njn63

iTrader: (4)

Run the exhaust outlet over the top of the diffuser and have it exit high on the bumper.

A access panel/hidden hitch might work. I'm waiting on a sawzall to be delivered so I can continue my lotus diffuser install.

k24madness

iTrader: (6)

Now that is something I would love to get my hands on.

NiklasFalk

iTrader: (1)

Standard part, but there are after market too
http://www.hangar111.com/shop/548-el...ser-black.html
http://shop.britishracinggroup.com/E...G-H111E3ED.htm
http://www.reverie.ltd.uk/diffuser_size_guide.php

greeenteeee

Here's what I took notice of airflow underneath the muffler.

Tested at 6-7^0 AOA in this vid, also tested at 9-10^0 AOA not in vid. Results were the same until you "fed" the diffuser well.

24"x26" center channel (no curve though)from lower edge of rear subframe-- bumper was cut 4-5" from body line. There is at least an estimated 2" gap (can't remember, have since removed this diffuser.

Vid is at 0 to 40, then to around 50 in the last seconds. Before 40mph, seems air is going where it should, and looks like not much activity above. @ +40mph, it is turbulent below and air is recirculating towards the diff. where that is from is TBD.



This was back in October 2011 and I've made quite a few more additions, but haven't received much feedback/criticism.

Continuing on what was mentioned already, track-time data is very difficult to differentiate unless you are a VERY consistent driver. I am not one lol.

To the OP, keep us updated. Would like to see some tests if you planned to do any. My rear end work isn't enough to offset the splitter I added, so I made it an airdam instead (reducing front dF).

mx5-kiwi

I suspect because you have way more 'actual' experience than many of us interested parties watching....

I know I have watched and waited for replies to your posts any time you have put anything up....but other than what I have read and concluded to "presume" your experiance trumps any of that.... by miles!

Don't be afraid to post more though. I am very interested in what you do and how it turns out good or bad ....

I'm undecided how far I want to go with my car but secretly desire a full front dam with splitter, vanes etc..a f--k off big rear wing and gi - normous diffuser as per the OP's....but feel self conscious even imagining it let alone building it and driving it...

FTNguyen

This is my 2 cents about air and cutting the bumper and what your seeing with airflow recirculating and why you don't want that gap in the bumper and why that airflow going back to the diff is a good thing. If you contained it even more.

and I don't have a career as an aerodynamicist so you can take this with a grain of salt.

Things i do know, air isn't infinitely compressible, especially at race speeds. Since this is true, the "parachute" affect is true the rear bumper does act as a parachute capturing air, there is a point there the high pressure created by this "parachute" can no longer collect air as air will always take the path of least resistance. Thus, having the parachute creates a high pressure zone in the under trunk cavity, we all know that the air under the car is relatively low pressure, thus the high pressure created by leaving the bumper in tack and sealed creates an environment where air can more easily pass under the car. This is really the same type of thing that the tailgate of a truck does for reducing drag.



(http://www.centurycaps.com/fuel-savings) Document on truck tailgates.

So on truck example
Bad - Tailgate down
Good - Tailgate up
Best - Bed covered

Thus...
Bad - Open area above diffuser
Good - Sealed area above diffuser
Best - Flat underpan leading up to diffuser.

also i think that the cut in the bumper to let air escape creates turbulent air behind the car as the air above/below the diffuser mixes, and that is the main reason you installed a diffuser in the first place.

hustler

iTrader: (6)

Is this the elusive "stalled wing" effect?

NiklasFalk

iTrader: (1)

<stupid suggestion>
Could the high pressure air in the "parachute" be led through the trunk (in 2 3" hoses) to the low pressure area behind the hardtop?
It might be a better place to release the air in an already turbulent stream, but the underside of the wing is the most important one...
</stupid suggestion>

damir130

Why doesn't anyone with a spare mapsensor make a quick measurement of said pressure? It would help a bunch in quantifying the effects.

A hardtop spoiler (one of those ugly things) seems to have really helped with the low pressure zone on the hardtop window. It made enough of a difference that the lexan will now stay put with just some duct-tape holding it in. Before it needed to be bolted in and would flex upwards visibly.
Again, this would be easy to measure with a mapsensor and the spoiler is easy to fab up with cardboard and ducttape.

triple88a

iTrader: (2)

See what you're missing is that high pressure air won't make your gaping hole by your diff a smooth lift for the other air to pass ..."especially at race speeds".

You know that's why real race cars don't have hack jobs and bunch of holes all over their bodies, instead they have a flat piece of sheet metal from the nose to the rear bumper and smooth as possible everywhere else. I could spend another half an hr writing an essay but whats the point? This thread is 5 pages long of people who know what they are doing telling you the same ----.

greeenteeee

This is interesting. Are you saying before the hardtop spoiler, the lexan window would want to pop off? (as in higher pressure inside the cabin?)

This is what I found:



This is with an R spoiler on the trunk.

~~~~~~~~



FTN: It seems then we need to figure out what the air is doing above the diffuser surface, but I'm willing to bet it is whether or not it is sealed from the undercarriage or open.

RE: your truck bed example, I believe the drag is simply from the drop off from the top of the roof to the floor of the trunk. With tail gate up, it creates an "eddy" (like the wake) of stagnant air in the truck bed so the air flowing above the car doesn't have a tendency to go into the truck bed area. The tonneau reduces this "wake." Using a fastback on a truck eliminates this wake and creates the BEST drag compromise for a truck (albeit not really useful).





What I'm confused about is when you said: "with airflow recirculating and why you don't want that gap in the bumper and why that airflow going back to the diff is a good thing. If you contained it even more. "

My video (diffuser only, no under panels) shows air going twd the diff. You say it is a good thing. My hypothesis is that everything between the trailing edge of diffuser and trunk lid is the wake (and in a wake, air is doing circles and random movements). Wake is what drags a car down and I presumed the strings were going back to the diff because of the wake and that gap.

In another video with more of my aero setup shown, the strings taped to the trailing edge do tend to curl up and if long enough, sit above the diffuser surface.

SOME MAY HAVE ALREADY SEEN THESE:

This is the first step I added:



You are right with everything you mentioned with the frame rails, I made it flush to block off the rear undercarriage since the entire bottom is not sealed (yet???) which happens to be in line with the bottom of the rear subframe.

I figured having more clearance in the rear, lets the air expand compared to the lowest point up front, helping to reduce rear lift. (3.75"F/4.00"R pinch height, front subframe jack point is 1/8" lower than my pinch weld).

Didn't like this gap:



So added more:



Have not looked at airflow above diffuser surface since doing this as it became less of a concern (and conclusions were drawn and realized).

I'm testing another diffuser layout right now, had to toss on a beater bumper and the way it is mounted right now (temporarily til I get figure out a solution to mount it with adjustability), it is closed between the diffuser and bumper. I may try to do some video above the diffuser with the sealed rear undercarriage in lieu of discussion in this thread.

misfit77

Fashion....Never Function

njn63

iTrader: (4)

You are only looking at drag in this example, not downforce. The two aren't necessarily proportional (hence the L/D ratio that is always present when evaluating aerodynamic improvements) and both have to be considered.

The reason a truck gets improved gas mileage with a tailgate up is because the tailgate creates a high pressure area in the bed of the truck and reduces the angle the angle of the path the air takes after the cab of the truck. I can pretty guarantee the Miata bumper with a diffuser is not creating a high pressure zone like this in the example you cited because it has open sides to release the air (unlike a truck bed). It's closer to a stake bed truck with a piece of plywood bolted to the tail fence... creating nothing but drag.
Which also reduces drag (suction behind the car). Any air after the diffuser is not creating downforce so as long as it's attached to the edge that is all that matters.

I'm not saying air flowing over the top of the diffuser is good (although the idea of underbody wings intrigues me) but I think it's better than the alternative. Honestly, I would probably leave the diffuser at home before I ran it on a track car without a flat bottom from the tail edge of the floor (at a minimum) because I'd be afraid of the amount of drag it's create.
#1. You're talking about solving a poorly designed system. It'd be much better to put a flat bottom on the car than try to duct the air up to the desk lid.
#2. Before I went through all of that, I'd get a fastback hardtop or take the back window out of the hardtop.

triple88a

iTrader: (2)

Great example... see that nasty spiny thing happening there?

That's drag. Worse yet that also creates more turbulence for your diffuser.

Savington

iTrader: (31)

I'm going to stop you right there - we don't all know that, because it's not true. If the air under the car were "relatively" low pressure, cars would naturally produce downforce, but in reality they naturally produce lift. Part of the reason they produce lift is because of things like the rear bumper of the Miata - it's a huge parachute that creates a large high-pressure zone that acts upon the bottom of the trunk to press up on the entire car.

There are significant differences between trucks and cars that you're completely ignoring.

First, the air pressure in the bed of a truck is acting downwards on the bed, but the high pressure zone you leave by having a sealed rear bumper in place is acting UP on the bottom of the trunk. The truck sees a small amount of downforce, while the car sees a small amount of lift.

Second, the cab of a truck is a huge drag-producing thing. If you drop the tailgate, you leave a low-pressure zone behind the cab, resulting in increased drag. Close the gate, and the bed fills with air, and the air coming off the cab has less of a drag effect on the cab.

With a car, you don't have a cab, so by allowing that air to escape all you're doing is reducing the pressure - there's no corresponding drag increase due to the interaction with the rest of the car.

Cutting the bumper lets air escape INTO a turbulent area - the area behind the car. Allowing air into that area will significantly reduce the drag effect of the entire rear end of the car, which is a good thing.

motormechanic

iTrader: (1)

Not to mention drag works on the vertical part of the rear bumper as well. Cut bumper = less surface area for drag to work on

greeenteeee

Yes, I know it's not the prettiest thing to look at and the first choice for the under panels would be aluminum, but this was much easier to work with... and especially cheaper, considering I wasn't sure of if it would all work out or not. Each 30x36" panel was $7-8 and I used 4 total so far. 2 in the back, one in the front and one under the tranny.

NiklasFalk

iTrader: (1)

What if we think of the Miata underside as three parts, the turbulent tunnel (which needs some cooling) and two relatively clean (or can be made clean/smooth without cooling loss) areas on either side.
There are plenty of cars that have diffusors where the middle part is a poor compromise to let exhaust and cooling air out, but this poorer part is often well divided from the less poor parts on either side.

This would require an exhaust with the last can straight behind the diff and ducting to relase this air a bit up in the turbulent wake.

The flat/smooth undersides would benefit from skirts against the turbulent tunnel, as they would benefit from skirts towards the outside...


But what do I know, I run (when I run) cage and no top (which make any aero thoughts redundant )...

Handy Man

Also, if you want to go to AL once you have it all worked out, they will make perfect stencils!

mx5-kiwi

Where did this all get in the end?

In particular the last bit about the bumper cut...

I am pondering this, because didn't Emilio used to say that he didn't believe the bumper was a "parachute" and thus didn't cut the bumper on the orange car?

I haven't cut mine yet because im not so sure it will help, it makes the suspension of the car open to view (ugly) and I have cool reflective signwriting on it

Emilio saying that (or my false (?) impression anyway) is part of my having not taken the plunge....