SEIDO

iTrader: (1)

How many people have a diffuser on their ride?
I’m looking to build one (or modify an existing one, off another vehicle); however I think I might have to sacrifice the spare tire well, and install run-flat tires from now on, instead.



I’ve seen some custom bumpers for the SC300, which include a diffuser; while they’re cool-looking, they aren’t very practical though, in my opinion. I’m a “functionality-junkie”; so I need things to serve a real purpose. Which is why with the exception of a front lip, and a vented hood, I don’t have many cosmetic modifications planned for this car. I think it’s beautiful as it is, and it really doesn’t need much more, other than weight-reduction, such as carbon-fiber panels. However, I wouldn’t change the body-style, or it’s shape, in other words. Even with CF replacements, I’d retain it’s OEM style.

That said, what a lot of people don’t know, is that it doesn’t matter if your back bumper has fins on it or not; that’s not how the effect of a diffuser takes place. So it may “look cool”, but it’s only an aesthetic upgrade; you aren’t gaining a whole lot (or as much as you “could be”) really.

The way a diffuser works, is by producing whats called: “The Venturi Effect”, in addition to Bernoulli’s Principle. In other words, for full-effect, you need a flat under-body. What happens is that, the area with an increased flow (faster air-travel), has the lowest amount of pressure. As a result, suction is produced in that area, because atmospheric pressure tends to impose upon it. This is how planes fly; the flow-over upon the top of the airfoil (the wing), produces “lift”. For guys like us, we want the opposite; so a cars wing, is just that; an airfoil turned upside-down.

I.e.



So with a flat under-body, you’re turning the whole car into a wing (essentially). Wings don’t only get “pushed down”, by flow-over (drag); they also get “sucked down” by flow-under … That said, when the air-flow escapes out the back of the car, that air-flow is turbulent, ‘cus it’s no longer guided or directional; it’s just wOoOosh! out into open space.

This is where the diffuser does it’s job.



The Venturi Effect, in fluid-dynamics (yes, air is a fluid; don’t confuse that with a “liquid”; although liquids are fluids also - Anything that “flows” … ), happens when you increase a flow in one area (dynamic pressure) it produces a vacuum … The air prior to that point (static pressure), gets caught in it’s momentum, and follows in that direction; so now the flow is equalized, and reduced in all (total pressure). The reduction in pressure overall, produces the desired downward suction, ultimately providing the performance gain that’s intended. The fins (or “fences”) on the other hand, serve as a boundary; they continue to provide guidance for that rear-end air that’s escaping, rather than it just flowing out all over the place, like a popped water balloon. This in effect reduces the turbulence, and allows the flow from front-to-back, to remain laminar (streamlined), and effectively faster. So by directing the final areas of air flow, the expulsion occurs in a more organized manner, therein producing that vacuum: The Venturi Effect. Now that everything’s flowing faster throughout the under-body, you have Bernoulli’s Principle functioning completely, beneath the car. It’s now being “sucked down” into the ground as you accelerate. As a result, you gain better traction, and effectively better handling - harder cornering capability.

That’s how a diffuser works.

So you can see, they’re not really as useful, without the under-body shield. The two parts correlate with one another, and function in unison.
In addition, the diffuser’s opening is just as important; because you want to expel the air as quickly as possible. So what’s been discovered (by super-car manufacturers, such as Ferrari) is that 8° of angle is the optimal shape.

Therefore, on the SC300, that spare tire well (pictured above) is in the way, I think. Granted, I haven’t lifted the car yet to measure it, but just from looking at it, I don’t think it’ll be sufficient; especially if your cars lowered like mine. I just rolled the fenders on Friday, so I’m dropping it another inch or two, which is the maximum I can go finally. So I need the clearance where the diffuser will be, in order to avoid any damage while daily-driving. I don’t want any impedance either, I need the right amount of angle on the device, with uninterrupted air-flow.

So I think that in addition to building a custom diffuser, I’m going to also develop an under-body shield, using the lightest material I can find, considering my budget; be it sheet-metal, or a strong plastic.



The only thing I won’t do, is cover he exhaust system as seen on this Honda S2000. It’s clean-looking, beautiful in fact; however, I don’t want to run the risk of increasing under-body temperatures, and melt the under-coating, etc. So I’ll design it around the exhaust pipe, leaving that section exposed, for adequate ventilation.

I.e.



Ideally, the final product will be a real eye-catcher as far as SC300’s go. There aren’t many cosmetic upgrades for that car (in my opinion) which truly enhance it’s appearance; it’s just too beautiful a car from the get-go. It doesn’t need body-kits, etc. However, a well developed diffuser in the rear, could indeed make a difference, and provide a somewhat contemporary style, to an already “timeless” sports-car design.

AnonymousG

iTrader: (2)

First of all are you even going to race your car competively or is it just something you want to do to say "you did it"?

I really don't see a reason to go through all the hassle and work if you car is still going to be a full interior,street car.

You won't need to delete it, I've seen people mount rear diffusers with it still installed. Also the rx7 rear diffuser from shine works on our cars.

SEIDO

iTrader: (1)

1. It is not a track car; although I may take it to the track at some point to test.

2. Stripping the interior is less beneficial than many people think; we're talking less than a second per -100 lbs.

3. It's about performance. I want an effective street car; both beautiful and capable.

4. I don't do things just to say I did it. Everything has a purpose.
I've topped my car out at 147; now I want it to go faster. Sure I can turbo it when I have the money (and I will), but there are other ways to achieve higher speeds in the meantime, which will still render themselves effective in the future. I mean, it's just a cut-out; then weld the hole shut. It's not gonna cost me anything. The trunks getting redone anyway; new fabric, tool box compartment, etc. So it doesn't need to be pretty at this point.

turbodremz

iTrader: (18)

If you still have a/c, removing the spare tire well is pointless. The a/c system weighs far more than the spare well does. I have mine removed to drop some of the drag under the car. Aside from drag cars, time attack or circuit cars where every lbs of downforce is needed, or doing something like the Texas Mile, diffusers and removing the spare well is pointless.

In addition to that, if you want the full effect, your going to have to remove a good portion of the lower section of the rear bumper, otherwise your adding more drag with the bumper under-hang. If you want a pic, let me know and I can take a clear shot of the rear of my SC with the spare well removed and the bumper shaped.

SEIDO

iTrader: (1)

I'm going to be doing straight runs; in attempts to exceed my top speed(s), each time.

The diffuser will be flush with the bumper; custom made.

I.e.

t2d2

iTrader: (8)

To answer the original question... If you're going to the trouble of cutting and welding the spare tire hole, how about chopping it down and putting a donut spare in there that doesn't need nearly as much depth?

2jznosht

I've been gone so long that I forgot how much I loved your threads

SEIDO

iTrader: (1)

... Because it's a donut; lol.

I'm too OCD; I need everything to be symmetrical and orderly. A donut would bother me.

net4n6

iTrader: (1)

@CityKnight. Just remove your passenger. She must be saved about 300 lbs.

oldManTan

do it, it's gonna be cool, the drag coefficient on this car is pretty damn low already so you're not gonna get much gain from adding aggressive aero addons, especially in terms of top speed. have fun with it though, can't wait to see some progress.

t2d2

iTrader: (8)

Wouldn't OCD prefer peace of mind over symmetry? Orderly is having it, not using it.

Biddles

iTrader: (2)

I think sheet metal would work good. Easier to transform into correct shapes. This will be difficult, and take a decent amount of time. Good luck though I want to see your progress.

Jrlewis05

iTrader: (1)

I say do it and post pics I'd be interested to see how it comes out.

HiPSI

iTrader: (2)

Studiogeek

iTrader: (6)

I love the forward thinking nature of your posts.
However....... Drive an Sc with run-flats before you commit to them forever though. I have 430 wheels with Blizzack run-flats for winter and I hate them more than face cancer. They made me think my suspension had been stolen from my car. My boy has them on his Vette and he loaned it to me once. I brought it back to him before the time he wanted it by. It was no fun at all in the city.