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PostPosted: Sun May 10, 2020 5:25 am 
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Posts: 108
All this blah-blah about flares and aerodynamics, yet a quick search on the board here and I can't find any discussion about something I'm curious.

Targa car: open or closed?

You're likely aware that targa cars can be run in Prod as either closed-top or open-top. Closed top and you bolt on the targa roof and keep the full windshield; closed top and you remove the top, rear glass, and windshield, and run similar to a convertible (with the targa roof bar still in place).

Most competitive convertibles prepped for Prod run open-top; Miata, for example. Sure, they're out there, but I can't think of the last time I saw a pointy-end Miata running a hard top. Is that because the open-top cars are stronger prep and being driven by stronger drivers, or is that actually faster? VW Rabbit convertibles seem to be the rage lately in HP; is that also better than the big box?

Back in the days when Porsche 914s were a rage in FP (mid-00s?) most of them were running closed-top though open-tops were speckled in. Was one better than the other? I've seen Fiat X-1/9s run both ways, too.

When I'm running vintage events on nice sunny days, I prefer to run the 914 with the top off. It makes getting in and out easier, plus I can more-easily wave at my adoring fans. But maybe I'm running Regionals and I feel the need for a tad more aero performance, I'll toss the top on and deal with the ingress and egress hassles. Of course, if I go out when it's raining, the top is on.

But "what if" I wanted to build a targa car to be as competitive as possible...? Which configuration, all else being equal, would be faster? What are some of the other factors to consider?

GA


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PostPosted: Sun May 10, 2020 6:12 am 
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I have no experience with convertibles or Targas but I would think that the lower CG would be an advantage without the top on.
Ken Kannard runs with a top on his Miata and finished 3rd at the Runoffs.

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PostPosted: Sun May 10, 2020 6:19 am 
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Location: Boston, Ma
Greg, you're a long time racer and aviator. I expect you know these answers.

Frontal area and rear facing shape. (there's a reason you guys sit in Cessnas for hours with your shoulders smashed into each other.)

I think you answered your performance question with the open top Miatas. (at least until you can make your roof line look like an Audi R18 or Mazda 787B)

With the windshield installed, the rear glass installed, and the top off, you are just asking for a high pressure area (drag) in front of each piece of glass and a low pressure area (drag) in back of each piece of glass. The cooling fan sucking on the back side of the rear glass just makes it worse. (if you sucked air from below the car in blew it out in the area behind the rear glass, that would better, but you'd have to run the numbers on the horsepower the fan took)

The only reason I chose a hard top car is safety. I have gotten tired of my face being the first thing that something can hit.

Any more detail than this and I'll have to charge you by the hour. :lol:

I swear there was a thread about open vs. closed Miata performance on here.

Do we need a rule giving open cars a weight penalty relative to their closed configuration??? :shock:

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PostPosted: Sun May 10, 2020 8:01 am 
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Location: Huntsville, AL
Cut the windshield and roof off all together. Full open top. 914s are sexy with no windshield and some flares.
Image

If you aren't force feeding air to the front of the fan, removing the roof will destroy the low pressure area behind the rear glass. Plus you get lift over 120ish with the standard configuration.

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PostPosted: Sun May 10, 2020 9:53 am 
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I'm going to go out on a limb and say "it depends". :)

Open will presumably have a lower frontal area and higher drag coefficient due to open top and dirty shape of tubes and such. I think it's anybody's guess without testing (hint hint) as to which will give a lower drag product, and I'd bet that it varies a lot car to car. With a VW Rabbit you are cutting off a LOT of square inches of a really boxy shape to go to a cabrio. With a Miata the size of the missing part (hardtop vs open/no windscreen) is far less, as is also true on the 914.

Open also has (presumably) a lower CG but the "missing roof" on some cars (like the 914 I'd presume) doesn't actually weigh much, ditto for the windows if polycarbonate. But if the driver feels safer he'll probably go faster.

Bottom line is that the 914 in particular looks much cooler open, like the pic above. OTOH, closed has a retro "914-6 at Sebring mid 70s" look. So take your pick!


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PostPosted: Sun May 10, 2020 10:36 am 
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"Back in the days when Porsche 914s were a rage in FP (mid-00s?) most of them were running closed-top though open-tops were speckled in. Was one better than the other? I've seen Fiat X-1/9s run both ways, too."

Greg,

With all respect, your observations are incorrect at least as it applies to the top 914's in the country.

At the 2005 Runoffs there were eight (8) 914's and seven (7) of them ran open tops including the eventual FP winner which was prepared by Dave Finch with an alleged unlimited budget. As an engineer to the automotive giants I'm quite certain that Mr. Finch would have and could have chosen a closed top version if it were deemed beneficial.

I think the choice almost becomes a personal one. Not unlike FP Miata's most of which have been built around the open top concept, there are exceptions
some of which have also proven to be competitive.

I once stumbled upon a wind tunnel test of a 914 scale model conducted by some university in CA. Using a variety of aero configurations I recall the most beneficial mods were running the hardtop with windows up and wheel opening covers both of which you know are not generally permitted.

In the early days of IMSA(70's) etc, closed tops seemed to be favored I suspect because of the long endurance races such as the likes of Daytona, Sebring and Le Mans.


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Tom Burdge


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PostPosted: Sun May 10, 2020 11:58 am 
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3 big leaf blowers , some yarn, a phone and you have aero data logging . Try to see how much air lands inside of the car without the roof. A lot depends on the windshield angle.

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PostPosted: Sun May 10, 2020 2:45 pm 
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Joined: Mon Feb 04, 2013 9:13 pm
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Before Ken Kannard had his current closed top F prod Miata he ran an open top one and also did quite well. Jessie Prather answered this question a while back on which was faster and the pros and cons.

Paul


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PostPosted: Mon May 11, 2020 5:03 am 
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Location: Atlanta Ga
Check this link. There is more on 914world.com. http://www.cassidy-online.com/porsche91 ... index.html

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FP 1989 Honda Civic 4dr


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PostPosted: Mon May 11, 2020 6:49 am 
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At Daytona, from the mid-point between NASCAR 3 and 4 through to the braking zone for Turn 1, Kennard's hardtop Miata pulled noticeably better than Campbell's and Prill's roadster Miata's. That's about the only apples to apples comparison I've ever really got to witness first hand. You can't argue that the roadster will have a lower CG, and IMO the hardtop will have better aero as speeds reach triple digits. For me personally, I just like having something substantial between my head and stuff hitting it.

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ruckracing honda's:
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