Aero enhancements..., What changes work besides huge wings? Options

[mgp4591]

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?

[damesandhotrods]

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

[johnhora]

here you go.....

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

[mgp4591]

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... (IMG:style_emoticons/default/idea.gif)
Think along the lines of the Ferrari 250 GTO with the cutouts behind the rear wheels.

[veekry9]

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. (IMG:style_emoticons/default/evilgrin.gif)



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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[FastFroggy]

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....

[Tom_T]

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! (IMG:style_emoticons/default/beerchug.gif)
Tom
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[damesandhotrods]

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…

[SirAndy]

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?

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

(IMG:style_emoticons/default/popcorn[1].gif)

[Andyrew]

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

[Andyrew]

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

[yeahmag]

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

[JmuRiz]

Never noticed it being bad at 110, but that was in a straight line (IMG:style_emoticons/default/biggrin.gif)
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.

[cwpeden]

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.

[jmill]

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

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

[Olympic 914]

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

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.... ........... (IMG:style_emoticons/default/idea.gif)









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[r_towle]

here you go.....

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

You can't do better than this report.

[mgp4591]

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!

[veekry9]

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

[jd74914]

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

You can't do better than this report.

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.