I know top speed isn't the goal with our cars, but I'm wondering what and who has made changes that address the lift issue associated with the negative pressure immediately behind the targa top? Someone once told me these cars lift badly over 100 mph. Any ideas / solutions besides huge aerodynamic add ons?

That sounds like a question for the sanctioning bodies rule book…

here you go.....

http://www.cassidy-online.com/porsche914/a...aids/index.html

I'm not looking at this from a race perspective- fuel mileage and stability are my goals and focusing on stability. Out west on longer roads we get crosswinds that play havoc with semis and high profile vehicles, but I've also been thrown on the motorcycle a few times with wind. A light car could use some aids in the stability department. Think of it as channeling the air for maximum efficiency...
Think along the lines of the Ferrari 250 GTO with the cutouts behind the rear wheels.

Virtually identical in every respect,tho a little heavier.
http://www.corvairforum.com/forum/viewtopi...f=165&t=766

The air ,it gets under the car,and a huge induced drag penalty in the low pressure volume behind the targa.
The Daytona Cobra worked good and was significantly lower drag,not ploughing like the 427 Roadster.
Take a good look at the behind of the 956/962,an effective method of evacuating the bottom.
I always liked the Hall Chaparrals,vacuuming the car down,brilliant,worked so good they outlawed them.
When I placed the flat fan on the rear decklid of the GT14-12,it was for more than cooling.

Click to view attachment

This too is a low drag shape,a four cylinder.
https://www.google.ca/search?q=Hall+Chaparr...cgVB_YQ_AUIBigB

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I love this subject.

If I may, I would suggest that you think about your goal. Efficiency will not give you downforce and the downforce is not efficiency.

Efficiency will not necessarily help with crosswind stability. In my book, crosswind stability has more to due with caster unless your making significant aero changes and driving fast(+90mph)

unfortunately you must choose:
a. downforce
b. low drag ( efficiency )

Just my rambling thoughts....

In 10 years & 130k+/- of DD & long trip use 75-85, I never really had any big stability problems with mine on the freeways out west at the 75-100+ mph range, although the chances to do so were few & far between, this included the crosswinds in CA, NV, AZ, UT & with Semi's.

If you're looking for a more stock back-in-the-day type accessory/option, then the small ducktail spoiler on the rear trunk was what most folks used ... probably more for looks. Jeff Bowlsby's classic 914 website has pix of them.

However, that requires drilling the rear decklid to mount it, thereby introducing a new path for the tin worm - so I'd personally prefer to be quick at the wheel for a few gusts/trucks, than opening the rust door!

PS - if mpg is your goal, then stick with the stock 165HR15 (/80) tires at proper cold inflation, since wider & under-inflated tires will rob you of far more mpg, than will any high speed drag which you describe.

BTW, I did 126 mph on I-15 through Virgin River Gorge on a late Friday night run to a weekend Rugby Tournament in Park City Utah in the late 70's on Semperit M601 "Dogbone" 165HR15 tires with no traction/steering-control nor stability problems (doubt you could do it today with the traffic though) - & 75-85 the rest of the run from SoCal/OC. I did so for several years running when we played up there in Sept.

Cheers!
Tom
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Having driven a 914 in storms on Interstates with semi-trucks I can tell you that you can drive it with 1 hand on the wheel with no problem. It is not like the Beetle, or the 911. The 914 is already pretty low drag with the roof on and the windows rolled up. If you are that worried you can add a front air dam which would divert air from going beneath the car and direct it around the car. But the downside is that this can have an adverse effect on engine cooling…

It's not just the low pressure behind the targa top, it's also the large amount of air getting under the car at those speeds.

- Add a front spoiler that directs air around the front instead of under the car
- Add a rear diffuser under the engine and transmission
- Add a rear wing to create more downforce
- Add louvers to the top of your fenders

My aero plans are as follows in this order

1. Rear diffuser from the cabin back
2. Side skirts that go from the front wheels all the way back
3. Front splitter 8" in front of bumper

And the cars start to lift above 100, but it gets really bad above 150.

One of the things I've read to work is popping the top up at the back to make a slot.

Never noticed it being bad at 110, but that was in a straight line
I'd be interested in the diffuser designs, not sure how to route the engine cooling air with a diffuser. Maybe a semi blown diffuser like they banned in F1 would work.

I wonder if Mitsubishi Evo-like shark fins on the rear of the targa would help.

If looks are as important as performance, an LE front spoiler is your best bet in my opinion.
For the limited time my 914 spends at 100 mph or above the stability difference was very noticeable the first time I drove with it.

I'm sure it would. Add some vortex generators on the rear of the top and add a ducktail and you've got something.

Hmmm.... ...........









.

You can't do better than this report.

I'm running water cooled Subaru power so the engine cooling isn't an issue. I plan to run a front spoiler with a modest splitter, a pan over the open parts to keep the underflow smooth and modest side splitters for the flow also in addition to a rear lower diffuser. A rear spoiler of sorts, being careful not to create paths for rust worms in the process. I'll be running flares of a different kind and not looking like much else out there but enough to give room for 225s and 245s. I realize the drag/efficiency battle but there's got to be a compromise somewhere that works. I was thinking about cutting channels is the back of the top between the reinforcement ribs at an angle so they would let air from the roof bleed through to the rear deck, filling in the vacuum that way. Still have to look at that. I have other ideas also including fans- and all these ideas and comments are giving me more ideas! Thanks!

Click to view attachment

The effectiveness of the vertical tailfins on the LMPs can be seen here,separating the helical flows.
Also useful for signage.

Except for CFD with moving wheels. In many cars airflow influenced by wheel movement is a really big deal (especially open wheel, but also closed wheel).

If you're putting a large spoiler of some kind on you absolutely need end plates like Veekry noted. Your downforce vs. drag efficiency is really affected by spillage off the sides of the wing.

https://www.youtube.com/watch?v=FW5HtJ5tou8


Feel the air.

Looking at their speed and the way the hill flattens at the bottom is giving my legs cramps from the compression!

Google "airtabs" and see vortex generators at work. I don't know if they work on a car but they add to the psych value.

Vortexes are drag. VGs can be useful to energize the boundary layer if you put them in just the right places, but for the most part they just add drag.

AirTabs seem to be one of the worse offenders, as they promise a lot and practically never deliver.

--DD

Lots of opinions on.this,topic, but this study has the most factual evidence I have ever seen regarding how the 914 behaves.
We are not talking about other cars here in this question.....

Wow, you're not kidding. They covered most of my ideas except for venting the wheelwells for the turbulence. They even covered my idea of slots in the rear pillars and got some good results there. The lengthening of the roof came off like I expected with the added length it only creates more drag. I wish they had addressed the rear diffuser possibilities and I have a couple of other ideas that follow along their lines of thinking- I'd love to get these folks in a room for a discussion culminating the sum of thought to arrive with the best compromise of results. A bit hard to get through after finishing my first three documents today- my eyes AND brains are slightly fried... Thank heavens for the weekend! And thanks for the link to those studies! And yeah, pretty damn good for a 45+ year old design...

Well,this is a big topic that needs great study to achieve any significant results.
The design feature of the greatest drag is the boat's prow front end,ploughing through the air and creating a giant pillow of high pressure on which the 914 rides.
Separating the air paths into smooth transitions with little turbulence is the task at hand.
This is a good place to start,and what a satisfying result you could make.
CFD for u.
https://www.google.ca/webhp?sourceid=chrome...8#q=linux%20cfd
Design your own ground effects 914.


Go ahead,make your clay.

Slots in the sides of the targa bar? Who knew? If the one made angled cuts to help channel air to the rear grill and relieving the low pressure zone could be the shizzle. Whose going first?

ralph did 170mph

True but I'm not a fan of driving on public roads with the Ginther style windscreen- this has to be a road legal DD.... and the vortex generators on GRNMNEE are questionable in their effectiveness if not really cool looking! I've always like the looks of that car!

I don't think they'd have to be as big as the ones in the illustration from their test either. Maybe smaller ones more horizontal than vertical and closer to the body line to take advantage of the airflow line in the stock configuration.

Very true, but, if you could energize the boundary layer to make the rear spoiler effective you can add a bit of down force on the rear of the car to add stability at speed. It's always a tradeoff. What's more important, limiting drag or down force?

It all depends on how much HP you have and how light your car gets at high speeds.

I still think the biggest thing we can do for increased rear stability is a proper diffuser and side skirts.

I plan on trying out an airport run when my car is all set and done and I will have the HP and gearing for 200mph, the question is aero, and specifically the rear.

That's what I'm thinking also. The engine and gearing I've got is good for 150 plus- not that I'll ever drive it that fast but I want it planted and relieved of the negative pressure that can upset it at speed. It would help with mileage also being easier to move through the air without looking like a Hot Wheels car. I think subtle changes could make a big difference.

But you cannot add downforce with a spoiler. It is called a spoiler because it spoils lift; if you are after downforce you need a wing…

It's called a spoiler because it "spoils" unfavorable air flow. I'm not going to argue semantics.

Click to view attachment

https://www.google.ca/search?q=porsche+race...tGs5e5_vl1SM%3A

Look to father Porsche for your aero ambitions,knowing they got it right.
An online course for the study of the science of fluid dynamics is available for your learning curve.
A faddish '70s applique of 'spoilers' which merely adds weight and trips the flow never did anything for me or the 914.
The IMSA 914 with the plywood looking plank fastened to the front doesn't do it either,never did.
The aero problem with the 914 is how do you make the airflow work like the 917's.
That is your mission ,if you so choose to accept it.
Get ready,get set.


Click to view attachment

http://racingcardynamics.com/race-car-aero...-air-lap-times/
http://www.tunersgroup.com/engineering/simon_mcbeath.html

Click to view attachment

UH-OH,this won't end well.Nuts,another wreck on my resume,a LeMans 'flyer'.

/

These are not semantics, these are engineering principles…

Click to view attachment
'72 LeMans Class Winner,aero worked great.

It is possible that a 914 could make the transformation,with some cutting and hacking.

Ok, I'll put it this way. Spoilers reduce up lift at the tail of the vehicle. I suppose an engineer would need to see a positive number on his/her spreadsheet before they would quantify the results as being down force. Although much improved with a spoiler, still being in the negative they would say the vehicle has less up lift.

My feeble mind believes if you have less of one you must have more of the other. What it's called is irrelevant to me when the net result is more weight over the rear of the car.

Click to view attachment

Smooth as a teardrop.
http://www.wired.com/2014/09/german-aerodynamic/
http://www.thetruthaboutcars.com/2010/02/a...ynamics-part-2/

Click to view attachment

A brick with a plywood plank up front.
Some racers didn't read the manual,probably not a sharp pencil nor a pilot.
Looking around for the fastest cars and what works,it is a matter of adopting a few features that will integrate with the 914.
The compromises that Porsche were constrained by can be disregarded for the speed/hp you would want.

https://www.google.ca/search?q=solar+racer&...S72BCMQ_AUIBigB
https://www.google.ca/search?q=fuel+efficie...reamliner+racer

You do realize your “Smooth as a teardrop” example is a perfect wing cross section. You now have designed in lift.

The “plywood plank up front” diverts air around the car. That is what you want. Until you are able to add tunnels under the car to speed up the under car air you are always going to have the air over car traveling faster than the air under the car. Air above the car moving faster than air beneath is lift, air above the car moving slower than air beneath the car is downforce…

Click to view attachment

Is this the fastest Porsche 914 hybrid/GT1?I like what they did,tho I believe it is too heavy.

https://books.google.ca/books?id=yMJFvrORtz...ity&f=false

Kicking a few ideas around,working with cadcam,cnc in many fields for four decades plus.
Robotics for manufacturing,aerospace hardware from the StearmanPT-17 and T-33,to contemporary.
Gonna build a clean fourteen machine in a way untried and I hope effective,we'll see.
The suspension for sure is due for an update,to exploit the new rubber of today.

http://www.914world.com/bbs2/index.php?showtopic=222522

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The drawing states the name of the designer,the date and the city.
Really,any person who has studied hydrodynamics has studied this drawing.
The beginning of aero theories as applied to automotive applications.


https://www.youtube.com/watch?v=OcnQuzpBkWQ

https://www.youtube.com/watch?v=fkZwGa50eqM
https://www.youtube.com/watch?v=PfcII0AIW0c
https://www.youtube.com/watch?v=myxMudtFX6g

As a personal test might I suggest you stand up in the passenger seat of your 914 holding a plank of similar size.
Have you wife or girlfriend drive up the Cabrillo hwy at a speed you think you can handle.
Good luck.

Click to view attachment

Nicki Lauda 3 time F1 Champ

Click to view attachment
Someone has scanned the R+T article from '79 or thereafter.
The BB was the Buchmann+Buchmann brothers,German hotrodders.


https://www.google.ca/search?q=isdera+comme...StQkcRfBgozM%3A

http://isdera.de/
Some 32 yrs ago.Very nice designs.

/

Here's an example of a "blogger" who hasn't done the simplest search and in the absence of effort and facts
made shit up,a blatant obfuscation,a lie.

http://blog.iso50.com/17004/1978-mercedes-benz-study-cw311/

Claiming to have designed a Pininfarina Ferrrari would have him prosecuted,as has happened before,not long ago.
Intellectual copyright laws exist to protect those rights,so you can't just attribute someone's work as another's.
All very cute,took photos that someone else took,and typed bullshit for some unknown motivation.

Here's another,without regard for the actual designer,labelling it with his own.

http://humster3d.com/2016/01/15/mercedes-benz-cw311-1979/

Fairly easy to hit a sitting duck,clean out of the water.
They might be morons.

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I wonder, has anyone ever taken a 914 to the salt flats for speed week?
What did they do for aerodynamics?

But very little drag. The teardrop shape (with a particular length-to-width ratio!) produces the lowest drag.

So the question is, what are you aiming for? Maximum downforce? Minimum drag? Some compromise between the two?

BTW, the stock-type engines really do need the forward air-flow along the rear deck; that is how cooling air get to the engine!

--DD

Click to view attachment
http://jnkdesignworks.blogspot.ca/

A lifting body has drag.The effective wing section in 1937 was..10mm wide,still is today.
A cd of 0.15 is why the fuel efficiency was so great,the largest significant factors were the R# and the wetted area.
The vehicle was light,an aircraft monocoque of aluminum.
Easy to test,make a 3' scale model in foam and epoxy glass and drive it the same way on the PCH.
Try to duplicate the conditions as closely as you can holding it up like the plywood.

https://www.google.ca/search?q=aerodynamic+...s+a+wind+tunnel