I should clarify some. I'm sure it provides some benefit, but it doesn't look like anything I've seen in racing and I wonder if it could be better if it followed conventional logic.

It can always be made better...
 

It can always be made better! The ideal degree I've heard (more than once) is 11, but that doesn't mean you won't gain downforce from any number of designs.

I've already hi-jacked this thread enough, but long story short is that the diffuser has more than one job, and needed to be blended into the whole design. For reference, the other pictures showed that we tried to keep the area around the wheels as tight as possible for best possible air flow. The diffuser has not been placed, and is actually lower than the pictures indicate.

I don't care as long as it stays on topic

I by no means have any idea about how or why diffusers with very steep AOAs work, but they are all over the place in pro class time attack. As far as I know, the better the underbody aero the more AOA the diffuser can run.

https://www.miataturbo.net/attachmen...ine=1359266985

https://www.miataturbo.net/attachmen...ine=1359266985

https://www.miataturbo.net/attachmen...ine=1359266985

https://www.miataturbo.net/attachmen...ine=1359266985

Would seem a lot of the fastest guys are starting to do some pretty wild things. Seems something this aggressive would require a lot of track time and revisions, some sort of CFD analysis, or wind tunnel time though.

I don't think there's any doubt those cars have much more downforce than they did before those mods. I'd really like to see what's happening with the airflow in those diffusers, though. Everything I can find indicates those are too steep and that airflow should be separating from them. But are they better than a non-flat underbody with no diffuser? Definitely.

Some of those time attack cars were designed by people that worked at the highest levels of motorsports like F1, ALMS etc...
The aero on that S13 was designed by some ex McLaren F1 aerodynamicist, there's a big article about this car in Moto IQ.

Good article. They have obviously spent a lot of time and effort on this car. I found it very interesting all of the aero work was done old school, with no CFD done at all.

I think those diffusers in particular are so long (like 6 ft!) that although their end angle is much steeper than conventional thinking says should work, the angle change is such a gradual curve that air may be able to stay attached...? Same thing but scaled down, I don't think there's any way it would work.
-Ryan

MOST of those time attack cars were built by people in sheds who want to be featured in speedhunters...

A lot of aero in time attack is done because they think it works, but really have no idea. It is still amateur racing without a team of engineers.

Some yes, most are not.

This. Some of the diffusers still have a ~10 degree angle that ramps as it continues down the car. This does allow a much steeper angle than conventional thought. Also some of the cars who were designed by aerodynamicist have other things going on to keep flow attached.

This. Most are not designed by aerodynamicist from high end racing.

I'm surprised they do not use a horizontal stabilizer to keep the air flow attached to the diffuser like on the Ferrari 360

https://www.miataturbo.net/attachmen...ine=1359314643

The better the WHOLE aerodynamic package, the larger the diffuser's AOA can be. Well designed wing profiles can 'drive' the air exiting a diffuser.

Plucas, I agree with almost everything you said, only this specific S13 that was given as the example is actually the exception.

:wavey:

Agreed :bigtu:

even old skook knew about flat bottoms
 

All things considered a flat bottom will benefit any build, as long as cooling, maintenance and the like are considered. Even the Mercedes 300SL had a flat bottom back in the mid-fifties. It's expensive and not expected, but the aero pays off.

As for diffusers and Time Attack, I think that you need to look at whatever the intent is for your car. If the lowest single lap time irrespective of all else if your goal, then your car is built for max downforce and max short-fuse horsepower. Endurance cars need downforce, but also need to balance it against fuel efficiency. It's all a balancing act, and some benefits will be sacrificed for others.

That's the beauty of building car your car your way...the end result should show the benefits of the decisions you made along the way, and your attention to detail getting there.

I had a skateboard ramp just like that as a kid. only mine was made out of plastic not fancy carbon fiber

Let me take a stab at describing the "tunnel" on the S13.
Many of the top time attack cars (NEMO, and the MCA S13 pictured above) are beginning to utilize tunnels. With tunnels, you have to stop thinking about the rear portion of the diffuser as a single component, and think of the shape that it creates on the underbody of the car, as a whole. If you take a look at the underside of the S13, it is completely flat at the front and begins to taper up in the tunnel at the back. If you think of the car in profile, from the front of the car, to the back of the tunnel, it looks like the underside profile of a wing. A wing with a decent angle of attack. So, in profile, you've essentially turned the entire center of the car into a wing, with a narrow span, but a very long chord. And that is why I don't think they worry about their AOA at the rear, or at least, the MCA S13. The tunnel sticks so far out, that it makes the wing profile nice and gradual.

Of course, I'm no aerodynamicist or engineer. So, take all of this with a grain of salt. It's just a different approach to thinking about it.

Vintagerust, I believe you are right. The tunnel the created here behaves like a venturi tube, with an inlet, throat, and outlet. Simon McBeath did state that the longer the throat and the steeper the inlet/outlet you can get away with while maintaining attached flow would benefit you most.

Flat bottom kit
 

It dawned on me that it would be fairly easy to turn my flat bottom drawings I have now into a flat bottom kit for Miatas; the wheelbase is the same, the width can be adjusted easily to the pinch seam, we're mounting the Catfish diffuser to a Miata now (90-05), and it wouldn't be difficult to change the front splitter shape to the Miata outline.

Would a pre-made flat bottom kit be interesting at all, or is this a purely DIY crowd?

If it was cheap as shit.