These pictures were taken in the dark, but they give you some idea of the basic shape it's taken. Each 'chamber' expands in width from front to rear, in addition to expanding in terms of height.

http://forums.pelicanparts.com/uploa...1112156028.jpg

http://forums.pelicanparts.com/uploa...1112156042.jpg

Goofy? Maybe.

Effective? Who knows?

I'll have to do some testing with it. I think a good way to start is the wool tufts (Can I use yarn? Maybe one black and one white strand in each tuft?) and a lipstick camera on an extended pole.

It has the makings of an effective setup IMO, based on sticking my head in the tunnels of wind at Boeing -

You did a nice job!

and um how does that affect the function of the diffuser?

I thought he was comparing the air flow of an aircooled to that of a water cooled. Can you not get by with less exhausting of hot air from the engine bay on a water pumper?

I would want some well placed makeup air in the back center. The GT 1's exhaust tips are in the license panel. Probably the lowest presssure spot at the rear. Good spot to divert engine air to.

When is the complete kit going to be available? It will fit an early narrow body car, right?

The diffuser may function the same.

Air flow to the engine compartment is important. Jack mentioned being nervous about cylinder head temps and I was making the corellation. The airflow concern is lessened with the later water-cooled 911's and the Corvette pictured. Should be simple. If the diffuser works, and doesn't affect engine temps, then it's a winner.

The purpose of diffuser is to exhaust any air that come under the car w/o increasing lift, the vast bulk of which has traveled under the front end and along the entire length of the car, air/ water cooling has nothing to do w/ it. Yes the air cooled pump additional hot air which also needs to be exhausted but it won't have the lift inducing velocity of the slipstream air.

I can't claim to be an aerodynamicist, and although I love it when rocket scientists debate laminar flow coefficients on these forums, I will digress....... Jack has enjoyed success in the past, and I'm sure he will tell us how it turns out.

Jack: Hope it gives you the edge (not that you didn't already have your share!)

E

Exactly, and if the factory, which does have genuine rocket scientists, hasn't seen fit to worry.......

So if it doesn't work as well as you want, can you lower it a couple of inches and force the air it scoops up to go over the top thereby creating downforce? An undercar wing?

Jack,
I'm not an aerodynamicist, but I stayed at a Howard Johnson's last night.

Not sure how much airflow you'll have flowing through the diffusers with the "fencing" around the perimeter. Purpose-built race cars supply diffuser air via ducts and holes in the bodywork to create the desired downforce. Or did you punch some holes in the front while we weren't looking? :)

Hope this works well. If it doesn't pan out, it doesn't matter. It's the thought process that's important.

Sherwood

I'm terribly interested to know the outcome, Jack. It does look darn cool.

And for all of the nay sayers, Jack does back his various experiments up with practical data. He logs laptimes.

Jack, yes, yarn would be fine. Other's have used yarn in the past.

An engine undertray on an aircooled hard working 911? Are you sure that's a good idea? Also how can you be sure that it won't create more lift?

I'm not worried so much about engine temps or lift, but more about drag, thats a lot of extra drag and weight being put onto the car, not to mention how it may change the handling characteristics. Jack you might have finally hit the point of diminishing return. But good luck, I hope I'm wrong. Worst comes to worst, it doesn't work out, you take it off and you're back to square one which is better than most peoples square two or three or fives :D

The issue regarding air cooled vs water cooled is a big deal.

The air cooled engine fan blows HUGE quantities of air through the cylinders and out the bottom of the engine. If this diffuser covers a portion of the bottom of the engine in such a way that it blocks the exit of this fan generated airflow the engine will not cool properly. Cylinder head temps could easily jump to dangerous levels. So the issue has nothing to do with the aero effects and is simply relative to cooling. Try having a friend hold your 911 at 5k revs then put your arm under the engine and see how much air is flowing out, you may be surprised at how much that fan pumps!

The Porsche factory removed the engine undertray on the 993 racecars.... if the aero benefits outweighed the cooling issues they would not have done this.

YMMV

I also vote that the use of an under-car diffuser is effective ( from an aero/downforce) standpoint...and contemporary race cars competing against the 964 and 993 series of cars used these. I presume Porsche understands the aero benefits yet couldn't use these for their AIR cooled cars...for cooling reasons.

Who knows why they're not used on 996/997 cars? We can't presume because they're not effective....

Wil

some race blueprints:
http://forums.pelicanparts.com/uploa...1112202994.jpg
http://forums.pelicanparts.com/uploa...1112203013.jpg

It is very difficult to install a truly functional rear diffuser on a rear engined car.
Just not enough room!

as far as my basic understanding on aerodynamics goes, i always thought that a diffuser only becomes interesting, if the bottom of the car has been taken care of , and is completely flat as a pan cake
then the airflow beneath the car , is allready aligned
something that's difficult to achieve on a rear engined car, with a less than flat bottom, and the engine blowing it's cooling air blowing down.

Yes! Although I wish it were otherwise.

Just to be clear, my expectations on the diffuser are nil.

I fabricated the more-aggressive splitter and side skirts, and I expect that by limiting air passage under the car, I'll see a benefit. There are loads of race cars out there doing the same, so this isn't that big a leap. My home-made skirts are probably less effective than commercially-available pieces, but mine are cheap and easy to remove for the drive home.

The flat plastic panel under my car was also an easy decision. My testing was by no means scientific, but it was less than $30, and produced an immediate reduction in my lap times. Remember that my car has an underbelly AC condenser, which (apparently) had a detrimental effect on the 911's under-car aerodynamics.

The diffuser was an afterthought. I already had the tools out.

My thinking is that if I'm able to come up with a fairly reliable way of measuring the effect of these pieces (as well as different wing settings and different tails) -- whether it be a couple of bicycle wheels or a set of strain gauges, wool tufts, or a combination of all three -- then I'll test everything I've got, including the diffuser.

But it's the one piece of the whole group that was done just for kicks.

It doesn't have to be as flat as a pancake (or as Kansas, which is now known to be flatter than a pnacake).... but the flow has to be pretty "flat" w/o many large eddies.

Now -- Jack how far are you willing to have stuff stick out behind the car? That will help with the issues above. You could even rename it "Langenblack."

Or if you don't like that one, "Black Booty II."

Some final pictures, before I take it back off:

http://forums.pelicanparts.com/uploa...1112209151.jpg

http://forums.pelicanparts.com/uploa...1112209167.jpg

Smooth, from head to tail:

http://forums.pelicanparts.com/uploa...1112209185.jpg

The next step is to find a way to monitor cylinder head temps, so that I can be sure the experiment doesn't do more harm than good.

Think about bashing the leading edge of those holes upwards -- for more air flow. Maybe NACA ducts would be good to use on the undertray also (or you can use the more advanced NASA ducts).

I think the Keebler elves could use that to slice cookie dough or something...

Jack, when do you sleep? or do you not work?

Jack - Seirously, how did you make it?

I actually angled up the trailing edge, which didn't make any sense to me (which might mean it has a chance of actually working). I copied the factory one I have as a model... sort of.

http://forums.pelicanparts.com/uploa...1112215421.jpg

I assume the factory did it this way in order to prevent a high-pressure pocket from forming?

http://forums.pelicanparts.com/uploa...1112215454.jpg

Do you think it might work? (Or am I more like a monkey who's found some old circuit boards and a suitcase, and thinks he can turn it all into a laptop computer?)

Jack, the slots on the sides of my 964 undertray are very important. They are there to let the air escape from the cylinder heads. The big middle openings are only for the cylinders.

You can see that I had drilled more holes in the undertray to aid in cooling before I simply took it off altogether.

That's neat looking - now you need a CF version...

Most of that area is already open. It's the part I labeled 'A' below. Should I drill out the corresponding area (labeled 'B') that remains?

http://forums.pelicanparts.com/uploa...1112217104.jpg

It's a cool idea, but I don't think the curve is gradual enough so I suspect the air is separating before it gets a chance to accelerate.

1987 Ferrari diffuser:

http://forums.pelicanparts.com/uploa...1112217291.jpg

Cool idea. I would recommend cutting up a kite (nylon, lightweight ripstop) and taping that to your car. Then I would get someone to follow me with a video camera in their car and record my car and the streamers on it. That will give you a good idea on laminar flow before you get to the track. There is an optimal curve for air (just like water) you might do a search on line, believe it a NACA prfile and you may be able to download profiles/templates before going further.

It makes sense now that i see the other side...

re monkey issues... Well, that is pretty much the way fluid dynamics works. Nothing is better than an educated guess b/c the fundamental equations have never been solved. All the models you see are simply numeric approximations...

Just be careful with the head temps. - I know you will be... And if not, your car will be even faster when the new 3.8L RS engine goes in...

put CHT's on the two heads that are not uncoverd... run it...(or all... to be safe)

i bet those will get the the most toasty...
you can always pit and remove the thing before the CHT's go way up...
or... bring a drill and add some hole's
then you can collect usefull data on just the sideskirts and the front...

but i think i am suffering from "not my car syndrome" or NMCS

or maybe "MWSFT"
(my wallet safe from this)
or maybe even "TSIBIDHTPTRBSIWTHTJDI"
(this sounds iffy but i dont have to pay the repair bill, so i will tell him to just do it)

I'm assuming that you mean since these racers don't have diffusers, why should Jack? I believe that they are not allowed to have diffusers in Le Mans GT classes, the bottom of the car must be flat. This is one of the reasons we have a maserati and not an Enzo in GT racing. If diffusers were allowed, I'm sure every one of them would have one.

Jack the diffuser looks great when do you going to test it?
about GT rules:
maserati have one
gt race car:
http://forums.pelicanparts.com/uploa...1112249440.jpg
http://forums.pelicanparts.com/uploa...1112249462.jpg
http://forums.pelicanparts.com/uploa...1112249498.jpg
st. car:
http://forums.pelicanparts.com/uploa...1112249610.jpg
enzo:
http://forums.pelicanparts.com/uploa...1112249715.jpg
ferrari CS:
http://forums.pelicanparts.com/uploa...1112249773.jpg
F50 LM
http://forums.pelicanparts.com/uploa...1112249835.jpg

Thanks. Track testing will be done on April 13th. I'll try to do some video with wool tufts sooner, but not before my new CHT gauge arrives.

Yellowbird,

After looking more, I think I'm half wrong. I remember hearing that the Maserati was a racing version of the Enzo and that they had to remove the diffuser, this is plainly wrong as you have pictures of them with diffusers. However, after looking at ALMS rules and finding some pictures it appears that the higher GTS class allows diffusers, but the GT class, which Bill's GT3RSR pictures were from does NOT allow diffusers. Check out the GT class 360 below. It doesn't have a diffuser. However, I'm sure if they allowed it, this car would have one and maybe the RSRs too.

So in summation: it seems GTS allows it, GT does not (from what I've gathered). Now GTS is called GT1 and GT is called GT2 (I think), I'm not sure if this policy with diffusers has continued.http://forums.pelicanparts.com/uploa...1112255541.jpg

I think you need a spot where you can exit hot engine/exhaust air (and the air you're scooping up from the openings under the engine)

If you look at the pics of the other cars, you can see they all have that (above the difuser)

How about putting some louvres (openings facing to the back) in the sloping part of your difuser?