Poor Man's Aero Package, Version 2

[Jack Olsen]

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.





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.

[Craig 930 RS]

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!

[Bill Verburg]

Umm.....well those cars are water cooled.....!?

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

[Zeke]

Originally posted by Bill Verburg
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.

[Tspringer]

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

[kaisen]

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

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.

[Bill Verburg]

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.

[kaisen]

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

[Bill Verburg]

I can't claim to be an aerodynamicist, and although I love it when rocket scientists debate laminar flow coefficients on these forums,

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

[Rob Channell]

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?

[911pcars]

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

[masraum]

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?

[ScottKelly911]

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

[Tspringer]

Originally posted by Bill Verburg
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.

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

[Wil Ferch]

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

[Yellowbird RS]

some race blueprints:

[Craig 930 RS]

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

[svandamme]

Originally posted by Craig911
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.

[Craig 930 RS]

Yes! Although I wish it were otherwise.