, Newbie....:biggrin
 
Watching the Discovery Channel special "Rebuilding a Race Car Legend" explains the need for the diffusers. It was either that or a wing to keep the car from flipping at speed. It takes the place of the wing and for the most part is out of sight.
 
It was my understanding that the diffuser actually creates a vacuum underneath the car by pulling the air apart (there is probably a more eloquent way to describe that). By using this method you get downforce without creating drag.
 
We need to update his status mods......
 
Good thread and factual as far as I can see. I will forward this note to Dr.Joseph Katz of SanDiego State University. He is a world renowned expert on vehicle aerodynamics and flight. He is working with us on the DevonGTX program for DevonMotorWorks.

Joe has an uncanny ability to explain the most difficult in layman's language.
I will see if he can 'chime-in' on this one.
AJB
AndyBenedict
 
Newbie Answers

From Newbie:
"My questions are, how much "drag" does the car have without an underwing and is the rear body designed to produce a simular effect of an underwing?"

Answer:
Technical questions on our car's aerodynamics routinely come up on this forum. As a minumum starting point you should first read the GT team's SAE paper on the topic "2005 Ford GT - Vehicle Aerodynamics - Updating a Legend" Number 2004-01-1254. This has a wealth of as much "non-proprietary" information the engineers could put in a public document. I will have my copy at the rally so look me up and I will let you read it.

Not sure what your question is about how much drag the car shape has "without an underwing"? The car does not have an "underwing". The aero "package" we have consists of the splitter, diffusers, airdams and spoiler extension. These were all taylored in many hours of wind tunnel testing to work in harmony to produce the desired aerodynamic targets of form lift, drag and downforce. See Figure 5 of the SAE report. Since aerodynamics plays such a key role in the 150+ mph regime our car is capable of, and the car was designed to operate in this regime with us non-skilled drivers, I would doubt little if any testing was done on a shape which did not have any of the aero package features as the car HAD to have these features by design intent. Also "drag" can be assessed as a "drag coefficient" or in pounds force. If you knew (for example) the car had a Cd = 0.89 or 1256 lbf drag at 203 mph, what could you do with this info?

Second question, the "rear body" interpeted to mean the rear shape of the GT (top and bottom ?) does not produce the "effect of an underwing"; the whole aerodynamic package (described above) beneath the car was designed to carefully accelerate flows under the car (producing negative pressures per Bernoulli's Principle) to "suck" the car down closer to the road surface at high speeds. This lowers drag, lowers Cg and aids in road/cornering stability.

Hope this helps.....
 
My last track day at Mosport, I did not have the engine belly pan on or the diffuser in place.
The rear of the car felt quite light at 145-150mph.
Yes they are back on for the mile:banana
Mike
 
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I know a lot of you guys won't drive in the rain, but the little vortices formed by the diffuser in the rain is quite the sight to see at high speed. See it, and you'll understand what they are there for.
 
I was told that if you place a tennis ball in the middle of the road, and drive over it so that is passes under the center of your car at high speed it will send the ball flying.:eek I was told this by a tech at a Ford dealer. I have not tried this or heard of this before, Has anyone else???

Mike
 
There is a great explination on Wikipedia but this link is not working. Do a search and pull it up.

http://en.wikipedia.org/wiki/Diffuser_(automotive)

Here's what they explain:
This article needs additional citations for verification.
Please help improve this article by adding reliable references. Unsourced material may be challenged and removed. (April 2009)

Top: Lateral view; the red circles mark the front air dam and rear diffuser. Bottom: Underbody view.A diffuser, in an automotive context, is a shaped section of the car underbody which improves the car's aerodynamic properties by enhancing the transition between the high-velocity airflow underneath the car and the much slower freestream airflow of the ambient atmosphere. It works by providing a space for the underbody airflow to decelerate and expand so that the boundary between the car's airflow and "external" airflow is less turbulent. It also provides a degree of "wake infill" (the wake being a turbulent area of low pressure that is caused by the passage of the vehicle through the air; this can cause pressure drag).

As the air enters towards the front of the car it accelerates and reduces pressure. There is a second suction peak at the transition of the flat bottom and diffuser. The diffuser then eases this high velocity air back to normal velocity and also helps fill in the area behind the car making the whole underbody a more efficient downforce producing device by reducing drag on the car and increasing downforce.

The aft part of a car underbody can be a diffuser. It works to bring the low pressure air below the car back to the ambient atmospheric pressure without inducing turbulence. It uses Bernoulli's principle, such that the pressure increases while the velocity decreases. Since the pressure below the car is lower than on the side and above the car, downforce is produced if implemented correctly. Injecting the exhaust into the rear diffuser can also help extract the air from below the car. The exhaust gasses effectively energize the boundary layer, helping to raise the pressure of the low-pressure, fast-moving airstream back to the ambient atmospheric pressure at the exit of the diffuser. This helps extract the air more efficiently from the underbody.

Note that the front of the car slows down the air without a diffuser making this the ideal place for an inlet. Instead, a splitter is commonly used here. The splitter serves to reduce the pressure below the front of the car and thereby increase the amount of downforce in that region. The airstream is brought to stagnation above the splitter, by an air dam, causing an area of high pressure. Below the splitter, air is accelerated, as stated above, which causes the pressure to drop and create downforce. Some race cars, such as the Toyota GT-One actually use a proper diffuser in this area to help create more downforce.[1]
 
I was told that if you place a tennis ball in the middle of the road, and drive over it so that is passes under the center of your car at high speed it will send the ball flying.:eek I was told this by a tech at a Ford dealer. I have not tried this or heard of this before, Has anyone else???

Mike

Sounds cool. Probably will do just that.
 
Nice post Stormy!

Probably a bit too technical for most, but absolutely accurate.

Aerodynamics on our GT's ARE a big deal although frequently misunderstood. We have options to install biger (than OEM) splitters and diffusers on our cars. Cars (as earlier posted) are even driven without the underskins/diffusers installed. Some think it makes the car "look" better or meaner.

Knowing the time, effort, testing and REtesting which went into refining our car's production aerodynamic package leads me to the obvious conslusion of not deviating from the Ford designed geometry. Who better knows our cars than those who actually conceived and produced the design.

Just my 2 cents.....
 
If that was accurate why do GT cars not duplicate factory aeros? Because they are a compromise IMO.
 
If that was accurate why do GT cars not duplicate factory aeros? Because they are a compromise IMO.

???
If "what" was accurate??
What are "factory aeros"???
 
If you feel the factory design is superior then leave yours stock :)
 
They change many of the cars characteristics on the race versions. I suspect that the air flow is altogether different on the two machines. The reason IndyGT advises you against changing the factory diffuser is because that is the advice given to us by the team in charge of the aerodynamics at Ford. The reasoning is that a larger diffuser would/might create more downforce on the rear of the vehicle and unintentionally lift the front. That would be unfortunate at 200mph, as mistakes at that speed can be painful.
 
Look up "Venturi Effect" at wikipedia

http://en.wikipedia.org/wiki/Venturi_effect
 
I'm an engineer and in fact I'm quite familiar with Bernoulli's principle as I used to design protein skimmers for many casinos in Las Vegas as well as the Scripts Institute of Oceanography. What I am suggesting is that all engineering is a compromise. To say that the factory specifications are superior to anything else out there is incorrect. The factory engineers had to make compromises in their design to meet user requirements. Selling a street going car to the public with 2" of ground clearance and a huge splitter and diffuser would not be practical. Did their design try and stake a balance? I'm sure. Do we want to upset that balance? No. Can you change out parts that will lesson the factory compromise and still maintain balance? Yes.
 
If you feel the factory design is superior then leave yours stock :)

I thought that is exactly what I said.... :)

And it is not that the OEM setup is "superior". You misundersand. All designs are, by nature, a compromise (cost, weight, front/rear downforce/lift, lateral balance, etc). The OEM setup IMO is targeted at the greatest majority of us driver/owners, with which I am very happy.

If you plan to operate your car say on the race track exclusively in a manner which the design team estimated less than 1% of us owners would operate and you have wind tunnel data or on-track telemetry to guide you in modifying the OEM aerodynamic package to produce "better" performance in your particular application....go for it!